In case of pure aloha, let the time taken for a frame to reach the receiver = T
Then the vulnerable time = 2*T
In general Transmission time = time taken to put a frame on the channel.
Propagation time = time taken by the frame (after transmission i.e., the frame is already put on the channel) to propagate and reach the receiver.
In case of pure aloha, we are talking about communication between machines which are using a direct link. These machines are not far apart. Hence channel length is not much, so propagation time is negligible. So we can say that the total time it takes for a frame to reach the receiver is = the transmission time only.
In vulnerable time expression, the T is the time taken by the frame to reach the receiver.
And because propagation time is neglected, this T is equal to the transmission time of the frame.
why we are not considering ??collision can happen during propagation time right??
@lalitver10 Yes because satellite communication uses TDMA method for accessing channel or other protocols such as TCP/IP that’s why it have high propagation time.