The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

4.2. Switched LAN Concepts

The original rationale for the development of LAN bridges was LAN extension, both in terms of distance and numbers of stations. With the advent of high-port-density bridges capable of wire-speed operation, a new paradigm for local networking has emerged: the switched LAN, whose primary rationale is LAN segmentation.

A switched LAN is an alternative to a traditional shared-bandwidth LAN. In terms of product deployment in a structured wiring environment, the only apparent difference is that the hub is a switching hub (bridge) rather than a shared hub (repeater). The behavior of the network changes considerably, however, between shared and switched LANs. In addition, a switched LAN offers the possibility of configurations that were not available to the shared LAN user. As usual, all of this comes at a price.

4.2.1. Separate Access Domains

In a shared Ethernet LAN, the CSMA/CD MAC algorithm is used to arbitrate for use of the shared channel. If two or more stations have frames queued for transmission at the same time, there will be a collision among them. The set of stations contending for access to a shared Ethernet LAN is known as a collision domain. Similarly, the set of stations contending for the use of a token on a token-passing LAN can be referred to as a token domain. Both are examples of access domains — the set of stations sharing a given LAN and arbitrating among themselves using whatever access control mechanism is appropriate for that LAN. ...