Chapter 1: Network Fundamentals Review (Part01)

The goal of this chapter is to introduce some fundamental concepts and terminology that are the foundation for the other material in the book. After a brief introduction to networks in general, we delve into the communication protocols that are used by network devices; this necessarily includes a discussion of the infamous Open Systems Interconnection (OSI) model. LANs and WANs are described, as are the various devices found in a network. This is followed by an introduction to TCP/IP, used extensively in the Internet. Routing and addressing, including IP addresses, are explored. The two types of switching—Layer 2 and Layer 3 switching—are described. Spanning Tree Protocol (STP) and its operation are introduced, followed by a discussion of VLANs. The chapter concludes with a comprehensive example, tying together many of the concepts covered. You are encouraged to review any of the material in this chapter that you are not familiar with before reading the rest of the book, because these ideas are critical to understanding the more complex technologies covered in the other chapters.

Introduction to Networks

In the 1960s and 1970s, before the PC was invented, a company would typically have only one central computer—a mainframe. Users connected to the mainframe through terminals on their desks. These terminals had no intelligence of their own—their only function was to display a text-based user interface provided by the mainframe. For this reason, they were usually called dumb terminals. The only network was the connection between the terminals and the mainframe.

In 1981, the IBM PC was released—an event that changed the industry significantly. The PC had intelligence of its own, allowing users to do tasks on their desktops that previously required a mainframe. Networks were introduced to interconnect these distributed PCs.

The term network is used in many ways. For example, people network with one another, telephones are networked in the public telephone system, and data networks connect different computers. These uses of the term have a common thread: Networks make it possible for people or devices to communicate with each other.

A data network is a network that allows computers to exchange data. The simplest data network is two PCs connected through a cable. However, most data networks connect many devices.

An internetwork is a collection of individual networks connected by networking devices and that function as a single large network. The public Internet is the most common example—it is a single network that connects millions of computers. Internetworking refers to the industry and products that are involved in the design, implementation, and administration of internetworks.

The first networks were LANs; they enabled multiple users in a relatively small geographic area to exchange files and messages and to access shared resources such as printers and disk storage. WANs were introduced to interconnect these LANs so that geographically dispersed users could also share information. The “LANs and WANs” section later in this chapter further describes these two types of networks.

Note

The “Acronyms and Abbreviations” appendix near the end of the book lists many of the acronyms that appear in this book.

Protocols and the OSI Model

This section describes the OSI model and protocols used in internetworking. As an introduction, imagine that you are in Toronto and you want to send an e-mail to your friend in San Francisco. Successfully sending and receiving e-mail involves doing many things, including the following:

• You must type the message in your e-mail application.

• You must address the message in your e-mail application.

• You must click the Send button in your e-mail application to start sending the message.

• You must use the correct type of connections and wires to connect your PC to your local network.

• Your PC must put the data on the wire.

• Your PC must be able to connect to the Internet, and you must provide any necessary login information.

• Network devices must find the best path through the Internet so that the e-mail is received by the right person.

The following section introduces the OSI model, a model that describes all these communication functions and their relationships with each other.

The OSI Model

The ISO standards committee created a list of all the network functions required for sending data (such as an e-mail) and divided them into seven categories. This model is known as the OSI seven-layer model. The OSI seven-layer model was released in 1984; it is illustrated in Figure 1-1.

Figure 1-1: Each of the Seven Layers of the OSI Model Represents Functions Required for Communication

Note

You might also have heard people talk about OSI Layers 8 and 9. Although they are not official, Layer 8 is commonly known as the political layer, and Layer 9 is the religious layer. These lightheartedly represent all the other issues you might encounter in an IT project.

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Key Point

The OSI model represents everything that must happen to send data. The important thing to remember is that the OSI model does not specify how these things are to be done, just what needs to be done. Different protocols can implement these functions differently. For example, the open-standard Internet Protocol (IP) and Novell’s Internetwork Packet Exchange (IPX) protocol are different implementations of the network layer.

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As also shown in Figure 1-1, the seven layers can be thought of in two groups: the upper layers and the lower layers. The term upper layers often refers to Layers 5 through 7, and the term lower layers often refers to Layers 1 through 4, although this terminology is relative. The term upper layer also refers to any layer above another layer.

