A Data Communication Model

Networks The number of computers in use worldwide is in the hundreds of millions. Moreover, the expanding memory and processing power of these computers means that users can put the machines to work on new kinds of applications and functions. Accordingly, the pressure from the users of these systems for ways to communicate among all these machines is irresistible. It is changing the way vendors think and the way all automation products and services are sold. One type of network that has become ubiquitous is the local area network (LAN. Indeed, the LAN
is to be found in virtually all medium- and large-size office buildings. As the number and power of computing devices have grown, so have the number and capacity of LANs to be found in an office.
Although standards have been developed that reduce somewhat the number of types of LANs, there are still half a dozen general types of local area networks to choose from. Furthermore, many offices need more than one such network, with the attendant problems of interconnecting and managing a diverse collection of networks, computers, and terminals.
Wide Area Networks Wide area networks generally cover a large geographical area, require the crossing of public right- of-ways, and rely at least in part on circuits provided by a common carrier. Typically, a WAN
consists of a number of interconnected switching nodes. A transmission from anyone device is routed through these internal nodes to the specified destination device. These nodes (including the boundary nodes) are not concerned with the content of the data rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination. Traditionally, WANs have been implemented using one of two technologies circuit switching and packet switching. More recently, frame relay and ATM networks have assumed major roles.

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