Network+ Guide to Networks, Chapter 3 Review
Transmission Basics and Networking Media
Transmission Basics
In data networking, the term transmit
means to issue signals along a network medium such as a cable. Transmission
refers to either the process of transmitting or the progress of signals after
they have been transmitted. In other words, you could say, “My NIC
transmitted a message, but because the network is slow, the transmission took
10 seconds to reach the server.” In fact, NICs both transmit and receive signals;
which means they are a type of transceiver. Long ago, people
transmitted information across distances via smoke or fire signals. Needless to
say, many different methods of data transmission have evolved since that time.
The transmission techniques in use on today’s networks are complex and varied.
In the following sections, you will learn about some fundamental
characteristics that define today’s data transmission. In later chapters, you
will learn about more subtle and specific differences between types of data
transmission.
Analog
and Digital Signaling
One important characteristic of data
transmission is the type of signaling involved. On a data network, information
can be transmitted via one of two signaling methods: analog or digital. Computers
generate and interpret digital signals as electrical current, the pressure of
which is measured in volts. The strength of an electrical signal is
directly proportional to its voltage. Thus, when network engineers talk
about the strength of a signal, they often refer to the signal’s voltage. Once
the electrical signal leaves a computer, it can travel over copper cabling as
electrical current, over fiber-optic cable as light pulses, or through the air
as electromagnetic waves. Analog data signals are also generated as voltage.
However, in analog signals, voltage varies continuously and appears as a wavy
line when graphed over time.
An analog signal, like other waveforms, is
characterized by four fundamental properties:
Amplitude - A wave’s amplitude
is a measure of its strength at any given point in time. On a wave
graph, the amplitude is the height of the wave at any point in time.
Frequency - Whereas
amplitude indicates an analog wave’s strength, frequency is the a, through its
highest amplitude and its lowest amplitude, and back to its starting point over
a fixed period of time. Frequency is expressed in cycles per second,
or hertz (Hz), named after German physicist Heinrich Hertz, who experimented
with electromagnetic waves in the late nineteenth century. Frequencies used to
convey speech over telephone wires fall in the 300 to 3300 Hz range. Humans can
hear frequencies between 20 and 20,000 Hz. An FM radio station may use a
frequency between 850,000 Hz (or 850 kHz) and 108,000,000 Hz (or 108 MHz) to
transmit its signal through the air. You will learn more about radio
frequencies used in networking later in this chapter.
Wavelength - The distance
between corresponding points on a wave’s cycle—for example, between one peak
and the next—is called its wavelength. Wavelengths can be expressed
in meters or feet. A wave’s wavelength is inversely proportional
to its frequency. In other words, the higher the frequency, the shorter the
wavelength. For example, a radio wave with a frequency of 1,000,000 cycles per
second (1 MHz) has a wavelength of 300 meters, whereas a wave with a frequency
of 2,000,000 Hz (2 MHz) has a wavelength of 150 meters.
Phase - The term phase
refers to the progress of a wave over time in relationship to a fixed point. Suppose two
separate waves have identical amplitudes and frequencies. If one wave starts at
its lowest amplitude at the same time the second wave starts at its highest
amplitude, these waves will have different phases. More precisely, they will be
180 degrees
out of phase (using the standard assignment of 360 degrees to one complete
wave). Had the second wave also started at its lowest amplitude, the two waves
would be in phase.
One
benefit to analog signals is that, because they are more variable than digital
signals, they can convey greater subtleties with less energy. For example,
think of the difference between your voice and a digitally simulated voice,
such as the automated service that some libraries as a poorer quality than your
own voice—that is, it sounds more like a machine. It can’t convey the full range of
tones and subtle inflections that you expect in a human voice. Only very
high-quality digital signals—for example, those used to record music on
compact discs—can achieve such accuracy. One drawback to analog signals is
that their voltage is varied and imprecise. Thus, analog transmission is more
susceptible to transmission flaws such as noise, or any type of interference
that may degrade a signal, than digital signals. If you have tried to
listen to AM radio on a stormy night, you have probably heard the crackle and
static of noise affecting the signal.
Digital
signals are composed of pulses of precise, positive voltages and zero voltages.
A pulse of positive voltage represents a 1. A pulse of zero voltage (in other
words, the lack of any voltage) represents a 0. The use of 1s and 0s to
represent information is characteristic of a binary system. Every pulse in the
digital signal is called a binary digit, or bit. A bit can have only
one of two possible values: 1 or 0. Eight bits together form a byte. In broad
terms, one byte carries one piece of information. For example, the byte
01111001 means 121 on a digital network.
Computers
read and write information—for example, program instructions, routing
information, and network addresses—in bits and bytes. When a number is
represented in binary form (for example, 01111001), each bit position, or
placeholder, in the number represents a specific multiple of 2. Because a byte
contains 8 bits, it has eight placeholders. When counting placeholders in a
byte, you move from right to left. The placeholder farthest to the right is
known as the zero position; the one to its left is in the first position, and
so on. The placeholder farthest to the left is in the seventh position.
To
find the decimal value of a bit, you multiply the 1 or 0 (whichever the bit is
set to) by 2x, where x equals the bit’s position. For example, the 1
or 0 in the zero position must be multiplied by 2 to the 0 power, or 20,
to determine its value. Any number (other than zero) raised to the power of 0
has a value of 1. Thus, if the zero-position bit is 1, it represents a value of
1×20, or 1 × 1, which equals 1. If a 0 is in the zero position, its
value equals 0 × 20
or
0 × 1, which equals 0. In every position, if a bit is 0, that position
represents a decimal number of 0.
To
convert a byte to a decimal number, determine the value represented by each
bit, and then add those values together. If a bit in the byte is 1 (in other
words, if it’s “on”), the bit’s numerical equivalent in the coding scheme is
added to the total. If a bit is 0, that position has no value and nothing is
added to the total.
The
binary numbering scheme may be used with more than eight positions. However, in
the digital world, bytes form the building blocks for messages, and bytes
always include eight positions. In a data signal, multiple bytes are combined
to form a message. If you were to peek at the 1s and 0s used to transmit an
entire e-mail message, for example, you might see millions of 0s and 1s passing
by. A computer can quickly translate these binary numbers into codes, such as
ASCII or JPEG, that express letters, numbers, and pictures. Converting between
decimal and binary numbers can be done by hand, as shown previously, or by
using a scientific calculator, such as the one available with any of the
Windows operating systems.
Take, for example, the number 131. To
convert it to a binary number:
1.
Click the Start button, select All Programs, select Accessories, and then select Calculator.
The
Calculator window opens.
2.
On a computer running Windows XP or Windows Vista, click View, then click Scientific.
On
a computer running Windows 7, click View,
then click Programmer. Verify
that the
Dec option button is selected.
3.
Type 131, and then click the Bin
option button. The binary equivalent of the number
131,
10000011, appears in the display window.
You
can reverse this process to convert a binary number to a decimal number. If
you’re connected to the Internet and using a Web browser, you can quickly
convert binary and decimal numbers by using Google calculator. Go to
www.google.com, then type in the number you want to convert, plus the format,
in the search text box. For example, to convert the decimal number 131 into
binary form, type “131 in binary” (without the quotation marks), and then press
Enter. You see the following result: 131 = 0b10000011. The prefix “0b”
indicates that the number is in binary format. To convert a binary number into
decimal form, type “0b” (without the quotation marks) before the binary number.
For example, entering “0b10000011 in decimal” (without the quotation marks)
would return the number 131.
Because digital transmission involves sending and
receiving only a pattern of 1s and 0s, represented by precise pulses, it is
more reliable than analog transmission, which relies on variable waves.
In addition, noise affects digital transmission less severely. On the other hand,
digital transmission requires many pulses to transmit the same amount of
information that an analog signal can transmit with a single wave.
Nevertheless, the high reliability of digital transmission makes this extra
signaling worthwhile. In the end, digital transmission is more efficient than
analog transmission because it results in fewer errors and, therefore, requires
less overhead
to compensate for errors.
Overhead is a term used by networking
professionals to describe the nondata information that must accompany data
for a signal to be properly routed and interpreted by the network. For
example, the Data Link layer header and trailer, the Network layer addressing
information, and the Transport layer flow-control information added to a piece
of data in order to send it over the network are all part of the transmission’s
overhead. It’s important to understand that in both the analog and digital
worlds, a variety of signaling techniques are used. For each technique,
standards dictate what type of transmitter, communications channel, and
receiver should be used.
For
example, the type of transmitter (NIC) used for computers on a LAN and the way
in which this transmitter manipulates electric current to produce signals is
different from the transmitter and signaling techniques used with a satellite
link. Although not all signaling methods are covered in this book, you will learn
about the most common methods used for data networking.
Data Modulation
Data relies almost exclusively on digital
transmission.
However, in some cases the type of connection your network uses may be capable
of handling only analog signals. For example, telephone lines are designed to
carry analog signals. If you connect to your ISP’s network via a
telephone line, the data signals issued by your computer must be converted into
analog form before they get to the phone line. Later, they must be converted
back into digital form when they arrive at the ISP’s access server. A modem
accomplishes this translation. The word modem reflects this device’s
function as a modulator/demodulator—that is, it modulates digital signals into analog
signals at the transmitting end, then demodulates analog signals into digital
signals at the receiving end.
Data
modulation is a technology used to modify analog signals to make them suitable
for carrying data over a communication path. In modulation, a simple wave,
called a carrier wave, is combined with another analog signal to produce a
unique signal that gets transmitted from one node to another. The carrier wave
has preset properties (including frequency, amplitude, and phase). Its purpose
is to help convey information; in other words, it’s only a messenger. Another
signal, known as the information or data wave, is added to the carrier wave.
When the information wave is added, it modifies one property of the carrier
wave (for example, the frequency, amplitude, or phase). The result is a new,
blended signal that contains properties of both the carrier wave and added
data. When the signal reaches its destination, the receiver separates the data
from the carrier wave.
Modulation
can be used to make a signal conform to a specific pathway, as in the case of FM
(frequency modulation) radio, in which the data must travel along a particular
frequency.
In
frequency modulation, the frequency of the carrier signal is modified by the
application of the data signal. In AM (amplitude modulation), the amplitude of
the carrier signal is modified by the application of the data signal.
Modulation may also be used to issue multiple signals to the same
communications channel and prevent the signals from interfering with one
another.
Simplex, Half-Duplex, and
Duplex
Data
transmission, whether analog or digital, may also be characterized by the direction in
which the signals travel over the media. In cases in which signals may
travel in only one direction, the transmission is considered simplex.
An example of simplex communication is a football coach calling out orders to his
team through a megaphone. In this example, the coach’s voice is the
signal, and it travels in only one direction—away from the megaphone’s mouthpiece
and toward the team. Simplex is sometimes called one-way, or
unidirectional, communication. In half-duplex transmission, signals
may travel in both directions over a medium but in only one direction at a time.
Half-duplex systems contain only one channel for communication, and that
channel must be shared for multiple nodes to exchange information. For example,
a
walkie-talkie or an apartment’s intercom system that requires you to press a
“talk” button to allow your voice to be transmitted uses half-duplex
transmission. If you visit a friend’s apartment building, you press the “talk”
button to send your voice signals to his apartment.
When
your friend responds, he presses the “talk” button in his apartment to send his
voice signal in the opposite direction over the wire to the speaker in the
lobby where you wait. If you press the “talk” button while he’s talking, you
will not be able to hear his voice transmission. In a similar manner, some
networks operate with only half-duplex capability.
When signals are free to travel in both directions
over a medium simultaneously, the transmission is considered full-duplex. Full-duplex
may also be called bidirectional transmission or, sometimes, simply duplex.
When you call a friend on the telephone, your connection is an example of a full-duplex
transmission because your voice signals can be transmitted to our friend at
the same time your friend’s voice signals are transmitted in the opposite direction
to you.
In other words, both of you can talk and hear each other simultaneously.
Full-duplex
transmission is also used on data networks. For example, Ethernet networks achieve
full-duplex transmission using multiple channels on the same medium. A channel
is a distinct communication path between nodes, much as a lane is a distinct
transportation path on a freeway. Channels may be separated either logically
or physically. You will learn about logically separate channels in the
next section. An example of physically separate channels occurs when
one wire within a network cable is used for transmission while another wire is
used for reception. In this example, each separate wire in the medium
allows half-duplex transmission. When combined in a cable, they form a medium
that provides full-duplex transmission. Full-duplex capability increases the speed
with which data can travel over a network. In some cases—for example, when
providing telephone service over the Internet—full-duplex data networks are a
requirement.
Many
network devices, such as modems and NICs, allow you to specify whether the
device should use half- or full-duplex communication. It’s important to know
what type of transmission a network supports before installing network devices
on that network. Modern NICs use full-duplex by default. If you configure a computer’s NIC to use half-duplex while the rest of
the network is using full-duplex, for example, that computer will not be able
to communicate on the network.
Multiplexing
A form of transmission that allows multiple signals
to travel simultaneously over one medium is known as multiplexing. To carry
multiple signals, the medium’s channel is logically separated into multiple smaller
channels, or subchannels. Many different types of multiplexing are
available, and the type used in any given situation depends on what the media, transmission,
and reception equipment can handle. For each type of multiplexing, a device that
can combine many signals on a channel, a multiplexer (mux), is required at the
transmitting end of the channel. At the receiving end, a demultiplexer (demux)
separates the combined signals and regenerates them in their original form. Networks
rely on multiplexing to increase the amount of data that can be transmitted in
a given time span over a given bandwidth.
