Chapter 2 - Review

Network+ Guide to Networks, Chapter 2 Review

Networking Standards and the OSI Model

When trying to grasp a new theoretical concept, it often helps to form a picture of that concept in your mind. In the field of chemistry, for example, even though you can’t see a water molecule, you can represent it with a simple drawing of two hydrogen atoms and one oxygen atom. Similarly, in the field of networking, even though you can’t see the communication that occurs between two nodes on a network, you can use a model to depict how the communication takes place. The model commonly used to describe network communications is called the OSI (Open Systems Interconnection) model.

Networking Standards Organizations

Standards are documented agreements containing technical specifications or other precise criteria that stipulate how a particular product or service should be designed or performed. Many different industries use standards to ensure that products, processes, and services suit their purposes. For example, the construction industry follows standards to ensure a building’s safety and accessibility, such as those defining the width and slope of wheelchair ramps. The airline industry adheres to standards that specify the precise contents of jet fuel. Because of the wide variety of hardware and software in use today, standards are especially important in the world of networking. Without standards, it would be very difficult to design a network because you could not be certain that software or hardware from different manufacturers would work together. For example, if one manufacturer designed a network cable with a 1-centimeter-wide plug and another company manufactured a wall plate with a 0.8-centimeter-wide opening, you would not be able to insert the plug into the wall plate. When purchasing networking equipment, therefore, you want to verify that equipment meets the standards your network requires. However, bear in mind that standards define the minimum acceptable performance of a product or service—not the ideal. So, for example, you might purchase two different network cables that comply with the minimum standard for transmitting at a certain speed, but one cable might exceed that standard, allowing for better network performance. In the case of network cables, exceeding minimum standards often follows from the use of quality materials and careful production techniques. Because the computer industry grew so quickly out of several technical disciplines, many different organizations evolved to oversee its standards. In some cases, a few organizations are responsible for a single aspect of networking. For example, both the American National Standards Institute (ANSI) and IEEE are involved in setting standards for wireless networks. Whereas ANSI prescribes the kind of NIC (network interface card) that the consumer needs to accept a wireless connection, IEEE prescribes, among other things, how the network will ensure that different parts of a communication sent through the atmosphere arrive at their destination in the correct sequence. A complete list of the standards that regulate computers and networking would fill an encyclopedia. Although you don’t need to know the fine points of every standard, you should be familiar with the groups that set networking standards and the critical aspects of standards required by your network.

ANSI:

ANSI (American National Standards Institute) is an organization composed of more than a thousand representatives from industry and government who together determine standards for the electronics industry and other fields, such as chemical and nuclear engineering, health and safety, and construction. ANSI also represents the United States in setting international standards. This organization does not dictate that manufacturers comply with its standards, but requests voluntarily compliance. Of course, manufacturers and developers benefit from compliance; because compliance assures potential customers that the systems are reliable and can be integrated with an existing infrastructure. New electronic equipment and methods must undergo rigorous testing to prove they are worthy of ANSI’s approval. You can purchase ANSI standards documents online from ANSI’s Web site (www.ansi.org) or find them at a university or public library. You need not read complete ANSI standards to be a competent networking professional, but you should understand the breadth and significance of ANSI’s influence.

EIA and TIA

Two related standards organizations are EIA and TIA. EIA (Electronic Industries Alliance) is a trade organization composed of representatives from electronics manufacturing firms across the United States. EIA not only sets standards for its members, but also helps write ANSI standards and lobbies for legislation favorable to the growth of the computer and electronics industries. In 1988, one of the EIA’s subgroups merged with the former United States Telecommunications Suppliers Association (USTSA) to form TIA (Telecommunications Industry Association). TIA focuses on standards for information technology, wireless, satellite, fiber optics, and telephone equipment. Both TIA and EIA set standards, lobby governments and industry, and sponsor conferences, exhibitions, and forums in their areas of interest. Probably the best known standards to come from the TIA/EIA alliance are its guidelines for how network cable should be installed in commercial buildings, known as the “TIA/EIA 568-B Series.”

IEEE

The IEEE (Institute of Electrical and Electronics Engineers), or “I-triple-E,” is an international society composed of engineering professionals. Its goals are to promote development and education in the electrical engineering and computer science fields. To this end, IEEE hosts numerous symposia, conferences, and local chapter meetings and publishes papers designed to educate members on technological advances. It also maintains a standards board that establishes its own standards for the electronics and computer industries and contributes to the work of other standards-setting bodies, such as ANSI. IEEE technical papers and standards are highly respected in the networking profession.

Among other places, you will find references to IEEE standards in the manuals that accompany NICs. You can purchase IEEE documents online from IEEE’s Web site (www.ieee.org) or find them in a university or public library.

ISO

ISO (International Organization for Standardization), headquartered in Geneva, Switzerland, is a collection of standards organizations representing 162 countries. ISO’s goal is to establish international technological standards to facilitate global exchange of information and barrier free trade. Given the organization’s full name, you might expect it to be called IOS, but ISO is not meant to be an acronym. In fact, iso is the Greek word for equal. Using this term conveys the organization’s dedication to standards.

ISO’s authority is not limited to the information-processing and communications industries. It also applies to the fields of textiles, packaging, distribution of goods, energy production and utilization, shipbuilding, and banking and financial services. The universal agreements on screw threads, bank cards, and even the names for currencies are all products of ISO’s work. In fact, fewer than 3000 of ISO’s more than 18,500 standards apply to computer-related products and functions. You can find out more about ISO at its Web site: www.iso.org.

ITU

The ITU (International Telecommunication Union) is a specialized United Nations agency that regulates international telecommunications, including radio and TV frequencies, satellite and telephony specifications, networking infrastructure, and tariffs applied to global communications. It also provides developing countries with technical expertise and equipment to advance those nations’ technological bases. The ITU was founded in Paris in 1865. It became part of the United Nations in 1947 and relocated to Geneva, Switzerland. Its standards arm contains members from 193 countries and publishes detailed policy and standards documents that can be found on its Web site: www.itu.int. Typically, ITU documents pertain more to global telecommunications issues than to industry technical specifications. However, the ITU is deeply involved with the implementation of worldwide Internet services. As in other areas, the ITU cooperates with several different standards organizations, such as ISOC (discussed next), to develop these standards.

ISOC

ISOC (Internet Society), founded in 1992, is a professional membership society that helps to establish technical standards for the Internet. Some current ISOC concerns include the rapid growth of the Internet and keeping it accessible, information security, and the need for stable addressing services and open standards across the Internet. ISOC’s membership consists of more than 44,000 Internet professionals from over 80 chapters around the world.

ISOC oversees groups with specific missions, such as the (Internet Architecture Board). IAB is a technical advisory group of researchers and technical professionals interested in overseeing the Internet’s design and management. As part of its charter, IAB is responsible for Internet growth and management strategy, resolution of technical disputes, and standards oversight. Another ISOC group is the IETF (Internet Engineering Task Force), the organization that sets standards or how systems communicate over the Internet—in particular, how protocols operate and interact. Anyone can submit a proposed standard for IETF approval. The standard then undergoes elaborate review, testing, and approval processes. On an international level, IETF works with the ITU to help give technical standards approved in the United States international acceptance. You can learn more about ISOC and its member organizations, IAB and IETF, at their Web site: www.isoc.org.

IANA and ICANN

You have learned that every computer on a network must have a unique address. On the Internet, this is especially important because millions of different computers must be available to transmit and receive data at any time. Addresses used to identify computers on the Internet and other TCP/IP-based networks are known as IP (Internet Protocol) addresses. To ensure that every Internet-connected device has a unique IP address, organizations across the globe rely on centralized authorities. In early Internet history, a nonprofit group called the IANA (Internet Assigned Numbers Authority) kept records of available and reserved IP addresses and determined how addresses were doled out. Starting in 1997, IANA coordinated its efforts with three RIRs (Regional Internet Registries): ARIN (American Registry for Internet Numbers), APNIC (Asia Pacific Network Information Centre), and RIPE (Reseaux IP Europeens). An RIR is a not-for-profit agency that manages the distribution of IP addresses to private and public entities. In the late 1990s, the United States Department of Commerce (DOC), which funded IANA, decided to overhaul IP addressing and domain name management. The DOC recommended the formation of ICANN (Internet Corporation for Assigned Names and Numbers), a private, nonprofit corporation. ICANN is now ultimately responsible for IP addressing and domain name management. Technically speaking, however, IANA continues to perform the system administration. Individuals and businesses do not typically obtain IP addresses directly from an RIR or IANA. Instead, they lease a group of addresses from their ISP (Internet service provider),a business that provides organizations and individuals with access to the Internet and often, other services, such as e-mail and Web hosting. An ISP, in turn, arranges with its RIR for the right to use certain IP addresses on its network. The RIR obtains its right to dole out those addresses from ICANN. In addition, the RIR coordinates with IANA to ensure that the addresses are associated with devices connected to the ISP’s network. You can learn more about IANA and ICANN at their Web sites, www.iana.org and www.icann.org, respectively.

