Network Standards

These access methods (CSMA/CD, CSMA/CA, and token passing) with their transmission medium (twisted-pair wire, coaxial cable, or fiber optic cable), are just one of several aspects (or levels) of an entire LAN structure. The topologies and network access methods just presented only establish a way to connect workstations or nodes together and how to pass along packets of data. These packets of data may be programs, data, system or personal messages, and so on. Above this hardware/software level are a number of other levels that are just as important in a LAN's design. These are the levels that define how the LAN system manages its resources, how a user like yourself is able to log onto another node's hard disk, how a common laser

Figure 1-7.\A ring network using the token passing access method.

printer is used by all nodes, how one file is passed among many users, and so on. If order and discipline are to be maintained on the network, standards or protocols must be established and adhered to. This allows the LAN to function in an efficient and effective manner.

Over the past few years, a number of network standards or protocols have been developed by the International Standards Organization (ISO). They provide some level of uniformity among computer manufacturers and network vendors. ISO is one of several governing organizations in this field that has developed a series of protocols (rules to live by) to ensure compatibility for the many different vendors who design network hardware and software products. IS0 has defined a seven-layer architecture. These seven layers of standards, shown in figure 1-8, define a generalized architecture called the Reference Model of open Systems Interconnection. It is also known as

the OSI reference model or OSI model. The primary purpose of the OSI model is to provide a basis for coordinating the development of standards that relate to the flexible interconnection of incompatible systems using data communications facilities.

The OSI model does NOT define any one vendor's particular network software as such, nor does it define detailed standards for any given software. It simply defines the broad categories of functions that each of the seven layers should perform. The OSI model can include different sets of standards at each layer that are appropriate for given situations. For example, in a very simple data communications system, one that uses a simple point-to-point link, the software at the higher-

Figure 1-8.\The OSI model showing the seven software layers.

level layers (say 5, 6, and 7) might be very simple or possibly nonexistent. However, in a very complex data communications system, all seven software layers may be implemented. Although there is no requirement for any hardware or software vendor to adhere to the principles set forth in the OSI model, there is a worldwide trend in the computer industry toward acceptance and conformance to these standards.

About now, you may be asking yourself, what are these seven software layers (shown in figure 1-8), and why all the need for protocols? Don't all computers work in binary? Do they not all have operating systems? If a computer wants to communicate with another system, do you not simply connect them together using some type of cable? The answers to these questions are yes, yes, and yes; however, the commonalities seem to stop there.

Ideally, if the hardware, network software, application software, and cabling were all supplied by the same manufacturer, we would have relatively few problems to contend with when we design and implement a network. Everything would work together rather smoothly. However, a computer manufacturer's architecture can make it difficult to interconnect hardware offered by other competing manufacturers/vendors. The protocols used by communications devices are also highly complex and are often completely different from one manufacturer to another. Then, there is the network software. Network software from one LAN vendor usually won't work on a competitor's network, nor will the application programs. Even the cabling must be selected for a specific local-area network.

We could go on and on explaining the many incompatibilities that exist within these different areas, but the good news is that many hardware and software manufacturers/vendors provide interfaces. These various types of interfaces (bridges, gateways, routers, and so on) allow networks to be compatible with one another. At this point, we briefly talk about the seven software layers defined in the OSI model to give you some idea of what they are and why they are needed. To illustrate how the OSI model works, we are using the analogy of sending a letter using the U.S. postal system.

Layer l\The physical layer is concerned with the transmission of the unstructured raw bit stream over a physical meduim. It addresses the electrical, mechanical, and functional interface to the carrier. It is the physical layer that carries the signals for all the higher layers, as follows:

Voltages and pulse encoding of bits

Media and media interface (cables, connectors, NIC, and so on)

Line discipline (full- or half-duplex)

Pin assignments

In our mail analogy, the mail truck and the highway provide the services of the physical layer.

Layer 2\The data link layer provides error-free transmission of information over the physical medium. This allows the next higher layer to assume virtually error-free transmission over the link. The data link layer is responsible for getting data packaged and onto the network cable. It manages the flow of the data bit stream into and out of each network node, as follows: . Creates and recognizes frame boundaries l Checks received messages for integrity . Manages channel access and flow control . Ensures correct sequence of transmitted data

The data link layer detects, and when possible, corrects errors that occur in the physical layer without using the functions of the upper layers. It also provides flow-control techniques to ensure link-buffer capacity is not exceeded. In our analogy, the data link layer is concerned with sending the mail trucks onto the highway and making sure they arrive safely.

Layer 3\The network layer decides which physical pathway the data should take, based on network conditions, priorities of service, and other factors. Software on the network interface card must

build the data packet so the network layer can recognize and route the data to the correct destination address. It relieves the upper layers of the need to know anything about the data transmission and switching technologies used to connect the systems. It is responsible for establishing, maintaining, and terminating connections across the intervening communications facility, as follows:

Addresses messages

Sets up the path between communicating nodes on possibly different networks

Routes messages among networks

Is concerned with the sequence delivery of data packets

* Controls congestion if too many packets are on the network

l Translates logical addresses or names into physical addresses

l Has accounting functions to count packets orbits sent by users to produce billing information This layer acts in our postal service analogy, like the regional mail distribution centers throughout the country. The trucks are directed to the centers and are routed along the best path to their final destinations.

Layer 4\The transport layer ensures data units are delivered error-free, in sequence, with no losses or duplications. It relieves higher layer protocols from any concern with the transportation of data between them, as follows: l l l l l

Message segmentation\accepts data from the session layer, splits it up into smaller units, and passes the units down to the network layer

Establishes and deletes host-to-host connections across the network

Multiplexes several message streams onto one channel and keeps track of which message belongs to which connection

Provides reliable end-to-end delivery with acknowledgment

Provides end-to-end flow control and window management

The transport layer functions are provided by the mail truck dispatcher, who takes over if there is a wreck out in the system. If the network goes down, the transport layer software will look for alternate routes or perhaps save the transmitted data until the network connection is reestablished.

Layer 5\The session layer allows users on different machines to establish sessions between them. It performs the functions that enable two applications to communicate across the network, performing security, name recognition, logging, administration, and other similar functions. Unlike the network layer, this layer is dealing with the programs in each machine to establish conversations between them, as follows:

Allows two applications processes on different machines to establish, use, and terminate a connection (or session)

Performs synchronization between end-user tasks by placing checkpoints in the data stream so if the network fails, only the data after the last checkpoint has to be retransmitted

Provides dialogue control (who speaks, when, how long, and so on)

The session layer in our postal agency recognizes different zip codes and reroutes letters.

Layer 6\The presentation layer formats data to be presented to the application layer. It can be viewed as the translator for the network. This layer provides a common representation for data that can be used between the application processes. The presentation layer relieves the applications from being concerned with data representation, providing syntax independence, as follows:

l Encodes data in a standard way (integers, floating point, ASCII, and so on)

l Provides data compression to reduce the number of bits that have to be transmitted

l Provides data encryption for privacy and authentication

This layer functions like a translator who translates a letter from French into English.

Layer 7\The application layer serves as the window for the application process to access the OSI environment. This layer represents the services that directly support users and application tasks. It contains a variety of commonly needed protocols for the following:

Network virtual terminals

File transfers

Remote file access

Electronic mail

Network management

In our analogy, the application layer is the person who writes or reads the letter.