One
type of multiplexing, TDM (time
division multiplexing), divides a channel into multiple intervals
of time, or time slots. It then assigns a separate time slot to every node on
the network and, in that time slot, carries data from that node. For example,
if five stations are connected to a network over one wire, five different time
slots are established in the communications channel. Workstation A may be
assigned time slot 1, workstation B time slot 2, workstation C time slot 3, and
so on. Time slots are reserved for their designated nodes regardless of
whether the node has data to transmit. If a node does not have data to
send, nothing is sent during its time slot. This arrangement can be inefficient
if some nodes on the network rarely send data.
Statistical multiplexing is similar to
time division multiplexing, but rather than assigning a separate slot to each
node in succession, the transmitter assigns slots to nodes according to priority and need. This
method is more efficient than TDM, because in statistical multiplexing time
slots are unlikely to remain empty. To begin with, in statistical multiplexing,
as in TDM, each node is assigned one time slot. However, if a node doesn’t use
its time slot, statistical multiplexing devices recognize that and assign its
slot to another node that needs to send data. The contention for slots may be
arbitrated according to use or priority or even more sophisticated factors,
depending on the network. Most important, statistical multiplexing maximizes
available bandwidth on a network.
FDM (frequency division multiplexing) is a type of
multiplexing that assigns a unique frequency band to each communications subchannel.
Signals are modulated with different carrier frequencies, and then multiplexed
to simultaneously travel over a single channel. The first use of FDM was in the
early twentieth century when telephone companies discovered they could send
multiple voice signals over a single cable. That meant that, rather than
stringing separate lines for each residence (and adding to the urban tangle of
wires), they could send as many as 24 multiplexed signals over a single
neighborhood line. Each signal was then demultiplexed before being brought
into the home. Now, telephone companies also multiplex signals on the phone
line that enters your residence. Voice communications use the frequency band of
300–3400 Hz (because this matches approximately the range of human hearing),
for a total bandwidth of 3100 Hz. But the potential bandwidth of one phone line
far exceeds this. Telephone companies implement FDM to subdivide and send
signals in the bandwidth above 3400 Hz. Because the frequencies can’t be heard,
you don’t notice the data transmission occurring while you talk on the telephone.
Different
forms of FDM exist. One type is used in cellular telephone transmission and another
by DSL
Internet access (you’ll learn more about DSL in Chapter 7). WDM (wavelength division multiplexing) is
a technology used with fiber-optic cable, which enables one fiber-optic
connection to carry multiple light signals simultaneously. Using
WDM,
a single fiber can transmit as many as 20 million telephone conversations at
one time. WDM can work over any type of fiber-optic cable. In the first
step of WDM, a beam of light is divided into up to 40 different carrier waves,
each with a different wavelength (and, therefore, a different color). Each
wavelength represents a separate transmission channel capable of transmitting
up to 10 Gbps. Before transmission, each carrier wave is modulated with a
different data signal. Then, through a very narrow beam of light, lasers issue
the separate, modulated waves to a multiplexer. The multiplexer combines all of
the waves, in the same way that a prism can accept light beams of different
wavelengths and concentrate them into a single beam of white light. Next,
another laser issues this multiplexed beam to a strand of fiber within a
fiber-optic cable. The fiber carries the multiplexed signals to a receiver,
which is connected to a demultiplexer. The demultiplexer
acts as a prism to separate the combined signals according to their different
wavelengths (or colors). Then, the separate waves are sent to their destinations
on the network. If the signal risks losing strength between the multiplexer and
demultiplexer, an amplifier might be used to boost it.
Most
modern fiber-optic networks use a type of WDM called DWDM
(dense wavelength division multiplexing). In DWDM, a
single fiber in a fiber-optic cable can carry between 80 and 160 channels.
It achieves this increased capacity because it uses more wavelengths for signaling.
In
other words, there is less separation between the usable carrier waves in DWDM
than there is in the original form of WDM. Because of its
extraordinary capacity, DWDM is
typically used on high-bandwidth or long-distance WAN links, such as
the connection between a large ISP and its (even larger) network service
provider.
Relationships between Nodes
So far you have learned about two important characteristics of data
transmission: the type of signaling (analog or digital) and the direction in
which the signal travels (simplex, half-duplex, full-duplex, or multiplex).
Another important characteristic is the number of senders and receivers, as
well as the relationship between them. In general, data communications may involve
a single transmitter with one or more receivers, or multiple transmitters with
one or more receivers. The remainder of this section introduces the most common
relationships between transmitters and receivers.
When a data transmission involves only one transmitter
and one receiver, it is considered a point-to-point transmission. An office building in Dallas exchanging data
with another office in St. Louis over a WAN connection is an example of point-to-point
transmission. In this case, the sender only transmits data that is
intended to be used by a specific receiver.
By contrast, point-to-multipoint
transmission involves one transmitter and multiple receivers.
Point-to-multipoint arrangements can be separated into two types: broadcast and nonbroadcast. Broadcast transmission involves one
transmitter and multiple, undefined receivers. For example, a radio
station indiscriminately transmitting a signal from its antenna on a tower to
thousands of cars with radio antennas uses broadcast transmission. A broadcast
transmission sends data to any and all receivers, without regard for which
receiver can use it. Broadcast
transmissions are frequently used on both wired and wireless networks because
they are simple and quick. They are used to identify certain nodes, to
send data to certain nodes (even though every node is capable of picking up the
transmitted data, only the destination node will actually do it), and to send
announcements to all nodes.
When more tailored data transfer is desired, a network
might use nonbroadcast point-to-multipoint transmission. In this scenario, a
node issues signals to multiple, defined recipients. For example, a network
administrator could schedule the LAN transmission of an instructional video
that only she and all of her team’s workstations could receive.
Throughput and Bandwidth
One data transmission characteristic often discussed and analyzed by
networking professionals is throughput.
Throughput
is the measure of how much data is transmitted during a given period of time. It may also be
called capacity or bandwidth (though as you will learn,
bandwidth is technically different from
throughput). Throughput is commonly expressed
as a quantity of bits transmitted per second, with prefixes used to
designate different throughput amounts. For example, the prefix kilo combined
with the word bit (as in kilobit) indicates 1000 bits per second. Rather than
talking about a throughput of 1000 bits per second, you typically say the throughput
was 1 kilobit per second (1 Kbps). The section below summarizes the terminology
and abbreviations used when discussing different throughput amounts.
|
Quantity
|
Prefix
|
Complete example
|
Abbreviation
|
|
1 bit per second
|
n/a
|
1 bit per second bps
|
bps
|
|
1000 bits per second
|
kilo
|
1 kilobit per second
|
Kbps
|
|
1,000,000 bits per second
|
mega
|
1 megabit per second
|
Mbps
|
|
1,000,000,000 bits per second
|
giga
|
1 gigabit per second
|
Gbps
|
|
1,000,000,000,000 bits per second
|
tera
|
1 terabit per second
|
Tbps
|
|
|
|
|
|
As an example, a residential broadband Internet connection might be rated
for a maximum throughput of 1.544 Mbps. A fast LAN might transport up to 10
Gbps of data. Contemporary networks commonly achieve throughputs of 10 Mbps,
100 Mbps, 1 Gbps, or higher. Applications
that require significant throughput include videoconferencing and telephone signaling.
By contrast, instant messaging and
e-mail, for example, require much less throughput.
Be careful not to
confuse bits and bytes when discussing throughput. Although data storage
quantities are typically expressed in multiples of bytes, data transmission
quantities (in other words, throughput) are more commonly expressed in
multiples of bits per second. When representing different data quantities, a
small b represents bits, whereas a capital B represents bytes. To put this into
context, a modem may transmit data at 56.6 Kbps (kilobits per second); a data
file may be 56 KB (kilobytes) in size. Another difference between data storage
and data throughput measures is that in data storage the prefix kilo means 2 to
the 10th power, or 1024, not 1000.
Often, the term bandwidth is
used interchangeably with throughput, and in fact, this may be the case on the
Network+ certification exam. Bandwidth and throughput are similar concepts,
but strictly speaking, bandwidth is a measure of
the difference between the highest and lowest frequencies that a medium can
transmit. This range of frequencies,
which is expressed in Hz, is directly related to
throughput. For example, if the Federal Communications Commission (FCC) told
you that you could transmit a radio signal between 870 and 880 MHz, your
allotted bandwidth (literally, the width of your frequency band) would be 10
MHz.
Baseband and Broadband
Baseband is a transmission form in which (typically)
digital signals are sent through direct current (DC) pulses applied to the
wire. This direct current requires exclusive use of the wire’s
capacity. As a result, baseband systems can transmit only one signal, or one
channel, at a time. Every device on a
baseband system shares the same channel. When one node is transmitting data on a baseband system, all other
nodes on the network must wait for that transmission to end before they can
send data. Baseband
transmission supports half-duplexing, which means that computers can both send
and receive information on the same length of wire. In some cases, baseband
also supports full duplexing.
Ethernet is an example of a baseband system found on many
LANs. In Ethernet, each
device on a network can transmit over the wire—but only one device at a time. For
example, if you want to save a file to the server, your NIC submits your
request to use the wire; if no other device is using the wire to transmit data
at that time; your workstation can go ahead. If the wire is in use, your
workstation must wait and try again later. Of course, this retrying process happens
so quickly that you don’t even notice the wait.
Broadband is a form of transmission in which signals
are modulated as radio frequency (RF) analog waves that use different frequency
ranges. Unlike baseband, broadband technology does not encode
information as digital pulses. As you may know, broadband transmission is used to bring cable TV to your home. Your cable TV connection can carry at least
25 times as much data as a typical baseband system (like Ethernet)
carries, including many different broadcast frequencies on different channels. In traditional broadband systems, signals
travel in only one direction—toward the user. To allow users to send
data as well, cable systems allot a
separate channel space for the user’s transmission and use amplifiers that can
separate data the user issues from data the network transmits. Broadband transmission is generally more
expensive than baseband transmission because of the extra hardware involved.
On the other hand, broadband systems
can span longer distances than baseband.
In the field of networking, some terms have more than one meaning, depending
on their context. Broadband is one of those terms. The broadband described in
this chapter is the transmission
system that carries RF signals across multiple channels on a coaxial cable,
as used by cable TV. This definition was the original meaning of broadband.
However, broadband has evolved to mean any
of several different network types that use digital signaling to transmit data
at very high transmission rates.
Transmission Flaws
Both analog and digital signals are susceptible to degradation between
the time they are issued by a transmitter and the time they are received. One
of the most common transmission flaws affecting data signals is noise.
Noise
As you learned earlier, noise is
any undesirable influence that may degrade or distort a signal. Many
different types of noise may affect transmission. A common source of noise is EMI
(electromagnetic interference), or waves that emanate from electrical devices or
cables carrying electricity. Motors, power lines, televisions, copiers,
fluorescent lights, microwave ovens, manufacturing machinery, and other sources
of electrical activity (including a severe thunderstorm) can cause EMI. One
type of EMI is RFI (radio frequency interference), or electromagnetic interference caused by radio waves. (Often,
you’ll see EMI referred to as EMI/RFI.) Strong broadcast signals from radio or
TV antennas can generate RFI. When EMI noise affects analog signals, this
distortion can result in the incorrect transmission of data, just as if static
prevented you from hearing a radio station broadcast. However, this type of noise affects digital signals
much less. Because digital signals do not depend on subtle amplitude or
frequency differences to communicate information, they are more apt to be
readable despite distortions caused by EMI noise.
Another form of noise that hinders data transmission is cross talk. Cross talk occurs when a signal traveling
on one wire or cable infringes on the signal traveling over an adjacent wire or
cable. When cross talk occurs between two cables, it’s called alien cross talk. When it occurs between
wire pairs near the source of a signal, it’s known as NEXT (near end cross talk).
One potential cause of NEXT is an improper termination—for example, one
in which wire insulation has been damaged or wire pairs have been untwisted too
far. If you’ve ever been on the phone and heard the conversation on your second
line in the background, you have heard the effects of cross talk. In this
example, the current carrying a signal on the second line’s wire imposes itself
on the wire carrying your line’s signal. The resulting noise, or cross talk, is
equal to a portion of the second line’s signal. Cross talk in the form of
overlapping phone conversations is bothersome, but does not usually prevent you
from hearing your own line’s conversation. In data networks, however, cross talk can be extreme enough to prevent
the accurate delivery of data.
In every signal, a certain amount of noise is unavoidable. However,
engineers have designed a number of ways to limit the potential for noise to
degrade a signal. One way is simply to ensure that the strength of the signal
exceeds the strength of the noise. Proper cable design and installation are
also critical for protecting against noise’s effects. Note that all forms of noise
are measured in decibels (dB).
Attenuation
Another transmission flaw is attenuation,
or the loss of a signal’s strength as it travels away from its source. Just as your voice
becomes fainter as it travels farther, so do signals fade with distance. To
compensate for attenuation, both analog and digital signals are boosted en
route. However, the technology used to boost an analog signal is different from
that used to boost a digital signal. Analog signals pass through an amplifier, an electronic device that increases the
voltage, or strength, of the signals. When an analog signal is
amplified, the noise that it has accumulated is also amplified. This indiscriminate amplification causes the
analog signal to worsen progressively. After multiple amplifications, an analog
signal may become difficult to decipher.
When digital signals are repeated, they are actually
retransmitted in their original form, without the noise they might have
accumulated previously. This process is known as regeneration.
