Chapter 8 - Review

Network+ Guide to Networks, Chapter 8 Review

Wireless Networking

The Wireless Spectrum

All wireless signals are carried through the air by electromagnetic waves. The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. On the spectrum, waves are arranged according to their frequencies, from lowest to highest. The wireless spectrum (as defined by the FCC, which controls its use) spans frequencies between 9 KHz and 300 GHz. Each type of wireless service can be associated with one area of the wireless spectrum. AM broadcasting, for example, sits near the low-frequency end of the wireless communications spectrum, using frequencies between 535 and 1605 KHz. Infrared waves belong to a wide band of frequencies at the high-frequency end of the spectrum, between 300 GHz and 300,000 GHz. Most wireless networks use frequencies around 2.4 GHz or 5 GHz and some use frequencies around 3.7 GHz. Figure 8-1 on page 345, shows the wireless spectrum and roughly identifies the range of frequencies associated with major wireless services. Later in this chapter, you will learn specifically which frequencies each technology uses. In the United States, the collection of frequencies available for communication—also known as “the airwaves”—is considered a natural resource available for public use. The FCC grants organizations in different locations exclusive rights to use each frequency. It also determines what frequency ranges can be used for what purposes. Of course, signals propagating through the air do not necessarily remain within one nation. Therefore, it is important for countries across the world to agree on wireless communications standards. ITU is the governing body that sets standards for international wireless services, including frequency allocation, signaling, and protocols used by wireless devices; wireless transmission and reception equipment; satellite orbits; and so on. If governments and companies did not adhere to ITU standards, chances are that a wireless device could not be used outside the country in which it was manufactured.

Characteristics of Wireless Transmission

In previous chapters, you learned about signals that travel over a physical medium, such as a copper or fiber-optic cable. Wired and wireless signals share many similarities, including use of the same Layer 3 and higher protocols, for example. However, the nature of the atmosphere makes wireless transmission vastly different from wired transmission. Because the air provides no fixed path for signals to follow, signals travel without guidance. Contrast this to guided media, such as UTP or fiber-optic cable, which do provide a fixed signal path. The lack of a fixed path requires wireless signals to be transmitted, received, controlled, and corrected differently from wired signals. Just as with wired signals, wireless signals originate from electrical current traveling along a conductor. The electrical signal travels from the transmitter to an antenna, which then emits the signal, as a series of electromagnetic waves, to the atmosphere. The signal propagates through the air until it reaches its destination. At the destination, another antenna accepts the signal, and a receiver converts it back to current. Figure 8-2 on page 346, illustrates this process. Notice that antennas are used for both the transmission and reception of wireless signals. As you would expect, to exchange information, two antennas must be tuned to the same frequency. In communications terminology, this means they share the same channel.

Antennas

Each type of wireless service requires an antenna specifically designed for that service. The service’s specifications determine the antenna’s power output, frequency, and radiation pattern. An antenna’s radiation pattern describes the relative strength over a three-dimensional area of all the electromagnetic energy the antenna sends or receives. A directional antenna issues wireless signals along a single direction. This type of antenna is used when the source needs to communicate with one destination, as in a point-to-point link. A satellite downlink (for example, the kind used to receive digital TV signals) uses directional antennas. In contrast, an omnidirectional antenna issues and receives wireless signals with equal strength and clarity in all directions. This type of antenna is used when many different receivers must be able to pick up the signal, or when the receiver’s location is highly mobile. TV and radio stations use omnidirectional antennas, as do most towers that transmit cellular signals. The geographical area that an antenna or wireless system can reach is known as its range. Receivers must be within the range to receive accurate signals consistently. Even within an antenna’s range, however, signals may be hampered by obstacles and rendered unintelligible.

Signal Propagation

Ideally, a wireless signal would travel directly in a straight line from its transmitter to its intended receiver. This type of propagation, known as LOS (line-of-sight), uses the least amount of energy and results in the reception of the clearest possible signal. However, because the atmosphere is an unguided medium and the path between a transmitter and a receiver is not always clear, wireless signals do not usually follow a straight line. When an obstacle stands in a signal’s way, the signal may pass through the object or be absorbed by the object, or it may be subject to any of the following phenomena: reflection, diffraction, or scattering. The object’s geometry governs which of these three phenomena occurs. Reflection in wireless signaling is no different from reflection of other electromagnetic waves, such as light. The wave encounters an obstacle and reflects—or bounces back—toward its source. For this reason, when assessing wireless transmission, reflection is also called bounce. A wireless signal will bounce off objects whose dimensions are large compared with the signal’s average wavelength. In the context of a wireless LAN, which may use signals with wavelengths between 1 and 10 meters, such objects include walls, floors, ceilings, and the Earth. In addition, signals reflect more readily off conductive materials, like metal, than insulators, like concrete. In diffraction, a wireless signal splits into secondary waves when it encounters an obstruction. The secondary waves continue to propagate in the direction in which they were split. If you could see wireless signals being diffracted, they would appear to be bending around the obstacle. Objects with sharp edges—including the corners of walls and desks—cause diffraction. Scattering is the diffusion, or the reflection in multiple different directions, of a signal. Scattering occurs when a wireless signal encounters an object that has small dimensions compared with the signal’s wavelength. Scattering is also related to the roughness of the surface a wireless signal encounters. The rougher the surface, the more likely a signal is to scatter when it hits that surface. In an office building, objects such as chairs, books, and computers cause scattering of wireless LAN signals. For signals traveling outdoors, rain, mist, hail, and snow may all cause scattering. Because of reflection, diffraction, and scattering, wireless signals follow a number of different paths to their destination. Such signals are known as multipath signals. Figure 8-3 on page 348, illustrates multipath signals caused by these three phenomena.

The multipath nature of wireless signals is both a blessing and a curse. On one hand, because signals bounce off obstacles, they have a better chance of reaching their destination. In environments such as an office building, wireless services depend on signals bouncing off walls, ceilings, floors, and furniture so that they may eventually reach their destination. Imagine how inconvenient and inefficient it would be, for example, to make sure you were standing within clear view of a transmitter to receive a text message. The downside to multipath signaling is that, because of their various paths, multipath signals travel different distances between their transmitter and a receiver. Thus, multiple instances of the same signal can arrive at a receiver at different times. This may cause signals to be misinterpreted, resulting in data errors. Error-correction algorithms will detect the errors and the sender will have to retransmit the signal. The more errors that occur, the slower the throughput; Environments such as manufacturing plants, which contain myriad reflective surfaces, experience greater throughput degradation than relatively open spaces, such as homes.

Signal Degradation

No matter what paths wireless signals take, they are bound to run into obstacles. When they do, the original signal issued by the transmitter will experience fading, or a variation in signal strength as a result of some of the electromagnetic energy being scattered, reflected, or diffracted after being issued by the transmitter. Multipath signaling is a significant cause of fading. Because of fading, the strength of the signal that reaches the receiver is lower than the transmitted signal’s strength. This makes sense because as more waves are reflected, diffracted, or scattered by obstacles, fewer are likely to reach their destination on time and without errors. Excessive fading can cause dropped calls or slow data transmission. As with wired signals, wireless signals also experience attenuation. After a signal is transmitted, the farther it moves away from the transmission antenna the more it weakens. Just as with wired transmission, wireless signals are amplified (if analog) or repeated (if digital) to strengthen the signal so that it can be clearly received. The difference is that the intermediate points through which wireless signals are amplified or repeated are transceivers connected to antennas. However, attenuation is not the most severe flaw affecting wireless signals. Wireless signals are also susceptible to noise. As you learned in Chapter 3, noise is also known as EMI (electromagnetic interference), or, in the context of wireless communications, interference. Interference is a significant problem for wireless communications because the atmosphere is saturated with electromagnetic waves. For example, wireless LANs may be affected by cellular phones, hands-free headsets, microwaves, machinery, or overhead lights. Interference can distort and weaken a wireless signal in the same way that noise distorts and weakens a wired signal. However, because wireless signals cannot depend on a conduit or shielding to protect them from extraneous EMI, they are more vulnerable to noise. The extent of interference that a wireless signal experiences depends partly on the density of signals within a geographical area. Signals traveling through areas in which many wireless communications systems are in use—for example, the center of a metropolitan area—are the most apt to suffer interference.

Frequency Ranges

For many years, wireless networks relied on frequencies in the range of 2.4–2.4835 GHz, more commonly known as the 2.4-GHz band, to send and receive signals. This band offers 11 communications channels that are unlicensed in the United States. An unlicensed frequency is one for which the FCC does not require users to register their service and reserve it for their sole use.

Because the 2.4-GHz band also carries cordless telephone and other types of signals, it is highly susceptible to interference. For example, on your home wireless network, your tablet computer might lose connectivity when your cordless telephone rings. One way to guard against this type of interference is to make sure your access point and cordless telephone use different channels within the 2.4-GHz band. Wireless LANs and WANs may instead use the 5-GHz band, which comprises four frequency bands: 5.1 GHz, 5.3 GHz, 5.4 GHz, and 5.8 GHz. It consists of 24 unlicensed bands, each 20-MHz wide. Because the 5-GHz band is also used by weather and military radar communications in the United States, WLAN equipment using this range of frequencies must be able to monitor and detect radar signals and, if one is detected, switch to a different channel automatically.

Narrowband, Broadband, and Spread-Spectrum Signals

Transmission technologies differ according to how much of the wireless spectrum their signals use. An important distinction is whether a wireless service uses narrowband or broadband signaling. In narrowband, a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies. In contrast to narrowband, broadband uses a relatively wide band of the wireless spectrum. Broadband technologies, as a result of their wider frequency bands, offer higher throughputs than narrowband technologies. The use of multiple frequencies to transmit a signal is known as spread-spectrum technology because the signal is spread out over the wireless spectrum. In other words, a signal never stays continuously within one frequency range during its transmission. One result of spreading a signal over a wide frequency band is that it requires less power per frequency than narrowband signaling. This distribution of signal strength makes spread-spectrum signals less likely to interfere with narrowband signals traveling in the same frequency band. Spread-spectrum signaling, originally used with military wireless transmissions in World War II, remains a popular way of making wireless transmissions more secure. Because signals are split across several frequencies according to a sequence known only to the authorized transmitter and receiver, it is much more difficult for unauthorized receivers to capture and decode spread-spectrum signals. To generic receivers, signals issued via spread-spectrum technology appear as unintelligible noise. One specific implementation of spread spectrum is FHSS (frequency hopping spread spectrum). In FHSS transmission, a signal jumps between several different frequencies within a band in a synchronization pattern known only to the channel’s receiver and transmitter, as shown in Figure 8-4 on page 350. FHSS devices in the U.S. split the 2.4–2.4835 band into 79 distinct frequencies. Another type of spread-spectrum signaling is called DSSS (direct-sequence spread spectrum). In DSSS, a signal’s bits are distributed over an entire frequency band at once, as shown in Figure 8-5 on page 351. Each bit is coded so that the receiver can reassemble the original signal upon receiving the bits. Most wireless LAN standards specify some form of DSSS modulation.

Fixed vs. Mobile

Each type of wireless communication falls into one of two categories: fixed or mobile. In fixed wireless systems, the locations of the transmitter and receiver do not move. The transmitting antenna focuses its energy directly toward the receiving antenna. This results in a point-to-point link. One advantage of fixed wireless is that, because the receiver’s location is predictable, energy need not be wasted issuing signals across a large geographical area. Thus, more energy can be used for the signal. Fixed wireless links are used in some data and voice applications. For example, a service provider may obtain data services through a fixed link with a satellite.

