Carrier Sense Multiple Access

ALOHA protocols do not listen to the channel before transmission and this leads to a high rate of packet collisions. This is the main reason of the relative inefficiency of the ALOHA schemes which equals at best 1/e.

Protocols in which stations listen for a carrier (i.e. transmission) and act accordingly are called carrier sense protocols. A number of them we will mention below.

The 1-persistent CSMA protocol is one in which each user terminal with data to transmit listens to the channel and waits for transmission until it determines the channel idle. The user terminal starts transmission with probability 1 as soon as the channel is available. After sending a packet, the user terminal waits for an acknowledgment (ACK). If no ACK is received in a specified time interval, the user will wait a random time interval and then resume listening to the channel until it finds the channel idle. At this time instant the packet is retransmitted immediately. The throughput characteristic of the protocol is better than that of either pure or slotted ALOHA.

In the non-persistent CSMA strategy after finding that the channel is busy a terminal waits a randomly selected interval of time before sensing the channel again. Here the randomized waiting times between channel sensings eliminate most of the collisions that would appear from multiple terminals transmitting simultaneously when the transition from busy to idle condition takes place. The throughput values are better than 1-persistent CDMA at high traffic loads. At low traffic loads the waiting strategy is of no benefits and the throughput results are poorer than that of 1-persistent CSMA.

P-persistent CSMA strategy is applicable to slotted channels. The user terminal with data to transmit senses the channel. If the channel is found idle, it transmits with probability p. If the transmission does not take place the procedure repeats in the next slot, i.e. if the channel is sensed idle the transmission takes place with the probability p. This procedure is repeated until either the packet has been transmitted or the channel is found busy. The user station senses the busy channel continuously and when it is found idle, it starts the described procedure again.