Analysis of Classic Ethernet

How much time does Ethernet “waste” on collisions? A paradoxical attribute of Ethernet is that raising the transmission-attempt rate on a busy segment can reduce the actual throughput. More transmission attempts can lead to longer contention intervals between packets, as senders use the transmission backoff algorithm to attempt to acquire the channel. What effective throughput can be achieved?

It is convenient to refer to the time between packet transmissions as the contention interval even if there is no actual contention, that is, even if the network is idle; we cannot tell if stations are not transmitting because they have nothing to send, or if they are simply waiting for their backoff timer to expire. Thus, a timeline for Ethernet always consists of alternating packet transmissions and contention intervals:

As a first look at contention intervals, assume that there are N stations waiting to transmit at the start of the interval. It turns out that, if all follow the exponential backoff algorithm, we can expect O(N) slot times before one station successfully acquires the channel; thus, Ethernets are happiest when N is small and there are only a few stations simultaneously transmitting. However, multiple stations are not necessarily a severe problem. Often the number of slot times needed turns out to be about N/2, and slot times are short. If N=20, then N/2 is 10 slot times, or 640 bytes. However, one packet time might be 1500 bytes. If packet intervals are 1500 bytes and contention intervals are 640 byes, this gives an overall throughput of 1500/(640+1500) = 70% of capacity. In practice, this seems to be a reasonable upper limit for the throughput of classic shared-media Ethernet.