Concepts

Network designers get excited when someone expresses interest in their rather esoteric field, which they invariable serve up as a layer cake. Far from a rich devil's food with a heavy chocolate frosting (with jimmies), they roll out a network model, something that will definitely disappoint your taste buds because it's entirely imaginary. They call their cake the Open Systems Interconnection Reference Model (or the OSI Model for short), and it's a layer cake the way software is layered. It represents the structure of a typical network with the most important functions each given its own layer.

Although the discussion of networks usually begins with this serving of cake, the OSI Model does not represent any particular network. In fact, networks are by no means obliged to follow the model. Although most have layers of functions, they are often stacked differently. And while the OSI Model preaches a seven-layer model, even the foremost networkers in the world—the Institute of Electrical and Electronic Engineers—slip in one more layer for good measure. Table 12.1 highlights the equivalence of different layers in several common networking systems, including the OSI model.

Table 12.1. Approximate Equivalence of Network Layers

Layer	OSI Model	Internet Model	Windows Model	Real World
1	Physical	Not specified	Adapter	Twisted-pair
2	Data Link	Not specified		NIC
3	Network	Internet Protocol	Protocol	Network operating system
4	Transport	Transport Control Protocol (TCP)
5	Session			Network driver
6	Presentation	FTP, HTTP	Service	Operating system
7	Application	Not specified	Client	Browser

What the OSI Model shows is how many different functions you need to make a network and how those functions interrelate to one another. It's a good place to begin, and it adds an academic aura to our network discussion, which just might make you believe you're learning something useful.

OSI Model

In 1984, the International Standards Organization laid out a blueprint to bring order to the nonsense of networking by publishing the Open Systems Interconnection Reference Model. You can imagine the bickering between hundreds of engineers trying to refine the right recipe for cooking up a network, then arguing over how many layers into which to divide their batter. The final, seven-layer model looks as if it might be the one and only way to build a network. These layers, ranging from the connecting wire to software applications, define functions and protocols that enable the wide variety of network hardware and software to work together. It just seems so logical, so compelling, that it is taught in most colleges and endorsed by major organizations such as IBM.

Regardless of where it fits in with your beliefs, philosophies, and diet, the OSI Model presents an excellent way to understand networks. The layering defined by the OSI Reference Model illustrates how the various elements of a network—from the wire running through your office ceiling to the Windows menu of your mail program—fit together and interact. Although few actual networks or network products exactly fit the model, the layers show how networks must be structured as well as the problems in building a network.

Physical

The first layer of the OSI Reference Model is the Physical layer. It defines the basic hardware of the network, which is the cable that conducts the flow of information between the devices linked by the network, or even the lack of a cable in wireless networking designs. This layer defines not only the type of wire (for example, coaxial cable, twisted-pair wire, and so on) but also the possible lengths and connections of the wire, the signals on the wire, and the interfaces of the cabling system—or the frequency and modulation of the radio devices that carry wireless network signals. This is the level at which the device that connects a computer to the network (the network host adapter) is defined.

Data Link

Layer 2 in a network is called the Data Link layer. It defines how information gains access to the wiring system. The Data Link layer defines the basic protocol used in the local network. This is the method used for deciding which computer can send messages over the cable at any given time, the form of the messages, and the transmission method of those messages.

This level defines the structure of the data that is transferred across the network. All data transmitted under a given protocol takes a common form called the packet or network data frame. Each packet is a block of data that is strictly formatted and may include destination and source identification as well as error-correction information. All network data transfers are divided into one or more packets, the lengths of which are carefully controlled.

Breaking network messages into multiple packets enables the network to be shared without interference and interminable waits for access. If you transfer a large file (say, a bitmap) across the network in one piece, you might monopolize the entire network for the duration of the transfer. Everyone would have to wait. By breaking all transfers into manageable pieces, everyone gets access in a relatively brief period, thus making the network more responsive.

Network

Layer 3 in the OSI Reference Model is the Network layer, which defines how the network moves information from one device to another. This layer corresponds to the hardware-interface function of the BIOS of an individual computer because it provides a common software interface that hides differences in underlying hardware. Software of higher layers can run on any lower-layer hardware because of the compatibility this layer affords. Protocols that enable the exchange of packets between different networks operate at this level.

Transport

Layer 4 controls data movement across the network. The Transport layer defines how messages are handled—particularly how the network reacts to packets that become lost as well as other errors that may occur.