The upper layers are concerned with application issues—for example, the interface to the user and the format of the data. The lower layers are concerned with transport issues—for example, how the data traverses the network and the physical characteristics of that network.

Protocols

A protocol is a set of rules. The OSI model provides a framework for the communication protocols used between computers. Just as we need rules of the road—for example, so that we know that a red light means stop and a green light means go—computers also need to agree on a set of rules to successfully communicate. Two computers must use the same protocol to communicate. Computers that try to use different protocols would be analogous to speaking in Italian to someone who understands only English—it would not work.

Many different networking protocols are in use, in a variety of categories. For example, LAN and WAN protocols (at the lower two OSI layers) specify how communication is accomplished across various media types. Routed protocols (at Layer 3) specify the data’s format and how it is carried throughout a network, and routing protocols (some of which also operate at Layer 3) specify how routers communicate with one another to indicate the best paths through the network.

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Key Point

Many protocol suites define various protocols that correspond to the functions defined in the seven OSI layers, including routed protocols, a selection of routing protocols, applications, and so forth. Protocol suites are also known as protocol stacks.

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The most widely used network protocol suite today is the TCP/IP suite, named after two of the protocols within the suite. This network protocol suite is used in many places, including the backbone of the Internet and within organization’s networks. Novell’s NetWare, Apple Corporation’s AppleTalk, and IBM’s System Network Architecture are other examples of network protocol suites.

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Key Point

The OSI protocol suite is yet another suite. Although the OSI protocol suite uses the same names for its seven layers as the OSI seven-layer model does, the two OSI items are different—one is a protocol suite, and the other is the model that is used as a point of reference for all of the protocol suites.

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The OSI Layers

The following sections briefly describe each of the seven layers of the OSI model, starting at the lowest layer. Appendix C, “Open System Interconnection (OSI) Reference Model,” delves deeper into the details of the OSI model.

Physical Layer—Layer 1

The OSI physical layer defines specifications such as the electrical and mechanical conditions necessary for activating, maintaining, and deactivating the physical link between devices. Specifications include voltage levels, maximum cable lengths, connector types, and maximum data rates. The physical layer is concerned with the binary transmission of data. This binary data is represented as bits (which is short for binary digits). A bit has a single binary value, either 0 or 1.

Data Link Layer—Layer 2

Layer 2, the data link layer, defines the format of data that is to be transmitted across the physical network. It indicates how the physical medium is accessed, including physical addressing, error handling, and flow control. The data link layer sends frames of data; different media have different types of frames.

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Key Point

A frame is a defined set of data that includes addressing and control information and is transmitted between network devices. A frame can contain a header field (in front of the data) and a trailer field (after the data); these two fields are said to “frame” the data.

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For LANs, the Institute of Electrical and Electronics Engineers (IEEE) split Layer 2 into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).

The LLC sublayer (defined by the IEEE 802.2 specification) allows multiple network layer (Layer 3) protocols to communicate over the same physical data link by allowing the Layer 3 protocol to be specified in the LLC portion of the frame.

Some examples of MAC sublayer protocols are IEEE 802.3 Ethernet and IEEE 802.5 Token Ring. The MAC sublayer specifies the physical MAC address that uniquely identifies a device on a network. Each frame that is sent specifies a destination MAC address; only the device with that MAC address should receive and process the frame. Each frame also includes the MAC address of the frame’s source.

Note

You might be interested in some IEEE trivia: The IEEE 802 committee was formed in February (the second month) of 1980, and thus was called “802.” The IEEE 802.3 standard, for example, was ratified in the IEEE annex building 3 in Geneva at that time.

Network Layer—Layer 3

The network layer is responsible for routing, which allows data to be properly forwarded across a logical internetwork (consisting of multiple physical networks). Logical network addresses (as opposed to physical MAC addresses) are specified at Layer 3. Layer 3 protocols include routed and routing protocols. The routing protocols determine the best path that should be used to forward the routed data through the internetwork to its destination.

The network layer sends datagrams (or packets); different routed protocols have different types of datagrams.

