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I have a doubt regarding the time a particular node will wait after collision.

I am using numerical values here so that one can easily understand my doubt.

Suppose in a network, we have 2 nodes A and B with propagation delay of 225 bit times.

If A and B sense channel at the same time and find it idle, they both will collide and will sense collision at 225 bit times .

If we assume jamming signal as 48 bit and IFS time 96 bit time(standard ones), then both A and B will send jamming signal to all other nodes by 225 + 48 = 273 bit time.

Now I want to know whether channel is really idle by time 273 bit times and can A start sending after IFS time or the channel still contains B's transmission and A needs to wait for another 225 bit times (propagation delay) until B's last bit reaches A and channel is cleared?

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  • You'll find your problem easier to solve by drawing a timeline diagram; usually done vertically, two lines for two stations, diagonal lines for signals. You might care to edit your question to clarify what you meant at 225 + 45 = 273 – jonathanjo Dec 1 '17 at 15:21
  • I am sorry. It's actually 48 and not 45. By the time 225 + 48, all nodes will receive the jamming signal. – Zephyr Dec 1 '17 at 15:44
  • My only doubt is will A wait until B's last bit reaches A and the channel gets cleared or not . Because in the actual algorithm flow diagram of CSMA/CD, I didn't find any such thing. – Zephyr Dec 1 '17 at 15:55
  • @jonathanjo Please check my edit for the diagram. – Zephyr Dec 1 '17 at 18:08
  • Can someone answer this ? – Zephyr Jan 8 '18 at 18:11
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First off, CSMA/CD is ancient and hopefully not in serious use any more.

The timing for coax (10BASE5/2) is 16 BT for the potential collision detection delay followed by 32 BT jam time for all PHYs. PHYs with distinct transmit/receive channels (-T, -F) detect collisions earlier than coax.

After that, the random pause is started, its size increasing with each unsuccessful transmission attempt. So yes, the channel must be idle after at most 273 BT (the collision started at 225 BT and was sensed at 241 BT). A could potentially transmit right away, after the IFS delay.

The jam signal could still be on its way to B but it's ahead of A's data and there's no collision.

The procedure is detailed in IEEE 802.3 Clause 4, especially in 4.1.

  • Please check my new edit to the question. See the diagram I have given link to. Is it correct? – Zephyr Dec 1 '17 at 18:04

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