In cases in which a long distance or difficult terrain must be traversed, fixed wireless links are more economical than cabling. Many types of communications are unsuited to fixed wireless, however. For example, a waiter who uses a wireless handheld computer to transmit orders to the restaurant’s kitchen could not use a service that requires him to remain in one spot to send and receive signals. Instead, wireless LANs, along with cellular telephone, paging, and many other services use mobile wireless systems. In mobile wireless, the receiver can be located anywhere within the transmitter’s range. This allows the receiver to roam from one place to another while continuing to pick up its signal. Now that you understand some characteristics of wireless transmission, you are ready to learn about the way most wireless LANs are structured. Later, you’ll learn about their access methods and how to install wireless connectivity devices.

WLAN (Wireless LAN) Architecture

Because they are not bound by cabling paths between nodes and connectivity devices, wireless networks are not laid out using the same topologies as wired networks. They have their own, different layouts. Smaller wireless networks, in which a small number of nodes closely positioned need to exchange data, can be arranged in an ad hoc fashion. In an ad hoc WLAN, wireless nodes, or stations, transmit directly to each other via wireless NICs without an intervening connectivity device, as shown in Figure 8-6 on page 352.  However, an ad hoc arrangement would not work well for a WLAN with many users or whose users are spread out over a wide area, or where obstacles could stand in the way of signals between stations. Instead of communicating directly with each other in ad hoc mode, nearly all WLANs use the infrastructure mode, which depends on an intervening connectivity device called a wireless access point. A wireless access point (WAP)—also known simply as an access point, or an AP—is a device that accepts wireless signals from multiple nodes and retransmits them to the rest of the network. Access points may also be known as base stations. Access points for use on small office or home networks often include routing functions. As such, they may also be called wireless routers or wireless gateways. To cover its intended range, an access point must have sufficient power and be strategically placed so that stations can communicate with it. For instance, if an access point serves a group of workstations in several offices on one floor in a building, it should probably be located in an open area near the center of that floor. And like other wireless devices, access points contain an antenna connected to their transceivers. An infrastructure WLAN is shown in Figure 8-7 on page 353. It’s common for a WLAN to include several access points. The number of access points depends on the number of stations a WLAN connects. The maximum number of stations each access point can serve varies from 10 to 100, depending on the wireless technology used. Exceeding the recommended maximum leads to a greater incidence of errors and slower overall transmission. Mobile networking allows wireless nodes to roam from one location to another within a certain range of their access point. This range depends on the wireless access method, the equipment manufacturer, and the office environment. As with other wireless technologies, WLAN signals are subject to interference and obstruction that cause multipath signaling. Therefore, a building with many thick, concrete walls, for example, will limit the effective range of a WLAN more severely than an open area divided into cubicles. In most WLAN scenarios, stations must remain within 300 feet of an access point to maintain optimal transmission speeds. In addition to connecting multiple nodes within a LAN, wireless technology can be used to connect two different parts of a LAN or two separate LANs. Such connections typically use a fixed link with directional antennas between two access points, as shown in Figure 8-8 on page 354.

Because point-to-point links only have to transmit in one direction, they can apply more energy to signal propagation than mobile wireless links. As a result of applying more energy to the signal, their maximum transmission distance is greater. In the case of connecting two WLANs, access points could be as far as 1000 feet apart. WLANs support the same protocols (for example, TCP/IP) and operating systems (for example, UNIX, Linux, or Windows) as wired LANs. This compatibility ensures that wireless and wired transmission methods can be integrated on the same network. Only the signaling techniques differ between wireless and wired portions of a LAN. However, techniques for generating and encoding wireless signals vary from one WLAN standard to another. The following section describes the most popular WLAN Physical and Data Link layer standards.

802.11 WLANs

Similar to the development of wired network access technologies, the evolution of wireless access methods did not follow one direct and cooperative path, but grew from the efforts of multiple vendors and organizations. Now, the industry accepts a handful of different wireless technologies. Each wireless technology is defined by a standard that describes unique functions at both the Physical and the Data Link layers of the OSI model. These standards differ in their specified signaling methods, geographic ranges, and frequency usages, among other things. Such differences make certain technologies better suited to home networks and others better suited to networks at large organizations. The most popular wireless standards used on contemporary LANs are those developed by IEEE’s 802.11 committee. IEEE released its first wireless network standard in 1997. Since then, its WLAN standards committee, also known as the 802.11 committee, has published several distinct standards related to wireless networking. Each IEEE wireless network access standard is named after the 802.11 task group (or subcommittee) that developed it. The four IEEE 802.11 task groups that have generated notable wireless standards are 802.11b, 802.11a, 802.11g, and 802.11n. Collectively, these four 802.11 standards are known as Wi-Fi, for wireless fidelity, and they share many characteristics. For example, although some of their Physical layer services vary, all four use half-duplex signaling. In other words, a wireless station using one of the 802.11 techniques can either transmit or receive, but cannot do both simultaneously (assuming the station has only one transceiver installed, as is usually the case). In addition, all 802.11 networks follow the same access method, as described in the following section.

Access Method

In Chapter 2, you learned that the MAC sublayer of the Data Link layer is responsible for appending physical addresses to a data frame and for governing multiple nodes’ access to a single medium. As with 802.3 (Ethernet), the 802.11 MAC services append 48-bit (or 6-byte) physical addresses to a frame to identify its source and destination. The use of the same physical addressing scheme allows 802.11 networks to be easily combined with other IEEE 802 networks, including Ethernet networks. However, because wireless devices are not designed to transmit and receive simultaneously, and, therefore, cannot quickly detect collisions, 802.11 networks use a different access method than Ethernet networks. 802.11 standards specify the use of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to access a shared medium. Using CSMA/CA, a station on an 802.11 network checks for existing wireless transmissions before it begins to send data. If the source node detects no transmission activity on the network, it waits a brief, random amount of time, and then sends its transmission.

If the source does detect activity, it waits a brief period of time before checking the channel again. The destination node receives the transmission and, after verifying its accuracy, issues an acknowledgment (ACK) packet to the source. If the source receives this acknowledgment, it assumes the transmission was properly completed. However, interference or other transmissions on the network could impede this exchange. If, after transmitting a message, the source node fails to receive acknowledgment from the destination node, it assumes its transmission did not arrive properly, and it begins the CSMA/CA process anew. Compared with CSMA/CD (Carrier Sense Multiple Access with Collision Detection), CSMA/CA minimizes, but does not eliminate, the potential for collisions. The use of ACK packets to verify every transmission means that 802.11 networks require more overhead than 802.3 networks. Therefore, a wireless network with a theoretical maximum throughput of 10 Mbps will, in fact, transmit less data per second than a wired Ethernet network with the same theoretical maximum throughput. In reality, most wireless networks tend to achieve between one-third and one-half of their theoretical maximum throughput. For example, one type of 802.11 network,

802.11g, is rated for a maximum of 54 Mbps; most 802.11g networks achieve between 20 and 25 Mbps. As described later in this chapter, however, the 802.11n standard includes several techniques for reducing overhead and making the technology’s actual throughput match its theoretical throughput. One way to ensure that packets are not inhibited by other transmissions is to reserve the medium for one station’s use. In 802.11, this can be accomplished through the optional RTS/CTS (Request to Send/Clear to Send) protocol. RTS/CTS enables a source node to issue an RTS signal to an access point requesting the exclusive opportunity to transmit. If the access point agrees by responding with a CTS signal, the access point temporarily suspends communication with all stations in its range and waits for the source node to complete its transmission. RTS/CTS is not routinely used by wireless stations, but for transmissions involving large packets (those more subject to damage by interference), RTS/CTS can prove more efficient. On the other hand, using RTS/CTS further decreases the overall efficiency of the 802.11 network. Figure 8-9 on page 356, illustrates the CSMA/CA process.

Association

Suppose you have just purchased a new laptop with a wireless NIC that supports one of the

802.11 wireless standards. When you bring your laptop to a local Internet café and turn it on, your laptop soon prompts you to log on to the café’s wireless network to gain access to the Internet. This seemingly simple process, known as association, involves a number of packet exchanges between the café’s access point and your computer. Association is another function of the MAC sublayer described in the 802.11 standard.  As long as a station is on and has its wireless protocols running, it periodically surveys its surroundings for evidence of an access point, a task known as scanning. A station can use either active scanning or passive scanning. In active scanning, the station transmits a special frame, known as a probe, on all available channels within its frequency range. When an access point finds the probe frame, it issues a probe response. This response contains all the information a station needs to associate with the access point, including a status code and station ID number for that station. After receiving the probe response, a station can agree to associate with that access point. The final decision to associate with an access point, at least for the first time, usually requires the consent of the user. Once association is complete, the two nodes begin communicating over the frequency channel specified by the access point. In passive scanning, a wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point.

The beacon frame contains information that a wireless node requires to associate itself with the access point. For example, the frame indicates the network’s transmission rate and the SSID (service set identifier), a unique character string used to identify an access point. After detecting a beacon frame, the station can choose to associate with that access point. The two nodes agree on a frequency channel and begin communicating. When setting up a WLAN most network administrators use the access point’s configuration utility to assign a unique SSID; rather than the default SSID provided by the manufacturer. This can contribute to better security and easier network management. For example, the access point used by employees in the Customer Service Department of a company could be assigned the SSID “CustSvc”. In IEEE terminology, a group of stations that share an access point are said to be part of one BSS (basic service set). The identifier for this group of stations is known as a BSSID (basic service set identifier). Some WLANs are large enough to require multiple access points. A group of access points connected to the same LAN are known collectively as an ESS (extended service set). BSSs that belong to the same ESS share a special identifier, called an ESSID (extended service set identifier). In practice, many networking professionals don’t distinguish between the terms SSID and ESSID. They simply configure every access point in a group or LAN with the same SSID. Within an ESS, a client can associate with any one of many access points that use the same ESSID. That allows users to roam about an office without losing wireless network service. In fact, roaming is the term applied to a station moving from one BSS to another without losing connectivity. Figure 8-10 on page 358, illustrates a network with only one BSS; Figure 8-11 on page 359, shows a network encompassing multiple BSSs that form an ESS. Clients running Windows 7 or modern versions of Linux will first attempt to associate with a known access point. For example, suppose the SSID for your access point at home is “SpaceInvader”. When you visit a café on the other side of the city, your laptop will recognize that the “SpaceInvader” SSID doesn’t exist in that location. Instead, your laptop’s operating system will detect the presence of other access points in the area. If the café has an access point, for example, it will offer you the option of associating with that access point. Suppose the café is in a busy metropolitan area where every business on the block has its own access point. In that case, the operating system (or the NetworkManager program, if you are running Linux) will present you with a list of all access points within range. Further, your client software will prioritize the access point with the strongest signal and the lowest error rate compared with others. Note that a station does not necessarily prioritize the closest access point. For example, suppose another user brings his own access point to the café and his access point has a signal that is twice as strong as the café’s access point. In that case, even if the new access point is farther away, your laptop will recognize the other user’s access point as the best option. When you are presented with this option, however, you would be wise to not confirm the association. If a client is configured to indiscriminately connect with the access point whose signal is strongest, that client is susceptible to being compromised by a powerful, rogue access point. If your system associates with this unauthorized access point, the person controlling that access point could steal your data or gain access to another network that trusts your system. Rogue access points can exist inadvertently, too, as when a user brings his own access point to work or uses software to turn his workstation into an access point. On a network with several authorized access points in an ESS, however, a station must be able to associate with any access point while maintaining network connectivity. Suppose that when you begin work in the morning at your desk, your laptop associates with an access point located in a telco room down the hall. Later, you need to give a presentation in the company’s main conference room on another floor of your building. Without your intervention, your laptop will choose a different access point as you travel to the conference room (perhaps more than one, depending on the size of your company’s building and network).

