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Access methods are used when computers share a common medium

100Base-T4 is star based

Why "the fourth pair is used for collision detection"?,when talking about the 100Base-T4 standard?

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    [tannoy announcement] A network archaeologist to the front desk, a network archaeologist, please. [/tannoy announcement] Sorry, could not quite resist. :-) 100Base-T4 is defunct, deprecated and newer saw wide adoption. For an attempt of an answer, see below. – Marc 'netztier' Luethi Apr 16 at 21:26
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http://ecomputernotes.com/computernetworkingnotes/communication-networks/fast-ethernet is the best in-a-nutshell explanation I managed to find.

100base-T4 is half duplex and uses four wire pairs, with one pair reserved for sending only (Rx-only at the other end), one pair used only for receiving (Tx-only at the other end) and two pairs swapping directions as needed, half duplex style.

Sending in one direction uses three (sic!) pairs, and uses some elaborate 8binary/6ternary encoding.

With that, carrier detection comes implicitely: If there is an (incoming) signal on the Rx-only wire pair, then the receiving station knows that the sender at the other end is "talking" and that it should be receiving on the two bidirectional pairs, too.

On the other hand, for collision detection, if a sending device, while "talking" on its Tx-only and the two bidi pairs, receives a signal on the fourth Rx-only pair, then chances are high that there must've been a collision on the two bidi pairs.

So the statement "fourth pair used for collision detection" is probably somewhat incomplete, while not actually wrong.

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