The OSI Model

In the early 1980s, ISO began work on a universal set of specifications that would enable computer platforms across the world to communicate openly. The result was a helpful model for understanding and developing computer-to-computer communications over a network. This model, called the OSI (Open Systems Interconnection) model, divides network communications into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. At each layer, protocols perform services unique to that layer. While performing those services, the protocols also interact with protocols in the layers directly above and below. In addition, at the top of the OSI model, Application layer protocols interact with the software you use (such as an e-mail or spreadsheet program). At the bottom, Physical layer services act on the networking cables and connectors to issue and receive signals. You have already learned that protocols are the rules by which computers communicate. A protocol is simply a set of instructions written by a programmer to perform a function or group of functions. Some protocols are included with a computer’s operating system. Others are files installed with software programs. The OSI model is a theoretical representation of what happens between two nodes communicating on a network. It does not prescribe the type of hardware or software that should support each layer. Nor does it describe how software programs interact with other software programs or how software programs interact with humans. Every process that occurs during network communications can be associated with a layer of the OSI model, so you should be familiar with the names of the layers and understand the key services and protocols that belong to each.

Application (Layer 7)	Provides interface between software applications and a network for interpreting applications’ requests and requirements.
Presentation (Layer 6)	Allows hosts and applications to use a common language; performs data formatting, encryption, and compression.
Session     (Layer 5)	Establishes, maintains, and terminates user connections.
Transport (Layer 4)	Ensures accurate delivery of data through flow control, segmentation and reassembly, error correction, and acknowledgment.
Network (Layer 3)	Establishes network connections; translates network addresses into their physical counterparts and determines routing
Data Link (Layer 2)	Packages data in frames appropriate to network transmission method.
Physical Layer 1	Manages signaling to and from physical network connections.

OSI Model Layers and Functions

The path that data takes from one computer to another through the OSI model is:

First, a user or device initiates a data exchange through the Application layer. The Application layer separates data into PDUs (protocol data units), or discrete amounts of data. From there, Application layer PDUs progress down through OSI model layers 6, 5, 4, 3, 2, and 1 before being issued to the network medium—for example, the wire. The data traverses the network until it reaches the second computer’s Physical layer. Then at the receiving computer the data progresses up the OSI model until it reaches the second computer’s Application layer. This transfer of information happens in milliseconds. Logically, however, each layer communicates with the same layer from one computer to another. In other words, the Application layer protocols on one computer exchange information with the Application layer protocols of the second computer. Protocols from other layers do not attempt to interpret Application layer data. In the following sections, the OSI model layers are discussed from highest to lowest, beginning with the Application layer, where the flow of information is initiated. Bear in mind that the OSI model is a generalized and sometimes imperfect representation of network communication. In some cases, network functions can be associated with more than one layer of the model, and in other cases, network operations do not require services from every layer.

Application Layer

The top, or seventh, layer of the OSI model is the Application layer. Contrary to what its name implies, the Application layer does not include software programs, such as Microsoft Word or Firefox. Instead, the Application layer facilitates communication between such programs and lower-layer network services. Services at this layer enable the network to interpret a program’s request and the program to interpret data sent from the network. Through Application layer protocols, programs negotiate their formatting, procedural, security, synchronization, and other requirements with the network. Note that not all these requirements are fulfilled by Application layer protocols. They are merely agreed upon at this stage. For example, when you choose to open a Web page in Firefox, an Application layer protocol called HTTP (Hypertext Transfer Protocol) formats and sends your request from your client’s browser (a software application) to the server. It also formats and sends the Web server’s response back to your client’s browser. Suppose you choose to view the Library of Congress’s Web site. You type www.loc.gov/index.html in Firefox and press Enter. At that point, Firefox’s API (application programming interface), a set of routines that make up part of the software, transfers your request to the HTTP protocol.  HTTP prompts lower-layer protocols to establish a connection between your computer and the Web server. Next, HTTP formats your request for the Web page and sends the request to the Web server. One part of the HTTP request includes a command that begins with “GET” and tells the server what page you want to retrieve. Other parts of the request indicate what version of HTTP you’re using, what types of graphics and what language your browser can accept, and what browser version you’re using, among other things. After receiving your computer’s HTTP request, the Web server responsible for www.loc.gov responds, also via HTTP. Its response includes the text and graphics that make up the Web page, plus specifications for the content contained in the page, the HTTP version used, the type of HTTP response, and the length of the page.However, if the Web page is unavailable, the host, www.loc.gov, sends an HTTP response containing an error message, such as “Error 404 - File Not Found.” After receiving the Web server’s response, your workstation uses HTTP to interpret this response so that Firefox can present the www.loc.gov/index.html Web page in a format you’ll recognize, with neatly arranged text and images. Note that the information issued by one node’s HTTP protocol is designed to be interpreted by the other node’s HTTP protocol. However, as you will learn in later sections, HTTP requests cannot traverse the network without the assistance of lower-layer protocols.

Presentation Layer

Protocols at the Presentation layer accept Application layer data and format it so that one type of application and host can understand data from another type of application and host. In other words, the Presentation layer serves as a translator. If you have spent any time working with computer graphics, you have probably heard of the GIF, JPG, and TIFF methods of compressing and encoding graphics. MPEG and QuickTime are two popular methods of compressing and encoding audio and video data. The popular audio format MP3, for example, uses MPEG compression. It can turn a music track that would require 30 MB of space on a CD into a file no larger than 3 MB—or even smaller, if lower quality were acceptable. In the previous example of requesting a Web page, the Presentation layer protocols would interpret the JPG files transmitted within the Web server’s HTTP response. Presentation layer services also manage data encryption (such as the scrambling of passwords) and decryption. For example, if you look up your bank account status via the Internet, you are using a secure connection, and Presentation layer protocols will encrypt your account data before it is transmitted. On your end of the network, the Presentation layer will decrypt the data as it is received.

Session Layer

Protocols in the Session layer coordinate and maintain communications between two nodes on the network. The term session refers to a connection for ongoing data exchange between two parties. Historically, it was used in the context of terminal and mainframe communications, in which the terminal is a device with little (if any) of its own processing or disk capacity that depends on a host to supply it with software and processing services. Today, the term session is often used in the context of a connection between a remote client and an access server or between a Web browser client and a Web server. When thinking in terms of the OSI model, however, this is misleading. Modern networks don’t make use of Session layer protocols for routine data exchange, such as Web page retrieval or file sharing. Yet applications that require precisely coordinated data exchanges, such as videoconferencing or voice (telephone) communication, still use Session layer protocols. Among the Session layer’s functions are establishing and keeping alive the communications link for the duration of the session, keeping the communication secure, synchronizing the dialogue between the two nodes, determining whether communications have been cut off, and, if so, figuring out where to restart transmission, and terminating communications. Session layer services also set the terms of communication by deciding which node communicates first and how long a node can communicate. If a connection is lost, the Session layer protocols will detect that and initiate attempts to reconnect. If they cannot reconnect after a certain period of time, they will close the session and inform your client software that communication has ended. Finally, the Session layer monitors the identification of session participants, ensuring that only the authorized nodes can access the session.

Transport Layer

Protocols in the Transport layer accept data from the Session layer and manage end-to-end delivery of data. That means they can ensure that the data are transferred from point A to point B reliably, in the correct sequence, and without errors. Without Transport layer services, data could not be verified or interpreted by its recipient. Transport layer protocols also handle flow control, which is the process of gauging the appropriate rate of transmission based on how fast the recipient can accept data. Dozens of different Transport layer protocols exist, but most modern networks, such as the Internet, rely on only a few. In the example of retrieving a Web page, a Transport layer protocol called TCP (Transmission Control Protocol) takes care of reliably transmitting the HTTP protocol’s request from client to server and vice versa.  Some Transport layer protocols take steps to ensure that data arrives exactly as it was sent. Such protocols are connection oriented because they establish a connection with another node before they begin transmitting data. TCP is one example of a connection-oriented protocol.In the case of requesting a Web page, the client’s TCP protocol first sends a SYN (synchronization) packet request for a connection to the Web server. The Web server responds with a SYN-ACK (synchronization-acknowledgment) packet, or a confirmation, to indicate that it’s willing to make a connection. Then, the client responds with its own ACK (acknowledgment).Through this three-step process, also known as a three-way handshake, a connection is established. Only after TCP establishes this connection does it transmit the HTTP request for a Web page.