Connecting to a different access point requires reassociation. Reassociation occurs when a mobile user moves out of one access point’s range and into the range of another, as described in the previous example. It might also happen if the initial access point is experiencing a high rate of errors. On a network with multiple access points, network managers can take advantage of the stations’ scanning feature to automatically balance transmission loads between those access points.

Frames

You have learned about some types of overhead required to manage access to the 802.11 wireless networks—for example, ACKs, probes, and beacons. For each function, the 802.11 standard specifies a frame type at the MAC sublayer. These multiple frame types are divided into three groups: control, management, and data. Management frames are those involved in association and reassociation, such as the probe and beacon frames. Control frames are those related to medium access and data delivery, such as the ACK and RTS/CTS frames. Data frames are those that carry the data sent between stations. An 802.11 data frame is illustrated in Figure 8-12 on page 360. (Details of control and management frames are beyond the scope of this book.) Glancing at the 802.11 data frame, its significant overhead—that is, the large quantity of fields added to the data field—becomes apparent. These fields are explained next. Compare the 802.11 data frame with the Ethernet data frame also shown in Figure 8-12. Notice that the wireless data frame contains four address fields, rather than two. These four addresses are the source address, transmitter address, receiver address, and destination address. The transmitter and receiver addresses refer to the access point or another intermediary device (if used) on the wireless network. The source and destination addresses have the same meaning as they do in the Ethernet II frame. Another unique characteristic of the 802.11 data frame is its Sequence Control field. This field is used to indicate how a large packet is fragmented—that is, subdivided into smaller packets for more reliable delivery. Recall that on wired TCP/IP networks, error checking occurs at the Transport layer of the OSI model and packet fragmentation, if necessary, occurs at the Network layer. However, in 802.11 networks, error checking and packet fragmentation is handled at the MAC sublayer of the Data Link layer. By handling fragmentation at a lower layer, 802.11 makes its transmission—which is less efficient and more error-prone—transparent to higher layers. This means 802.11 nodes are more easily integrated with 802.3 networks and prevent the 802.11 segments of an integrated network from slowing down the 802.3 segments. The Frame Control field in an 802.11 data frame holds information about the protocol in use, the type of frame being transmitted, whether the frame is part of a larger, fragmented packet, whether the frame is one that was reissued after an unverified delivery attempt, what type of security the frame uses, and so on. Security is a significant concern with WLANs because access points are typically more vulnerable than devices on a wired network. Wireless security is discussed in detail along with other network security topics in Chapter 11. Although 802.11b, 802.11a, 802.11g, and 802.11n share all of the MAC sublayer characteristics described in the previous sections, they differ in their modulation methods, frequency usage, and ranges. In other words, each varies at the Physical layer. In addition, 802.11n modifies the way frames are used at the MAC sublayer. The following sections summarize those differences.

802.11b

In 1999, the IEEE released its 802.11b standard, which uses the 2.4–2.4835-GHz frequency range, better known as the 2.4-GHz band, and separates it into 22-MHz channels. 802.11b provides a theoretical maximum of 11-Mbps throughput; actual throughput is typically around 5 Mbps. To ensure this throughput, wireless nodes must stay within 100 meters (or approximately 330 feet) of an access point or each other, in the case of an ad hoc network. Among all the 802.11 standards, 802.11b was the first to take hold. It is also the least expensive of all the 802.11 WLAN technologies. However, most network administrators have replaced 802.11b with a faster standard, such as 802.11n.

802.11a

Although the 802.11a task group began its standards work before the 802.11b group, 802.11a was released after 802.11b. The 802.11a standard differs from 802.11b and 802.11g in that it uses channels in the 5-GHz band and provides a maximum theoretical throughput of 54 Mbps, though its effective throughput falls generally between 11 and 18 Mbps. 802.11a’s high throughput is attributable to its use of higher frequencies, its unique method of modulating data, and more available bandwidth. Perhaps most significant is that the 5-GHz band is not as congested as the 2.4-GHz band. Thus, 802.11a signals are less likely to suffer interference from microwave ovens, cordless phones, motors, and other (incompatible) wireless LAN signals. However, higher-frequency signals require more power to transmit, and they travel shorter distances than lower-frequency signals. The average geographic range for an 802.11a antenna is 20 meters, or approximately 66 feet. As a result, 802.11a networks require a greater density of access points between the wired LAN and wireless clients to cover the same distance that 802.11b networks cover. The additional access points, as well as the nature of 802.11a equipment, make this standard more expensive than either 802.11b or 802.11g. For this and other reasons, 802.11a is rarely preferred.

802.11g

IEEE’s 802.11g WLAN standard is designed to be just as affordable as 802.11b while increasing its maximum theoretical throughput from 11 Mbps to 54 Mbps through different data modulation techniques. The effective throughput of 802.11g ranges generally from 20 to 25 Mbps.

An 802.11g antenna has a geographic range of 100 meters (or approximately 330 feet).

802.11g, like 802.11b, uses the 2.4-GHz frequency band. In addition to its high throughput,

802.11g benefits from being compatible with 802.11b networks. Thus, if a network administrator installed 802.11b access points on her LAN three years ago, this year she could add 802.11g access points and laptops, and the laptops could roam between the ranges of the 802.11b and 802.11g access points without an interruption in service.

802.11n

In 2009, IEEE ratified the 802.11n standard. However, it was in development for years before that, and as early as mid-2007, manufacturers were selling 802.11n-compatible transceivers in their networking equipment. The primary goal of IEEE’s 802.11n committee was to create a wireless standard that provided much higher effective throughput than the other 802.11 standards. By all accounts, they succeeded. 802.11n boasts a maximum throughput of 600 Mbps, making it a threat to Fast Ethernet and a realistic platform for telephone and video signals. IEEE also specified that the 802.11n standard must be backward compatible with the 802.11a, b, and g standards.

802.11n may use either the 2.4-GHz or 5-GHz frequency range. It employs the same data modulation techniques used by 802.11a and 802.11g. However, it differs dramatically from the other three 802.11 standards in how it manages frames, channels, and encoding. These differences, which allow 802.11n to achieve its high throughput, include the following innovations:

MIMO (multiple input-multiple output) - In 802.11n, multiple antennas on an access point may issue a signal to one or more receivers. As you learned earlier, signals issued by an omnidirectional antenna will propagate in a multipath fashion. Therefore, multiple signals cannot be expected to arrive at the same receiver in concert. To account for this, in MIMO the phases of these signals are adjusted when they reach a receiving station, and the strength of the multiple signals are summed. To properly adjust phases, MIMO requires stations to update access points with information about their location.  Among 802.11 equipment this function is only available with 802.11n-capable transceivers. In addition to increasing the network’s throughput, MIMO can increase an access point’s range. Figure 8-13 on page 362, shows an 802.11n access point with three antennas.

Channel bonding - In 802.11n, two adjacent 20-MHz channels can be combined, or bonded, to make a 40-MHz channel, as shown in Figure 8-14 on page 363.  In fact, bonding two 20-MHz channels more than doubles the bandwidth available in a single 20-MHz channel; that’s because the small amount of bandwidth normally reserved as buffers against interference at the top and bottom of the 20-MHz channels can be assigned to carry data instead. Because the 5-GHz band contains more channels and is less crowded (at least, for now), it’s better suited to channel bonding than the 2.4-GHz band.

Higher modulation rates - As mentioned earlier, 802.11n uses the same type of data modulation used by 802.11a and 802.11g. This modulation technique allows for a single channel to be subdivided into multiple, smaller channels. Simply put, 802.11n makes more efficient use of these smaller channels and is capable of choosing from different encoding methods. 802.11n also allows for shortening the period of time transceivers wait between issuing each bit of data (which is necessary to prevent interference).

Frame aggregation - 802.11n networks can use one of two techniques for combining multiple frames (of the type shown in Figure 8-12) into one larger frame. Combining multiple frames reduces overhead. Suppose four small data frames are combined into one larger frame. Each larger frame will have only one copy of the same addressing information that would appear in the smaller frames. Proportionally, the data field takes up more of the aggregated frame’s space. In addition, replacing four small frames with one large frame means an access point and station will have to exchange one-quarter the number of statements to negotiate media access and error control. To take advantage of frame aggregation, the maximum frame size for 802.11n is 64 KB, compared with the maximum 802.11a, b, and g frame size of 4 KB. The potential disadvantage with using larger frames is the increased probability of errors when transmitting larger blocks of data. Figure 8-15 on page 363, illustrates the relatively low overhead of an aggregated 802.11n frame.

Note that not all of the techniques listed here will be used in every 802.11n implementation.

Further, reaching maximum throughput depends on the number and type of these strategies used. It also depends on whether the network uses the 2.4-GHz or 5-GHz band. Considering these factors, an 802.11n network’s actual throughputs will vary between 65 to 500 Mbps.

As mentioned earlier, 802.11n is compatible with all three earlier versions of the 802.11 standard. However, in mixed environments, some of the new standard’s techniques for improving throughput will not be possible. To ensure the fastest data rates on your 802.11n LAN, it’s optimal to use only 802.11n-compatible devices. To qualify for Network+ certification, you need to understand the differences between the 802.11 wireless standards. A summary of these WLAN standards is shown in Table 8-1.

Table 8-1 Wireless standards

Standard	Frequency range	Theoretical maximum throughput	Effective throughput (approximate)	Average geographic range
802.11b	2.4 GHz	11 Mbps	5 Mbps	100 meters (or approximately 330 feet)
802.11a	5 GHz	54 Mbps	11–18 Mbps	20 meters (or approximately 66 feet)
802.11g	2.4 GHz	54 Mbps	20–25 Mbps	100 meters (or approximately 330 feet)
802.11n	2.4 GHz or 5 GHz	65 to 600 Mbps	65 to 600 Mbps	Up to 400 meters (or approximately 1310 feet) if MIMO is used

The actual geographic range of any wireless technology depends on several factors, including the power of the antenna, physical barriers or obstacles between sending and receiving nodes, and interference in the environment. Therefore, although a technology is rated for a certain average geographic range, it may actually transmit signals in a shorter or longer range.

Implementing a WLAN

Now that you understand how wireless signals are exchanged, what can hinder them, and which Physical and Data Link layer standards they may follow, you are ready to put these ideas into practice. This section first describes how to design small WLANs, the types you might use at home or in a small office. It also describes how larger, enterprise-wide WANs are formed. Next it walks you through installing and configuring access points and clients. Finally, it details the pitfalls of implementing WLANs and how to avoid them.

Determining the Design

You have learned that WLANs may be arranged as ad hoc or infrastructure networks. You also know that infrastructure WLANs is far more common. This section assumes your WLAN follows the infrastructure model, and as such, will include access points. A home or small office network might call for only one access point. In this case, the access point, often combined with switching and routing functions, connects wireless clients to the LAN and acts as their gateway to the Internet. Note that the access point functions independently from the Internet access technology.

