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I don't know if this is the right place to post this question, but here it goes: I was wondering how a random time is generated when a collision is detected in CSMA/CD?


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up vote 2 down vote accepted

The Ethernet standard doesn't specify which random number generator to use, only that one should be used. It specifies that the random number generator should have a uniform output distribution but, beyond that, it doesn't specify any specific algorithm to use.

Stop and think about it for a moment. As long as the output of the random number generator is uniformly distributed it doesn't matter what the algorithm is, from an operational perspective. If I showed you the output of two random number generators, both of which produced uniformly distributed output, you wouldn't be able to tell if they were generated by the same algorithm or different algorithms.

Protocol standards are concerned with specifying the operational characteristics of the protocol in great detail. The functional specifications necessary to implement the protocol are typically left up to the implementor. As long as your implementation of a protocol has the correct operational characteristics, per the standard, the implementation is irrelevant. Further I would argue that a protocol that requires a particular implementation in order to produce the correct operational result is a flawed protocol.

Great answer Evan, also I think that the quality of the generator isn't important for that purpose, because they don't specify any metric to compare... Thanks! – Joaquín L. Robles May 17 '11 at 12:10

CSMA/CD is defined in IEEE Standard 802.3. These Standards are free to download:

EDIT: I think Section One Page 67 is what you are looking for.

Thanks duenni, but there it says "The number of slot times to delay before the nth retransmission attempt is chosen as a uniformly distributed random integer r in the range" so which random generator to use is not mentioned... – Joaquín L. Robles May 15 '11 at 18:09
Mhh, I would try to get in contact with them via mail. – duenni May 16 '11 at 16:35

Maybe its not mentioned so that manufacturers can choose one at random?

In a manner of speaking, yes. – Evan Anderson May 17 '11 at 4:21

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