From: Hansang Bae (hbae@xxxxxxxxxx)
Date: Fri Aug 23 2002 - 19:28:49 GMT-3
This was discussed to death over at comp.dcom.lans.ethernet. So I thought I'd
post the answer from Rich Seifert (author of The Switch Book, Gigabit Ethernet
, and chair/editor of few 802.x IEEE groups).
HTH.
hsb
-------------- BEGIN QUOTE ---------
Let's separate some issues. Ethernet is *bit* synchronous--there is a clock sig
nal sent from transmitter to receiver that the receiver uses to determine the a
ppropriate sampling point. This is unlike RS-232, where the receiver uses a com
pletely independent clock to sample the received signal. It is because of the l
ack of a receive clock synchronous with the transmitted signal that RS-232 must
issue a "start" bit every byte. That is, RS-232 is *bit*-asychronous, but *byt
e*-synchronous. The "start" bit is the "byte-clock", in a general sense. Howeve
r, there is no "master clock"; each transmitter provides its own clock to the r
eceiver. The clocks of all stations are not synchronized to any common referenc
e phase. The clock signal is embedded in the transmitted stream, using Manchest
er code (10 Mb/s), 4B/5B code (100 Mb/s), 8B/10B code (1000BASE-X), etc. In 10
Mb/s Ethernet, the preamble bits are used by the receiver to phase-lock the clo
ck. The receiver has a free-running clock!
at th
e nominal operating frequency, and only needs phase (and slight frequency) adju
stment to properly decode the rest of the stream. This is because the channel i
s completely idle between frame transmissions. Note that collision detection is
a completely separate matter; it does not depend at all on clock synchronizati
on, and uses signals besides the preamble for this purpose. In 100/1000 Mb/s Et
hernet, the preamble is superfluous. There is a continuous signal transmitted f
rom sender to receiver, even in-between frames, and the receiver is in phase/fr
equency lock within milliseconds following power-up. It does not re-lock on a f
rame-by-frame basis. Now, it can be said that while Ethernet is *bit*-synchrono
us, it is also *frame*-asynchronous. That is, a frame can begin at any time (wi
thin the constraints of the bit clock). This contrasts with systems like ATM, w
hich are both bit- and cell-synchronous. ATM cells occur at precise intervals,
unlike Ethernet frames.
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