Vulnerable time in ALOHA depends on the frame transmission time (Tfr), but in CSMA it depends on frame propagation time

Question

Q: Why the vulnerable time in ALOHA depends on the frame transmission time (Tfr), but in CSMA it depends on frame propagation time (Tp) -->I've understood that Vulnerable time is the time when there is a possibility for collision There is no proper explanation about Transmission time and propagation time anywhere. Please help

Answer 1

This is because of the difference in the magnitude of Transmission time and Propogation time. Most propogation is done through EM waves and thus for most distances the propogation delay is very negligible as compared to transmission time which is limited by the Bandwidth of a channel. In the case of ALOHA, both these times contribute to the vulnerable time and thus the dominant factor is retained , however incase of CSMA the transmission time factor is avoided and thus only the propogation delay in considered although its very small , it must be taken into account to validate the efficiencies of CSMA protocols.

Answer 2

I would pin it to medium and probing behaviour of csma:

• ALOHA is mainly a wireless protocol, which means that information travels at the speed of light, making propagation delay negligible.
• CSMA on the contrary is used in physical links, so propagation delay is not negligible. The reason transmission delay is not accounted for is because a node senses the link before starting the transmission, i.e. the only moment the channel is vulnerable is if the first bit transmitted hasn't propagated to the farthest node.