In other words, configuring your home or small office WLAN follows the same principles no matter whether you connect to the Internet using broadband cable or DSL. Figure 8-16 on page 365, illustrates the typical arrangement of a home or small office WLAN. Notice that the access point (or wireless router) is connected to the cable or DSL modem using an RJ-45 cable. The cable is inserted into the access point’s WAN port, which is set apart from the other data ports and might be labeled “Internet” or remain unlabeled. The additional ports on the access point allow for wired access to the router. An access point that does not include routing or switching functions would lack these extra ports and act much like a wireless hub. Placement of an access point on a WLAN must take into account the typical distances between the access point and its clients. If your small office spans three floors, for instance, and clients are evenly distributed among the floors, you might choose to situate the access point on the second floor. Recall that 802.11b and g signals can extend a maximum of 330 feet and still deliver data reliably, while 802.11n signals can, using MIMO, extend a maximum of 1310 feet. Also consider the type and number of obstacles between the access point and clients. For example, if your three-story building is constructed like a bunker with massive concrete floors, you might consider installing a separate access point on each floor. For best signal coverage, place the access point in a high spot, such as on a shelf or rack or in a drop ceiling. Also, make sure it’s not close to potential sources of interference, including cordless phones and microwave ovens. Larger WLANs warrant a more systematic approach to access point placement. Before placing access points in every telco room, it’s wise to conduct a site survey. A site survey assesses client requirements, facility characteristics, and coverage areas to determine an access point arrangement that will ensure reliable wireless connectivity within a given area. For example, suppose you are the network manager for a large organization whose wireless clients are distributed over six floors of a building. On two floors, your organization takes up 2000 square feet of office space, but on the other four floors, your offices are limited to only 200 square feet. In addition, clients move between floors regularly. Other building occupants are also running wireless networks. As part of a site survey, you should study building blueprints to help identify potential obstacles and clarify the distances your network needs to span on each floor. The site survey will indicate whether certain floors require multiple access points. Visually inspecting the floors will also help determine coverage areas and best access point locations. Measuring the signal coverage and strength from other WLANs will inform your decision about the optimal strength and frequency for your wireless signals. A site survey also includes testing proposed access point locations. In testing, a “dummy” access point is carried from location to location while a wireless client connects to it and measures its range and throughput. (Some companies sell software specially designed to conduct such testing.) Most important is testing wireless access from the farthest corners of your space. Also, testing will reveal unforeseen obstacles, such as EMI issued from lights or heavy machinery. After a site survey has identified and verified the optimal quantity and location of access points, you are ready to install them. Recall that to ensure seamless connectivity from one coverage area to another, all access points must belong to the same ESS and share an ESSID. Configuring access points, including assigning ESSIDs, is described in the next section. Figure 8-17 on page 367, shows an example of an enterprise-wide WLAN.

When designing an enterprise-wide WLAN, you must consider how the wireless portions of

the LAN will integrate with the wired portions. Access points connect the two. But an access

point may perform other functions as well. It may provide security features, by, for example,

including and excluding certain clients. It may participate in VLANs, allowing mobile clients

to move from one access point’s range to another while belonging to the same virtual LAN.

Every wireless client’s MAC address can be associated with an access point and each access

point can be associated with a port on a switch.

When these ports are grouped together in a VLAN, it doesn’t matter with which access point a client associates. Because the client stays in the same grouping, it can continue to communicate with the network as if it had remained in one spot.

Configuring Wireless Connectivity Devices

You have learned that access points provide wireless connectivity for mobile clients on an infrastructure WLAN. Access points vary in which wireless standards they support, antenna strength, and optional features such as support for voice signals or the latest security measures.

You can find a small access point or wireless router suitable for home or small-office use for less than $50. More sophisticated or specialized access points—for example, those designed for rugged outdoor use, as on city streets or at train platforms—cost much more. However, as wireless networking has become commonplace, sophistication in even the least expensive devices has increased. Each access point comes with an installation program on CD-ROM or DVD that guides you through the setup process. The process for installing such devices is similar no matter the manufacturer or model.

The variables you will set during installation include:

·         Administrator password

·         SSID

·         Whether or not DHCP is used; note that most network administrators do not configure their wireless access point as a DHCP server and, in fact, doing so when another DHCP server is already designated will cause addressing problems on the network.

·         Whether or not the SSID is broadcast.

·        Security options such as which type, and, for each type, what credentials are necessary to associate with the access point.

In the Hands-On Projects at the end of this chapter, you will have the chance to install and configure one popular wireless router/access point. If something goes awry during your wireless router configuration, you can force all of the variables you changed to be reset. Wireless routers feature a reset button on their back panel. To reset the wireless router, first unplug it. Then, using the end of a paper clip, depress the reset button while you plug it in. Continue holding down the button for at least 30 seconds (this time period varies among manufacturers; check your wireless router’s documentation for the duration yours requires). At the end of this period, the wireless router’s values will be reset to the manufacturer’s defaults. After successfully configuring your access point/wireless router, you are ready to introduce it to the network. In the case of a small office or home WLAN, this means using a patch cable to connect the devices WAN port and your cable or DSL modem’s LAN port. Afterward, clients should be able to associate with the access point and gain Internet access. The following section describes how to configure clients to connect to your WLAN.

Configuring Wireless Clients

Wireless access configuration varies from one type of client to another. In general, as long as an access point is broadcasting its SSID, clients in its vicinity will detect it and offer the user the option to associate with it. If the access point uses encryption, you will need to know the type of encryption and provide the right credentials to associate with it successfully. In the Hands-On Projects at the end of this chapter, you’ll have the chance to explore wireless client configuration on a computer running Windows 7.

As with Windows operating systems, most Linux and UNIX clients provide a graphical interface for configuring their wireless interfaces. Because each version differs somewhat from the others, describing the steps required for each graphical interface is beyond the scope of this book. However, iwconfig, a command-line function for viewing and setting wireless interface parameters, is common to nearly all versions of Linux and UNIX. Following is a basic primer for using the iwconfig command. For more detailed information, type man iwconfig at any Linux or UNIX command-line prompt. Before using iwconfig, make sure your wireless NIC is installed and that your Linux or UNIX workstation is within range of a working access point. You must also be logged in as root or a user with root-equivalent privileges. (Root on UNIX or Linux systems is comparable to an administrative user on Windows systems.) Next, open a terminal session (i.e., Command Prompt window), type iwconfig at the prompt, and then press Enter. The iwconfig output should look similar to that shown in Figure 8-18 on page 369. Notice that in this example, “eth0” represents an interface that is not wireless (that is, a wired NIC), while “eth1” represents the wireless interface. The “lo” portion of the output indicates the loopback interface. On your computer, the wireless NIC might have a different designation. Also notice that iwconfig reveals characteristics of your access point’s signal, including its frequency, power, and signal and noise levels. Using the iwconfig command, you can modify the SSID of the access point you choose to associate with, as well as many other variables. Some examples are detailed below.

The syntax of the following examples assumes your workstation has labeled your wireless NIC “eth1”:

·         iwconfig eth1 essid CLASS_1—This command instructs the wireless interface to associate with an access point whose SSID (or ESSID, as shown in this command) is CLASS_1.

·         iwconfig eth1 mode Managed—This command instructs the wireless interface to operate in infrastructure mode (as opposed to ad hoc mode).

·         iwconfig eth1 channel auto—This command instructs the wireless interface to automatically select the best channel for wireless data exchange.

·         iwconfig eth1 freq 2.422G—This command instructs the wireless interface to communicate on the 2.422-GHz frequency.

·         iwconfig eth1 key 6e225e3931—This command instructs the wireless interface to use the hexadecimal number 6e225e3931 as its key for secure authentication with the access point. (6e225e3931 is only an example; on your network you will choose your own key.)

In this and the previous section, you have learned how to configure wireless clients and access points. The following section summarizes some key points about setting up wireless networks properly.

Avoiding Pitfalls

You might have had the frustrating experience of not being able to log on to a network, even though you were sure you’d typed in your username and password correctly. Maybe it turned out that your Caps Lock key was on, changing your case-sensitive password. Or maybe you were trying to log on to the wrong server. On every type of network, many variables must be accurately set on clients, servers, and connectivity devices in order for communication to succeed. Wireless networks add a few more variables.

As a reminder, following are some wireless configuration pitfalls to avoid:

SSID mismatch - Your wireless client must specify the same SSID as the access point it’s attempting to associate with. As you have learned, you may instruct clients to search for any available access point (or clients might be configured to do this by default). However, if the access point does not broadcast its SSID, or if your workstation is not configured to look for access points, you will have to enter the SSID during client configuration. Also bear in mind that SSIDs are case sensitive. That is, CLASS_1 does not equal Class_l. SSID mismatch will result in failed association.

Incorrect encryption -Your wireless client must be configured to (a) use the same type of encryption as your access point, and (b) use a key or passphrase that matches the access point’s. If either of these is incorrect, your client cannot authenticate with the access point.

Incorrect channel or frequency - You have learned that the access point establishes the channel and frequency over which it will communicate with clients. Clients, then, automatically sense the correct channel and frequency. However, if you have instructed your client to use only a channel or frequency different from the one your access point uses, association will fail to occur.

Standard mismatch (802.11 a/b/g/n) - If your access point is set to communicate only via

802.11g, even if the documentation says it supports 802.11b and 802.11g, clients must also follow the 802.11g standard. Clients may also be able to detect and match the correct type of 802.11 standards. However, if they are configured to follow only one standard, they will never find an access point broadcasting via a different standard.

Incorrect antenna placement - On a network, many factors can cause data errors and a resulting decrease in performance. As you have learned, the most popular WLAN standards require clients to be within 330 feet of an access point’s antenna for reliable data delivery. Beyond that distance, communication might occur, but data errors become more probable. Also remember to place your antenna in a high spot for best signal reception.

Interference - If intermittent and difficult-to-diagnose wireless communication errors occur, interference might be the culprit. Check for sources of EMI, such as fluorescent lights, heavy machinery, cordless phones, and microwaves in the data transmission path.

Wireless WANs

The best 802.11n signal can travel approximately a quarter of a mile. But other types of wireless networks can connect stations over longer distances. For example, in some large cities dozens of surveillance cameras trained on municipal buildings and parks beam video images to a central public safety headquarters. Meanwhile, in developing countries, wireless signals deliver lectures and training videos to students in remote, mountainous regions. In rural areas of the U.S., elderly patients at home wear medical monitoring devices, such as blood pressure sensors and blood glucose meters, which use wireless networks to convey the information to their doctors hundreds of miles away. Such networks can even alert paramedics in case of an emergency. All of these are examples of wireless WANs. Unlike wireless LANs, wireless WANs are designed for high-throughput, long-distance digital data exchange. As in asymmetrical wired broadband, on wireless WANs downstream data transmission is typically faster than upstream transmission. Downstream, also called downlink in the context of wireless transmission, represents the connection between a carrier’s antenna and a client’s transceiver—for example, a smartphone. Upstream, also called uplink in the context of wireless transmission, refers to the connection between a client’s transceiver and the carrier’s antenna. The following sections describe a variety of ways wireless clients can communicate across a city or state.

802.16 (WiMAX)

In 2001, IEEE standardized a new wireless technology under its 802.16 (wireless MAN) committee. Since that time, IEEE has released several versions of the 802.16 standard. Collectively, the 802.16 standards are known as WiMAX, which stands for Worldwide Interoperability for Microwave Access, the name of a group of manufacturers, including Intel and Nokia, who banded together to promote and develop 802.16 products and services. WiMAX was envisioned as a wireless alternative to DSL and T-carrier services for homes and businesses. It achieves much faster throughput than T-carriers at a lower cost for end users.

Notable features of this standard include:

·         Line-of-sight transmission between two antennas for use with fixed clients or non-line-of-sight transmission between multiple antennas for use with mobile clients.

·         Use of frequencies in the 2 to 11 GHz range or the 11 to 66 GHz, either licensed or nonlicensed; most WiMAX installations in the U.S. use the 2.3-, 2.5-, or 3.65-GHz bands.

·         Use of MIMO

·         Ability to transmit and receive signals up to 50 km, or approximately 30 miles, when antennas are fixed or up to 15 km, or approximately 10 miles, when they are mobile.

·         QoS (quality of service) provisions.

To date, the most popular IEEE 802.16 version is 802.16e, which was approved in 2005.

