Newsgroups: comp.dcom.lans.ethernet

Lauren asks about the effect of using 75 ohm cable in an Ethernet:
    > |>My ham backround tells me that the main problem would be SWR, and
    > |>suggests a 50 ohm termination would be correct even on that 75 ohm
    > |>segment.  But I don't have a handle on how bad the overall effect
    > |>would be both for the connected workstation and for the rest of the
    > |>network.
    > 
Paul responds:
    > SWR is exactly the issue.  Collision detection (the /CD from CSMA/CD)
    > is a very simple process: it involves looking at the signal on the
    > coax and comparing it with the signal the transmitter is putting out.
    > If the two are the same, then only this transmitter is talking; if the
    > two differ, then what's seen on the cable is the sum of this transmitter's
    > output and that of another -- in other words, a collision.

and then Rich responds:
    Not at all. In coaxial Ethernet, collision detect is performed by 
    looking at the DC voltage on the wire. Each transmitter is a current
    source, and multiple transmitters will draw additional current from
    the cable (through the terminators). It is only the terminator value
    and the DC resistance of the cable system that will affect collision
    detection.

    Not that the proposed 75 ohm cable should be used, however. It will
    cause data errors in many cases, some of which may be difficult to
    find, as the reflections will cause failures differently as seen at
    different points in the network (as the reflections add in or out
    of phase). 

    The problem is with lost packets, not collisions.


Both Rich and Paul are somewhat right.  Collision detection is done by
looking at DC levels on the coax.  The drivers are current sources and
send the data signal superimposed on a DC current of about 40 mA which
causes a DC level of nominally 1 V when one transceiver transmits 
(40 mA * 25 ohms = 1 V; it is 25 ohms because there are two 50 ohm
terminators in parallel).  A collision will nominally cause twice that
voltage.  That seems like a lot of margin, but when you analyse all the
variations allowed in components, there is only about 175 mV between
the maximum non-collision DC voltage and the minimum collision DC voltage.

If you terminate the cable in 75 ohms, you are very likely to get false
collisions.  Even if you terminate with 50 ohms, you can get false collisions
because the level on the cable is elevated until the reflection propogates
back from the end of the cable.  Also, the larger data signal and the
impulse response of the low-pass filter used in the collision signal
can cause false collision detects.

You can also get data errors because of the reflections if you mix 50 ohm
and 75 ohm cable or if you terminate with 75 ohm.  You can't
generalize your experience with SWR and, say, a packet station to this
baseband system.  In the case of running a ham packet station, the round
trip delay of the reflection is short compared to the symbol time.  The
SWR may be reducing your transmit power but it isn't causing one symbol
to interfere with another noticably.  In the case of an Ethernet segment,
the delay can be several bit times, so the reflection from one bit is
effecting a later bit.  It can cause enough jitter to cause errors.
That is why cable impedance and sectioning is controlled in 802.3.

Pat Thaler