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Key Point

A datagram is a defined set of data that includes addressing and control information and is routed between the data’s source and destination.

If a datagram needs to be sent across a network that can handle only a certain amount of data at a time, the datagram can be fragmented into multiple packets and then reassembled at the destination. Therefore, a datagram is a unit of data, whereas a packet is what physically goes on the network. If no fragmentation is required, a packet is a datagram; the two terms are often used interchangeably.

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Transport Layer—Layer 4

Layer 4, the transport layer, is concerned with end-to-end connections between the source and the destination. The transport layer provides network services to the upper layers.

Connection-oriented reliable transport establishes a logical connection and uses sequence numbers to ensure that all data is received at the destination. Connectionless best-effort transport just sends the data and relies on upper-layer error detection mechanisms to report and correct problems. Reliable transport has more overhead than best-effort transport.

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Key Point

Best-effort delivery means that the protocol will not check to see whether the data was delivered intact; a higher-level protocol, or the end user, must confirm that the data was delivered correctly

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Multiplexing allows many applications to use the same physical connection. For example, data is tagged with a number that identifies the application from which it came. Both sides of the connection then can interpret the data in the same way.

The transport layer sends segments.

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Key Point

A segment is a defined set of data that includes control information and is sent between the transport layers of the sender and receiver of the data.

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Upper Layers—Layers 5 Through 7

From the lower layers’ perspective, the three upper layers represent the data that must be transmitted from the source to the destination; the network typically neither knows nor cares about the contents of these layers. For completeness, the following briefly describes the functions of these layers:

• The session layer, Layer 5, is responsible for establishing, maintaining, and terminating communication sessions between applications running on different hosts.

• The presentation layer, Layer 6, specifies the format, data structure, coding, compression, and other ways of representing the data to ensure that information sent from one host’s application layer can be read by the destination host.

• Finally, the application layer, Layer 7, is the closest to the end user; it interacts directly with software applications that need to communicate over the network.

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Key Point

The OSI application layer is not the application itself; rather, the OSI application layer provides the communication services to the application.

For example, your e-mail application might use two OSI application layer protocols—Simple Mail Transfer Protocol (SMTP) and Post Office Protocol version 3 (POP3)—to send and retrieve e-mail messages.

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Communication Among OSI Layers

This section describes how communication among the seven OSI layers is accomplished. When you send an e-mail from Toronto to your friend in San Francisco, you can think of your e-mail application sending a message to the e-mail application on your friend’s computer. In OSI model terms, information is exchanged between peer OSI layers—the application layer on your computer is communicating with the application layer on your friend’s computer. However, to accomplish this, the e-mail must go through all the other layers on your computer; for example, it must have the correct network layer address, be put in the correct frame type, and so on. The e-mail must then go over the network, and then go back through all the layers on your friend’s computer, until it finally arrives at your friend’s e-mail application.

Control information from each layer is added to the e-mail data before it passes to lower layers; this control information is necessary to allow the data to go through the network properly. Thus, the data at each layer is encapsulated, or wrapped in, the information appropriate for that layer, including addressing and error checking. The right side of Figure 1-2 illustrates the following encapsulation process:

• At Layer 4, the e-mail is encapsulated in a segment.

• At Layer 3, this segment is encapsulated in a packet.

• At Layer 2, this packet is encapsulated in a frame.

• Finally, at Layer 1, the frame is sent out on the wire (or air, if wireless is used) in bits.

Figure 1-2: Data Is Encapsulated as It Goes Down Through the Layers and Is Unencapsulated as It Goes Up

The grouping of data used to exchange information at a particular OSI layer is known as a protocol data unit (PDU). Thus, the PDU at Layer 4 is a segment, at Layer 3 is a packet, and at Layer 2 is a frame.

Notice how the overall size of the information increases as the data goes down through the lower layers. When data is received at the other end of the network, this additional information is analyzed and then removed as the data is passed to the higher layers toward the application layer. In other words, the data is unencapsulated, or unwrapped; this process is shown on the left side of Figure 1-2.

Note

Cisco sometimes uses the word decapsulate instead of unencapsulate.