This was the first version of the standard that allowed for mobile clients. 802.16e connections can theoretically reach throughputs of up to 70 Mbps. A newer version of WiMAX, based on the 802.16m standard, also known as WiMAX 2, was released in 2011.

With higher throughput, less latency, and better support for IP telephony than previous WiMAX versions, 802.16m is positioned to compete favorably with cellular data services. And because it is backward compatible with 802.16e equipment, customers and carriers can easily transition to the newer version. Its maximum downlink throughput is 120 Mbps and its maximum uplink throughput is 60 Mbps. Future improvements to WiMAX are in the works, with throughputs of 1 Gbps predicted. In practice, since WiMAX is a shared technology, actual throughputs for all versions are lower than the published maximums. For example, a typical non-line-of-sight client using 802.16e actually experiences 4-Mbps downlink throughput instead of 70 Mbps. Also, as you would expect, the highest throughput is possible only over the shortest distances between transceivers in a line-of-sight arrangement. For example, 802.16e will not achieve its maximum 70 Mbps across a 30-mile span. Furthermore, some service providers cap the maximum bandwidth each customer is allowed to use and might offer a maximum downlink rate of 1 Mbps. Still, WiMAX provides much greater throughput than Wi-Fi and T1s. Also, its range extends much farther than any of the 802.11 standards. For these reasons, WiMAX is considered more appropriate for use on MANs and WANs. It offers an alternative to DSL and broadband cable for business and residential customers who want high-speed Internet access or business customers who want an alternative to T-carriers. It’s well suited to rural customers, for example, who might be in an area lacking copper or fiber-optic cabling infrastructure. WiMAX also provides network access for mobile computerized devices, including smartphones, laptops, and PDAs in metropolitan areas. Finally, WiMAX can act as the backhaul link, or an intermediate connection between subscriber networks and a telecommunications carrier’s network. Figure 8-19 on page 372, illustrates three uses for WiMAX, including a residential customer, mobile users, and a backhaul link. As shown in Figure 8-19, in residential or small business WiMAX, the carrier installs a small antenna on the roof or even inside the building. This antenna is connected to a device similar to a cable or DSL modem for clients to access the LAN. The connectivity device could be incorporated along with the antenna in the same housing or might be separate. If separate, the device typically attaches to the antenna with coaxial cable. It’s often combined with a router. The customer’s antenna communicates in a non-line-of-sight fashion with the service provider’s antenna. If the service provider’s facility is far away, it might use multiple antennas on towers that communicate in a line-of-sight manner, as shown in Figure 8-19. Figures 8-20 and 8-21 on page 373, depict the type of antenna used at a customer’s location and the type of antenna used by service providers on their towers. In some installations, as when a WiMAX provider serves a metropolitan area, the customer’s antenna and connectivity device are eliminated. Instead, each computer communicates directly via its on-board WiMAX transceiver with an antenna such as the one shown in Figure 8-21. In the United States, companies such as Clearwire have established WiMAX networks in several cities using licensed frequency bands. For example, Clearwire has registered with the FCC for the sole use of channels in the 2.5-GHz band in the Chicago metropolitan area. Because the band is licensed, it suffers little interference, and Clearwire can guarantee a certain level of service. In Tokyo, WiMAX will provide Internet access to riders on fast-moving subway trains. On a citywide network, WiMAX makes more sense than Wi-Fi. However, WiMAX has received significant competition from quickly evolving cellular data services, described next.

Cellular

Cellular networks were initially designed to provide analog phone service. However, since the first mobile phones became available to consumers in the 1970s, cellular services have changed dramatically. In addition to voice signals, cellular networks now deliver text messages, Web pages, music, and videos to smartphones and handheld devices. This section describes current cellular data technology and explains the role it plays in wide area networking. To put today’s services in context, it’s useful to understand that each leap in cellular technology has been described as a new generation. Each successive generation has brought a greater range of services, better quality, and higher throughputs. For example, first-generation, or 1G, services from the 1970s and 1980s were analog. Second-generation or 2G, services, which reigned in the 1990s, used digital transmission and paved the way for texting and media downloads on mobile devices. Still, data transmission on 2G systems didn’t exceed 240 Kbps. With the third generation, or 3G, released in the early 2000s, data rates rose to 384 Kbps and data (but not voice) communications used packet switching. The latest generation is 4G. It is characterized by an all-IP, packet-switched network for both data and voice transmission. 4G standards, released in 2008, also specify throughputs of 100 Mbps for fast-moving mobile clients, such as those in cars, and 1 Gbps for slow-moving mobile clients, such as pedestrians. WiMAX, though not strictly a cellular-based technology, is considered 4G because of its high-speed, packet-switched characteristics. Later in this section, you will learn about other 3G and 4G systems. Although their access methods and features might differ, all cellular networks share a similar infrastructure in which coverage areas are divided into cells. Each cell is served by an antenna and its base station, or cell site. At the base station, a controller assigns mobile clients frequencies and manages communication with them. In network diagrams, cells are depicted as hexagons. Multiple cells share borders to form a network in a honeycomb pattern, as shown in Figure 8-22. Antennas are positioned at three corners of each cell, radiating and providing coverage over three equidistant lobes. When a client passes from one coverage area to another, his mobile device begins communicating with different antenna. His communication might change frequencies or even carriers between cells. The transition, which normally happens without the user’s awareness, is known as handoff. Cell sizes vary from roughly 1000 feet to 12 miles in diameter. The size of a cell depends on the network’s access method and the region’s topology, population, and amount of cellular traffic. An urban area with dense population and high volume of data and voice traffic might use cells with a diameter of only 2000 feet, their antennas mounted on tall buildings. In sparsely populated rural areas, with antennas mounted on isolated hilltop towers, cells might span more than 10 miles. In theory, the division of a network into cells provides thorough coverage over any given area. In reality, cells are misshapen due to terrain, EMF, and antenna radiation patterns. Some edges overlap and others don’t meet up, leaving gaps in coverage. As shown in Figure 8-22 on page 375, each base station is connected to an MSC (mobile switching center), also called an MTSO (mobile telecommunications switching office) by a wireless link or fiber-optic cabling. The MSC might be located inside a telephone company’s central office or it might stand alone and connect to the central office via another fiber-optic cabling or a microwave link. At the MSC, the mobile network intersects with the wired network. Equipment at an MSC manages mobile clients, monitoring their location and usage patterns, and switches cellular calls. It also assigns each mobile client an IP address. With 4G cellular services, a client’s IP address remains the same from cell to cell and from one carrier’s territory to another. In 3G cellular services, however, client IP addresses may change when the user transitions to a different carrier’s service area.

From the switching center, packets sent from cellular networks are routed to wired data networks through the PSTN or private backbones using WAN technologies you learned about in Chapter 7. Cellular networking is a complex topic, with rapidly evolving encoding and access methods, changing standards, and innovative vendors vying to dominate the market. This chapter does not detail the various encoding and access methods used on cellular networks. However, to qualify for Network+ certification, you should understand the basic infrastructure of a cellular network and the cellular technologies frequently used for data networking, beginning with HSPA+. HSPA+ (High Speed Packet Access Plus) is a 3G technology released in 2008 that uses MIMO and sophisticated encoding techniques to achieve a maximum 84-Mbps downlink throughput and 11-Mbps uplink throughput in its current release. Soon, the downlink data rate is expected to increase to 336 Mbps. To achieve such speeds, HSPA+ will use limited channels more efficiently and incorporate more antennas in MIMO transmission. Approximately 170 HSPA+ networks exist around the globe. However, faster and more flexible technologies, such as LTE, are likely to overtake HSPA+ in popularity. LTE (Long Term Evolution) is a 4G technology that uses a different access method than HSPA+ to achieve downlink data rates of up to 1 Gbps and uplink rates up to 500 Mbps. LTE is currently the fastest wireless broadband service available in the U.S. While Sprint embraced WiMAX early on for its wireless broadband services, other carriers, such as AT&T and Verizon, passed on WiMAX to adopt LTE service. WiMAX 2 and LTE now coexist and compete for market share in the U.S. Table 8-2 summarizes wireless WAN services described in this section. Note that this does not represent a complete list of wireless broadband services—for example, older 2G and 3G cellular technologies are excluded.

Table 8-2 Characteristics of some wireless WAN services

Technology	Voice switching	Data switching	Maximum theoretical downlink throughput	Maximum theoretical uplink throughput
3G–HSPA+	Circuit	Packet	84 Mbps (with promises of 336 Mbps)	11 Mbps
(N/A)–WiMAX (802.16e)	Packet	Packet	70 Mbps	70 Mbps
4G–WiMAX 2 (802.16m)	Packet	Packet	120 Mbps (with promises of 1 Gbps for fixed clients)	60 Mbps
4G–LTE	Packet	Packet	1 Gbps	500 Mbps

Satellite

In 1945, Arthur C. Clarke (the author of 2001: A Space Odyssey) wrote an article in which he described the possibility of communication between manned space stations that continually orbited the Earth. Other scientists recognized the worth of using satellites to convey signals from one location on Earth to another. By the 1960s, the United States was using satellites to transmit telephone and television signals across the Atlantic Ocean. Since then, the proliferation of this technology and reductions in its cost, have made satellite transmission appropriate and available for transmitting consumer voice, video, music, and data. For many years, satellites have been used to transmit live broadcasts of events happening around the world. Satellites are also used to deliver digital television and radio signals, voice and video signals, and cellular and paging signals.

More recently, they have become a means of providing data services to mobile clients, such as travelers in flight or on ships at sea, who are beyond the reach of WiMAX, HSPA+, or LTE.

Satellite Orbits

Most satellites circle the Earth 22,300 miles above the equator in a geosynchronous orbit. Geosynchronous earth orbit (GEO) means that satellites orbit the Earth at the same rate as the Earth turns. Consequently, at every point in their orbit, the satellites maintain a constant distance from a specific point on the Earth’s equator. Because satellites are generally used to relay information from one point on Earth to another, information sent to Earth from a satellite first has to be transmitted to the satellite from Earth in an uplink from an Earth-based transmitter to an orbiting satellite. Often, the uplink signal information is scrambled (in other words, its signal is encoded) before transmission to prevent unauthorized interception. At the satellite, a transponder receives the uplink signal, and then transmits it to an Earth-based receiver in a downlink. A typical satellite contains 24 to 32 transponders. Each satellite uses unique frequencies for its downlink. These frequencies, as well as the satellite’s orbit location, are assigned and regulated by the FCC. Back on Earth, the downlink is picked up by a dish-shaped antenna. The dish shape concentrates the signal so that it can be interpreted by a receiver. Figure 8-23 on page 377, provides a simplified view of satellite communication. Geosynchronous earth orbiting satellites are the type used by the most popular satellite data service providers. This technology is well established, and is the least expensive of all satellite technology. Also, because they remain in a fixed position relative to the Earth’s surface, stationary receiving dishes on Earth can be counted on to receive satellite signals reliably.

Satellite Frequencies

Satellites transmit and receive signals in any of following five frequency bands, which are roughly defined as:

·         L-band—1.5–2.7 GHz

·         S-band—2.7–3.5 GHz

·         C-band—3.4–6.7 GHz

·         Ku-band—12–18 GHz

·        Ka-band—18–40 GHz

Within each band, frequencies used for uplink and downlink transmissions differ. This variation helps ensure that signals traveling in one direction (for example from a satellite to the Earth) do not interfere with signals traveling in the other direction (for example, signals from the Earth to a satellite).

