Network Standards

Token Passing Token passing is an orderly access method (figure 1-7). Each workstation passes on the opportunity to transmit to its closest neighbor, until a station is found with a message to send. This permission to transmit is called a token. When a workstation with data to send is handed a token, part of the token is changed, indicating it is carrying a message, and then data is transmitted with the token. The token is then passed around the network, and every station checks to see if the message is intended for them. The receiving station copies the message from the token but then passes the unchanged token along the network. When the transmitting station receives the same token, it knows the message has been passed around the network. The transmitting station erases the message and puts the empty token back into circulation on the network. The amount of information that may be transmitted during possession of the token is limited so that all workstations can share the cable equally. 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. 1-13