Acknowledgments are also used in subsequent communications to ensure that data was properly delivered. For every data unit a node sends, its connection-oriented protocol expects an acknowledgment from the recipient. For example, after a client’s TCP protocol issued an HTTP request, it would expect to receive an acknowledgment from the Web server proving that the data arrived. If data isn’t acknowledged within a given time period, the client’s protocol assumes the data was lost and retransmits it. To ensure data integrity further, connection-oriented protocols such as TCP use a checksum. A checksum is a unique character string that allows the receiving node to determine if an arriving data unit exactly matches the data unit sent by the source. Checksums are added to data at the source and verified at the destination. If at the destination a checksum doesn’t match what the source predicted, the destination’s Transport layer protocols ask the source to retransmit the data. As you will learn, protocols at other layers of the OSI model also use checksums. Not all Transport layer protocols are concerned with reliability. Those that do not establish a connection before transmitting and make no effort to ensure that data is delivered free of errors are called connectionless protocols. A connectionless protocol’s lack of sophistication makes it more efficient than a connection-oriented protocol and renders it useful in situations in which data must be transferred quickly, such as live audio or video transmissions over the Internet. In these cases, connection-oriented protocols—with their acknowledgments, checksums, and flow control mechanisms—would add overhead to the transmission and potentially bog it down. In a video transmission, for example, this could result in pictures that are incomplete or aren’t updated quickly enough to coincide with the audio. In addition to ensuring reliable data delivery, Transport layer protocols break large data units received from the Session layer into multiple smaller units, called segments. This process is known as segmentation.  On certain types of networks, segmentation increases data transmission efficiency. In some cases, segmentation is necessary for data units to match a network’s MTU (maximum transmission unit), the largest data unit it will carry. Every network type specifies a default MTU (though its size can be modified to some extent by a network administrator). For example, by default, Ethernet networks cannot accept packets with data payloads larger than 1500 bytes. Suppose an application wants to send a 6000-byte unit of data. Before this data unit can be issued to an Ethernet network, it must be segmented into units no larger than 1500 bytes. To learn a network’s MTU size (and thereby determine whether it needs to segment packets), Transport layer protocols perform a discovery routine upon establishing a connection with the network. Thereafter, the protocols will segment each data unit as necessary until closing the connection. Segmentation is similar to the process of breaking down words into recognizable syllables that a child uses when learning to read. Reassembly is the process of reconstructing the segmented data units. To continue the reading analogy, when a child understands the separate syllables, he can combine them into a word—that is, he can reassemble the parts into a whole. To learn how reassembly works, suppose that you asked this  in history class: “Ms. Jones? How did poor farming techniques contribute to the Dust Bowl?” but that the words arrived at Ms. Jones’s ear as “poor farming techniques Ms. Jones? how did to the Dust Bowl? contribute.” On a network, the Transport layer recognizes this kind of disorder and rearranges the data pieces so that they make sense. Sequencing is a method of identifying segments that belong to the same group of subdivided data. Sequencing also indicates where a unit of data begins, as well as the order in which groups of data were issued and, therefore, should be interpreted. While establishing a connection, the Transport layer protocols from two devices agree on certain parameters of their communication, including a sequencing scheme. For sequencing to work properly, the Transport layer protocols of two nodes must synchronize their timing and agree on a starting point for the transmission.

Network Layer

The primary function of protocols at the Network layer, the third layer in the OSI model, is to translate network addresses into their physical counterparts and decide how to route data from the sender to the receiver. Addressing is a system for assigning unique identification numbers to devices on a network. Each node has two types of addresses. One type of address is called a network address. Network addresses follow a hierarchical addressing scheme and can be assigned through operating system software. They are hierarchical because they contain subsets of data that incrementally narrow down the location of a node, just as your home address is hierarchical because it provides a country, state, zip code, city, street, house number, and person’s name. Network layer address formats differ depending on which Network layer protocol the network uses. Network addresses are also called Network layer addresses, logical addresses, or virtual addresses. The second type of address assigned to each node is called a physical address. For example, a computer running on a TCP/IP network might have a Network layer address of 10.34.99.12 and a physical address of 0060973E97F3. In the classroom example, this addressing scheme is like saying that “Ms. Jones” and “United States citizen with Social Security number 123-45-6789” are the same person.  Even though there may be other people named “Ms. Jones” in the United States, only one person has the Social Security number 123-45-6789. Within the confines of your classroom, however, there is only one Ms. Jones, so you can be certain the correct person will respond when you say, “Ms. Jones?”  There’s no need to use her Social Security number. Network layer protocols accept the Transport layer segments and add logical addressing information in a network header. At this point, the data unit becomes a packet. Network layer protocols also determine the path from point A on one network to point B on another network by factoring in:

·         Delivery priorities (for example, packets that make up a phone call connected through the Internet might be designated high priority, whereas a mass e-mail message is low priority)

·         Network congestion

·         Quality of service (for example, some packets may require faster, more reliable delivery)

·         Cost of alternative routes

·          

The process of determining the best path is known as routing. More formally, to route means to intelligently direct data based on addressing, patterns of usage, and availability. Because the Network layer handles routing, routers—the devices that connect network segments and direct data—belong in the Network layer. Although there are numerous Network layer protocols, one of the most common, and the one that underlies most Internet traffic, is the IP (Internet Protocol). In the example of requesting a Web page, IP is the protocol that instructs the network where the HTTP request is coming from and where it should go. On TCP/IP-based networks (such as the Internet), Network layer protocols can perform an additional function called fragmentation. In fragmentation, a Network layer protocol (such as IP) subdivides the segments it receives from the Transport layer into smaller packets. If this process sounds familiar, it’s because fragmentation accomplishes the same task at the Network layer that segmentation performs at the Transport layer. It ensures that packets issued to the network are no larger than the network’s maximum transmission unit size. However, if a Transport layer protocol performs segmentation, fragmentation may not be necessary. For greater network efficiency, segmentation is preferred. Not all Transport layer protocols are designed to accomplish segmentation. If a Transport layer protocol cannot perform segmentation, Network layer protocols will perform fragmentation, if needed.