Satellite Internet Services

A handful of companies offer high-bandwidth Internet access via GEO satellite links. Each subscriber uses a small satellite antenna and receiver, or satellite modem, to exchange signals with the service provider’s satellite network. Clients may be fixed, such as rural dwellers who are too remote for DSL, or mobile subscribers, such as travelers on ocean-going yachts. Clients are able to exchange signals with satellites as long as they have a line-of-sight path, or an unobstructed view of the sky. To establish a satellite Internet connection, each subscriber must have a dish antenna, which is approximately two feet high by three feet wide, installed in a fixed position. In North America, these dish antennas are pointed toward the Southern Hemisphere (because the geosynchronous satellites travel over the equator).

The dish antenna’s receiver is connected, via cable, to a modem. This modem uses either a PCI or USB interface to connect with the subscriber’s computer. As with several other wireless WAN technologies, satellite services are typically asymmetrical and bandwidth is shared among many subscribers. Throughputs vary and are controlled by the service provider. Typical downlink rates range from 1 to 2 Mbps and uplink rates reach approximately 300 Kbps. Compared with other wireless WAN options, satellite services are slower and suffer more latency. In addition, client equipment is more expensive than that required by WiMAX, HSPA+, or LTE. Given these drawbacks, satellite data service is preferred only in circumstances that allow few alternatives or in cases where satellite receiving equipment is already installed.

Chapter Summary

■ The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. Each type of wireless service can be associated with one area, or frequency band, of the wireless spectrum.

■ Most cordless telephones and many WLANs (wireless LANs) use frequencies in the 2.4-GHz band. Other WLANs use a range of frequencies near 5 GHz. The 5-GHz band offers more unlicensed channels and less potential interference.

■ Wireless signals originate from electrical current traveling along a conductor. The electrical signal travels from the transmitter to an antenna, which then emits the signal, as a series of electromagnetic waves, to the atmosphere. The signal propagates through the air until it reaches its destination. At the destination, another antenna accepts the signal, and a receiver converts it back to current.

■ To exchange information, two antennas must be tuned to the same frequency. In communications terminology, this means they share the same channel.

■ The geographical area that an antenna or wireless system can reach is known as its range. Receivers must be within the range to receive accurate signals consistently.

■ Wireless transmission is susceptible to interference from EMI. Signals are also affected by obstacles in their paths, which cause them to reflect, diffract, or scatter. A large number of obstacles can prevent wireless signals from reaching their destination.

■ Because of reflection, diffraction, and scattering, wireless signals follow a number of different paths to their destination. Such signals are known as multipath signals.

■ Each type of wireless communication falls into one of two categories: fixed or mobile. In fixed wireless systems, the locations of the transmitter and receiver do not move. In mobile wireless, the receiver can be located anywhere within the transmitter’s range. This allows the receiver to roam from one place to another while continuing to pick up its signal.

■ In an ad hoc WLAN, wireless nodes, or stations, transmit directly to each other via wireless NICs without an intervening connectivity device.

■ Modern WLANs operate in infrastructure mode. They rely on access points that transmit and receive signals to and from wireless stations and connectivity devices. Access points may connect stations to a LAN or multiple network segments to a backbone. They are often combined with routers.

■ Wireless standards vary by frequency, methods of signal, and geographic range. The IEEE 802.11 committee has ratified four notable wireless standards: 802.11b, 802.11a, 802.11g, and 802.11n.

■ All four 802.11 standards share characteristics at the MAC sublayer level, including the CSMA/CA access method, frame formats, and methods of association between access points and stations.

■ 802.11b operates in the 2.4-GHz band, uses DSSS (direct-sequence spread spectrum), and is characterized by a maximum theoretical throughput of 11 Mbps (though actual throughput is typically half of that).

■ 802.11a, ratified after 802.11b, operates in the 5-GHz band and is incompatible with802.11b or g. It’s characterized by a maximum theoretical throughput of 54 Mbps, though actual throughput is much less.

■ 802.11g, which operates in the 2.4-GHz band and is compatible with 802.11b, is characterized by a maximum theoretical throughput of 54 Mbps, though actual throughput is much less.

■ 802.11n offers significantly faster throughput than any previously adopted 802.11 standard. Techniques such as MIMO (multiple input-multiple output), channel bonding, frame aggregation, and higher modulation rates allow 802.11n to achieve between 65 and 600 Mbps actual throughput. MIMO also dramatically increases the range of 802.11n access points. 802.11n is backward compatible with 802.11b, a, and g, though mixed 0environments cannot take advantage of all of 802.11n’s speed enhancements.

■ Most home and small office WLANs depend on a single access point, which should be

centrally located to provide reliable wireless service to clients at any location.

■ Designing an enterprise-wide WLAN involves choosing the appropriate quantity of

access points and knowing where to position them. A site survey helps by assessing

the network’s client requirements, facility characteristics, and coverage areas.

■ You can install and configure an access point using setup software provided by the manufacturer, or by directly connecting to the device’s operating software. At a minimum, you should change the access point’s SSID (service set identifier) and administrator password. If the access point acts as a router, or Internet gateway, you must provide your Internet account credentials. In addition, you probably want to choose a method of secure authentication, modify the LAN TCP/IP properties, and perhaps change the channel and mode of communication.

■ Client setup for WLANs can be very simple, if you allow the client to find an access point and choose default values for associating with it. If the access point requires secure authentication, you must at least configure the client to meet those credentials.

■ For correct functioning on your WLAN, make sure clients and access points agree on an SSID, security settings, and channels. Also make sure access points are positioned far from sources of interference and that client locations do not exceed the maximum range for the type of wireless technology you use.

■ IEEE 802.16 (WiMAX) is a wireless broadband technology designed for residential or business subscribers who may be fixed or mobile. The 802.16e standard can use frequencies from 2 to 66 GHz and may issue signals in a line-of-sight or non-line-of-sight manner. WiMAX can achieve throughputs of up to 70 Mbps at the shortest ranges. Its signals can travel up to 30 miles in a line-of-sight arrangement.

■ WiMAX 2, specified in IEEE’s 802.16m standard, is a 4G technology that achieves theoretical throughputs of 330 Mbps with lower latency and better quality for VoIP applications than previous WiMAX versions. 802.16m has been approved as a true 4G technology. Manufacturers expect it to reach throughputs of 1 Gbps in the near future.

■ Cellular networks provide data services to mobile clients over packet-switched networks. Though there are many types of cellular networks, all share an infrastructure in which coverage areas are divided into cells that are serviced by antennas and base stations. Base stations communicate with MSCs (mobile switching centers), which connect the cellular network with the PSTN and other WANs.

■ Two types of high-speed cellular data services are currently vying for market share: HSPA+ (High Speed Packet Access Plus) and LTE (Long Term Evolution). Both are considered 4G technologies. LTE can already achieve downlink throughputs of up to 1 Gbps, which makes it more attractive than WiMAX as well.

■ Geosynchronous satellites are used to provide wireless data services to mobile or fixed clients. Satellite-based services are most appropriate for rural clients who cannot receive DSL or cable broadband or by users on planes or ships at sea, where other wireless broadband services can’t reach.

Key Terms

1G - The first generation of mobile phone services, popular in the 1970s and 1980s, which were entirely analog.

2.4-GHz band - The range of radio frequencies from 2.4 to 2.4835 GHz. The 2.4-GHz band, which allows for 11 unlicensed channels, is used by WLANs that follow the popular 802.11b and 802.11g standards. However, it is also used for cordless telephone and other transmissions making the 2.4-GHz band more susceptible to interference than the 5-GHz band.

2G - Second-generation mobile phone service, popular in the 1990s. 2G was the first standard to use digital transmission, and as such, it paved the way for texting and media downloads on mobile devices.

3G - Third-generation mobile phone service, released in the early 2000s, that specifies throughputs of 384 Kbps and packet switching for data (but not voice) communications.

4G - Fourth-generation mobile phone service that is characterized by an all-IP, packet switched network for both data and voice transmission. 4G standards, released in 2008, also specify throughputs of 100 Mbps for fast-moving mobile clients, such as those in cars, and 1 Gbps for slow-moving mobile clients, such as pedestrians.

5-GHz band - A range of frequencies that comprises four frequency bands: 5.1 GHz, 5.3 GHz, 5.4 GHz, and 5.8 GHz. It consists of 24 unlicensed bands, each 20-MHz wide. The 5-GHz band is used by WLANs that follow the 802.11a and 802.11n standards.

802.11a - The IEEE standard for a wireless networking technique that uses multiple frequency bands in the 5-GHz frequency range and provides a theoretical maximum throughput of 54 Mbps. 802.11a’s high throughput, compared with 802.11b, is attributable to its use of higher frequencies, its unique method of encoding data, and more available bandwidth.

802.11b - The IEEE standard for a wireless networking technique that uses DSSS (direct-sequence spread spectrum) signaling in the 2.4–2.4835-GHz frequency range (also called the 2.4-GHz band). 802.11b separates the 2.4-GHz band into 14 overlapping 22-MHz channels and provides a theoretical maximum of 11-Mbps throughput.

802.11g - The IEEE standard for a wireless networking technique designed to be compatible with 802.11b while using different encoding techniques that allow it to reach a theoretical maximum capacity of 54 Mbps. 802.11g, like 802.11b, uses the 2.4-GHz frequency band.

802.11n - The IEEE standard for a wireless networking technique that may issue signals in the 2.4- or 5-GHz band and can achieve actual data throughput between 65 and 600 Mbps.  It accomplishes this through several means, including MIMO, channel bonding, and frame aggregation. 802.11n is backward compatible with 802.11a, b, and g.

802.16 - An IEEE standard for wireless MANs. 802.16 networks may use frequencies between 2 and 66 GHz. Their antennas may operate in a line-of-sight or non-line-of-sight manner and cover 50 kilometers (or approximately 30 miles). 802.16 connections can achieve a maximum throughput of 70 Mbps, though actual throughput diminishes as the distance between transceivers increases. Several 802.16 standards exist. Collectively, they are known as WiMAX.

802.16e - Currently, the most widely implemented version of WiMAX. With 802.16e, IEEE improved the mobility and QoS characteristics of the technology, making it better suited to VoIP and mobile phone users. 802.16e is capable of 70-Mbps throughput, but because bandwidth is shared and service providers cap data rates, most users actually experience 1–4 Mbps throughput.

802.16m - Also known as WiMAX 2, the IEEE standard for a version of 802.16 that achieves theoretical throughputs of 330 Mbps with lower latency and better quality for VoIP applications than previous WiMAX versions. 802.16m has been approved as a true 4G technology. Manufacturers expect it to reach throughputs of 1 Gbps in the near future.

access point  - A device used on wireless LANs that transmits and receives wireless signals to and from multiple nodes and retransmits them to the rest of the network segment. Access points can connect a group of nodes with a network or two networks with each other. They may use directional or omnidirectional antennas.

active scanning - A method used by wireless stations to detect the presence of an access point. In active scanning, the station issues a probe to each channel in its frequency range and waits for the access point to respond.

ad hoc - A type of wireless LAN in which stations communicate directly with each other (rather than using an access point).

AP - See access point.

Association - In the context of wireless networking, the communication that occurs between a station and an access point to enable the station to connect to the network via that access point.

backhaul - An intermediate connection between subscriber networks and a telecommunications carrier’s network.

base station - See access point.

basic service set - See BSS.

basic service set identifier - See BSSID.

beacon frame - In the context of wireless networking, a frame issued by an access point to alert other nodes of its existence.

bounce - See reflection.

BSS (basic service set) - In IEEE terminology, a group of stations that share an access point.

BSSID (basic service set identifier) - In IEEE terminology, the identifier for a BSS (basic service set).

