ir transmissions, wait a random amount of time, and try again. This technique has the wonderful name of truncated binary exponential back- off. It's one of those phrases you just have to commit to memory because it sounds so good when you casually let it roll off the tongue in conversa- tion. Ultimately, each station will get a turn to transmit, although how long they may have to wait is based on how busy the LAN is. Contention- based systems are characterized by what is known as unbounded delay, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Premises Technologies Premises Technologies Figure 5-19 A collision on a contention- based LAN because there is no upward limit on how much delay a station can incur as it waits to use the shared medium. As the LAN gets busier and traffic increases, the number of stations vying for access to the shared medium -which only allows a single station at a time to use it, by the way-also goes up, which naturally results in more collisions. Collisions translate into wasted bandwidth, SO LANs do everything they can to avoid them. We will discuss techniques for this in the contention world a bit later in this chapter. The protocol that contention-based LANs employ is called carrier sense, multiple access with collision detection (CSMA/CD). In CSMA/CD, a station observes the following guidelines when attempting to use the shared network. First, it listens to the shared medium to determine whether it is in use or not-that's the "carrier sense" part of the name. If the LAN is available (not in use), it begins to transmit, but continues to listen while it is transmitting, knowing that another station could also choose to transmit at the same time-that's the "multiple access part." In the event that a collision is detected, usually indicated by a dramatic increase in the signal power measured on the shared LAN, both 