n BIU-Baseband interface unit HSP-High-speed printer Figure 11.1b A typical ring network. A star network is illustrated in Figure 11.1c. At the center of the star is a switching device. This could be a switching hub. Users can be paired, two at a time, three at a time, or all at a time, segmented into temporary families of users depending on the configuration of the switch at that moment in time. Such a concept lends itself particularly ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS Server Switching hub or controller- Comp. processor Server Server Figure 11.1c A star network. Comp. Comp.-Computer WS-Workstation BIU-Baseband interface unit HSP-High-speed printer Figure 11.1d A typical tree configuration. well to ATM (asynchronous transfer mode). Each user is connected to the switch on a point-to-point basis. A tree network is illustrated in Figure 11.1d. 11.4 BASEBAND LAN TRANSMISSION CONSIDERATIONS A baseband LAN is a point-to-point or point-to-multipoint network. Two transmission problems arise as a result. The first deals with signal level and signal-to-noise (S/N) ratio, and the second deals with standing waves. Each access on a common medium must have sufficient signal level and S/N such that copied signals have a BER in the 11.5 OVERVIEW OF ANSI/IEEE LAN PROTOCOLS range of 1 X 10-8 to 1 x 10-12. If the medium is fairly long in extension and there are many accesses, the signal level must be high for a transmitting access to reach its most distant destination. The medium is lossy, particularly at the higher bit rates, and each access tap has an insertion loss. This leads to very high signal levels. These may be rich in harmonics and spurious emissions, degrading bit error rate. On the other hand, with insufficient level, the S/N ratio degrades, which will also degrade error performance. A good level balance must be achieved for all users. Every multipoint connectivity must be examined. The number of multipoint connectivities can be expressed by n(n 1), where n is the number of accesses. If, on a particular LAN, 10