Data Link Layer

In the second layer of the OSI model, the Data Link layer, protocols divide data they receive from the Network layer into distinct frames that can then be transmitted by the Physical layer. A frame is a structured package for moving data that includes not only the raw data, or “payload,” but also the senders and receiver’s network addresses, and error checking and control information. The addresses tell the network where to deliver the frame, whereas the error checking and control information ensure that the frame arrives without any problems. To understand the function of the Data Link layer fully, pretend for a moment that computers communicate as humans do. Suppose you are in Ms. Jones’s large classroom, which is full of noisy students, and you need to ask the teacher a . To get your message through, you might say, “Ms. Jones? Can you explain more about the effects of railroads on commerce in the mid-nineteenth century?” In this example, you are the sender (in a busy network) and you have addressed your recipient, Ms. Jones, just as the Data Link layer addresses another computer on the network. In addition, you have formatted your thought as a , just as the Data Link layer formats data into frames that can be interpreted by receiving computers. What happens if the room is so noisy that Ms. Jones hears only part of your ? For example, she might receive “on commerce in the late-nineteenth century?” This kind of error can happen in network communications as well (because of wiring problems, for example). The Data Link layer protocols find out that information has been dropped and ask the first computer to retransmit its message—just as in a classroom setting Ms. Jones might say, I didn’t hear you. Can you repeat the ?” The Data Link layer accomplishes this task through a process called error checking.  Error checking is accomplished by a 4-byte FCS (frame check sequence) field, whose purpose is to ensure that the data at the destination exactly match the data issued from the source. When the source node transmits the data, it performs an algorithm (or mathematical routine) called a CRC (cyclic redundancy check). CRC takes the values of all of the preceding fields in the frame and generates a unique 4-byte number, the FCS. When the destination node receives the frame, its Data Link layer services unscramble the FCS via the same CRC algorithm and ensure that the frame’s fields match their original form. If this comparison fails, the receiving node assumes that the frame has been damaged in transit and requests that the source node retransmit the data. Note that the receiving node, and not the sending node, is responsible for detecting errors. In addition, the sender’s Data Link layer waits for acknowledgment from the receiver’s Transport layer that data was received correctly. If the sender does not get this acknowledgment within a prescribed period of time, its Data Link layer gives instruction to retransmit the information. The Data Link layer never tries to figure out what went wrong. Similarly, as in a busy classroom, Ms. Jones will probably say, “Pardon me?” rather than, “It sounds as if you might have a  about railroads, and I heard only the last part of it, which dealt with commerce, so I assume you are asking about commerce and railroads; is that correct? Obviously, the former method is more efficient.Another communications mishap that might occur in a noisy classroom or on a busy network is a glut of communication requests. For example, at the end of class, 20 people might ask Ms. Jones 20 different s at once. Of course, she can’t pay attention to all of them simultaneously. She will probably say, “One person at a time, please,” then point to one student who asked a . This is just like what the Data Link layer does for the Physical layer. One node on a network (a Web server, for example) may receive multiple requests that include many frames of data each. The Data Link layer controls the flow of this information, allowing the NIC to process data without error. In fact, the IEEE has divided the Data Link layer into two sublayers. The reason for this change was to allow higher-layer protocols (for example, those operating in the Network layer) to interact with Data Link layer protocols without regard for Physical layer specifications. The upper sublayer of the Data Link layer, called the LLC (Logical Link Control) sublayer, provides an interface to the Network layer protocols, manages flow control, and issues requests for transmission for data that have suffered errors. The MAC (Media Access Control) sublayer, the lower sublayer of the Data Link layer, manages access to the physical medium. It appends the physical address of the destination computer onto the data frame. The physical address is a fixed number associated with a device’s network interface. It is assigned to each NIC at the factory and stored in the NIC’s on-board memory. Because this address is appended by the MAC sublayer of the Data Link layer, it is also known as a MAC address or a Data Link layer address. Sometimes, it’s also called a hardware address. You can find a NIC’s physical address through your computer’s protocol configuration utility or by simply looking at the NIC. The physical address will be stamped directly onto the NIC’s circuit board or on a sticker attached to some part of the NIC. Physical addresses contain two parts. The first part, known as the OUI (Organizationally Unique Identifier), is a character sequence assigned by IEEE that identifies the NIC’s manufacturer. For example, a series of Ethernet NICs manufactured by the 3Com Corporation begins with the hexadecimal characters “00608C,” while a series of Ethernet NICs manufactured by Intel begins with “00AA00.” Some manufacturers have several different OUIs. IEEE also uses the term company_id to refer to the OUI. Traditionally, this portion of a physical address is sometimes called the block ID. The remaining characters in a physical address, known as the extension identifier, identify the interface. Vendors such as 3Com and Intel assign each NIC a unique extension identifier, based on the NIC’s model and manufacture date. By assigning unique extension identifiers, companies ensure that no two NICs share the same physical address. Extension identifiers may also be known as device IDs. In traditional physical addressing schemes, the OUI is six characters (or 24 bits) long and the extension identifier is also six characters long. Together, the OUI and extension identifier form a whole physical address. For example, IBM might assign one of its NICs the extension identifier 005499. The combination of the IBM OUI and this extension identifier result in a unique, 12-character, or 48-bit address of 00608C005499.  Physical addresses are frequently depicted as hexadecimal numbers separated by colons—for example,  0:60:8C:00:54:99.Whereas the traditional MAC addressing scheme assigns interfaces a 48-bit address, IEEE’s newer EUI-64 (Extended Unique Identifier-64) standard calls for a 64-bit physical address. In the EUI-64 standard, the OUI portion is 24 bits in length. A 40-bit extension identifier makes up the rest of the physical address to total 64 bits. Hexadecimal, or base 16, is a numeral system that uses 0 through 9 to represent its first 10 numbers, and then uses the letters A through F to represent the next six numbers.

Decimal: 0123456789101112131415

Hexadecimal: 0123456789 A B C D E F

In hexadecimal notation, the decimal number 12 is represented by the letter C, for example. Starting with the decimal number 16, hexadecimal notation uses a 1 to represent the previous 15 digits and begins counting again at 0. In other words, a decimal number 16 is represented as 10 in hexadecimal and a decimal number 32 is represented as 20 in hexadecimal, or 2 x 16 + 0 x 1. You can convert a hexadecimal number to a decimal number by multiplying the decimal equivalent of the digit in each position by its hexadecimal value for that position. Each value is an exponential value of 16. For instance, the value at position 3 equals 16 ³ or 4096. The value associated with positions is shown below (note that positions can extend beyond the 4 Hexadecimal position: 4 3 2 1 0^th position): Hexadecimal value: 65536 4096 256 16 1 The decimal equivalent of the hexadecimal number C0F is 12 x 256 + 0 x 16 + 15 x 1, or 3072 + 0 + 15, or 3087. In computer science, hexadecimal notation (sometimes called, simply, “hex”) is used as a shorter, readable version of the binary numbers that computers interpret. If you know a computer’s physical address, you can determine which company manufactured its NIC by looking up its block ID. IEEE maintains a database of block IDs and their manufacturers, which is accessible via the Web. At the time of this writing, the database search page could be found at: http://standards.ieee.org/regauth/oui/index.shtml. Because of their hardware addressing function, NICs can be said to perform in the Data Link layer of the OSI model. However, they also perform services in the Physical layer, which is described next.

The Physical layer is the lowest, or first, layer of the OSI model. Protocols at the Physical layer accept frames from the Data Link layer and generate signals as changes in voltage at the NIC. (Signals are made of electrical impulses that, when issued in a certain pattern, represent information.) When the network uses copper as its transmission medium, these signals are also issued over the wire as voltage. In the case of fiber-optic cable, signals are issued as light pulses. When a network uses wireless transmission, the signals are sent from antennas as electromagnetic waves. When receiving data, Physical layer protocols detect and accept signals, which they pass on to the Data Link layer. Physical layer protocols also set the data transmission rate and monitor data error rates. However, even if they recognize an error, they cannot perform error correction. When you install a NIC in your desktop PC and connect it to a cable, you are establishing the foundation that allows the computer to be networked. In other words, you are providing a Physical layer. Simple connectivity devices such as hubs and repeaters operate at the Physical layer. NICs operate at both the Physical layer and at the Data Link layer. As you would expect, physical network problems, such as a severed wire or a broken connectivity device, affect the Physical layer. Similarly, if you insert a NIC but fail to seat it deeply enough in the computer’s main circuit board, your computer will experience network problems at the Physical layer. Most of the functions that network administrators are most concerned with happen in the first four layers of the OSI model: Physical, Data Link, Network, and Transport. Software programmers, on the other hand, are more apt to be concerned with what happens at the Application, Presentation, and Session layers.

Communication between Two Systems

Based on what you have learned about the OSI model, it should be clear to you that data issued from a software application are not in the same form as the data that your NIC sends to the network. At each layer of the OSI model, some information—for example, a format specification or a network address—is added to the original data. After it has followed the path from the Application layer to the Physical layer, data are significantly transformed. The following paragraphs describe this process in detail. To understand how data changes, it is useful to trace the steps in a typical client/server exchange, such as retrieving a mail message from a mail server. Suppose that you connect to your company’s network from your home computer via a broadband Internet connection, log on, start your e-mail application, and then click a button in the e-mail application to retrieve your mail from the server. At that point, Application layer services on your computer accept data from your mail application and formulate a request meant for the mail server software. They add an application header to the data that the program wants to send. The application header contains information about the e-mail application’s requirements, so that the mail server can fulfill its request properly. The Application layer transfers the request to the Presentation layer, in the form of a protocol data unit (PDU). The Presentation layer first determines whether and how it should format or encrypt the data request received from the Application layer. For example, if your mail client requires encryption, the Presentation layer protocols will add that information to the PDU in a presentation header. If your e-mail message contains graphics or formatted text, that information will also be added.  Then, the Presentation layer sends its PDU to the Session layer, which adds a session header that contains information about how your home computer communicates with the network. For example, the session header might indicate that your Internet connection can only transmit and receive data at 512 Kbps. The Session layer then passes the PDU to the Transport layer. At the Transport layer, the PDU—your request for mail and the headers added by previous layers—is broken down into smaller pieces of data, or segments. The segments’ maximum size is dictated by the type of network transmission method in use (for example, Ethernet). Suppose your mail request PDU is too large to be a single segment. In that case, Transport layer protocols subdivide it into two or more smaller segments and assign sequence identifiers to all of the smaller segments. This information becomes part of the transport header. Protocols also add checksum, flow control, and acknowledgment data to the transport header. The Transport layer then passes these segments, one at a time, to the Network layer. Next, Network layer protocols add logical addressing information to the segments, so that your request will be properly routed to the mail server and the mail server will respond to your computer. This information is contained in the network header. With the addition of network address information, the pieces of data are called packets. The Network layer then passes the packets to the Data Link layer. At the Data Link layer, protocols add a header to the front of each packet and a trailer to the end of each packet to make frames. (The trailer indicates where a frame ends.) In other words, the Data Link layer protocols encapsulate the Network layer packets. Encapsulation is frequently compared with placing an envelope within a larger envelope. This analogy conveys the idea that the Data Link layer does not attempt to interpret any information added in the Network layer, but simply surrounds it. Using frames reduces the possibility of lost data or errors on the network because built into each frame is a way of checking for errors. After verifying that the data have not been damaged, the Data Link layer then passes the frames to the Physical layer. Finally, your request for mail, in the form of many frames, hits the NIC at the Physical layer. The Physical layer does not interpret the frames or add information to the frames; it simply transmits them over the broadband connection to your LAN, across your office network, and to the mail server after the binary digits (bits), or ones and zeroes, have been converted to electrical pulses. As the frames arrive at the mail server, the server’s Physical layer accepts the frames and transfers them to the Data Link layer. The mail server begins to unravel your request, reversing the process just described, until it responds to your request with its own transmission, beginning from its Application layer.

