CSMA persistence

A carrier-sense/multiple-access transmission strategy is said to be nonpersistent if, when the line is busy, the sender waits a randomly selected time. A strategy is p-persistent if, after waiting for the line to clear, the sender sends with probability p≤1. Ethernet uses 1-persistence. A consequence of 1-persistence is that, if more than one station is waiting for line to clear, then when the line does clear a collision is certain. However, Ethernet then gracefully handles the resulting collision via the usual exponential backoff. If N stations are waiting to transmit, the time required for one station to win the backoff is linear in N.

When we consider the Wi-Fi collision-handling mechanisms, we will see that collisions cannot be handled quite as cheaply: for one thing, there is no way to detect a collision in progress, so the entire packet-transmission time is wasted. In the Wi-Fi case, p-persistence is used with p<1.

An Ethernet broadcast storm was said to occur when there were too many transmission attempts, and most of the available bandwidth was tied up in collisions. A properly functioning classic Ethernet had an effective bandwidth of as much as 50-80% of the nominal 10Mbps capacity, but attempts to transmit more than this typically resulted in successfully transmitting a good deal less.