Carrier Sense Multiple Access with Collision Avoidance - See CSMA/CA.

cell - In a cellular network, an area of coverage serviced by an antenna and base station.

channel bonding - In the context of 802.11n wireless technology, the combination of two 20-MHz frequency bands to create one 40-MHz frequency band that can carry more than twice the amount of data that a single 20-MHz band could. It’s recommended for use only in the 5-GHz range because this band has more available channels and suffers less interference than the 2.4-GHz band.

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) - A network access method used on 802.11 wireless networks. In CSMA/CA, before a node begins to send data it checks the medium. If it detects no transmission activity, it waits a brief, random amount of time, and then sends its transmission. If the node does detect activity, it waits a brief period of time before checking the channel again. CSMA/CA does not eliminate, but minimizes, the potential for collisions.

Diffraction - In the context of wireless signal propagation, the phenomenon that occurs when an electromagnetic wave encounters an obstruction and splits into secondary waves. The secondary waves continue to propagate in the direction in which they were split. If you could see wireless signals being diffracted, they would appear to be bending around the obstacle. Objects with sharp edges—including the corners of walls and desks—cause diffraction.

direct-sequence spread spectrum - See DSSS.

directional antenna - A type of antenna that issues wireless signals along a single direction, or path.

downlink - In the context of wireless transmission, the connection between a carrier’s antenna and a client’s transceiver—for example, a smartphone.

DSSS (direct-sequence spread spectrum) - A transmission technique in which a signal’s bits are distributed over an entire frequency band at once. Each bit is coded so that the receiver can reassemble the original signal upon receiving the bits.

ESS (extended service set) - A group of access points and associated stations (or basic service sets) connected to the same LAN.

ESSID (extended service set identifier) - A special identifier shared by BSSs that belong to the same ESS.

extended service set -  See ESS.

extended service set identifier - See ESSID.

fading - A variation in a wireless signal’s strength as a result of some of the electromagnetic energy being scattered, reflected, or diffracted after being issued by the transmitter.

FHSS (frequency hopping spread spectrum) - A wireless signaling technique in which a signal jumps between several different frequencies within a band in a synchronization pattern known to the channel’s receiver and transmitter.

fixed - A type of wireless system in which the locations of the transmitter and receiver are static. In a fixed connection, the transmitting antenna focuses its energy directly toward the receiving antenna. This results in a point-to-point link.

frequency hopping spread spectrum - See FHSS.

GEO (geosynchronous earth orbit) - The term used to refer to a satellite that maintains a constant distance from a point on the equator at every point in its orbit. Geosynchronous orbit satellites are the type used to provide satellite Internet access.

geosynchronous earth orbit - See GEO.

handoff -The transition that occurs when a cellular network client moves from one antenna’s coverage area to another.

High Speed Packet Access Plus - See HSPA+.

HSPA+ (High Speed Packet Access Plus) - A 3G mobile wireless technology released in 2008 that uses MIMO and sophisticated encoding techniques to achieve a maximum 84-Mbps downlink throughput and 11-Mbps uplink throughput in its current release. Advances in more efficiently using limited channels and incorporating more antennas in MIMO promise to push the maximum downlink data rate to 336 Mbps.

infrastructure WLAN - A type of WLAN in which stations communicate with an access point and not directly with each other.

Iwconfig -  A command-line utility for viewing and setting wireless interface parameters on Linux and UNIX workstations.

line-of-sight - See LOS.

Long Term Evolution - See LTE.

LOS (line-of-sight) - A wireless signal or path that travels directly in a straight line from its transmitter to its intended receiver. This type of propagation uses the least amount of energy and results in the reception of the clearest possible signal.

LTE (Long Term Evolution) - A 4G cellular network technology that achieves downlink data rates of up to 1 Gbps and uplink rates up to 500 Mbps. AT&T and Verizon have adopted LTE for their high-speed wireless data networks.

MIMO (multiple input-multiple output) - In the context of 802.11n wireless networking, the ability for access points to issue multiple signals to stations, thereby multiplying the signal’s strength and increasing their range and data-carrying capacity. Because the signals follow multipath propagation, they must be phase-adjusted when they reach their destination.

Mobile -  A type of wireless system in which the receiver can be located anywhere within the transmitter’s range. This allows the receiver to roam from one place to another while continuing to pick up its signal.

mobile switching center - See MSC.

mobile telecommunications switching office - See MSC.

MSC (mobile switching center) - A carrier’s facility to which multiple cellular base stations connect. An MSC might be located inside a telephone company’s central office or it might stand alone and connect to the central office via fiber-optic cabling or a microwave link. Equipment at an MSC manages mobile clients, monitoring their location and usage patterns, and switches cellular calls. It also assigns each mobile client an IP address.

MTSO (mobile telecommunications switching office) - See MSC.

multipath  - The characteristic of wireless signals that follow a number of different paths to their destination (for example, because of reflection, diffraction, and scattering).

multiple input-multiple output - See MIMO.

narrowband - A type of wireless transmission in which signals travel over a single frequency or within a specified frequency range.

omnidirectional antenna - A type of antenna that issues and receives wireless signals with equal strength and clarity in all directions. This type of antenna is used when many different receivers must be able to pick up the signal, or when the receiver’s location is highly mobile.

passive scanning - In the context of wireless networking, the process in which a station listens to several channels within a frequency range for a beacon issued by an access point.

Probe -  In 802.11 wireless networking, a type of frame issued by a station during active scanning to find nearby access points.

radiation pattern - The relative strength over a three-dimensional area of all the electromagnetic energy an antenna sends or receives.

range -The geographical area in which signals issued from an antenna or wireless system can be consistently and accurately received.

reassociation - In the context of wireless networking, the process of a station establishing a connection (or associating) with a different access point.

reflection - In the context of wireless, the phenomenon that occurs when an electromagnetic wave encounters an obstacle and bounces back toward its source. A wireless signal will bounce off objects whose dimensions are large compared with the signal’s average wavelength.

Request to Send/Clear to Send - See RTS/CTS.

Roaming -  In wireless networking, the process that describes a station moving between BSSs without losing connectivity.

RTS/CTS (Request to Send/Clear to Send) - An exchange in which a wireless station requests the exclusive right to communicate with an access point and the access point confirms that it has granted that request.

scanning - The process a wireless station undergoes to find an access point. See also active scanning and passive scanning.

scattering - The diffusion of a wireless signal that results from hitting an object that has smaller dimensions compared with the signal’s wavelength. Scattering is also related to the roughness of the surface a wireless signal encounters. The rougher the surface, the more likely a signal is to scatter when it hits that surface.

service set identifier - See SSID.

site survey - In the context of wireless networking, an assessment of client requirements, facility characteristics, and coverage areas to determine an access point arrangement that will ensure reliable wireless connectivity within a given area.

spread spectrum - A type of wireless transmission in which lower-level signals are distributed over several frequencies simultaneously. Spread-spectrum transmission is more secure than narrowband.

SSID (service set identifier) - A unique character string used to identify an access point on

an 802.11 network.

station - An end node on a network; used most often in the context of wireless networks.

transponder - The equipment on a satellite that receives an uplinked signal from Earth, amplifies the signal, modifies its frequency, then retransmits it (in a downlink) to an antenna on Earth.

Uplink - In the context of wireless transmission, the connection between a client’s transceiver and a carrier’s antenna.

WAP (wireless access point) - See access point.

Wi-Fi - See 802.11.

WiMAX - See 802.16.

WiMAX 2 - See 802.16m.

wireless - A type of signal made of electromagnetic energy that travels through the air.

wireless access point - See access point.

wireless gateway -  An access point that provides routing functions and is used as a gateway.

wireless LAN - See WLAN.

wireless router - An access point that provides routing functions.

wireless spectrum - A continuum of electromagnetic waves used for data and voice communication. The wireless spectrum (as defined by the FCC, which controls its use) spans frequencies between 9 KHz and 300 GHz. Each type of wireless service can be associated with one area of the wireless spectrum.

WLAN (wireless LAN) - A LAN that uses wireless connections for some or all of its transmissions.

Worldwide Interoperability for Microwave Access (WiMAX) - See 802.16a.

Review Questions

1.   To transmit and receive signals to and from multiple nodes in a three-story office building, what type of antenna should an access point use?

a.      Omnidirectional

b.      Unidirectional

c.       Bidirectional

d.      Tridirectional

2.   Which of the following is not true about multipath signaling?

a.   The more obstacles a wireless signal reflects or diffracts off, the better chance it has of reaching its destination.

b.  Multipath signaling uses less energy and results in clearer reception than line- of-sight signaling.

c.   Given that they follow multiple paths to their destination, signals will arrive at the same destination at slightly different times.

d.   The more obstacles between a wireless transmitter and receiver, the more signal fading will occur.

3.   You are setting up a WLAN for an insurance agency. The network includes 32 clients, three printers, two servers, and a DSL modem for Internet connectivity. What type of WLAN architecture would best suit this office?

a.   Ad hoc

b.   Interstitial

c.   Infrastructure

d.   Round robin

4.   Which of the following 802.11 transmission requirements contributes to its inefficiency?

a.     Before it can associate, a station must listen for an access point’s beacon on every channel within its frequency range.

b.      A source node must regularly ping the access point to ensure it is still available for transmitting data to the rest of the stations.

c.       A destination node must issue an acknowledgment for every packet that is received intact.

d.      Before transmitting, a source node must check to ensure the access point has not changed its SSID.

5. In the 802.11 standard, IEEE specifies what type of access method?

a.      Beacon passing

b.      Demand priority

c.       CSMA/CD

d.      CSMA/CA

6. Suppose a user on your office network has changed the channel on which his wireless NIC communicates. Assuming the wireless connection is his only access to the LAN, what will happen when he next tries to send an e-mail?

a.   The e-mail program will take longer than usual to send his message.

b.  The e-mail program will respond with a message indicating it could not connect to the mail server.

c.   The e-mail program will send the message without problems.

d.   The e-mail program will request the user to supply his logon credentials again before sending the message.

7.   What frequency band is used by 802.11b, 802.11g, and 802.11n?

a.      1.5 GHz

b.      2.4 GHz

c.       5 GHz

d.      11 GHz

8. Your office currently runs a mix of 802.11b and 802.11g clients. Rumor has it that your company is about to merge with another company that uses a different wireless technology. Which of the following would be compatible with what your WLAN currently runs?

a. 802.11a

b. 802.11n

c. Bluetooth

d. WiMAX

9. If your wireless stations are configured to perform passive scanning, what do they need from an access point to initiate association?

a.   A request to send

b.   An alert frame

c.   A beacon frame

d.   Nothing; they will find the access point on their own

10. You’re working on a school district’s 802.11n WLAN. Within each school, several access points serve students, teachers, and administrators. So that users can move about the school with their laptops and not lose network connectivity, each of the access points must share which of the following?

a.   The same ESSID

b.   The same make and model

c.   The same average distance to the client

d.   The same location

11. When a mobile WLAN user roams from access point A’s range into access point B’s range, what does it do automatically to maintain network connectivity?

a.  Associate with access point B in order to communicate with access point A

b. Reassociate with access point B

c. Reestablish its connection with access point A on another channel

d. Nothing; the user must reestablish network connectivity manually

12. Which two of the following techniques help to reduce overhead in 802.11n wireless transmission?

a. CSMA/CA

b. Asynchronous communication

c. Frame aggregation

d. Spread-spectrum signaling

e. Channel bonding

13. Your organization is expanding and plans to lease 3000 square feet of space in a nearby building. Your supervisor asks you to conduct a site survey of the space. If conducted properly, which of the following will your site survey reveal?

a. The optimal quantity and locations of access points for the WLAN

b. All potential sources of EMI

c. The distance between each workgroup area and telco room

d. All of the above

14. Which of the following wireless technologies boasts the highest maximum theoretical throughput?

a. LTE

b. HSPA+

c. 802.11g

d. 802.11n

15. Which of the following will help an access point’s transmissions reach farther?

a. Limiting the number of stations that may associate with the access point

b. Boosting its signal strength

c. Using the highest possible channel in the frequency band

d. Configuring it to use 802.11n only

16.  What part of a cellular network manages handoff?

a.   The client

b.   The base station

c.   The MSC

d.   The central office

17. Suppose you work for a telecommunications carrier who is looking into providing WiMAX in a suburb of a large city. A colleague suggests that your company reserve licensed frequencies from the FCC for your service. Why?

a.   Licensed frequencies will suffer less interference than unlicensed frequencies.

b.   Licensed frequencies allow users to roam farther than unlicensed frequencies.

c.   Licensed frequencies can use multiple areas of the wireless spectrum at once, thus increasing potential throughput.

d.   Licensed frequencies require less expensive equipment to transmit and receive than unlicensed frequencies.