The terms frame, packet, datagram, and PDU are sometimes used interchangeably to refer to a small piece of data formatted for network transmission. Technically, however, a packet is a piece of information that contains network addressing information, and a frame is a piece of data enclosed by a Data Link layer header and trailer. Datagram is synonymous with packet. PDU generically refers to a unit of data at any layer of the OSI model. However, networking professionals sometimes use the term packet to refer to frames, PDUs, and Transport layer segments alike.

Frame Specifications

You have learned that frames are composed of several smaller components, or fields. The characteristics of these components depend on the type of network on which the frames run and on the standards that they must follow. By far, the most popular type of networking technology in use today is Ethernet, which uses Ethernet frames. You’ll learn much more about Ethernet in Chapter 5, but the following serves as an introduction, as well as a comparison between this favored network type and its historical rival, token ring. Ethernet is a networking technology originally developed at Xerox in the early 1970s and improved by Digital Equipment Corporation, Intel, and Xerox. There are four different types of Ethernet frames. The most popular form of Ethernet is characterized by the unique way in which devices share a common transmission channel, described in the IEEE 802.3 standard. A much less-common networking technology, token ring, was developed by IBM in the 1980s. It relies upon direct links between nodes and a ring topology. Nodes pass around tokens, special control frames that indicate to the network when a particular node is about to transmit data. Although this networking technology is nearly obsolete, there is a remote chance that you might work on a token ring network. The IEEE has defined token ring technology in its 802.5 standard. Ethernet frames are different from token ring frames, and the two will not interact with each other on a network. In fact, most LANs do not support more than one frame type because devices cannot support more than one frame type per physical interface, or NIC. (NICs can, however, support multiple protocols.) Although you can conceivably transmit both token ring and Ethernet frames on a network, Ethernet interfaces cannot interpret token ring frames, and vice versa. Normally, LANs use either Ethernet or token ring, and almost all contemporary LANs use Ethernet.

It is important to know what frame type (or types) your network environment requires. You will use this information when installing network operating systems, configuring servers and client workstations, installing NICs, troubleshooting network problems, and purchasing network equipment.

IEEE Networking Specifications

In addition to frame types and addressing, IEEE networking specifications apply to connectivity, networking media, error-checking algorithms, encryption, emerging technologies, and more. All of these specifications fall under the IEEE’s Project 802, an effort to standardize physical and logical elements of a network. IEEE developed these standards before the OSI model was standardized by ISO, but IEEE’s 802 standards can be applied to the layers of the OSI model. The section below describes just some of the IEEE 802 specifications. The Network+ certification exam includes questions about IEEE 802 specifications, with an emphasis on the technologies described by 802.3 and 802.11.

802.1 - Bridging and Management; Routing, bridging, and network-to-network communications

802.2 - Logical Link Control; Error and flow control over data frames

802.3 - Ethernet; All forms of Ethernet media and interfaces

802.5 - Token Ring LAN; All forms of token ring media and interfaces

802.11- Wireless LANs; Standards for wireless networking for many different broadcast frequencies and usage techniques.

802.15 - Wireless PANs; The coexistence of wireless personal area networks with other wireless devices in unlicensed frequency bands

802.16 - Broadband Wireless MANs; The atmospheric interface and related functions associated with broadband wireless connectivity; also known as WiMAX

802.17 - Resilient Packet Rings; Access method, physical layer specifications, and management of shared packet-based transmission on resilient rings (such as SONET)

802.20 - Mobile Broadband; Wireless Access; Packet handling and other specifications for multivendor, mobile high-speed wireless transmission, nicknamed “mobile WiMAX”

802.22 -Wireless Regional Area Networks; Wireless, broadcast-style network to operate in the UHF/VHF frequency bands formerly used for TV channels

Chapter Summary

• Standards are documented agreements containing precise criteria that are used as guidelines to ensure that materials, products, processes, and services suit their purpose. Standards also help to ensure interoperability between software and hardware from different manufacturers.

•  Some of the significant standards organizations are ANSI, EIA/TIA, IEEE, ISO, ITU, ISOC, IANA, and ICANN.

• ISO’s OSI (Open Systems Interconnection) model represents communication between two computers on a network. It divides networking architecture into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has its own set of functions and interacts with the layers directly above and below it.

• Protocols in the Application layer, the seventh layer of the OSI model, enable software programs to negotiate their formatting, procedural, security, synchronization, and other requirements with the network.

• Protocols in the Presentation layer, the sixth OSI model layer, serve as translators between the application and the network, using a common language for different hosts and applications to exchange data.

• Protocols in the Session layer, the fifth OSI model layer, coordinate and maintain links between two devices for the duration of their communication. They also synchronize dialogue, determine whether communications have been cut off, and, if so, figure out where to restart transmission.

• The primary function of protocols in the Transport layer, the fourth OSI model layer, is to oversee end-to-end data delivery. In the case of connection-oriented protocols, this means data are delivered reliably. They verify that data are received in the same sequence in which they were sent. They are also responsible for flow control, segmentation, and reassembly of packets. Connectionless Transport layer protocols do not offer such guarantees.

• Protocols in the Network layer, the third OSI model layer, manage logical addressing and determine routes based on addressing, patterns of usage, and availability. Routers belong to the Network layer because they use this information to intelligently direct data from sender to receiver.

• Network layer addresses, also called logical or virtual addresses, are assigned to devices through operating system software. They are composed of hierarchical information, so they can be easily interpreted by routers and used to direct data to their destination.

• The primary function of protocols at the Data Link layer, the second layer of the OSI model, is to organize data they receive from the Network layer into frames that contain error-checking routines and can then be transmitted by the Physical layer.

• The Data Link layer is subdivided into the Logical Link Control and MAC sublayers. The LLC sublayer ensures a common interface for the Network layer protocols. The MAC sublayer is responsible for adding physical address data to frames.

• Physical addresses (also known as MAC addresses) are 48- or 64-bit unique identifiers assigned to each network interface. They consist of an OUI (Organizationally Unique Identifier), which is assigned to manufacturers by IEEE, and an extension identifier, a unique number assigned to each NIC by the manufacturer.

• Protocols at the Physical layer generate and detect signals so as to transmit and receive data over a network medium. These protocols also set the data transmission rate and monitor data error rates, but do not provide error correction.

• A data request from a software program is received by the Application layer protocols and is transferred down through the layers of the OSI model until it reaches the Physical layer (the network cable, for example). At that point, data is sent to its destination over the network medium, and the Physical layer protocols at the destination send it back up through the layers of the OSI model until it reaches the Application layer.

• Data frames are small blocks of data with control, addressing, and handling information attached to them. Frames are composed of several fields. The characteristics of these fields depend on the type of network on which the frames run and the standards that they must follow. Ethernet and token ring networks use different frame types, and one type of network cannot interpret the others’ frames.

• In addition to frame types and addressing schemes, the IEEE networking specifications apply to connectivity, networking media, error-checking algorithms, encryption, emerging technologies, and more. All of these specifications fall under the IEEE’s Project 802, an effort to standardize the elements of networking.

