yes, Ethernet controllers really can hear their own output


Charles Hedrick (aramis.rutgers.edu!athos.rutgers.edu!hedrick@rutgers.edu)
18 Aug 88 00:52:46 GMT


adam@etive.ed.ac.uk (A Hamilton) tells us that Ethernet transceivers
"CANNOT listen during their own transmission." but must use software
to simulate doing so. The version 2 Ethernet spec in section 7.2.1.2
requires that "In the case of a station transmitting without collision
interference, the station's own transmit transitions on the coaxial
cable will also appear on the receive pair, after a delay due to
propagation through the transceiver." So there is no problem with the
transceiver. However most controllers can't receive at the same time
as they transmit. I just took at look at the description for the
Intel Ethernet chips. These are among the most common chips used in
controllers (and happen to be the only ones I have details on). The
82501, which does the signal processing, is full duplex, and can
handle send and receive at the same time. The controller chip, 82586,
can do so only when it is in loopback mode. One gets the impression
that they explicitly disallow simultaneous sending and receiving
because the chip can't handle the throughput of doing both at once.
(I am guessing that this is the limit because they say that in
loopback mode, where they do send and receive at the same time, the
size of packet must be limited in order to avoid overruning the
receiver portion.)

The message from Bill Westfield saying that some controllers can hear
their own packets is describing an actual controller made by Bill's
company (cisco). It really does hear its own output. I don't think
this is the first controller with this property. (At the very least,
I think Stanford's MEIS had it.)



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