18. On your Linux workstation, you open a terminal window and type at the command prompt iwconfig eth0 key 5c00951b22. What have you done?

a.   Established the wireless interface’s mode of transmission

b.  Established the strength with which the wireless interface will transmit data

c.   Established the credentials the wireless interface will use to communicate securely with the access point

d.  Established the SSID with which the wireless interface will attempt to associate

19. As the network manager for a small business, you have been asked to evaluate high-speed, packet-switched wireless data services that your company’s users can use at their desks, in their cars, and at their homes within your metropolitan area. Which two of the following meet those criteria, and are therefore candidates for evaluation?

a.   802.11g

b.   802.11n

c.  WiMAX 2

d.  HSPA+

e.  LTE

20. Which of the following types of satellites is used to provide satellite data services?

a. Geosynchronous orbit

b. Low Earth orbit

c. Medium Earth orbit

d. High Earth orbit

Sample Quiz

1. On the wireless spectrum, waves are arranged according to their ________ , from lowest to highest.

a. wavelengths           

b. strengths    

c. amplitudes

d. frequencies            

2. An antenna's ________ describes the relative strength over a three-dimensional area of all the electromagnetic energy the antenna sends or receives.

a. range          

b. reflection   

c. radiation pattern    

d. spread-spectrum    

3. ________ is the diffusion, or the reflection in multiple different directions, of a signal.

a. Diffraction  

b. Reflection   

c. Fading         

d. Scattering   

4. In a(n) ________ WLAN, wireless nodes, or stations, transmit directly to each other via wireless NICs without an intervening connectivity device.

a. mobile        

b. fixed           

c. ad hoc         

d. infrastructure         

5. Each IEEE wireless network access standard is named after the ________ .

a. the company who first sold it        

b. the person who invented it            

c. 802.11 task group that developed it          

d. the technology it uses        

6. The 802.11 standards specify the use of ________ to access a shared medium.

a. CSMA/CA   

b. CSMA/CD   

c. RTS/CTS      

d. RTS/CA       

7. In active scanning, the station transmits a special frame, known as a ________ , on all available channels within its frequency range.

a. routing metric        

b. broadcast signal     

c. cell  

d. probe          

8. A(n) ________ assesses client requirements, facility characteristics, and coverage areas to determine an access point arrangement that will ensure reliable wireless connectivity within a given area.

a. site survey  

b. audit report            

c. wiring closet diagram        

d. infrastructure map            

9. Which statement is true with regard to WLANs?

a. Wireless access configuration is largely identical across clients.            

b. Proposed access point locations should be tested prior to a site survey.

c. Incorrect encryption is usually the culprit with intermittent and difficult-to-diagnose wireless communication errors.          

d. Infrastructure WLANs are far more common than ad-hoc WLANs.         

10. Which statement is true with regard to wireless WANs?

a. Satellites maintain a constant distance from a specific point on the Earth's equator.    

b. Upstream data transmission is typically faster than downstream transmission.

c. Cellular networks were initially designed to provide digital phone service.        

d. HSPA+ is currently the fastest wireless broadband service available in the U.S.

11. Wireless signals originate from electrical current traveling along a conductor.

a. True

b. False           

12. Wireless networks are laid out using the same topologies as wired networks.

a. True

b. False           

13. 802.11n is compatible with all three earlier versions of the 802.11 standard.

a. True

b. False           

14. Placement of an access point on a WLAN must take into account the typical distances between the access point and its clients.

a. True

b. False           

15. WiMAX provides fewer throughputs than Wi-Fi and T1s.

a. True

b. False           

Practice Test

1. A(n) ____ is the creation of a communications channel for a transmission from an Earth-based transmitter to an orbiting satellite.

a.      downlink

b.      transponder

c.       directional antenna

d.      uplink

2.  ____ networking allows wireless nodes to roam from one location to another within a certain range of their access point.

a.      LOS

b.      Extended

c.       Fixed

d.      Mobile

 

 3. The downside to multipath signaling is that, because of their various paths, multipath signals travel different distances between their transmitter and a receiver.

                       

    True

                       

    False

 

 4

    The IEEE 802.11 standard specifies communication between two wireless nodes, or stations, and between a station and an access point. It also specifies how two access points should communicate.

                       

    True

                       

    False

 

 5

    802.11 networks use the same access method as Ethernet networks.

    True

    False

 

 6

    Access points for use on small office or home networks often include routing functions.

                       

    True

                       

    False

 

 7

    ____ occurs when a mobile user moves out of one access point's range and into the range of another.

                       

    Fading

                       

    Low earth orbiting

                       

    Reassociation

                       

    Passive scanning

 

 8

    In ____ transmission, a signal jumps between several different frequencies within a band in a synchronization pattern known only to the channel's receiver and transmitter.

                       

    star

                       

    bus

                       

    mesh

                       

    FHSS

 

 9

    If governments and companies did not adhere to ITU standards, chances are that a wireless device could not be used outside the country in which it was manufactured.

                       

    True

                       

    False

 

 10

    ____ signals follow a number of different paths to their destination because of reflection, diffraction, and scattering.

                       

    Multipath

                       

    Opened

                       

    Closed

                       

    Variable

 

 11

    Access points are also known as ____.

base stations

 

 12

    Using the ____ command, you can modify the SSID of the access point you choose to associate with, as well as many other variables.

                       

    ping

                       

    ipconfig

                       

    iwconfig

                       

    nbtstat

 

 13

    Spread-spectrum signaling is a popular way of making wireless transmissions more secure.

    True

    False

 

 14

    ____ is the governing body that sets standards for international wireless services, including frequency allocation, signaling and protocols used by wireless devices, wireless transmission and reception equipment, satellite orbits, and so on.

                       

    ITU

                       

    IEEE

                       

    CSMA/CA

                       

    LEO

 

 15

    In a(n) ____ arrangement, a subscriber receives data from the Internet via a satellite downlink transmission, but sends data to the satellite via an analog modem (dial-up) connection.

Dial Return

 

 16

    WiMAX provides Internet access to mobile computerized devices, including cell phones, laptops, and PDAs in metropolitan areas.

                       

    True

                       

    False

 

 17

    Data ____ are those that carry the data sent between stations.

frames

 18

    The four 802.11 standards (802.11b, 802.11a, 802.11g, 802.11n) are collectively known as ____.

                       

    WiMAX

                       

    Wi-Fi

                       

    Bluetooth

                       

    Open

 

 19

    One advantage of  ____ wireless is that because the receiver's location is predictable, energy need not be wasted issuing signals across a large geographical area.

fixed

 

 20

    A site survey also includes testing proposed access point locations.

                       

    True

                       

    False

Chapter Test

1

    Clients are able to exchange signals with satellites as long as they have a ____ path.

            a.        

    view

            b.        

    directional

            c.        

    proprietary

            d.        

    line-of-sight

 

 2

    802.11b and g signals can extend a maximum of ____________________ feet and still deliver data reliably.

330

 

 3

    ____________________ means that satellites orbit the Earth at the same rate as the Earth turns.

GEO

 

 4

    If intermittent and difficult-to-diagnose wireless communication errors occur, ____ might be the culprit.

            a.        

    incorrect encryption

            b.        

    incorrect antenna placement

            c.        

    SSID mismatch

            d.        

    interference

 

 5

    WiMAX is defined by the IEEE 802.11 standard.

    True

    False

 

 6

    Wireless signals experience attenuation.

    True

    False

 

 7

    Most satellites circle the Earth ____  miles above the equator in a geosynchronous orbit.

            a.        

    100

            b.        

    1240

            c.        

    6,000

            d.        

    22,300

 

 8

    ____ satellites are the type used by the most popular satellite data service providers.

            a.        

    Low earth orbiting

            b.        

    Transponder

            c.        

    Medium earth orbiting

            d.        

    Geosynchronous earth orbiting

 

 9

    The average geographic range for an 802.11a antenna is ____ meters.

            a.        

    20

            b.        

    100

            c.        

    330

            d.        

    600

 

 10

    ____ is a significant problem for wireless communications because the atmosphere is saturated with electromagnetic waves.

            a.        

    Interference

            b.        

    Attenuation

            c.        

    Diffraction

            d.        

    Fading

 

 11

    The ____________________ is a unique character string used to identify an access point.

SSID

 

 12

    An 802.11g antenna has a geographic range of ____ meters.

            a.        

    20

            b.        

    100

            c.        

    300

            d.        

    330

 

 13

    Which satellites transmit and receive signals in the 12 to 18 GHz band?

            a.        

    Ku-

            b.        

    S-

            c.        

    L-

            d.        

    Ka-

 

 14

    In ____ scanning, the station transmits a special frame, known as a probe, on all available channels within its frequency range.

            a.        

    fixed

            b.        

    passive

            c.        

    active

            d.        

    open

 

 15

    In the case of connecting two WLANs, access points could be as far as ____ feet apart.

            a.        

    500

            b.        

    800

            c.        

    1000

            d.        

    1200

 

 16

    ____ may use either the 2.4-GHz or 5-GHz frequency range.

            a.        

    802.11a

            b.        

    802.11b

            c.        

    802.11g

            d.        

    802.11n

 

 17

    A(n) ____________________ assesses client requirements, facility characteristics, and coverage areas to determine an access point arrangement that will ensure reliable wireless connectivity within a given area.

site survey

 

 18

    The use of multiple frequencies to transmit a signal is known as ____________________ technology.

 

 19

    ____ is an 802.11n feature that allows two adjacent 20-MHz channels to be combined to make a 40-MHz channel.

            a.        

    Channel bonding

            b.        

    Channel aggregation

            c.        

    Frame bonding

            d.        

    Frame aggregation

 

 20

    An ad hoc arrangement would work well for a WLAN with many users.

    True

    False

 

 21

    In ____, a wireless signal splits into secondary waves when it encounters an obstruction.

            a.        

    bounce back

            b.        

    scattering

            c.        

    reflection

            d.        

    diffraction

 

 22

    Spread spectrum is a popular way of making wireless transmissions more secure.

    True

    False

 

 23

    ____ is a command-line function for viewing and setting wireless interface parameters and it is common to nearly all versions of Linux and UNIX.

            a.        

    config

            b.        

    ivconfig

            c.        

    iwconfig

            d.        

    ipconfig

 

 24

    In ____ wireless systems, the transmitting antenna focuses its energy directly toward the receiving antenna which results in a point-to-point link.

            a.        

    open

            b.        

    variable

            c.        

    mobile

            d.        

    fixed

 

 25

    To establish a satellite Internet connection, each subscriber must have a ____.

            a.        

    dish pathway

            b.        

    dish antenna

            c.        

    line-of-sight antenna

            d.             source pathway