• Significant IEEE 802 standards are 802.3, which describes Ethernet; 802.11, which describes wireless networking; and 802.16, which describes broadband wireless metropolitan area networks.

Keyterms

802.2

The IEEE standard for error and flow control in data frames. 

802.3

The IEEE standard for Ethernet networking devices and data handling (using the CSMA/CD access method). 

802.5

The IEEE standard for token ring networking devices and data handling. 

802.11

The IEEE standard for wireless networking. 

ACK

A response generated at the Transport layer of the OSI model that confirms to a sender that its frame was received. This packet is the third of three in the three-step process of establishing a connection. 

acknowledgment

See ACK. (Spelled Out) 

American National Standards Institute

See. ANSI. (Spelled Out) 

ANSI

An organization composed of more than 1000 representatives from industry and government who together determine standards for the electronics industry in addition to other fields, such as chemical and nuclear engineering, health and safety, and construction. 

API

A set of routines that make up part of a software application. 

Application layer

The seventh layer of the OSI model. This layer protocols enable software programs to negotiate formatting, procedural, security, synchronization, and other requirements with the network. 

application programming interface

See API. (Spelled Out) 

block ID

See OUI, company_id. 

checksum

A method of error checking that determines if the contents of an arriving data unit match the contents of the data unit sent by the source. 

company_id

See OUI, block ID. 

connection oriented

A type of Transport layer protocol that requires the establishment of a connection between communicating nodes before it will transmit data. 

connectionless

A type of Transport layer protocol that services a request without requiring a verified session and without guaranteeing delivery of data. 

CRC

An algorithm (or mathematical routine) used to verify the accuracy of data contained in a data frame. 

cyclic redundancy check

See CRC. (Spelled Out) 

Data Link layer

The second layer in the OSI model. This layer bridges the networking media with the Network layer. Its primary function is to divide the data it receives from the Network layer into frames that can then be transmitted by the Physical layer. 

Data Link layer address

See MAC address, physical address, hardware address. 

device ID

See extension identifier. 

EIA

A trade organization composed of representatives from electronics manufacturing firms across the United States that sets standards for electronic equipment and lobbies for legislation favorable to the growth of the computer and electronics industries. 

Electronic Industries Alliance

See EIA. (Spelled Out) 

Encapsulate

The process of wrapping one layer's PDU with protocol information so that it can be interpreted by a lower layer. For example, Data Link layer protocols _____ Network layer packets in frames. 

Ethernet

A networking technology originally developed at Xerox in the 1970s and improved by Digital Equipment Corporation, Intel, and Xerox. _____, which is the most common form of network transmission technology, follows the IEEE 802.3 standard. 

EUI-64

The IEEE standard defining 64-bit physical addresses. In the ____ scheme, the OUI portion of an address is 24 bits in length. A 40-bit extension identifier makes up the rest of the physical address to total 64 bits. 

Extended Unique Identifier-64

See EUI-64. (Spelled Out) 

Extension identifier

A unique set of characters assigned to each NIC by its manufacturer. In the traditional, 48-bit physical addressing scheme, it is 24 bits long. In EUI-64, it is 40 bits long. 

FCS

The field in a frame responsible for ensuring that data carried by the frame arrives intact. It uses an algorithm, such as CRC, to accomplish this verification. 

Flow control

A method of gauging the appropriate rate of data transmission based on how fast the recipient can accept data. 

fragmentation

A Network layer service that subdivides segments it receives from the Transport layer into smaller packets. 

Frame

A package for data that includes not only the raw data, or "payload," but also the sender's and recipient's addressing and control information. They are generated at the Data Link layer of the OSI model and are issued to the network at the Physical layer. 

Frame check sequence

See FCS. (Spelled Out) 

Hardware address

See MAC address, physical address, Data Link layer address. 

HTTP

An Application layer protocol that formulates and interprets requests between Web clients and servers. 

Hypertext Transfer Protocol

See HTTP. (Spelled Out) 

IAB

A technical advisory group of researchers and technical professionals responsible for Internet growth and management strategy, resolution of technical disputes, and standards oversight. 

IANA

A nonprofit, US government funded group established at University of Southern California and charged with managing IP address allocation and the DNS. Oversight for many of the organizations functions was given to ICANN in 1998; ____ continues to perform Internet addressing and DNS administration. 

ICANN

The nonprofit corporation currently designated by the United States government to maintain and assign IP addresses. 

IEEE

An international society composed of engineering professionals. Its goals are to promote development and education in the electrical engineering and computer science fields. 

IETF

An organization that sets standards for how systems communicate over the Internet (for example, how protocols operate and interact). 

Institute of Electrical and Electronics Engineers

See IEEE. (Spelled Out) 

International Organization for Standardization

See ISO. (Spelled Out) 

International Telecommunication Union

See ITU. (Spelled Out) 

Internet Architecture Board

See IAB. (Spelled Out) 

Internet Assigned Numbers Authority

See IANA. (Spelled Out) 

Internet Corporation for Assigned Names and Numbers

See ICANN. (Spelled Out) 

Internet Engineering Task Force

See IETF. (Spelled Out) 

Internet Protocol

See IP. (Spelled Out) 

Internet Protocol address

See IP address. (Spelled Out)

Internet service provider

See ISP. (Spelled Out) 

Internet Society

See ISOC. (Spelled Out)

IP

A core protocol in the TCP/IP suite that operates in the Network layer of the OSI model and provides information about how and where data should be delivered. The subprotocol that enables TCP/IP to internetwork. 

IP address

The Network layer address assigned to nodes to uniquely identify them on a TCP/IP network. IPv4 addresses consist of 32 bits divided into four octets, or bytes. IPv6 addresses are composed of eight 16-bit fields, for a total of 128 bits.

ISO

A collection of standards organizations representing 162 countries with headquarters located in Geneva, Switzerland. Its goal is to establish international technological standards to facilitate the global exchange of information and barrier-free trade. 

ISOC

A professional organization with members from 90 chapters around the world that helps to establish technical standards for the Internet. 

ISP

A business that provides organizations and individuals with Internet access and often, other services, such as e-mail and Web hosting. 

ITU

A United Nations agency that regulates international telecommunications and provides developing countries with technical expertise and equipment to advance their technological bases. 

LLC sublayer

The upper sublayer in the Data Link layer. Provides a common interface and supplies reliability and flow control services. 

logical address

See network address, Network layer address, virtual address. 

Logical Link Control sublayer

See LLC sublayer. (Spelled Out) 

MAC address

See physical address, Data Link Layer address, hardware address. 

MAC sublayer

The lower sublayer of the Data Link layer. Appends the physical address of the destination computer onto the frame.

maximum transmission unit

See MTU. (Spelled Out) 

Media Access Control sublayer

See MAC sublayer. (Spelled Out) 

MTU

The largest data unit a network (for example, Ethernet or token ring) will accept for transmission. 

Network address

A unique identifying number for a network node that follows a hierarchical addressing scheme and can be assigned through operating system software. Added to data packets and interpreted by protocols at the Network layer of the OSI model. 

Network layer

The third layer in the OSI model. Protocols in this layer translate network addresses into their physical counterparts and decide how to route data from the sender to the receiver. 

Network layer address

See network address, logical address, virtual address. 

Open Systems Interconnection model

See OSI model. (Spelled Out) 

Organizationally Unique Identifier

See OUI. (Spelled Out) 

OSI model

A model for understanding and developing computer-to-computer communication developed in the 1980s by ISO. It divides networking functions among seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. 

OUI

A 24-bit character sequence assigned by IEEE that appears at the beginning of a network interface's physical address and identifies the NIC's manufacturer. 

PDU

A unit of data at any layer of the OSI model. 

physical address

A 48- or 64-bit network interface identifier that includes two parts: the OUI, assigned by IEEE to the manufacturer, and the extension identifier, a unique number assigned to each NIC by the manufacturer. 

Physical layer

The lowest, or first, layer of the OSI model. Protocols in this layer generate and detect signals so as to transmit and receive data over a network medium. These protocols also set the data transmission rate and monitor data error rates, but do not provide error correction. 

Presentation layer

Sixth layer of OSI model. Protocols translate between application and network. Data are formatted in a schema that the network can understand, format varying according to type of network used. Also manages data encryption and decryption, such as scrambling of system passwords. 

protocol data unit

See PDU. (Spelled Out) 

reassembly

The process of reconstructing data units that have been segmented. 

