Comparing Party Line and Ethernet Operations Step

In a party line, people occasionally speak over each other. When the party line is loaded with more callers, the more often people attempt to speak at the same time. It is the same with Ethernet collisions. Because users share Ethernet bandwidth and are part of the same collision domain, it is often referred to as shared media or shared Ethernet. (See Figure 1-4.) The efficiency of shared Ethernet is proportional to the number of devices attempting to communicate at the same time. As more devices are added, the efficiency decreases.

The algorithm in CSMA/CD used after a collision is Truncated Binary Exponential Backoff algorithm. When a collision occurs, the device must wait a random number of slot times before attempting to retransmit the packet. The slot time is contingent upon the speed of the link. For instance, slot time will be different for 10 Mbps Ethernet versus 100 Mbps Ethernet. Table 1-3 shows an example for a 10 Mbps Ethernet link. Cisco switches uses a more aggressive Max Wait Time than what is illustrated in this example. The purpose of the example is to give you a feel for how Truncated Binary Exponential Backoff works