#Self Doubt

Question

When we say that vulnerable time in pure aloha will be 2*frame transmission time.So what this frame transmission time??

My doubt is does frame transmission time= propagation time+transmission time …

collision can also happen during propagation time ??

Answer 1

In case of pure aloha, let the time taken for a frame to reach the receiver = T
Then the vulnerable time  = 2*T

Important points; 
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.
 

Answer 2

First of all in Aloha we don’t consider propagation time, i mean in wireless network Tp is very less. so time will be Tt in Tt collision may occur or may not it totally depends other station as in pure aloha transmission will be possible in any time whenever they (station) want.

Comments

@Nisha Bharti

why we are not considering ??collision can happen during propagation time right??

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due to wireless network propagation time is less..

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In Satellite Communication we have high Propagation time right ??

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@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.