CN: Aloha vs CSMA

Efficiency Comparison of Aloha and CSMA

In Aloha, a station never senses before transmission.

Collisions are possible in two ways –

1. A station starts transmitting when some frame is already propagating through the
medium.

2. A station transmits when a frame is about to reach that station before it completes its
transmission.

Let K be the no. of frames generated by the system of stations out of which X frames
were transferred successfully, hence,

Aloha - Efficiency = X / K.

Let no. of collisions = a+b

where a= case 1 collisions, b= case 2 collisions
X=k - (a+b).

hence, Aloha - efficiency = [K - (a + b)] / K

Where as in CSMA the medium is sensed, thus now the collisions of case 1 will never
happen in CSMA. Plus such frames will also not be generated in the network system.

hence, CSMA - Efficiency = [K - (a + b)]/ (K - b).
Clearly, [CSMA - efficiency] > [Aloha - efficiency]