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The time required to send a frame is called frame time. Vulnerable time is the time during which no transmission should be done to avoid any collision.

My question is what kind of problem will be created if vulnerable time was equal to frame time?

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  • $\begingroup$ This is an excellent question for you to ponder. $\endgroup$ – Yuval Filmus Oct 16 '17 at 7:54
  • $\begingroup$ @YuvalFilmus The stations participating in aloha don't need to transmit frame starting at the initial time t=0. But the vulnerable time(T) is added to initial time to create some kind of check points (t, t+T, t+2T,...). So if a station decides to send a frame somewhere between (t, t+T), the transmission time will cross the t+T point and use some of the time between t+2T. In order to avoid collision we have to include the next duration (t+T, t+2T) also into vulnerable time because another station might decide to transfer between time period (t+T, t+2T) causing collision. $\endgroup$ – Sahil Oct 16 '17 at 8:29
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    $\begingroup$ Great! So you can write an answer now. $\endgroup$ – Yuval Filmus Oct 16 '17 at 8:30
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    $\begingroup$ @YuvalFilmus thanks sir for confirming that my reasoning was correct. I'll write the answer. $\endgroup$ – Sahil Oct 16 '17 at 8:33
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If vulnerable time was equal to frame time, there is a chance to collide with a frame as there is no gap. Let me give an example :

If there is a car in road and it maintains a gap of 2 meter from front and back there is less chance to hit with another car on the road, or if each and every car in a road maintains 2 meter gap there will be less hit if some one push brake.

But if vulnerable time was equal to frame time that is, if each car on the road maintains zero distance there is maximum possibility to get hit. I think it's clear now.

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  • $\begingroup$ But why is it exactly twice the frame time? $\endgroup$ – Yuval Filmus Jun 1 '18 at 9:35
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The stations participating in aloha don't need to transmit frame starting at the initial time t=0.

But the vulnerable time(T) is added to initial time to create some kind of check points (t, t+T, t+2T,...).

So if a station decides to send a frame somewhere between (t, t+T), the transmission time will cross the t+T point and use some of the time between t+2T.

In order to avoid collision we have to include the next duration (t+T, t+2T) also into vulnerable time because another station might decide to transfer between time period (t+T, t+2T) causing collision

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In aloha vulnerable time is twice frame time. To see why this is, suppose you transmit the msg bit in 10sec(FT) to the receiver. So when you start transmission of msg bit and before completing 10 sec if anyone is transmit another msg bit than there will be collision occur same as 10sec before you transmit the msg bit than less collision occur. So we can say total vulnerable time is twice of frame time.

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  • $\begingroup$ I tried to improve the answer a bit but it's still hard to understand. Can you edit it to write in complete sentences? $\endgroup$ – 6005 Apr 9 at 15:01

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