Regional Internet Registry

See RIR. (Spelled Out) 

RIR

A not-for-profit agency that manages the distribution of IP addresses to private and public entities. ARIN covers North, Central, and South America and sub-Saharan Africa. APNIC covers Asia and the Pacific region. RIPE covers Europe and North Africa. 

route

To intelligently direct data between networks based on addressing, patterns of usage, and availability of network segments. 

router

A device that connects network segments and directs data based on information contained in the data packet. 

segments

Units of data that result from subdividing a larger protocol data unit. 

segmentation

The process of decreasing the size of data units when moving data from a network that can handle larger data units to a network that can handle only smaller data units. 

sequencing

The process of assigning a placeholder to each piece of a data block to allow the receiving node's Transport layer to reassemble the data in the correct order. 

session

A connection for data exchange between two parties. The term may be used in the context of Web, remote access, or terminal and mainframe communications, for example. 

Session layer

The fifth layer in the OSI model. This layer establishes and maintains communication between two nodes on the network. It can be considered the "traffic cop" for communications, such as videoconferencing, that require precisely coordinated data exchange. 

standard

A documented agreement containing technical specifications or other precise criteria that are used as guidelines to ensure that materials, products, processes, and services suit their intended purpose. 

SYN

The packet one node sends to request a connection with another node on the network. This packet is the first of three in the three-step process of establishing a connection. 

SYN-ACK

The packet a node sends to acknowledge to another node that it has received a SYN request for connection. This packet is the second of three in the three-step process of establishing a connection. 

synchronization

See SYN. (Spelled Out) 

synchronization-acknowledgment

See SYN-ACK. (Spelled Out) 

Telecommunication Industry Association

See TIA. (Spelled Out) 

terminal

A device with little (if any) of its own processing or disk capacity that depends on a host to supply it with applications and data-processing services. 

three-way handshake

A three-step process in which Transport layer protocols establish a connection between nodes. The three steps are: Node A issues a SYN packet to node B, node B responds with SYN-ACK, and node A responds with ACK. 

TIA

A subgroup of the EIA that focuses on standards for information technology, wireless, satellite, fiber optics, and telephone equipment. Best known standard __/EIA alliance are its guidelines for how network cable should be installed in commercial buildings, known as "__/EIA 568-B Series." 

token

A special control frame that indicates to the rest of the network that a particular node has the right to transmit data. 

token ring

A networking technology developed by IBM in the 1980s. It relies upon direct links between nodes and a ring topology, using tokens to allow nodes to transmit data. 

Transport layer

The transport layer is the fourth layer of the OSI model. In this layer, protocols ensure that data are transferred from point A to point B reliably and without errors. This layer services include flow control, acknowledgment, error correction, segmentation, reassembly, and sequencing. 

virtual address

See network address, logical address, network layer address.

Review Questions

1.   Your supervisor has asked you to correct several cable management problems that might be slowing down the network. Which organization’s standards will guide you in assessing your firm’s current cabling situation?

a.   ISO

b.   ITU

c.   TIA/EIA

d.   IEEE

2.   Which technology does the IEEE 802.11 specification describe?

a.   Network security

b.   Ethernet LANs

c.   Logical Link Control

d.  Wireless networks

3.   You are configuring clients to communicate over an Ethernet LAN. Which of the following IEEE specifications will identify which frame type your client should use?

a.  801.2

b.  802.3

c.  802.11

d.  801.16

4. Which layer of the OSI model is responsible for issuing acknowledgments (ACKs)?

a.   Application layer

b.   Data Link layer

c.   Network layer

d.  Transport layer

5.   Suppose your network is connected to another network via a router. Which OSI model layer provides the information necessary to direct data between the two networks ?

a.   Data Link layer

b.   Physical layer

c.   Network layer

d.   Session layer

6.  In which two layers of the OSI model do NICs belong?

a.   Presentation and Application layers

b.   Transport and Network layers

c.   Network and Data Link layers

d.  Physical and Data Link layers

7.   Which OSI model layer is responsible for keeping open a communications path between your computer and the server when you dial in to a remote access server?

a.   Session layer

b.   Data Link layer

c.   Presentation layer

d.   Physical layer

8.  Under what circumstances would the Transport layer use segmentation?

a.   When too many data frames are flooding into a receiving node’s NIC

b.   When more than 10 percent of transmitted frames are damaged

c.   When the destination node cannot accept the size of the data blocks transmitted by the source node

d.   When the source node requests that data blocks be segmented for faster processing

9.  Which OSI model layer generates and detects voltage so as to transmit and receive signals carrying data?

a.   Physical layer

b.   Data Link layer

c.   Network layer

d.   Transport layer

10. An IP address is an example of what type of address?

a.   Physical layer

b.  Network layer

c.   MAC sublayer

d.   Data Link sublayer

11. If the TCP protocol did not receive an acknowledgment for data it transmitted, what would it do?

a.   Issue its own acknowledgment, indicating to the recipient that it did not receive the acknowledgment it expected

b.   Issue a warning frame to tell the recipient it would retransmit the data if it did not receive the acknowledgment within a certain time frame

c.   Reestablish the connection with the recipient

d.  Retransmit the data to the recipient

12. Which part of a MAC address is unique to each manufacturer?

a.   The destination ID

b.  The OUI

c.   The physical node ID

d.   The SYN

13. What is the purpose of the trailer field added to a frame in the Data Link layer?

a.   To indicate the sum of the error-checking algorithm

b.   To signal the rate at which a node can receive the data

c.   To mark the end of a frame

d.   To represent the frame’s sequence number

14. Which layer of the OSI model encapsulates Network layer packets?

a.   Physical layer

b.   Session layer

c.   Data Link layer

d.   Transport layer

15.  At what OSI model layer do protocols manage data delivery priorities?

a.   Presentation layer

b.   Session layer

c.   Transport layer

d.  Network layer

16.  What are the sublayers of the Data Link layer as defined in the IEEE 802 standards?

a.   Logical Link Control sublayer and Media Access Control sublayer

b.   Transport Control sublayer and Media Access Control sublayer

c.   Logical Link Control sublayer and Physical Addressing sublayer

d.   Transport Control sublayer and Data Link Control sublayer

17. Suppose that, at the receiving node, a frame’s FCS doesn’t match the FCS it was issued at the transmitting node. What happens as a result?

a.   The receiving node’s Transport layer assesses the error and corrects it.

b.   The transmitting node’s Data Link layer assesses the error and corrects it.

c.   The receiving node’s Data Link layer requests a retransmission.

d.   The transmitting node’s Transport layer immediately issues a replacement frame.

18. In which of the following situations would it be most desirable to use a connectionless Transport layer protocol?

a.   When retrieving a spreadsheet from a busy file server

b.  When viewing a movie on the Web

c.   When connecting to a graphics-intensive Web site

d.   When sending an e-mail message to a long list of recipients

19. Which of the following would be found in a Data Link layer header?

a.   The packet’s fragmentation offset

b.   The packet’s sequence number

c.   The source’s logical address

d.  The source’s physical address

20. By default, what is the largest data payload that packets on an Ethernet network can accept?

a.   64 bytes

b.   128 bytes

c.   1500 bytes

d.   2400 bytes

Sample Quiz

1. By far, the most popular type of networking technology in use today is ________ .

a. Frame Relay           

b. Ethernet     

c. Broadband

d. Token Ring            

2. Simple connectivity devices such as hubs and repeaters operate at the ________ layer.

a. Network     

b. Physical      

c. Transport    

d. Data Link   

3. Within the OSI Model Transport layer, ________ is the process of gauging the appropriate rate of transmission based on how fast the recipient can accept data.

a. load balancing        

b. flow control           

c. capacity planning   

d. throughput management    

4. What organization is responsible for IP addressing and domain name management?

a. ISOC (Internet Society)     

b. ICANN (Internet Corporation for Assigned Names and Numbers)         

c. ISO (International Organization for Standardization)       

d. IETF (Internet Engineering Task Force)   

5. Data issued from a software application are not in the same form as the data that a NIC sends to the network.

a. True

b. False           

6. IEEE developed the EEE 802 specifications after the OSI model was standardized by ISO.

a. True

b. False           

7. Most LANs support more than one frame type.

a. True

b. False

8. The OSI model is a theoretical representation of what happens between two nodes communicating on a network.

a. True

b. False     

9. TCP is an example of a connection-oriented protocol.

a. True

b. False

  

10. The top, or seventh, layer of the OSI model is the ________ layer.

a. Application

b. Physical      

c. Transport    

d. Presentation

  