A device that regenerates a digital signal is called a repeater. Amplifiers
and repeaters belong to the Physical layer of the OSI model. Both are used to
extend the length of a network. Because
most networks are digital, they typically use repeaters.
Latency
In an ideal world, networks could transmit data instantaneously between sender
and receiver, no matter how great the distance between the two. However, in the
real world every network is subjected
to a delay between the transmission of a signal and its eventual receipt.
For example, when you press a key on your computer to save a file to a network
server, the file’s data must travel through your NIC, the network wire, one or
more connectivity devices, more cabling, and the server’s NIC before it lands
on the server’s hard disk. Although electrons travel rapidly, they still have
to travel, and a brief delay takes place between the moment you press the key
and the moment the server accepts the data. This delay is called latency. The length of the
cable involved affects latency, as does the existence of any intervening connectivity
device, such as a router. Different devices affect latency to different
degrees. For example, modems, which must modulate both incoming and outgoing
signals, increase a connection’s latency far more than hubs, which simply
repeat a signal. The most common way to measure latency on data networks is by calculating
a packet’s RTT (round-trip time), or the length of
time it takes for a packet to go from sender to receiver, then back from
receiver to sender. RTT is
usually measured in milliseconds.
Latency causes problems only when a receiving node is expecting some type
of communication, such as the rest of a data stream it has begun to accept. If
that node does not receive the rest of the data stream within a given time
period, it assumes that no more data are coming. This assumption may cause
transmission errors on a network. When
you connect multiple network segments and thereby increase the distance between
sender and receiver, you increase the network’s latency. To constrain
the latency and avoid its associated errors, each type of cabling is rated for
a maximum number of connected network segments, and each transmission method is
assigned a maximum segment length.
Common Media Characteristics
Now that you are familiar with data-signaling characteristics, you are
ready to learn more about the physical and atmospheric paths that these signals
traverse. When deciding which kind of
transmission media to use, you must match your networking needs with the
characteristics of the media. This section describes the characteristics
of several types of physical media, including throughput, cost, noise
immunity, size and scalability, and connectors
and media converters. The medium used
for wireless transmission, the atmosphere, is discussed in detail in Chapter 8.
Throughput
Perhaps the most significant factor in choosing a
transmission method is its throughput. All media are
limited by the laws of physics that prevent signals from traveling faster than
the speed of light. Beyond that, throughput is limited by the signaling and
multiplexing techniques used in a given transmission method. Using fiber-optic cables allows faster
throughput than copper or wireless connections. Noise and devices
connected to the transmission medium can further limit throughput. A noisy circuit spends more time
compensating for the noise and, therefore, has fewer resources
available for transmitting data.
Cost
Cost is another significant factor in choosing a network medium. However,
the precise costs of using a
particular type of cable or wireless connection can be difficult to pinpoint.
For example, although a vendor might quote you the cost per foot for new
network cabling, you might also have to upgrade some hardware on your network
to use that type of cabling. Thus, the cost of upgrading your media would
actually include more than the cost of the cabling itself. Not only do media costs depend on the
hardware that already exists in a network, but they also depend on the size of
your network and the cost of labor in your area (unless you plan to install the
cable yourself). The following variables can all influence the final
cost of implementing a certain type of media:
·
Cost of
installation—Can you install the media yourself, or must you hire
contractors to do it? Will you need to move walls or build new conduits or
closets? Will you need to lease lines from a service provider?
·
Cost of new
infrastructure versus reusing existing infrastructure—Can you use
existing wiring? In some cases, for example, installing all new Category 6 UTP
wiring may not pay off if you can use existing Category 5 UTP wiring. If you
replace only part of your infrastructure, will it be easily integrated with the
existing media?
·
Cost of maintenance
and support—Reuse of an existing cabling infrastructure does not
save any money if it is in constant need of repair or enhancement. Also, if you
use an unfamiliar media type, it may cost more to hire a technician to service
it. Will you be able to service the media yourself, or must you hire
contractors to service it?
·
Cost of a lower
transmission rate affecting productivity—If you save money by reusing existing slower
connections, are you incurring costs by reducing productivity? In other words,
are you making staff wait longer to save and print reports or exchange e-mail?
·
Cost of downtime—If a cabling system
is poorly installed and needs fixing, or if an inexpensive solution needs to be
replaced in just a few years, at least some of your organization’s users will
not be able to work. Even at a small office, a few hours of downtime can cost a
business several thousand dollars in lost productivity. For an organization
that relies on its network for taking orders or providing services, the toll could
be much higher.
·
Cost of
obsolescence—Are you choosing media that may become passing fads,
requiring rapid replacement? Will you be able to find reasonably priced connectivity
hardware that will be compatible with your chosen media for years to come?
When planning a new cabling installation or replacing existing
infrastructure, a good rule of thumb is to choose a solution that will last at
least 10 years. A long-term solution, such as choosing the latest high-quality
cabling that can handle throughput 10 times faster than your existing network
uses, might cost more than a short-term solution initially, but will likely
cost less over its life of use.
Noise Immunity
As you learned earlier, noise can distort data signals. The extent to
which noise affects a signal depends partly on the transmission media. Some
types of media are more susceptible to noise than others. The type of media least susceptible to noise
is fiber-optic cable because it does not use electric current, but light waves,
to conduct signals. On most networks, noise is an ever-present threat,
so you should take measures to limit its impact on your network. For example,
install cabling well away from powerful electromagnetic forces. If your
environment still leaves your network vulnerable, choose a type of transmission
media that helps to protect the signal from noise. For example, wireless
signals are more apt to be distorted by EMI/RFI than signals traveling over a
cable. It is also possible to use antinoise algorithms to protect data from
being corrupted by noise. If these measures don’t ward off interference, in the
case of wired media, you may need to use a metal conduit, or pipeline, to
contain and further protect the cabling.
Size and Scalability
Three specifications determine the size and scalability of networking
media: maximum nodes per segment,
maximum segment length, and maximum network length. In cabling,
each of these specifications is based on the physical characteristics of the
wire and the electrical characteristics of data transmission. The maximum number of
nodes per segment depends on attenuation
and latency. Each device added to a network segment causes a slight
increase in the signal’s attenuation and latency. To ensure a clear, strong,
and timely signal, you must limit the number of nodes on a segment. The maximum segment
length
depends on attenuation and latency plus the segment type. A network can include two types of segments: populated and unpopulated. A populated segment is a part of a network that contains end nodes. For
example, a switch connecting users in a classroom is part of a populated
segment. An unpopulated segment, also known as a link segment, is a part of the
network that does not contain end nodes, but simply connects two networking
devices such as routers. Segment
lengths are limited because after a certain distance, a signal loses so
much strength that it cannot be accurately interpreted. The maximum distance a
signal can travel and still be interpreted accurately is equal to a segment’s maximum
length. Beyond this length, data loss is apt to occur.
As with the maximum number of nodes per segment, maximum
segment length varies between different cabling types. The same principle
of data loss applies to maximum network length, which is the sum of the
network’s segment lengths.
Connectors and Media
Converters
Connectors are the pieces of
hardware that connect the wire to the network device, be it a file server,
workstation, switch, or printer. Every networking medium requires a specific kind
of connector. The type of connectors you use will affect the cost of installing
and maintaining the network, the ease of adding new segments or nodes to the
network, and the technical expertise required to maintain the network. The
connectors you are most likely to encounter on modern networks are illustrated
throughout this chapter.
Connectors are specific to a particular media type, but
that doesn’t prevent one network from using multiple media. Some connectivity
devices are designed to accept more than one type of media. If you are working
with a connectivity device that can’t, you can integrate the two media types by
using media converters. A media converter is
a piece of hardware that enables networks or segments running on different
media to interconnect and exchange signals. For example,
suppose a segment leading from your company’s data center to a group of
workstations uses fiber-optic cable, but the workgroup hub can only accept
twisted pair (copper) cable. In that case, you could use a media converter to
interconnect the hub with the fiber-optic cable. The media converter completes the
physical connection and also converts the electrical signals from the copper cable
to light wave signals that can traverse the fiber-optic cable, and vice versa.
The terms wire and
cable are used synonymously in some situations. Strictly speaking, however,
wire is a subset of cabling, because the cabling category may also include
fiber-optic cable, which is almost never called wire. The exact meaning of the
term wire depends on context. For example, if you said, in a somewhat casual
way, “We had 6 gigs of data go over the wire last night,” you would be
referring to whatever transmission media helped carry the data—whether fiber,
radio waves, coax, or UTP.
Now that you understand data transmission and the factors to consider
when choosing a transmission medium, you are ready to learn about different
types of transmission media. To qualify for Network+ certification, you must
know the characteristics and limitations of each type of media, how to install
and design a network with each type, how to troubleshoot networking media
problems, and how to provide for future network growth with each option.
Coaxial Cable
Coaxial cable, called “coax”
for short, was the foundation for Ethernet networks in the
1980s and remained a popular transmission medium for many years. Over time, however, twisted pair and
fiber-optic cabling have replaced coax in modern LANs. If you work on long-established
networks or cable systems, however, you might have to work with coaxial cable. Coaxial cable consists
of a central metal core (often copper) surrounded by an insulator, a braided
metal shielding, called braiding or shield, and an outer cover, called the
sheath or jacket. The core may be constructed of one solid metal wire
or several thin strands of metal wire. The core carries the electromagnetic
signal, and the braided metal shielding acts as both a shield against noise and
a ground for the signal. The insulator
layer usually consists of a plastic material such as PVC (polyvinyl chloride)
or Teflon.
It protects the core from the metal shielding because if the two made
contact, the wire would short-circuit. The sheath, which protects the cable
from physical damage, may be PVC or a more expensive, fire-resistant plastic. Because of its shielding, most coaxial cable
has a high resistance to noise. It
can also carry signals farther than twisted pair cabling before amplification
of the signals becomes necessary (although not as far as fiber-optic
cabling). On the other hand, coaxial
cable is more expensive than twisted pair cable because it requires
significantly more raw materials to manufacture.
Coaxial cabling comes in hundreds of specifications, although you are
likely to see only two or three types of coax in use on data networks. All
types have been assigned an RG specification number. (RG stands for radio
guide, which is appropriate because coaxial cabling is used to guide radio
frequencies in broadband transmission.) The
significant differences between the cable types lie in the materials used for
their shielding and conducting cores, which in turn influence their
transmission characteristics, such as impedance (or the resistance that
contributes to controlling the signal, as expressed in ohms), attenuation, and
throughput. Each type of coax is suited to a different purpose. When
discussing the size of the conducting core in a coaxial cable, we refer to its American Wire
Gauge (AWG) size. The larger the AWG size, the smaller the diameter
of a piece of wire.
Following is a list
of coaxial cable specifications used with data networks:
RG-6—A type of coaxial
cable that is characterized by an impedance of 75 ohms and contains an 18 AWG
conducting core. The core is usually made of solid copper. RG-6 coaxial cables
are used, for example, to deliver broadband cable Internet service and cable
TV, particularly over long distances. If a service provider such as Comcast or
Charter supplies you with Internet service, the cable entering your home is
RG-6.
RG-8—A type of coaxial
cable characterized by a 50-ohm impedance and a 10 AWG core. RG-8 provided the
medium for the first Ethernet
networks, known as Thicknet. You will never find Thicknet on new networks, but
you might find it on older networks.
RG-58—A type of coaxial
cable characterized by a 50-ohm impedance and a 24 AWG core. RG-58 was a
popular medium for Ethernet LANs in the 1980s. With a smaller diameter than
RG-8, RG-58 is more flexible and easier to handle and install. Its core is
typically made of several thin strands of copper. The Ethernet standard that relies on RG-58 coax is called Thinnet
because it is thinner than Thicknet cables. Like Thicknet, Thinnet is almost never used
on modern networks, although you might encounter it on networks
installed in the 1980s.
RG-59—A type of coaxial
cable characterized by a 75-ohm impedance and a 20 or 22 AWG core, usually made
of braided copper. Less expensive but
suffering from greater attenuation than the more common RG-6 coax, RG-59 is
still used for relatively short connections, for example, when
distributing video signals from a central receiver to multiple monitors within
a building.
The two coaxial cable types commonly used in networks today, RG-6 and RG-59, can terminate
with one of two connector types: an F-Type connector or a BNC connector.
F-Type connectors attach to coaxial cable so that the pin
in the center of the connector is the conducting core of the cable. Therefore, F-Type
connectors require that the cable contain a solid metal core. After being
attached to the cable by crimping or compression, connectors are threaded and
screw together like a nut-and-bolt assembly.
A male F-Type connector, or plug, is attached to a coax cable, a
corresponding female F-Type connector, or jack, would be coupled with the male
connector. F-Type connectors are most
often used with RG-6 cables.
BNC stands for Bayonet Neill-Concelman, a term
that refers to both a style of connection and its two inventors.
(Sometimes the term British Naval Connector is also used.) A BNC connector
is crimped, compressed, or twisted onto a coaxial cable. It connects to another
BNC connector via a turning-and-locking mechanism—this is the bayonet
coupling referenced in its name. Unlike
an F-Type connector, male BNC connectors do not use the central conducting core
of the coax as part of the connection, but provide their own conducting pin. BNC
was once the standard for connecting coaxial-based Ethernet segments. Today, though, you’re more likely to find
BNC connectors used with RG-59 coaxial cable. Less commonly, they’re also used with RG-6.
