 section will focus on the more practical aspects of network Often the geographic distribution of the users and the information design giving attention to performance, configuration, routing, traf- fic requirements, expansion and topology. Some of these areas are technology resources dictates the basic topology of the network interrelated and there is a degree of overlap in functionality. which serves to interconnect them. A typical network topology comprises a set of very high speed low intelligence backbone switches, such as synchronous TDMs, which serve to interconnect 40.8.1 Specification lower speed high functionality packet switching nodes, or statistical multiplexers, to which LAN users are connected (via modems or The first consideration when specifying any data communications kilostream links) through bridges. network is to establish the nature and rates of traffic which the network will be expected to support both in the short and the long term. This is crucial to all network design and is the starting point 40.8.4 Performance requirements of all network decisions. If errors are made here, the network cannot be expanded to meet new (and possibly unexpected) requirements. A common misconception is that performance is all about line rates. This is only a part of the equation. Hop counts, accumulative transit 40.8.2 Traffic types delays, turn around times, congestion level, congestion handling and the actual end systems all contribute to the performance of the network as a whole. There are numerous different types of traffic, some of which are: The performance requirements of a network depend on the types 1. Stop-start traffic in the form of lots of short packets travelling of traffic running over it, the time of day traffic requirements, the in one direction often with slightly longer packets in the reverse distribution of the traffic, the cost of the resources, the reliability direction. A characteristic of this type of traffic is that it is often requirements. associated with a requirement for very shor