11. What dictates a segments' maximum size?

a. The type of network transmission method in use  

b. The NIC manufacturer's specifications      

c. The network operating system       

d. The type of transmission media in use

12. What document contains technical specifications or other precise criteria stipulating how a particular product or service should be designed or performed?

a. Contracts    

b. Policies       

c. Guidelines  

d. Standards

13. Which IEEE 802 specification technologies are emphasized in the Network+ certification exam?

a. 802.3 (Ethernet) and 802.5 (Token Ring LAN)    

b. 802.5 (Token Ring LAN) and 802.11 (Wireless LANs)   

c. 802.11 (Wireless LANs) and 802.20 (Mobile Broadband Wireless Access)         

d. 802.3 (Ethernet) and 802.11 (Wireless LANs)

14. What OSI model layer manages data encryption and decryption?

a. Application

b. Presentation           

c. Transport    

d. Physical

15. Technically, a ________ is a piece of data enclosed by a Data Link layer header and trailer.

a. datagram    

b. packet         

c. frame          

d. PDU

Practice Quiz

 1.  Which standards organization requests voluntary compliance with their standards?

a.       IANA

b.      ISO

c.       ITU

d.      ANSI

2. Simple connectivity devices such as hubs and repeaters operate at the ____.

a.       Application layer

b.      Network layer

c.       Data Link Layer

d.      Physical layer

3. The Application layer transfers the request to the Presentation layer, in the form of a ____.

a.       protocol data unit

b.      packet

c.       datagram

d.      frame

 4. The top, or seventh, layer of the OSI model is the ____.

a.       Presentation layer

b.      Network layer

c.       Transport layer

d.      Application layer

 5. The ____ standard refers to packet handling and other specifications for multivendor, mobile high-speed wireless transmission, nicknamed "mobile WiMAX".

a.       802.20

b.      mobile broadband wireless network

 

 6. The Physical layer interprets and adds information to frames.

a.       True

b.      False

 

 7. The IP (Internet Protocol) operates in the Transport layer.

a.       True

b.      False

 

 8. A checksum is a unique character string that allows the receiving node to determine if an arriving data unit exactly matches the data unit sent by the source.

a.       True

b.      False

  9. Which standards organization is responsible for providing the OSI model?

a.       ISOC

b.      ANSI

c.       IEEE

d.      ISO

 

 10. A connectionless protocol's lack of sophistication makes it more efficient than a connection-oriented protocol and renders it useful in situations in which data must be transferred quickly, such as live audio or video transmissions over the Internet.

a.       True

b.      False

11. Standards define maximum acceptable performance.

a.       True

b.      False

12. Most LANs do not support more than one frame type, because devices cannot support more than one frame type per physical interface, or NIC.

a.       True

b.      False

 13. Which standards organization’s technical papers and standards are highly respected in the networking profession?

a.       ICANN

b.      ANSI       

c.       IEEE        

d.      ISO

 

 14. The ____ provides developing countries with technical expertise and equipment to advance those nations' technological bases.

a.       ITU

b.      IEEE

c.       ISO

d.      ANSI

15. Standards help to ensure interoperability between software and hardware from different manufacturers.

a.       True

b.      False

 

 16. The ____ layer serves as a translator.

a.       application

b.      presentation

c.       transport

d.      physical

 

 17. ITU oversees groups with specific missions, such as the IAB (Internet Architecture Board). IAB is a technical advisory group of researchers and technical professionals interested in overseeing the Internet's design and management.

a.       True

b.      False

   18. ____ is frequently compared to placing an envelope within a larger envelope.

a.       Standards

b.      Datagram

c.       Encapsulation

d.      Packet

 19. The ____ standard refers to the routing, bridging, and network-to-network communications.

a.       802.2        

b.      802.1

c.       802.3                    

d.      802.11

20. IEEE's 802 standards can be applied to the layers of the OSI model.

a.       True                      

b.      False

21. Which standards organization is a professional membership society that helps to establish technical standards for the Internet?

a.       ISOC

b.      ANSI       

c.       IEEE        

d.      ISO

22.  ____ not only sets standards for its members, but also helps write ANSI standards and lobbies for legislation favorable to the growth of the computer and electronics industries.

a.       ANSII

b.      EIA

c.       ISO

d.      ITU

23. By far, the most popular type of networking technology in use today is ____.

a.       checksum

b.      CRC

c.       IANA

d.      Ethernet

24. NICs operate at both the Physical layer and at the ____ layer.

a.       Session

b.      Transport

c.       Application

d.      Data Link

25. The Application layer includes software applications.

a.       True

b.      False

26. Which statement accurately describes the OSI model?

a.       It describes how software programs interact with humans.

b.      It prescribes the type of hardware or software that should support each layer.

c.       It describes how software programs interact with other software programs.

d.      It describes a theoretical representation of what happens between two nodes communicating on a network.

27. A network’s ____________________ represents the largest data unit the network will carry.

a.       SYN

b.      MTU

c.       IP

d.      ACK

28.  ____ is the process of reconstructing segmented data.

a.       Realigning

b.      Reassembly

c.       Reengineering

d.      Resegmenting

29. The ____ is a fixed number associated with a device’s NIC.

a.       LLC address

b.      frame address

c.       logical address

d.      physical address

30.  ____________________ are documented agreements containing technical specifications or other precise criteria that stipulate how a particular product or service should be designed or performed.

a.       Standards

b.      Sessions

c.       Packets

d.      Frames

31. ____ oversees the IAB (Internet Architecture Board).

a.       ISOC

b.      ISO

c.       EIA

d.      ICANN

32. Which IEEE standard describes Ethernet?

a.       802.1

b.      802.3

c.       802.5

d.      802.11

33. Network functions are associated with only one layer of the OSI model.

a.       True

b.      False

34. The Application layer separates data into ____________________, or discrete amounts of data.

a.       segments

b.      protocol data units (PDUs)

c.       frames

d.      packets

35. Each network node has ____ types of addresses.

a.       two

b.      three

c.       four

d.      five

36. ____________________ is the process of gauging the appropriate rate of transmission based on how fast the recipient can accept data.

a.       Encapsulate

b.      Flow Control

c.       Addressing

d.      Routing

37. In which OSI model layer(s) do NICs operate?

a.       Physical and Data Link

b.      Data Link

c.       Physical

d.      Network and Physical

38. Transport layer protocols break large data units into ____.

a.       block IDs

b.      frames

c.       segments

d.      PDUs

39. Which OSI model layer manages data encryption?

a.       Session

b.      Application

c.       Physical.   

d.      Presentation

40. Standards define the ____ performance of a product or service.

a.       maximum acceptable

b.      minimum acceptable

c.       most acceptable

d.      ideal

41. Not all Transport layer protocols are concerned with reliability.

a.       True

b.      False

42. The ____ is a specialized United Nations agency that provides developing countries with technical expertise and equipment to advance those nations’ technological bases.

a.       ISOC

b.      ANSI

c.       ITU

d.      ISO

43. Which type of protocol is useful when data must be transferred quickly?

a.       IP

b.      connection-oriented

c.       TCP

d.      connectionless

44. Which Data Link sublayer manages access to the physical medium?

a.       Addressing layer

b.      LLC

c.       Management layer

d.      MAC

45. In which OSI model layer do hubs operate?

a.       Physical and Data Link

b.      Network

c.       Data Link

d.      Physical

46. ____________________ protocols establish a connection with another node before they begin transmitting data.

a.       connection oriented

b.      connectionless

c.       wireless

d.      IP

47. Which OSI model layer initiates the flow of information?

a.       Presentation

b.      Application

c.       Physical

d.      Session

48. Which IEEE standard describes specifications for wireless transmissions?

a.       802.1

b.      802.3

c.       802.5

d.      802.11

49. The process of determining the best path from Point A on one network to Point B on another is known as ____.  

a.       reconfiguring

b.      enhancing 

c.       routing

d.      mapping

50. The goal of ____ is to establish international technological standards to facilitate the global exchange of information and barrier free trade.

a.       ITU

b.      ISO

c.       ISOC

d.      LLC

56. Which OSI model layer does TCP operate?    

a.       Physical

b.      Data Link

c.       Application

d.      Transport

57. Which Data Link sublayer manages flow control?

a.       LLC

b.      MAC

58. ______________ is a method of identifying segments that belong to the same group of subdivided data.

a.       Segmenting

b.      Sequencing

c.       Routing

d.      Addressing