When sourcing
connectors for coaxial cable, you need to specify the type of cable you are
using. For instance, when working with RG-6 coax, choose an F-Type connector
made specifically for RG-6 cables. That way, you’ll be certain that the
connectors and cable share the same impedance rating. If impedance ratings
don’t match, data errors will result and network performance will suffer.
Next, you will learn about a medium you are more likely to find on modern
LANs, twisted pair cable.
Twisted Pair Cable
Twisted pair cable consists of color-coded pairs of
insulated copper wires, each with a diameter of 0.4 to 0.8 mm (approximately
the diameter of a straight pin). Every two wires are twisted
around each other to form pairs, and all the pairs are encased in a plastic
sheath.. The number of pairs in a
cable varies, depending on the cable type. The more twists per foot in a pair
of wires, the more resistant the pair will be to cross talk. Higher-quality,
more-expensive twisted pair cable contains more twists per foot. The number of
twists per meter or foot is known as the twist ratio. Because twisting the wire pairs more tightly
requires more cable, however, a high twist ratio can result in greater
attenuation. For optimal performance, cable manufacturers must strike a
balance between minimizing cross talk and reducing attenuation.
Because twisted pair is used in such a wide variety of environments and
for a variety of purposes, it comes in hundreds of different designs. These
designs vary in their twist ratio, the number of wire pairs that they contain, the grade of copper used, the type of
shielding
(if any), and the materials used for shielding, among other
things. A twisted pair cable may
contain from 1 to 4200 wire pairs. Modern networks typically use cables that
contain four wire pairs, in which one pair is dedicated to sending data and
another pair is dedicated to receiving data.
In 1991, two standards organizations, the TIA/EIA, finalized their specifications for twisted pair wiring
in a standard called “TIA/EIA 568.”
Since then, this body has continually revised the international standards for
new and modified transmission media. Its standards now cover cabling media,
design, and installation specifications. The TIA/EIA 568 standard divides twisted
pair wiring into several categories. The types of twisted pair wiring you will
hear about most often are Cat
(category) 3, 5, 5e, 6, and 6a, and Cat 7. All of the category cables fall
under the TIA/EIA 568 standard. Modern
LANs use Cat 5 or higher wiring.
Twisted pair cable is relatively inexpensive, flexible,
and easy to install, and it can span a significant distance before requiring a
repeater (though not as far as coax). Twisted pair cable easily accommodates
several different topologies, although it is most often implemented in star or
star-hybrid topologies. All twisted pair cable falls into one of two
categories: STP (shielded twisted pair) or UTP (unshielded
twisted pair).
STP (Shielded Twisted Pair)
STP (shielded twisted pair) cable consists of
twisted-wire pairs that are not only individually insulated, but also
surrounded by a shielding made of a metallic substance such as foil. Some STP use a
braided copper shielding. The shielding acts as a barrier to external
electromagnetic forces, thus preventing them from affecting the signals traveling
over the wire inside the shielding. It also contains the electrical energy of
the signals inside. The shielding may be grounded to enhance its protective
effects. The effectiveness of STP’s shield depends on the level and type of
environmental noise, the thickness and material used for the shield, the grounding
mechanism, and the symmetry and consistency of the shielding.
UTP (Unshielded Twisted Pair)
UTP (unshielded twisted pair) cabling consists of one or
more insulated wire pairs encased in a plastic sheath. As its name
implies, UTP does not contain additional shielding for the twisted pairs. As a
result, UTP is both less expensive and
less resistant to noise than STP.
Earlier, you learned that the TIA/EIA consortium designated standards for
twisted pair wiring. To manage network cabling, you need to be familiar with
the standards for use on modern networks, particularly Cat 5 or higher. Note that Cat 4 cabling exists, too, but it
is rarely used.
Cat 3 (Category 3)—A form of UTP that contains four
wire pairs and can carry up to 10 Mbps of data with a possible bandwidth of 16
MHz. Cat 3 was used for 10-Mbps Ethernet or 4-Mbps token ring networks. You
will rarely find it on any modern network, however.
Cat 5 (Category 5)—A form of UTP that contains
four wire pairs and supports up to
1000 Mbps
throughput and a 100-MHz signal rate. Figure 3-22 depicts a typical Cat
5 UTP cable with
its twisted pairs untwisted, allowing you to see their matched color coding.
For example, the wire that is colored solid orange is twisted around the wire that
is part orange and part white to form the pair responsible for transmitting
data.
Cat 5e (Enhanced Category 5)—A higher-grade
version of Cat 5 wiring that contains high-quality copper offers a high twist
ratio and uses advanced methods for reducing cross talk. Cat 5e can support a
signaling rate as high as 350 MHz, more than triple the capability of regular
Cat 5.
Cat 6 (Category 6)—A twisted pair cable that
contains four wire pairs, each wrapped in foil insulation. Additional foil
insulation covers the bundle of wire pairs, and a fire-resistant plastic sheath
covers the second foil layer. The foil insulation provides excellent resistance
to cross talk and enables Cat 6 to support a 250-MHz signaling rate and at least
six times the throughput supported by regular Cat 5.
Cat 6a (Augmented Category 6)—A higher-grade
version of Cat 6 wiring that reduces attenuation and cross talk and allows for
potentially exceeding traditional network segment length limits. Cat 6a is
capable of a 500-MHz signaling rate and can reliably transmit data at
multi-gigabit per second rates. Cat 6a cabling is backward compatible with Cat
5, Cat 5e, and Cat 6 cabling, which means that it can replace lower-level
cabling without requiring connector or equipment changes.
Cat 7 (Category 7)—A twisted pair cable that
contains multiple wire pairs, each surrounded by its own shielding, then
packaged in additional shielding beneath the sheath. One advantage to Cat 7
cabling is that it can support signal rates up to 1 GHz. However, it requires
different connectors than other versions of UTP because its twisted pairs must
be more isolated from each other to ward off cross talk. Because of its added shielding,
Cat 7 cabling is also larger and less flexible than other versions of UTP
cable. Cat 7 is less common than Cat 5, Cat 6, or Cat 6a on modern networks.
Technically, because Cat 6 and Cat 7 contain wires that are individually
shielded, they are not unshielded twisted pair. Instead, they are more similar
to shielded twisted pair. UTP cabling may be used with any one of several IEEE
Physical layer networking standards that specify throughput maximums of 10,
100, 1000, and even 10,000 Mbps.
It can be difficult
to tell the difference between four-pair Cat 3 cables and four-pair Cat 5 or
Cat 5e cables. However, some visual clues can help. On Cat 5 cable, the jacket
is usually stamped with the manufacturer’s name and cable type, including the
Cat 5 specification. A cable whose jacket has no markings is more likely to be
Cat 3. Also, pairs in Cat 5 cables have a significantly higher twist ratio than
pairs in Cat 3 cables. Although Cat 3 pairs might be twisted as few as three
times per foot, Cat 5 pairs are twisted at least 12 times per foot. Other
clues, such as the date of installation (old cable is more likely to be Cat 3),
looseness of the jacket (Cat 3’s jacket is typically looser than Cat 5’s), and the
extent to which pairs are untwisted before a termination (Cat 5 can tolerate
only a small amount of untwisting) are also helpful, though less definitive.
Comparing STP and UTP
STP and UTP share several characteristics. The following list highlights
their similarities and differences:
Throughput—STP and UTP can both transmit data at 10 Mbps, 100
Mbps, 1 Gbps, and 10 Gbps, depending on the grade of cabling and the
transmission method in use.
Cost—STP and UTP vary in cost, depending on the grade of
copper used, the category rating, and any enhancements. Typically, STP is more
expensive than UTP because it contains more materials and it has a lower
demand. It also requires grounding, which can lead to more expensive
installation. High-grade UTP can be expensive, too, however. For example, Cat
6a costs more per foot than Cat 5 cabling.
Connector—STP and UTP use RJ-45 (registered jack 45) modular
connectors and data jacks, which look similar to analog telephone connectors
and jacks. However, telephone connections follow the RJ-11 (registered jack 11)
standard. All types of Ethernet that rely on twisted pair cabling use RJ-45
connectors.
Noise immunity—Because of its shielding, STP
is more noise resistant than UTP. On the other hand, signals transmitted over
UTP may be subject to filtering and balancing techniques to offset the effects
of noise.
Size and scalability—The maximum segment length for
both STP and UTP is 100 m, or 328 feet, on Ethernet networks that support data
rates from 1 Mbps to 10 Gbps. These accommodate a maximum of 1024 nodes.
(However, attaching so many nodes to a segment is very impractical, as it would
slow traffic and make management nearly impossible.)
Terminating Twisted Pair Cable
Imagine you have been sent to one of your employer’s remote offices and
charged with upgrading all the old Cat 3 patch cables in a data closet with
new, Cat 6a patch cables. A patch
cable is a relatively short (usually between 3 and 25 feet) length of cabling
with connectors at both ends. Based on the company’s network
documentation, you brought 50 premade cables with RJ-45 plugs on both ends,
which you purchased from an online cable vendor. At the remote location,
however, you discover that its data closet actually contains 60 patch cables
that need replacing. No additional premade cables are available at that office,
and you don’t have time to order more. Luckily, you have brought your
networking tool kit with spare RJ-45 plugs and a spool of Cat 6a cable. Knowing
how to properly terminate Cat
6a cables allows you to make all the new patch cables you need and complete
your work. Even if you are never faced with this situation, it’s likely that at
some point you will have to replace an RJ-45 connector on an existing cable.
This section describes how to terminate twisted pair cable.
Proper cable termination is a basic requirement for two nodes on a
network to communicate. Beyond that, however, poor terminations can lead to
loss or noise—and consequently, errors—in a signal. Closely following
termination standards, then, is critical. TIA/EIA has specified two different
methods of inserting twisted pair wires into RJ-45 plugs: TIA/EIA 568A and TIA/EIA 568B. Functionally, there is
no difference between the standards. You only have to be certain that you use
the same standard on every RJ-45 plug and jack on your network, so that data is
transmitted and received correctly.
If you terminate the RJ-45 plugs at both ends of a patch cable
identically, following one of the TIA/EIA 568 standards, you will create a straight-through cable. A straight-through cable is so named because it
allows signals to pass “straight through” from one end to the other. This is the type used to connect a
workstation to a router, for example. However, in some cases you may
want to reverse the pin locations of some wires—for example, when you want to connect two workstations
without using a connectivity device or when you want to connect two hubs
through their data ports. This can be accomplished through the use of a
crossover
cable, a patch
cable in which the termination locations of the transmit and receive wires on
one end of the cable are reversed.
Modern NICs and
switches are equipped with an autosense
function that enables them to detect the way wires are terminated in a
plug and adapt their transmit and receive signaling accordingly. Therefore,
crossover cables are only necessary if you are connecting computers or
connectivity devices with older interfaces that lack the autosense feature.
Connecting two brand-new computers with a straight-through patch cable enables
them to communicate directly.
The tools you’ll need to terminate a twisted pair cable with an RJ-45
plug are a wire cutter, wire stripper, and crimping tool. (In fact, you can find a single device that contains
all three of these tools.)
Following are the steps to create a straight-through patch cable using
Cat 5e twisted pair cable. The process of fixing wires inside the connector is
called crimping, and it is a skill that requires practice—so don’t be
discouraged if the first cable you create doesn’t reliably transmit and receive
data. You’ll get to practice making cables in the end-of-chapter
Hands-On Projects:
1. Using the wire cutter, make a clean cut at both ends of the twisted
pair cable.
2. Using the wire stripper, remove the sheath off of one end of the
twisted pair cable, beginning at approximately 1 inch from the end. Be careful
to neither damage nor remove the insulation that’s on the twisted pairs inside.
3. In addition to the four wire pairs, inside the sheath you’ll find a
string. Cut the string, then separate the four wire pairs in the full 1 inch of
exposed cabling. Carefully unwind each pair and straighten each wire.
4. To make a straight-through cable, align all eight wires on a flat
surface, one next to the
other, ordered according to their colors and positions listed below (It
might be helpful first to “groom”—or pull steadily across the length of—the
unwound section of each wire to straighten it out and help it stay in place.)
|
Pin #
|
Color
|
Pair
#
|
Function
|
|
1
|
White with orange stripe
|
2
|
Transmit +
|
|
2
|
Orange
|
2
|
Transmit
|
|
3
|
White with green stripe
|
3
|
Receive +
|
|
4
|
Blue
|
1
|
Unused
|
|
5
|
White with blue stripe
|
1
|
Unused
|
|
6
|
Green
|
3
|
Receive
|
|
7
|
White with brown stripe
|
4
|
Unused
|
|
8
|
Brown
|
4
|
Unused
|
5. Measure ½" from the end of the wires, and cleanly cut the wires
at this length. Keeping the wires in line and in order, gently slide them into
their positions in the RJ-45 plug.
(The sheath should extend into the plug about 3/8".)
6. After the wires are fully inserted, place the RJ-45 plug in the
crimping tool and press firmly to crimp the wires into place. (Be careful not
to rotate your hand or the wire as you do this, otherwise only some of the
wires will be properly terminated.) Crimping causes the internal RJ-45 pins to
pierce the insulation of the wire, thus creating contact between the two
conductors.
7. Now remove the RJ-45 connector from the crimping tool. Examine the end
and see whether each wire appears to be in contact with the pin. It may be
difficult to tell simply by looking at the connector. The real test is whether
your cable will successfully transmit and receive signals.
8. Repeat Steps 2 through 7 for the other end of the cable. After
completing Step 7 for the other end, you will have created a straight-through
patch cable.