Session

Layer 5 of the OSI Reference Model defines the interaction between applications and hardware, much as a computer BIOS provides function calls for programs. By using functions defined at this Session layer, programmers can create software that will operate on any of a wide variety of hardware. In other words, the Session layer provides the interface for applications and the network. Among computers, the most common of these application interfaces is IBM's Network Basic Input/Output System (NetBIOS).

Presentation

Layer 6, the Presentation layer, provides the file interface between network devices and the computer software. This layer defines the code and format conversions that must take place so that applications running under a computer operating system, such as DOS, OS/2, or Macintosh System 7, can understand files stored under the network's native format.

In classic networking, this function would be served by the computer's BIOS. The first personal computers lacked any hint of network connectibility, but in 1984 IBM introduced a trend-setting system called the IBM PC Network, which has been the foundation for small computer networking ever since. The critical addition was a set of new codes to the system BIOS developed by Sytek. This set of new codes—specifically, the Interrupt 5C(hex) routines—has become known as the Network BIOS, or NetBIOS.

The NetBIOS serves as a low-level application program interface for the network, and its limitations get passed on to networks built around it. In particular, the NetBIOS imposed requires that each computer on the network wear a unique name up to 15 characters long. This limits the NetBIOS to smaller networks. Today's operating systems use driver software that takes the place of the NetBIOS.

Application

Layer 7 is the part of the network that you deal with personally. The Application layer includes the basic services you expect from any network, including the ability to deal with files, send messages to other network users through the mail system, and control print jobs.

Internet Model

The Internet does not neatly fit the seven-layer OSI Model for the simple reason that the Internet is not meant to be a network. It describes a network of networks. Consequently, the Internet standards do not care about, nor do they describe, a "physical" layer or an "application" layer. (The World Wide Web standards arguably define a "presentation" layer.) When you use the Internet, however, the computer network you use to tie into the Internet becomes a physical layer, and your browser acts as an application layer.

Similarly, Ethernet does not conform to a seven-layer model because its concerns are at only the Physical through Session layers. Note that Ethernet breaks the Data Link layer into its own two-layer system.

Windows Model

When you work across a network using Windows, all the OSI layers come into play. Windows sees networks as layered somewhere between the two-tier practical approach and the seven layers of OSI. Microsoft assigns four levels of concern when configuring your network, using your network or operating system software. Under Windows, these levels include the adapter, protocol, service, and client software, as shown in the Windows Select Network Component Type dialog box (see Figure 12.1).

In this model, the network has four components—the client, the adapter, the protocol, and the service. Each has a distinct role to play in making the connection between computers.

Adapter

The adapter is the hardware that connects your computer to the network. The common term network interface card, often abbreviated as NIC, is one form of host adapter, a board you slide into an expansion slot. The host adapter function also can be integrated into the circuitry of the motherboard, as it often is in business-oriented computers.

No matter its form or name, the adapter is the foundation of the physical layer of the network, the actual hardware that makes the network work with your computer. It translates the parallel bus signals inside your computer into a serial form that can skitter through the network wiring. The design and standards that the adapter follows determine the form and speed of the physical side of the network.

From a practical perspective, the network adapter is generally the part of the network that you must buy if you want to add network capabilities to a computer lacking them. You slide a network host adapter or NIC into an expansion slot in your computer to provide a port for plugging into the network wire.

Protocol

The protocol is the music of the packets, the lyrics that control the harmony of the data traffic through the network wiring. The protocol dictates not only the logical form of the packet—the arrangement of address, control information, and data among its bytes—but also the rules on how the network deals with the packets. The protocol determines how the packet gets where it is going, what happens when it doesn't, and how to recover when an error appears in the data as it crosses the network.

Support for the most popular and useful protocols for small networks is included with today's operating systems. It takes the form of drivers you install to implement a particular networking system. Windows, for example, includes several protocols in its basic package. If you need to, you can add others as easily as installing a new software driver.

Service

The service of the network is the work the packets perform. The services are often several and always useful. Network services include exchanging files between disk drives (or making a drive far away on the network appear to be local to any or every computer in the network), sharing a printer resource so that all computers have access to a centralized printer, and passing electronic mail from a centralized post office to individual machines.

Most networking software includes the more useful services as part of the basic package. Windows includes file and printer sharing as its primary services. The basic operating system also includes e-mail support. Again, new services are as easy to add as new driver software.

Client

To the network, the client is not you but rather where the operating system of your computer and the network come together. It's yet another piece of software, the one that brings you the network resources so that you can take advantage of the services. The client software allows the network to recognize your computer and to exchange data packets with it.