Note

For simplicity, Figure 1-2 shows only two systems, one in San Francisco and one in Toronto, and does not show the details of e-mail protocols or e-mail servers. Later sections in this chapter describe what happens when intermediate devices, such as routers, are encountered between the two systems.

At each layer, different protocols are available. For example, the packets sent by IP are different from those sent by IPX because different protocols (rules) must be followed. Both sides of peer layers that are communicating must support the same protocol.

LANs and WANs

LANs were first used between PCs when users needed to connect with other PCs in the same building to share resources. A LAN is a high-speed, yet relatively inexpensive, network that allows connected computers to communicate. LANs have limited reach (hence the term local-area network), typically less than a few hundred meters, so they can connect only devices in the same room or building, or possibly within the same campus.

A LAN is an always-on connection—in other words, you don’t have to dial up or otherwise connect to it when you want to send some data. LANs also usually belong to the organization in which they are deployed, so no incremental cost is typically associated with sending data. A variety of LAN technologies are available, some of which are shown in the center of Figure 1-3 and briefly described here:

• Ethernet and IEEE 802.3, running at 10 megabits per second (Mbps), use a carrier sense multiple access collision detect (CSMA/CD) technology. When a CSMA/CD device has data to send, it listens to see whether any of the other devices on the wire (multiple access) are transmitting (carrier sense). If no other device is transmitting, this device starts to send its data, listening all the time in case another device erroneously starts to send data (collision detect).

• Fast Ethernet (at 100 Mbps), covered by the IEEE 802.3u specification, also uses the CSMA/CD technology.

• Gigabit Ethernet (running at 1 gigabit per second [Gbps]) is covered by the IEEE 802.3z and 802.3ab specifications and uses the CSMA/CD technology.

• Wireless LAN (WLAN) standards, defined by the IEEE 802.11 specifications, are capable of speeds up to 54 Mbps under the 802.11g specification. (A new standard, 802.11n, planned to be ratified in 2007, will be capable of higher speeds.) WLANs use a carrier sense multiple access collision avoidance (CSMA/CA) mechanism (versus the CSMA/CD mechanism used by the wired Ethernet standards).

Figure 1-3: A Variety of LAN and WAN Standards

WANs interconnect devices that are usually connected to LANs and are located over a relatively broad geographic area (hence the term wide-area network). Compared to a LAN, a typical WAN is slower, requires a connection request when you want to send data, and usually belongs to another organization (called a service provider). You pay the service provider a fee (known as a tariff) for the use of the WAN; this fee could be a fixed monthly amount, or it could be variable based on usage and distance.

Just as you find many types of LANs, many types of WANs are also available, some of which are illustrated on the right side of Figure 1-3. Like LANs, WANs function at the lower two layers of the OSI model. A few, such as ISDN, also function at Layer 3. The service you use depends on many factors, including what is available where you are and, of course, the cost of the service. Some of the common WAN technologies include the following:

• Packet-switched network: A network that shares the service provider’s facilities. The service provider creates permanent virtual circuits and switched virtual circuits that deliver data between subscribers’ sites. Frame Relay is an example of a packet-switched network.

• Leased line: A point-to-point connection reserved for transmission. Common data link layer protocols used in this case are PPP and High-Level Data Link Control (HDLC).

• Circuit-switched network: A physical path reserved for the duration of the connection between two points. ISDN Basic Rate Interface (BRI) is an example of this type of network.

Two other technologies, digital subscriber line (DSL) and cable, connect residential and business premises to service providers’ premises:

• DSL: Uses unused bandwidth on traditional copper telephone lines to deliver traffic at higher speeds than traditional modems allow. The most common DSL implementation is asymmetric DSL (ADSL). It is called asymmetric because the download speed is faster than the upload speed, reflecting the needs of most users and more efficiently using the available bandwidth on standard two-wire telephone lines. ADSL allows regular telephone traffic to simultaneously share the line with high-speed data traffic so that only one telephone line is required to support both high-speed Internet and normal telephone services.

• Cable: Uses unused bandwidth on cable television networks to deliver data at higher speeds than traditional modems allow.

Note

These and other WAN technologies are discussed in Chapter 5, “Designing Remote Connectivity.”