Even after you feel confident making your own cables, it’s a good idea to
verify that they can transmit and receive data at the necessary rates using a
cable tester. Cable testing is discussed in Chapter 13.
Now that you have learned about transmission media that use copper wires
to conduct signals, you are ready to learn how signals are transmitted over
glass fibers.
Fiber-Optic Cable
Fiber-optic cable, or simply fiber, contains one or
several glass or plastic fibers at its center, or core. Data is transmitted via pulsing light sent
from a laser (in the case of 1- and 10-gigabit technologies) or an LED
(light-emitting diode) through the central fibers. Surrounding the fibers is a
layer of glass or plastic
called cladding. The cladding has a different density from the
glass or plastic in the strands. It reflects light back to the core in patterns
that vary depending on the transmission mode. This reflection allows the fiber
to bend around corners without diminishing the integrity of the light-based
signal. Outside the cladding, a plastic buffer protects the cladding and core. Because the buffer
is opaque, it also absorbs any light that might escape. To prevent the cable
from stretching, and to protect the inner core further, strands of Kevlar (a polymeric
fiber) surround the plastic buffer. Finally, a plastic sheath covers the strands of Kevlar.
Like twisted pair and coaxial cabling, fiber-optic cabling comes in a
number of different varieties, depending on its intended use and the
manufacturer. For example, fiber-optic cables used to connect the facilities of
large telephone and data carriers may contain as many as 1000 fibers and be
heavily sheathed to prevent damage from extreme environmental conditions. At the
other end of the spectrum, fiber-optic patch cables for use on LANs may contain
only two strands of fiber and be pliable enough to wrap around your hand. Because each strand of glass in a
fiber-optic cable transmits in one direction only—in simplex fashion—two strands are
needed for full-duplex communication. One solution is to use a zipcord cable, in which two strands are
combined side by side in conjoined jackets. You’ll find zipcords where
fiber-optic cable spans relatively short distances, such as connecting a server
and switch. A zipcord may come with one of many types of connectors on its
ends, as described later in this section.
Fiber-optic cable provides the following benefits over
copper cabling:
·
Extremely high throughput
·
Very high resistance to noise
·
Excellent security
·
Ability to carry signals for much longer distances
before requiring repeaters than copper cable
·
Industry standard for high-speed networking
The most significant drawback to the use of fiber is that
covering a certain distance with fiber-optic cable is more expensive than using
twisted pair cable. Also, fiber-optic cable requires special equipment
to splice, which means that quickly repairing a fiber-optic cable in the field
(given little time or resources) can be difficult.
Fiber’s
characteristics are summarized in the following list:
Throughput—Fiber has proved
reliable in transmitting data at rates that can reach 100 gigabits (or 100,000
megabits) per second per channel. Fiber’s amazing throughput is partly due to
the physics of light traveling through glass. Unlike electrical pulses
traveling over copper, the light experiences virtually no resistance.
Therefore, light-based signals can be transmitted at faster rates and with
fewer errors than electrical pulses. In fact, a pure glass strand can accept up
to 1 billion laser light pulses per second. Its high throughput capability
makes it suitable for network backbones and for serving applications that
generate a great deal of traffic, such as video or audio conferencing.
Cost—Fiber-optic cable
is the most expensive transmission medium. Because of its cost, most
organizations find it impractical to run fiber to every desktop. Not only is the
cable itself more expensive than copper cabling, but fiber-optic transmitters
and connectivity equipment can cost as much as five times more than those
designed for UTP networks. In addition, hiring skilled fiber cable installer’s
costs more than hiring twisted pair cable installers.
Connectors—With fiber cabling,
you can use any of 10 different types of connectors. Figures 3-32, 3-33, 3-34,
and 3-35 show four of the most common connector types: the
SC (subscriber connector or standard connector), ST (straight tip), LC
(local connector), and MT-RJ (mechanical transfer registered jack). Existing
fiber networks might use ST or SC connectors. However, LC and MT-RJ connectors
are used on the very latest fiber-optic technology. LC and MT-RJ connectors are
preferable to ST and SC connectors because of their smaller size, which allows
for a higher density of connections at each termination point. The MT-RJ
connector is unique because it contains two strands of fiber in a single
ferrule, which is a short tube within a connector that encircles the fiber and
keeps it properly aligned. With two strands in each ferrule, a single MT-RJ
connector provides for full-duplex signaling. Linking devices that require
different connectors is simple because you can purchase fiber-optic cables with
different connector types at each end.
Noise
immunity—Because fiber does not conduct electrical current to
transmit signals, it is unaffected by EMI. Its impressive noise resistance is
one reason why fiber can span such long distances before it requires repeaters
to regenerate its signal. Size and scalability—Depending on the type of
fiber-optic cable used, segment lengths vary from 150 to 40,000 meters. This
limit is due primarily to optical loss, or the degradation of the light signal
after it travels a certain distance away from its source (just as the light of
a flashlight dims after a certain number of feet). Optical loss accrues over
long distances and grows with every connection point in the fiber network. Dust
or oil in a connection (for example, from people handling the fiber while
splicing it) can further exacerbate optical loss. Some types of fiber-optic
cable can carry signals 40 miles while others are suited for distances under a
mile. The distance a cable can carry light depends partly on the light’s
wavelength. It also depends on whether the cable is single mode or multimode.
SMF (Single-Mode Fiber)
SMF (single-mode fiber) consists of a narrow core of 8 to 10 microns in
diameter. Laser- generated light
travels a single path over the core, reflecting very little. Because it
reflects little, the light does not disperse as the signal travels along the
fiber. This continuity allows single-mode fiber to accommodate the highest
bandwidths and longest distances (without requiring repeaters) of all network
transmission media. Single-mode fiber can carry signals many miles
before the signals require repeating. Therefore, it is preferred for connecting WAN locations and service provider facilities,
for example. The Internet backbone
depends on single-mode fiber. However, because of its relatively high
cost, single-mode fiber is rarely used
for shorter connections, such as those between a server and switch.
MMF (Multimode Fiber)
MMF (multimode fiber) contains a core with a larger diameter than
single-mode fiber, usually 50 or 62.5 microns, over which many pulses of light
generated by a laser or LED travel at different angles. Signals traveling over multimode fiber experience greater attenuation
than those traversing single-mode fibers. Therefore, multimode fiber is not
suited to distances longer than a few miles. On the other hand,
multimode fiber is less expensive to install. It is often found on cables that
connect a router to a switch or a server on the backbone of a network. In cases where fiber connects desktop
workstations to the network, multimode fiber is preferred because of its lower
cost.
Fiber-Optic Converters
Converters are required to connect networks using multimode fiber with
networks using single-mode fiber. They are also required to connect the fiber-
and copper-based parts of a network. The bidirectional converter accepts the
signal from one part of the network and regenerates before issuing it to the
next part.
Serial Cables
So far you have learned about the kinds of physical media used between
connectivity devices and with nodes on a LAN or WAN. This section describes a
type of cable that can be used to connect directly to a device such as a
router, server, or switch. Serial refers to a style of data transmission in which the
pulses that represent bits follow one another along a single transmission line.
In other words, they are issued
sequentially, not simultaneously. A serial cable is one that carries serial transmissions. Several types of
serial cables exist. EIA/TIA has codified a popular serial data transmission
method known as RS-232 (Recommended Standard 232). This Physical layer
standard specifies, among other things, signal voltage and timing, plus the
characteristics of compatible interfaces. Different
connector types comply with this standard, including RJ-45 connectors, DB-9
connectors, and DB-25 connectors. You are already familiar with RJ-45 plugs. The
arrangement of the pins on both connectors resembles a sideways letter D. Also a
DB-9 connector contains 9 contact points and a DB-25 connector contains 25.
For many years, serial cables were used to connect external modems with
workstations.
However, as an administrator on
today’s networks, you’re more likely to use an RS-232 connection between a
workstation and a router to make your workstation act as a console for
configuring and managing that router.
In fact, a higher-end router designed for use in your data center (not
the kind of router you’d use at home) usually comes with an RS-232-compatible
cable. The serial interface on the
back of the connectivity device is often labeled “Console.” This is not
to say that a serial cable is the only way of connecting to a router for
configuring and managing it. However, if the router is brand new, if the network
is malfunctioning, or if for some other reason the device cannot obtain an IP address,
you need to access it directly and not via a network connection. You can find
RS-232 cables with different types of connectors at either end. For example, many
Cisco routers come with a console port that’s RJ-45 compliant. If you wanted to
connect such a router to your laptop’s DB-9 serial port, you could find an
RS-232 cable with an RJ-45 plug on one end and a DB-9 plug on the other.
The fact that a serial cable terminates in an RJ-45
connector does not mean it will work if plugged into a device’s RJ-45 Ethernet
port!
When using a serial cable with an RJ-45 connector, be certain to plug it
into the appropriate serial interface. In addition to using different connector
types, the termination points on RS-232 cables can be arranged in various ways,
depending on the cable’s purpose. Earlier you learned about the difference
between straight-through and crossover cables in the context of terminating twisted
pair cables. An RS-232 cable, whether
it uses DB-9, DB-25, or RJ-45 connectors, can also be straight-through.
You also have the option of reversing
the transmit and receive pins on one end, thereby making it into a crossover
cable. Among other things, you could use such a crossover cable to
directly connect two routers via their serial interfaces. You’ll learn more
about connectivity devices such as routers and switches in Chapter 6. The following
section describes how to arrange physical networking media between end users
and connectivity devices on a LAN or WAN.
Structured
Cabling
Organizations that pay attention to their cable plant—the hardware that makes up the
enterprise-wide cabling system—are apt to experience fewer Physical
layer network problems, smoother network expansions, and simpler network
troubleshooting. Following the cabling standards and best practices described
in this chapter can help. If you were to tour hundreds of data centers and
equipment rooms at established enterprises, you would see similar cabling
arrangements. That’s because most organizations follow a cabling standard. One
popular standard is TIA/EIA’s joint 568 Commercial Building Wiring
Standard, also known as structured cabling, for uniform,
enterprise-wide, multivendor cabling systems. The standard suggests how
networking media can best be installed to maximize performance and minimize
upkeep. Structured cabling applies no
matter what type of media or transmission technology a network uses.
(It does, however assume a network based on the star topology.) In other words,
it’s designed to work just as well for 10-Mbps networks as it does for 10-Gbps
networks. Structured cabling is based
on a hierarchical design that begins where a telecommunications company’s
service enters a building and ends at a user’s workstation.
Detailed descriptions of the components used in
structured cabling follow:
Entrance facilities—The
facilities necessary for a service provider (whether it is a local phone
company, Internet service provider, or long-distance carrier) to connect with
another organization’s LAN or WAN. Entrance facilities may
include fiber-optic cable and multiplexers, coaxial cable, UTP, satellite or
wireless transceivers, and other devices or cabling. If the entrance facilities
are supplied by a telecommunications carrier and rely on UTP, they may come in
the form of 25-pair wire.
As the name suggests, 25-pair wire is a bundle of 25 wire pairs. As you
might expect, 100-pair wire contains 100 twisted wire pairs. More commonly, however, entrance facilities
depend on fiber-optic cable. The
entrance facility designates where the telecommunications service provider
accepts responsibility for the (external) connection. The point of
division between the service provider’s network and the internal network is
also known as the demarcation point (or demarc).
MDF (main distribution
frame)—Also known as the main cross-connect, the first point of interconnection between
an organization’s LAN or WAN and a service provider’s facility. An MDF
typically includes connectivity devices, such as switches and routers, and
media, such as fiber-optic cable, capable of the greatest throughput. Often, it
also houses an organization’s main servers. In an enterprise-wide network, equipment
in an MDF connects to equipment housed in another building’s IDF. Sometimes the
MDF is simply known as the computer room or equipment room.
Cross-connect facilities—The
points where circuits interconnect with other circuits. For example, when
an MDF accepts UTP from a service provider, the wire pairs terminate at a punch-down block. A punch-down block is
a panel of data receptors into which twisted air wire is inserted, or punched
down, to complete a circuit. Punch-down blocks were for many
years the standard method of terminating telephone circuits. The type used on
data networks is known as the 110 block. 110 blocks are available in several
different capacities. That is, “110” does not represent the number of wire
pairs the block can terminate. From a punch-down block, wires are distributed
to a patch panel, a wallmounted panel of data receptors. A patch panel allows the
insertion of patch cables. Note that
cross-connect facilities are not limited to the MDF and may be used in other
equipment rooms that are part of a building’s cable infrastructure.
IDF (intermediate
distribution frame)—A junction point between the MDF and
concentrations of fewer connections—for example, those that
terminate in a telecommunications closet. Backbone wiring—The cables or
wireless links that provide interconnection between entrance facilities and
MDFs, MDFs and IDFs, and IDFs and telecommunications closets. One component of the backbone is given a
special term: vertical cross-connect.
A vertical cross-connect runs between a building’s
floors. For example, it might connect an MDF and IDF or
IDFs and telecommunications closets (described next) within a building. The
TIA/EIA standard designates distance limitations for backbones of varying cable
types. On modern networks, backbones
are usually composed of fiber-optic or UTP cable.
Telecommunications closet—Also
known as a “telco room,” it contains connectivity for groups of
workstations in its area, plus cross-connections to IDFs or, in smaller
organizations, an MDF. Large organizations may have several telco rooms per
floor, but the TIA/EIA standard specifies at least one per floor.
Telecommunications closets typically house patch panels, punch-down blocks, and
connectivity devices for a work area. Because telecommunications closets are
usually small, enclosed spaces, good cooling and ventilation systems are
important to maintaining a constant temperature.
