Carrier Sense Multiple Access

The ALOHA solution has been enhanced by CSMA. As you read this section, be able to describe CSMA and how it works to share computer network resources.

6.2.3 Carrier Sense Multiple Access

ALOHA and slotted ALOHA can easily be implemented, but unfortunately, they can only be used in networks that are very lightly loaded. Designing a network for a very low utilisation is possible, but it clearly increases the cost of the network. To overcome the problems of ALOHA, many Medium Access Control mechanisms have been proposed which improve channel utilization. Carrier Sense Multiple Access (CSMA) is a significant improvement compared to ALOHA. CSMA requires all nodes to listen to the transmission channel to verify that it is free before transmitting a frame [KT1975]. When a node senses the channel to be busy, it defers its transmission until the channel becomes free again. The pseudo-code below provides a more detailed description of the operation of CSMA.

# persistent CSMAN=1while N<= max:  wait(channel_becomes_free)  send(frame)  wait(ack or timeout)  if ack:    break # transmission was successful  else:    # timeout    N=N+1# end of while loop  # Too many transmission attempts

The above pseudo-code is often called persistent CSMA [KT1975] as the terminal will continuously listen to the channel and transmit its frame as soon as the channel becomes free. Another important variant of CSMA is the non-persistent CSMA [KT1975]. The main difference between persistent and non-persistent CSMA described in the pseudo-code below is that a non-persistent CSMA node does not continuously listen to the channel to determine when it becomes free. When a non-persistent CSMA terminal senses the transmission channel to be busy, it waits for a random time before sensing the channel again. This improves channel utilization compared to persistent CSMA. With persistent CSMA, when two terminals sense the channel to be busy, they will both transmit (and thus cause a collision) as soon as the channel becomes free. With non-persistent CSMA, this synchronisation does not occur, as the terminals wait a random time after having sensed the transmission channel. However, the higher channel utilization achieved by non-persistent CSMA comes at the expense of a slightly higher waiting time in the terminals when the network is lightly loaded.

# Non persistent CSMAN=1while N<= max:  listen(channel)  if free(channel):    send(frame)    wait(ack or timeout)    if received(ack):      break # transmission was successful    else:      # timeout      N=N+1  else:    wait(random_time)# end of while loop  # Too many transmission attempts

[KT1975] analyzes in detail the performance of several CSMA variants. Under some assumptions about the trans- mission channel and the traffic, the analysis compares ALOHA, slotted ALOHA, persistent and non-persistent CSMA. Under these assumptions, ALOHA achieves a channel utilization of only 18.4% of the channel capacity. Slotted ALOHA is able to use 36.6% of this capacity. Persistent CSMA improves the utilization by reaching 52.9% of the capacity while non-persistent CSMA achieves 81.5% of the channel capacity.