ing from within that location and only 20% was exchanged with other locations or other entities. This let businesses know that for the majority of their communications, they needed networks that had limited geographical span, and hence evolved the local area network (LAN). LANs were defined as serving a business address-a given building or at most a campus environment. A shift began to occur in how the network needed to accommodate data. In the mainframe environment, with its single-terminal-to-host communications, traffic volumes were predictable. The traffic levels between a given terminal and its host were known, so it was possible to make some fairly adequate assumptions about the amount of capacity to provision between those two points. However, in the LAN environment, the traffic patterns were very unpredictable. For example, in a business with 100 PCs on one LAN and 50 PCs on another LAN, the level of traffic on each LAN might change throughout the day. Sometimes it was extremely high volume, sometimes there was nothing going on, and sometimes it was a steady, average stream. This unpredictability introduced a requirement for network ser- vices that could be flexible in how they addressed bandwidth requirements (i.e., services that could introduce bandwidth on demand). Frame Relay, which is dis- cussed in Chapter 7, "Wide Area Networking," is one such network service. Frame Relay has the capability to provide more bandwidth than the user subscribes to, but because the traffic patterns fluctuate, the overall usage should balance out at the end of the day. Chapter 5 Data Communications Basics Throughout the mid- to late 1980s, the major design emphasis was on deploy- ing LANs, which help to speed up corporate communications, to make the work- force more productive, and to reduce costs associated with sharing software and hardware resources. LAN Internetworking As LANs were popping up in enterprises all over, it became necessary to come up with a tool for internetworking them. Otherwise, islands of knowledge 