Horizontal wiring—This
is the wiring that connects workstations to the closest telecommunications
closet. TIA/EIA recognizes three possible cabling types
for horizontal wiring: STP, UTP, or fiber-optic cable. The maximum allowable
distance for horizontal wiring is 100 m. This span includes 90 m to connect a
data jack on the wall to the telecommunications closet plus a maximum of 10 m
to connect a workstation to the data jack on the wall.
Work area—An
area that encompasses all patch cables and horizontal wiring necessary to
connect workstations, printers, and other network devices from their NICs to
the telecommunications closet. The TIA/EIA standard calls for
each wall jack to contain at least one voice and one data outlet. Realistically,
you will encounter a variety of wall jacks. For example, in a student computer
lab lacking phones, a wall jack with a combination of voice and data outlets is
unnecessary.
Knowing the standards for cabling a building or enterprise is key, but
until you have practiced terminating, running, and testing cables, this
knowledge is only theoretical. The following section provides some practical
information that you can apply when working with physical networking media.
TIA/EIA specifications for backbone cabling
|
Cable type
|
Cross-connects to
telecommunications
closet
|
MDF or IDF to
telecommunications
closet
|
Cross-connects to
IDF or MDF
|
|
UTP
|
800 m (voice specification)
|
500 m
|
300 m
|
|
Single-mode fiber
|
3000 m
|
500 m
|
1500 m
|
|
Multimode fiber
|
2000 m
|
500 m
|
1500 m
|
Best Practices for Cable Installation and
Management
So far, you have read about the variety of cables used in networking and
the limitations inherent in each. You may worry that with hundreds of varieties
of cable, choosing the correct one and making it work with your network is next
to impossible. The good news is that if you follow both the manufacturers’
installation guidelines and the TIA/EIA standards, you are almost guaranteed
success. Many network problems can be
traced to poor cable installation techniques. For example, if you don’t crimp twisted pair wires in the
correct position in an RJ-45 connector, the cable will fail to transmit or
receive data (or both—in which case, the cable will not function at
all). Installing the wrong grade of
cable can either cause your network to fail or render it more susceptible to
damage.
The art of proper cabling could fill an entire book. If you plan to
specialize in cable installation, design, or maintenance, you should invest in
a reference dedicated to this topic. As a network professional, you will likely
occasionally add new cables to a room or telecommunications closet, repair
defective cable ends, or install a data outlet.
Following are some
cable installation tips that will help prevent Physical layer failures:
·
Do not untwist twisted pair cables more than
one-half inch before inserting them into the punch-down block.
·
Do not leave more than 1 inch of exposed (stripped)
cable before a twisted pair termination. Doing so will increase the possibility
for cross talk and data errors.
·
Pay attention to the bend radius limitations for the type of cable you are
installing. Bend radius is the radius of the maximum arc into which
you can loop a cable before you will impair data transmission. Generally, a
twisted pair cable’s bend radius is equal to or greater than four times the
diameter of the cable. Be careful not to exceed it.
·
Use a cable tester to verify that each segment of
cabling you install transmits data reliably. This practice will prevent you
from later having to track down errors in multiple, long stretches of cable.
Chapter 13, which covers troubleshooting network problems, explains the tools
and methods needed to test cable continuity.
·
Avoid cinching cables so tightly that you squeeze
their outer covering, a practice that leads to difficult-to-diagnose data
errors.
·
Avoid laying cable across the floor where it might
sustain damage from rolling chairs or foot traffic. If you must take this tack,
cover the cable with a cable protector.
·
Install cable at least 3 feet away from fluorescent
lights or other sources of EMI. This will reduce the possibility for noise to
affect your network’s signals.
·
Always leave some slack in cable runs. Stringing
cable too tightly risks connectivity and data transmission problems.
·
If you run cable in the plenum, the area above the ceiling tile or below the
subflooring, make sure the cable sheath is plenum-rated, and consult
with local electric installation codes to be certain you are installing it
correctly. A plenum-rated cable is more fire resistant, and if burned, produces
less smoke than other cables.
·
Pay attention to grounding requirements and follow
them religiously.
·
Adhering to structured cabling hierarchies is only
part of a smart cable management strategy. You or your network manager should
also specify standards for the types of cable used by your organization and
maintain a list of approved cabling vendors.
·
Keep a supply room stocked with spare parts so that
you can easily and quickly replace defective parts.
·
Create documentation for your cabling plant,
including the locations, installation dates, lengths, and grades of installed
cable. Label every data jack, punch-down block, and connector. Use color-coded
cables for different purposes (cables can be purchased in a variety of sheath
colors). For example, you might want to use pink for patch cables, green for
horizontal wiring, and gray for vertical (backbone) wiring. Be certain to
document your color schemes.
·
Keep your cable plant documentation in a centrally
accessible location and be certain to update it as you change the network. The
more you document, the easier it will be to move or add cable segments.
·
Finally, create a plan for expanding your cabling
plant. For example, if your organization is rapidly enlarging, consider
replacing your backbone with fiber and leave plenty of space in your
telecommunications closets for more racks.
Chapter Summary
·
Information
can be transmitted via two methods: analog or digital. Analog signals are continuous waves that
result in variable and inexact transmission. Digital signals are based on
electrical or light pulses that represent information encoded in binary form.
·
In
half-duplex transmission, signals can travel in both directions over a medium
but in only one direction at a time.
When signals can travel in both directions over a medium simultaneously, the
transmission is considered full-duplex.
·
A form of
transmission that allows multiple signals to travel simultaneously over one medium
is known as multiplexing. In
multiplexing, the single medium is logically separated into multiple channels,
or subchannels.
·
Throughput
is the amount of data that the medium can transmit during a given period of
time. Throughput is usually measured
in bits per second and depends on the physical nature of the medium.
·
Baseband is
a form of transmission in which digital signals are sent through direct current
pulses applied to the wire.
Baseband systems can transmit only one signal, or one channel, at a time.
Broadband, on the other hand, uses modulated analog frequencies to transmit
multiple signals over the same wire.
·
Noise is
interference that distorts an analog or digital signal. It may be caused by electrical sources, such
as power lines, fluorescent lights, copiers, and microwave ovens, or by
broadcast signals.
·
Analog and
digital signals both suffer attenuation, or loss of signal, as they travel farther
from their sources. To
compensate, analog signals are amplified, and digital signals are regenerated
through repeaters.
·
Every
network is susceptible to a delay between the transmission of a signal and its receipt. This delay is called latency. The length
of the cable contributes to latency, as does the presence of any intervening
connectivity device.
·
Coaxial cable consists of a central metal
conducting core (often copper) surrounded by a plastic insulator, a braided
metal shielding, and an outer plastic cover called the sheath.
The conducting core carries the
electromagnetic signal, and the shielding acts as both a protection against
noise and a ground for the signal. The insulator layer protects the copper core
from the metal shielding. The sheath protects the cable from physical damage.
·
Most
networks no longer rely on coaxial cable; however, if you obtain Internet service from a cable
company, the cable that enters your home will be a type of coax known as RG-6.
·
Twisted pair
cable consists of color-coded pairs of insulated copper wires, each with a diameter of 0.4 to 0.8 mm,
twisted around each other and encased in plastic coating. STP (shielded twisted
pair) cable consists of twisted-wire pairs that are not only individually
insulated, but also surrounded by a shielding made of a metallic substance such
as foil to reduce the effects of noise on the signal.
·
UTP
(unshielded twisted pair) cabling consists of one or more insulated wire pairs encased
in a plastic sheath. As its name
suggests, UTP does not contain additional shielding for the twisted pairs. As a
result, UTP is both less expensive and less resistant to noise than STP.
·
Fiber-optic
cable contains one or several glass or plastic fibers in its core. Data is transmitted via pulsing light sent
from a laser or light-emitting diode through the central fiber(s). Outside the
fiber(s), cladding reflects light back to the core in different patterns that
vary depending on the transmission mode.
·
Fiber-optic
cable provides the benefits of very high throughput, very high
resistance to noise, and excellent security.
·
Fiber cable
variations fall into two categories: single mode and multimode.
Single-mode fiber uses a small-diameter core, over which light travels mostly
down its center, reflecting very few times. This allows single-mode fiber to
accommodate high throughput over long distances without requiring repeaters.
·
MMF
(multimode fiber) uses a core with a larger diameter, over which many pulses of light travel at
different angles. Multimode fiber is less expensive than SMF (single-mode
fiber).
·
Serial
communication is often used on short connections between devices when a network is not available. For
example, you might use an RS-232 serial cable to connect your laptop to a
router so that you can configure the router from your laptop.
·
TIA/EIA’s
568 Commercial Building Wiring Standard, also known as structured cabling,
provides guidelines for uniform, enterprise-wide, multivendor cabling systems. Structured cabling is based on a
hierarchical design that begins with a service provider’s facilities and ends
at users’ workstations.
·
The best
practice for installing cable is to follow the TIA/EIA 568 specifications and the
manufacturer’s recommendations. Be
careful not to exceed a cable’s bend radius, untwist wire pairs more than
one-half inch, or remove more than one inch of insulation from copper wire.
Install plenum-rated cable in ceilings and floors, and run cabling away from
where it might suffer physical damage. Maintain clear, comprehensive
documentation on your cable plant.
Key Terms
1
gigabit per second (Gbps)
1,000,000,000
bits per second.
1
kilobit per second (Kbps)
1000
bits per second.
1
megabit per second (Mbps)
1,000,000
bits per second.
1
terabit per second (Tbps)
1,000,000,000,000
bits per second.
100-pair
wire
UTP
supplied by a telecommunications carrier that contains 100 wire pairs.
110
block
Part
of an organization's cross-connect facilities, a type of punch-down block
designed to terminate Cat 5 or better twisted pair wires.
25-pair
wire
UTP
supplied by a telecommunications carrier that contains 25 wire pairs.
alien
cross talk
EMI
interference induced on one cable by signals traveling over a nearby cable.
AM
(amplitude modulation)
A
modulation technique in which the amplitude of the carrier signal is modified
by the application of a data signal.
American
Wire Gauge
See
AWG
amplifier
A
device that boosts, or strengthens, an analog signal.
amplitude
A
measure of a signal's strength.
amplitude
modulation
See
AM.
analog
A
signal that uses variable voltage to create continuous waves, resulting in an
inexact transmission.
attenuation
The
extent to which a signal has weakened after traveling a given distance.
augmented
Category 6
See
Cat 6a.
AWG
(American Wire Gauge)
A
standard rating that indicates the diameter of a wire, such as the conducting
core of a coaxial cable.
bandwidth
A
measure of the difference between the highest and lowest frequencies that a
medium can transmit.
baseband
A
form of transmission in which digital signals are sent through direct current
pulses applied to a wire. This direct current requires exclusive use of the
wire's capacity, so baseband systems can transmit only one signal, or one
channel, at a time.
bend
radius
The
radius of the maximum arc into which you can loop a cable before you will cause
data transmission errors. Generally, a twisted pair cable's bend radius is
equal to or greater than four times the diameter of the cable.
binary
A
system founded on using 1s and 0s to encode information.
bit
(binary digit)
Equals
a single pulse in the digital encoding system. It may have only one of two
values: 0 or 1.
BNC
(Bayonet Neill-Concelman, or British Naval Connector)
A
standard for coaxial cable connectors named after its coupling method and its
inventors.
BNC
connector
A
coaxial cable connector type that uses a twist-and-lock (or bayonet) style of
coupling. It may be used with several coaxial cable types, including RG-6 and
RG-59.
braiding
A
braided metal shielding used to insulate some types of coaxial cable.
broadband
A
form of transmission in which signals are modulated as radio frequency analog
pulses with different frequency ranges. It does not involve binary encoding.
The use of multiple frequencies enables a system to operate over several
channels.
broadcast
A
transmission that involves one transmitter and multiple, undefined receivers.
byte
Eight
bits of information. In a digital signaling system, broadly speaking, 1 ___
carries one piece of information.
cable
plant
The
hardware that constitutes the enterprise-wide cabling system.
capacity
See
throughput.
Cat
Abbreviation
for the word category when describing a type of twisted pair cable. For
example, Category 5 unshielded twisted pair cable may also be called Cat 5.
Cat
3 (Category 3)
A
form of UTP that contains four wire pairs and can carry up to 10 Mbps, with a
possible bandwidth of 16 MHz. Cat 3 was used for 10-Mbps Ethernet or 4-Mbps
token ring networks.
Cat
5 (Category 5)
A
form of UTP that contains four wire pairs and supports up to 100-Mbps
throughput and a 100-MHz signal rate.
Cat
5e (Category 5e)
A
higher-grade version of Cat 5 wiring that contains highquality copper, offers a
high twist ratio, and uses advanced methods for reducing cross talk. Enhanced
Cat 5 can support a signaling rate of up to 350 MHz, more than triple the
capability of regular Cat 5.
Cat
6 (Category 6)
A
twisted pair cable that contains four wire pairs, each wrapped in foil
insulation. Additional foil insulation covers the bundle of wire pairs, and a
fire-resistant plastic sheath covers the second foil layer. The foil insulation
provides excellent resistance to cross talk and enables Cat 6 to support a
signaling rate of 250 MHz and at least six times the throughput supported by
regular Cat 5.
Cat
6a (Category 6a)
A
higher-grade version of Cat 6 wiring that further reduces attenuation and cross
talk and allows for potentially exceeding traditional network segment length
limits. Cat 6a is capable of a 500-MHz signaling rate and can reliably transmit
data at multi-gigabit per second rates.
