Carrier sensing random multiple access with collision detection protocol (CSMA/CD) can solve competition effectively and provide high throughput and low delay, so it has become an attractive protocol to the designers of local area net (LAN). Based on LAN’s CSMA/CD protocol analysis, the user oriented availability model of Ethernet is put forward. By defining the workstation operating mode as congesting model and idle mode, the availability model based on bandwidth and time slot is established. The probability of LAN sending packets successfully in specified time is defined as the availability measure of Ethernet. By analyzing the effect of different parameters which include the number of workstations in Ethernet, average length of packets, packets arrival rate and retransmission rate on availability, it is show that the LAN’s availability will decrease synchronously with workstation number in network increasing. With packets arrival rate increasing, availability will improve to maximum firstly, then decrease subsequently. It also shows that increasing retransmission rate can improve LAN’s availability. By setting parameters reasonably, the higher LAN’s availability can be obtained.
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