Cat
7 (Category 7)
A
twisted pair cable that contains multiple wire pairs, each separately shielded
then surrounded by another layer of shielding within the jacket. Cat 7 can
support up to a 1-GHz signal rate. But because of its extra layers, it is less
flexible than other forms of twisted pair wiring.
Category
3
See
Cat 3.
Category
5
See
Cat 5.
Category
6
See
Cat 6.
Category
7
See
Cat 7.
channel
A
distinct communication path between two or more nodes, much like a lane is a
distinct transportation path on a freeway. Channels may be separated either
logically (as in multiplexing) or physically (as when they are carried by
separate wires).
cladding
The
glass or plastic shield around the core of a fiber-optic cable. Cladding
reflects light back to the core in patterns that vary depending on the
transmission mode. This reflection allows fiber to bend around corners without
impairing the light-based signal.
coaxial
cable
A
type of cable that consists of a central metal conducting core, which might be
solid or stranded and is often made of copper, surrounded by an insulator, a
braided metal shielding, called braiding, and an outer cover, called the sheath
or jacket. Coaxial cable, called "coax" for short, was the foundation
for Ethernet networks in the 1980s. Today it's used to connect cable Internet
and cable TV systems.
conduit
The
pipeline used to contain and protect cabling. Conduit is usually made from
metal.
connectors
The
pieces of hardware that connect the wire to the network device, be it a file
server, workstation, switch, or printer.
core
The
central component of a cable designed to carry a signal. The core of a
fiber-optic cable, for example, consists of one or several glass or plastic
fibers. The core of a coaxial copper cable consists of one large or several
small strands of copper.
crossover
cable
A
twisted pair patch cable in which the termination locations of the transmit and
receive wires on one end of the cable are reversed.
cross
talk
A
type of interference caused by signals traveling on nearby wire pairs
infringing on another pair's signal.
DB-9
connector
A
type of connector with nine pins that's commonly used in serial communication
that conforms to the RS-232 standard.
DB-25
connector
A
type of connector with 25 pins that's commonly used in serial communication
that conforms to the RS-232 standard.
demarc
See
demarcation point.
demarcation
point (demarc)
The
point of division between a telecommunications service carrier's network and a
building's internal network.
demultiplexer
(demux)
A
device that separates multiplexed signals once they are received and
regenerates them in their original form.
demux
See
demultiplexer.
dense
wavelength division multiplexing
See
DWDM.
digital
As
opposed to analog signals, digital signals are composed of pulses that can have
a value of only 1 or 0.
duplex
See
full-duplex
DWDM
(dense wavelength division multiplexing)
A
multiplexing technique used over single-mode or multimode fiber-optic cable in
which each signal is assigned a different wavelength for its carrier wave. In
DWDM, little space exists between carrier waves in order to achieve
extraordinary high capacity.
electromagnetic
interference
See
EMI.
EMI
(electromagnetic interference)
A
type of interference that may be caused by motors, power lines, televisions,
copiers, fluorescent lights, or other sources of electrical activity.
Enhanced
Category 5
See
Cat 5e.
entrance
facilities
The
facilities necessary for a service provider (whether it is a local phone
company, Internet service provider, or long-distance carrier) to connect with
another organization's LAN or WAN.
FDM
(frequency division multiplexing)
A
type of multiplexing that assigns a unique frequency band to each
communications subchannel. Signals are modulated with different carrier
frequencies, then multiplexed to simultaneously travel over a single channel.
ferrule
A
short tube within a fiber-optic cable connector that encircles the fiber strand
and keeps it properly aligned....
fiber-optic
cable
A
form of cable that contains one or several glass or plastic fibers in its core.
Data is transmitted via pulsing light sent from a laser or light-emitting diode
(LED) through the central fiber (or fibers). Fiber-optic cables offer
significantly higher throughput than copper-based cables. They may be
single-mode or multimode and typically use wavedivision multiplexing to carry multiple
signals.
FM
(frequency modulation)
A
method of data modulation in which the frequency of the carrier signal is
modified by the application of the data signal.
frequency
The
number of times that a signal's amplitude changes over a fixed period of time,
expressed in cycles per second, or hertz (Hz).
frequency
division multiplexing
See
FDM.
frequency
modulation
See
FM.
F-Type
connector
A
connector used to terminate coaxial cable used for transmitting television and
broadband cable signals.
full-duplex
A
type of transmission in which signals may travel in both directions over a
medium simultaneously. May also be called, simply, "duplex."
half-duplex
A
type of transmission in which signals may travel in both directions over a
medium, but in only one direction at a time.
hertz
(Hz)
A
measure of frequency equivalent to the number of amplitude cycles per second.
IDF
(intermediate distribution frame)
A
junction point between the MDF and concentrations of fewer connections-for
example, those that terminate in a telecommunications closet.
impedance
The
resistance that contributes to controlling an electrical signal. Impedance is
measured in ohms.
intermediate
distribution frame
See
IDF.
latency
The
delay between the transmission of a signal and its receipt.
LC
(local connector)
A
connector used with single-mode or multimode fiber-optic cable.
link
segment
See
unpopulated segment
local
connector
See
LC.
main
cross-connect
See
MDF.
main
distribution frame
See
MDF.
MDF
(main distribution frame)
Also
known as the main cross-connect, the first point of interconnection between an
organization's LAN or WAN and a service provider's facility.
mechanical
transfer registered jack
See
MT-RJ.
media
converter
A
device that enables networks or segments using different media to interconnect
and exchange signals.
MMF
(multimode fiber)
A
type of fiber-optic cable that contains a core with a diameter between 50 and
100 microns, through which many pulses of light generated by a lightemitting
diode (LED) travel at different angles.
modem
A
device that modulates analog signals into digital signals at the transmitting
end for transmission over telephone lines, and demodulates digital signals into
analog signals at the receiving end.
modulation
A
technique for formatting signals in which one property of a simple carrier wave
is modified by the addition of a data signal during transmission.
MT-RJ
(mechanical transfer registered jack)
A
connector used with single-mode or multimode fiber-optic cable.
multimode
fiber
See
MMF.
multiplexer
A
device that separates a medium into multiple channels and issues signals to
each of those subchannels.
multiplexing
A
form of transmission that allows multiple signals to travel simultaneously over
one medium.
near
end cross talk
See
NEXT.
NEXT
(near end cross talk)
Cross
talk, or the impingement of the signal carried by one wire onto a nearby wire,
that occurs between wire pairs near the source of a signal.
noise
The
unwanted signals, or interference, from sources near network cabling, such as
electrical motors, power lines, and radar.
nonbroadcast
point-to-multipoint transmission
A
communications arrangement in which a single transmitter issues signals to
multiple, defined recipients.
optical
loss
The
degradation of a light signal on a fiber-optic network.
overhead
The
nondata information that must accompany data for a signal to be properly routed
and interpreted by the network.
patch
cable
A
relatively short section (usually between 3 and 25 feet) of cabling with
connectors on both ends.
patch
panel
A
wall-mounted panel of data receptors into which cross-connect patch cables from
the punch-down block are inserted.
phase
A
point or stage in a wave's progress over time.
plenum
The
area above the ceiling tile or below the subfloor in a building.
point-to-multipoint
A
communications arrangement in which one transmitter issues signals to multiple
receivers. The receivers may be undefined, as in a broadcast transmission, or
defined, as in a nonbroadcast transmission.
point-to-point
A
data transmission that involves one transmitter and one receiver.
populated
segment
A
network segment that contains end nodes, such as workstations.
punch-down
block
A
panel of data receptors into which twisted pair wire is inserted, or punched
down, to complete a circuit.
radio
frequency interfaerence
See
RFI.
Recommended
Standard 232
See
RS-232.
regeneration
The
process of retransmitting a digital signal. Regeneration, unlike amplification,
repeats the pure signal, with none of the noise it has accumulated.
registered
jack 11
See
RJ-11.
registered
jack 45
See
RJ-45.
repeater
A
device used to regenerate a signal.
RFI
(radio frequency interference)
A
kind of interference that may be generated by broadcast signals from radio or
TV antennas.
RG-6
A
type of coaxial cable with an impedance of 75 ohms and that contains an 18 AWG
core conductor. RG-6 is used for television, satellite, and broadband cable
connections.
RG-8
A
type of coaxial cable characterized by a 50-ohm impedance and a 10 AWG core.
RG-8 provided the medium for the first Ethernet networks, which followed the
now obsolete 10BASE-5 standard.
RG-58
A
type of coaxial cable characterized by a 50-ohm impedance and a 24 AWG core.
RG-58 was a popular medium for Ethernet LANs in the 1980s, used for the
now-obsolete 10BASE-2 standard.
RG-59
A
type of coaxial cable characterized by a 75-ohm impedance and a 20 or 22 AWG
core, usually made of braided copper. Less expensive but suffering greater
attenuation than the more common RG-6 coax, RG-59 is used for relatively short
connections.
RJ-11
(registered jack 11)
The
standard connector used with unshielded twisted pair cabling (usually Cat 3 or
Level 1) to connect analog telephones.
RJ-45
(registered jack 45)
The
standard connector used with shielded twisted pair and unshielded twisted pair
cabling.
round-trip
time
See
RTT.
RS-232
(Recommended Standard 232)
A
Physical layer standard for serial communications, as defined by EIA/TIA.
RTT
(round-trip time)
The
length of time it takes for a packet to go from sender to receiver, then back
from receiver to sender. RTT is usually measured in milliseconds.
SC
(subscriber connector or standard connector)
A
connector used with single-mode or multimode fiber-optic cable.
serial
A
style of data transmission in which the pulses that represent bits follow one
another along a single transmission line. In other words, they are issued
sequentially, not simultaneously.
serial
cable
A
cable, such as an RS-232 type, that permits serial data transmission.
sheath
The
outer cover, or jacket, of a cable.
shield
See
braiding.
shielded
twisted pair
See
STP.
simplex
A
type of transmission in which signals may travel in only one direction over a
medium.
single-mode
fiber
See
SMF.
SMF
(single-mode fiber)
A
type of fiber-optic cable with a narrow core that carries light pulses along a
single path data from one end of the cable to the other end. Data can be
transmitted faster and for longer distances on single-mode fiber than on
multimode fiber. However, single-mode fiber is more expensive.
ST
(straight tip)
A
connector used with single-mode or multimode fiber-optic cable.
standard
connector
See
SC.
statistical
multiplexing
A
method of multiplexing in which each node on a network is assigned a separate
time slot for transmission, based on the node's priority and need.
STP
(shielded twisted pair)
A
type of cable containing twisted-wire pairs that are not only individually
insulated, but also surrounded by a shielding made of a metallic substance such
as foil.
straight-through
cable
A
twisted pair patch cable in which the wire terminations in both connectors
follow the same scheme.
straight
tip
See
ST.
structured
cabling
A
method for uniform, enterprise-wide, multivendor cabling systems specified by
the TIA/EIA 568 Commercial Building Wiring Standard. Structured cabling is
based on a hierarchical design using a high-speed backbone.
subchannel
One
of many distinct communication paths established when a channel is multiplexed
or modulated.
subscriber
connector
See
SC.
TDM
(time division multiplexing)
A
method of multiplexing that assigns a time slot in the flow of communications
to every node on the network and, in that time slot, carries data from that
node.
telecommunications
closet
Also
known as a "telco room," the space that contains connectivity for
groups of workstations in a defined area, plus cross-connections to IDFs or, in
smaller organizations, an MDF. Large organizations may have several
telecommunications closets per floor, but the TIA/EIA standard specifies at
least one per floor.
Thicknet
An
IEEE Physical layer standard for achieving a maximum of 10-Mbps throughput over
coaxial copper cable. Thicknet is also known as 10Base-5. Its maximum segment
length is 500 meters, and it relies on a bus topology.
Thinnet
An
IEEE Physical layer standard for achieving 10-Mbps throughput over coaxial
copper cable. Thinnet is also known as 10Base-2. Its maximum segment length is
185 meters, and it relies on a bus topology.
throughput
The
amount of data that a medium can transmit during a given period of time.
Throughput is usually measured in megabits (1,000,000 bits) per second, or
Mbps. The physical nature of every transmission media determines its potential
throughput.
time
division multiplexing
See
TDM.
transceiver
A
device that transmits and receives signals.
transmission
In
networking, the application of data signals to a medium or the progress of data
signals over a medium from one point to another.
Review Questions
1. What is different about the method used to boost a digital
signal’s strength, compared with the method of boosting an analog signal’s strength?
a. A digital
signal
requires an
amplifier, which
increases the strength of both the noise and the signal, and an analog signal
requires a repeater, which retransmits
the signal in its original
form.
b. A digital
signal
requires a repeater, which
increases
the strength of both the signal and
the noise it has accumulated,
and an analog signal
requires an amplifier,
which retransmits
the
signal in its original
form.
c.
A digital
signal
requires
a repeater,
which
retransmits
the signal in its original
form, and an analog
signal requires
an amplifier,
which
increases the
strength of both the signal and the noise
it has accumulated.
d. A digital
signal
requires an
amplifier, which
introduces noise into
the signal, and an analog signal
requires a repeater, which retransmits
the
signal in its original form.
2. Which of the following
decimal numbers corresponds to the binary number 0001001?
a.
5
b.
7
c.
9
d.
3
3. A wave with
which of the following frequencies would have the longest wavelength?
a. 10 MHz
b. 100 MHz
c. 1 GHz
d. 100 GHz
4. Ethernet
relies on which of
the following transmission types?
a. simplex
b. half-simplex
c. half-duplex
d. full-duplex
5.
In wavelength division multiplexing,
two modulated signals are guaranteed to
differ
in what characteristic?
a. Throughput
b. Color
c. Amplitude
d. Phase
6. Which of the following can increase latency on a network?
a. An EMI source,
such as fluorescent
lighting
b. The use of full-duplex transmission
c. The
use of multiple protocols
d. Adding 50 meters to the length
of the network
7. What part of a
cable protects
it against environmental
damage?
a. Sheath
b. Braiding
c. Plenum
d. Cladding
8. You are helping to install a cable
broadband system
in your
friend’s
home.
She
wants to bring the signal
from where the
service provider’s
cable enters the house to
a room on another floor, which means you
have to attach a new
cable
to the existing one.
What
type of
cable
should this be?
a. RG-6
b. RG-8
c. RG-58
d. RG-59
9. With everything else being equal, a network
using which of the following UTP types will
suffer the most cross talk?
a.
Cat 3
b. Cat
5
c. Cat 6a
d. Cat
7
10. Which of the following is not a source of EMI?
a. Power line
b. Megaphone
c. Microwave oven
d. Fluorescent lights
11. Which
of the following network
transmission media offers the highest
potential throughput over
the longest distances?
a. MMF
b. SMF
c. UTP
d. STP
12. In which
of the following network
links might you use MT-RJ connectors?
a. A coaxial
connection between a cable modem and a server
b. A UTP
connection between
a workstation
and a
hub
c. A wireless connection between a handheld computer and a
desktop computer
d. A fiber-optic connection between a
server and router
13. Fiber-optic cable
networks can
span much longer distances than copper-based networks before requiring repeaters because:
a. they use wavelength
division multiplexing.
b. their signals
never
require amplification.
c. they are resistant
to noise
such as EMI.
d. their signals
do not suffer degradation
no matter how small the cable’s bend
radius.
14. What
is the purpose of cladding in
a fiber-optic cable?
a.
It reflects the signal
back to the core.
b. It
protects
the
inner core from damage.
c. It shields the signal
from EMI.
d. It
concentrates
the signal and helps
keep it from fading.
15. Which of the following is a potential drawback to using fiber-optic cable for LANs?
a. It is expensive.
b. It
cannot
handle high-bandwidth
transmissions.
c. It
can
carry transmissions
using only TCP/IP.
d. It
is not yet an accepted standard for
high-speed networking.
16. In what
part of a structured cabling system would you find users’ desktop
computers?
a. Telco
room
b. MDF
c. IDF
d. Work area
17. Every user at one location
on your company’s
WAN
is unable to reach the Internet.
Your colleague on the network management team suspects a critical
router is unable to connect to
the network. Because you’re in the building affected by the outage, she asks you
to troubleshoot the router.
You confirm that the router
is not responding to any requests over
the network. Which
of the following do you use
to connect directly from your laptop
to the router to diagnose it?
a. An RG-6
cable
b. An RG-58
cable
c. A serial cable
d. An SMF
cable
18. What is the maximum distance specified in the structured
cabling standard for a
horizontal wiring subsystem?
a. 10 m
b. 90 m
c. 100 m
d. 200 m
19. How many wire pairs can a single 110 block terminate?
a. 55
b. 110
c. 200
d. It depends on the type of 110
block.
20. Your campuswide WAN is
experiencing slow Internet
response times. When you
call
your Internet
service provider to ask if they can
troubleshoot the problem from
their end, they warn you
that their responsibilities end
at the demarc. What do
they
mean?
a. They will not
diagnose problems
beyond your
organization’s
MDF.
b. They will not
diagnose problems
beyond your organization’s
entrance facilities.
c. They will not
diagnose problems
beyond your
organization’s IDF.
d. They will not
diagnose problems
beyond your
organization’s
telco rooms.
Sample Quiz
1. A
form of transmission that allows multiple signals to travel simultaneously over
one medium is known as ________ .
a. amplitude modulation
b. multiplexing
c. bilateral broadcasting
d. segmenting
2.
Connectivity devices are designed to accept only one type of media.
a. True
b. False
3.
What measurement identifies the difference between the highest and lowest
frequencies that a medium can transmit?
a. Bandwidth
b. Attenuation
c. Latency
d. Frequency
4.
In fiber-optic cable, data is transmitted via ________ through the central
fibers.
a. pulsing light sent from
a laser
b. pulsing electromagnetic signals
c. microwave signals
d. streaming radar signals
5.
The use of 1s and 0s to represent information is a characteristic of a(n)
________ system.
a. decimal
b. duplex
c. binary
d. analog
6. In twisted pair cable, a high twist ratio can result in
________.
a. greater attenuation
b. more repairs
c. cross talk
d. noise
7. ________ cable was the foundation for Ethernet networks in the
1980s and remained a popular transmission medium for many years.
a. Unshielded twisted
pair
b. Coaxial
c. Telephone
d. Shielded twisted
pair
8. The strength of an electrical signal is directly proportional
to its voltage.
a. True
b. False
9. Many network problems can be traced to ________ .
a. poor cable installation techniques
b. poor documentation
c. the use of too
many segments
d. improperly
installed application programs
10. What three specifications determine the size and scalability
of networking media?
a. Maximum nodes per segment, maximum segment length, and
maximum network length
b. Maximum nodes per
segment, maximum network length, and segment media
c. Maximum connectors
per segment, maximum nodes per segment, and maximum segment length
d. Maximum connectors
per segment, maximum segment length, and segment media
11. As an administrator on today's networks, you're more likely to
use an RS-232 connection between a workstation and a router to make your
workstation act as a console for configuring and managing that router.
a. True
b. False
12. Structured cabling is based on a hierarchical design that
begins where a telecommunications company's service enters a building and ends
at a user's workstation.
a. True
b. False
13. What transmission type involves one transmitter and multiple,
undefined receivers?
a. Nonbroadcast
point-to-multipoint
b. Broadcast
c.
Point-to-multipoint
d. Point-to-point
14. Broadband technology encodes information as digital pulses.
a. True
b. False
15. What is a benefit of analog signal transmission?
a. Analog signal
transmission is more reliable than digital transmission.
b. Analog signal
transmission is less susceptible to transmission flaws such as noise.
c. Analog signal transmission can convey greater
subtleties with less energy.
d. Analog signal
transmission voltage is varied.
Practice Quiz
1. Which type of modulation occurs when the
amplitude of the carrier signal is modified by the application of the data
signal?
a.
Statistical modulation
b.
Frequency modulation
c.
Amplitude
modulation
d.
Data modulation
2. ____ , also known as the main cross-connect,
is the first point of interconnection between an organization's LAN or WAN and
a service provider's facility.
a.
Cross-connect facilities
b.
Main
distribution frame
c.
Backbone wiring
d.
Ethernet
3. The term phase refers to the progress of a
wave over time in relationship to a fixed point.
a.
True
b.
False
4. ____ is a type of coaxial cable characterized
by a 75-ohm impedance and a 20 or 22 AWG core, usually made of braided copper.
a.
RG-59
b.
RG-45
5. EMI (electromagnetic interference) is a
latency issue.
a.
True
b.
False
6. A ____ is a piece of hardware that enables
networks or segments running on different media to interconnect and exchange
signals.
a.
local connector
b.
plenum
c.
repeater
d.
media
converter
7. Broadband technology encodes information
as digital pulses.
a.
True
b.
False
8. In ____ multiplexing, the transmitter
assigns slots to nodes according to priority and need.
a.
statistical
b.
frequency division
c.
wavelength division
d.
dense wavelength division
9. Computers generate and interpret digital
signals as electrical current, the pressure of which is measured in ____.
a.
volts
b.
gigabit
10. A____ is a panel of data receptors into
which twisted pair wire is inserted, or punched down, to complete a circuit.
a.
punch-down
block
11. ____ is a higher-grade version of Cat 6
wiring that reduces attenuation and cross talk, and allows for potentially
exceeding traditional network segment length limits.
a.
Cat 6a
b.
Enhanced Category 6
12. The term ____ refers to the number of
times that a wave's amplitude cycles from its starting point, through its highest
amplitude and its lowest amplitude, and back to its starting point over a fixed
period of time
a.
frequency
b.
bend radius
c.
ferrule
d.
impedance
13. Which term represents an analog wave’s
strength?
a.
Phase
b.
Wavelength
c.
Frequency
d.
Amplitude
14. The reduction of noise interference is a
benefit of digital signal transmissions.
a.
True
b.
False
15. A vertical cross-connect runs between a
building's floors.
a.
True
b.
False
16. Which term means to issue signals along a
network medium such as a cable?
a.
Transmission
b.
Transmit
c.
Transceiver
d.
Transistor
17. RG-8 is a type of coaxial cable that is
characterized by an impedance of 75 ohms and contains an 18 AWG conducting
core.
a.
True
b.
False
18. ____ is a form of UTP that contains four
wire pairs and can support up to 16 Mbps throughput.
a.
Cat 4
b.
Cat 5
c.
Cat 6
d.
Cat 7
19. Noise and devices connected to the
transmission medium can further limit throughput.
a.
True
b.
False
20. A____ is crimped, compressed, or twisted
onto a coaxial cable. It connects to another BNC connector via a turning and
locking mechanism — this is the bayonet coupling referenced in its name.
a.
punch-down block
b.
rollover cable
c.
BNC
connector
d.
patch panel
21. A____ is a part of the network that does
not contain end nodes, but simply connects two networking devices such as
routers.
a.
populated segment
b.
unpopulated
segment
22. The more twists per foot in a pair of
wires, the more resistant the pair will be to cross talk.
a.
True
b.
False
CHAPTER QUIZ
1. The
distance between corresponding points on a wave’s cycle is called its ____.
a.
phase
b.
amplitude
c. wavelength
d.
frequency
2. Many network problems can be traced to
poor cable ____ techniques.
a.
manufacturing
b.
planning
c. installation
d.
engineering
3. Which connector is used in RS-232
transmissions?
a.
Cat-5
b.
BNC
c.
ST
d. DB-25
4. ____ describes wiring that connects
workstations to the closest telecommunications closet.
a.
Work area
b. Horizontal wiring
c.
Simple wiring
d.
Backbone wiring
5. ____________________ cabling consists of one
or more insulated wire pairs encased in a plastic sheath.
UTP
6. Which standard is also known as structured
cabling?
a.
TIA/EIA 568A Standard
b.
TIA/EIA RS-232 (Recommended Standard 232)
c.
TIA/EIA 568BA Standard
d. TIA/EIA Commercial Building
Wiring Standard
7. Serial refers to a style of data
transmission in which the pulses that represent bits follow one another along a
____ transmission line.
a.
long
b. single
c.
short
d.
secondary
8. One of the most common transmission flaws
affecting data signals is ____.
a.
attenuation
b. noise
c.
latency
d.
throughput
9. A digital signal composed of a pulse of
positive voltage represents a(n) ____.
a.
0
b. 1
c.
4
d.
8
10. When signals are free to travel in both
directions over a medium simultaneously, the transmission is considered ____.
a. full-duplex
b.
simplex
c.
multiplex
d.
half-duplex
11. In modulation, a simple wave called a(n)
____ wave, is combined with another analog signal to produce a unique signal
that gets transmitted from one node to another.
a.
data
b.
FM
c. carrier
d.
information
12. ____________________ is a term used by
networking professionals to describe the nondata information that must
accompany data for a signal to be properly routed and interpreted by the
network.
Overhead
13. The data transmission characteristic most
frequently discussed and analyzed by networking professionals is ____.
a.
scalability
b.
cost
c.
noise
d. throughput
14. The most significant factor in choosing a
transmission method is its ____.
a.
attenuation
b.
noise
c.
latency
d. throughput
15. Ethernet is an example of a baseband
system found on many LANs.
a.
True
b.
False
16. ____________________ transmission involves one
transmitter and multiple receivers.
point-to-multipoint
17. The maximum distance a signal can travel
and still be interpreted accurately is equal to a segment’s maximum length.
a.
True
b.
False
18. Which term identifies a room containing
connectivity for groups of workstations in its area?
a. telecommunications closet
b.
entrance facilities
c.
work area
d.
MDF (main distribution frame)
19. ____________________ are the pieces of
hardware that connect the wire to the network device.
Connectors
20. _________ describes a popular serial data
transmission method.
a.
EIA/TIA Commercial Building Wiring Standard
b. EIA/TIA RS-232 (Recommended
Standard 232)
c.
EIA/TIA 568A standard
d.
EIA/TIA 568BA standard
21. The more twists per foot in a pair of
wires, the more resistant the pair will be to ____.
a.
latency
b.
attenuation
c. cross talk
d.
throughput
22. The points where circuits interconnect
with other circuits is known as ____.
a.
telecommunications closet
b.
IDF (intermediate distribution frame)
c. cross-connect facilities
d.
entrance facilities
23. The serial interface on the back of the
connectivity device is often labeled “____.”
a.
Connector
b.
Board
c.
Port
d. Console
24. ____ is a technology used with
fiber-optic cable, which enables one fiber-optic connection to carry multiple
light signals simultaneously.
a. WDM (wavelength division
multiplexing)
b.
FDM (frequency division multiplexing)
c.
TDM (time division multiplexing)
d.
Statistical multiplexing
25. The byte 00001110 means ____ on a digital
network.
a.
3
b.
6
c. 14
d.
30
