Newsgroups: comp.dcom.lans.ethernet

In comp.dcom.lans.ethernet, jdm1@vogon1.pasen.gov (Jonathan D McCown) writes:

    I am curious as to the degree of "weakening" the 10BaseT spec underwent
    in the committee process.  Other vendors would stand a much better chance
    with us if their gear could "go the distance."

The short answer is that 100 m is the number in the 10BASE-T standard
because that is the longest length (of voice grade DIW aka Category 3)
that can be shown to work worst case.  That is, if you put together
a 10BASE-T segment where all the components were at the worst edge of
the spec (transmit level, cable attenuation, crosstalk, etc) it would
work.  Philisophically, IEEE 802.3 has operated under the assumption that
if you set up a network with components that are all in spec, it should
work.

With enough cable to cause 11.5 dB of insertion loss (the 10BASE-T standard
actually specs insertion loss, not cable length; if you use a lower loss
cable such as Category 5, you can go a bit farther), the minimum signal
level is just under 600 mV.  The peak crosstalk voltage is 264 mV; this
allows for crosstalk from multiple 10BASE-T links in bundles of 25 or
more pairs, but the 4 pair cable crosstalk is about the same.  If you
went 100 more meters, the worst case crosstalk would be larger than the
signal.  You won't see that level of crosstalk every time you set up a 
link, but if you test enough pair combinations on enough cables you will
get near it.

One of the biggest concerns is to not allow a bad link to take down the
network.  If crosstalk is high enough to be seen as data, false collisions
will be detected.  One 10BASE-T link detecting crosstalk or external
noise as data could hurt the efficiency of the entire network.  To protect
against that the 10BASE-T standard requires that the receivers have a 
squelch which prevents signals less than 300 mV from being detected as
incoming data.  As you increase the attenuation of the cable, the received
signal level drops and becomes insufficient to trigger the squelch.  This
is normally the first thing that stops a 10BASE-T link from functioning
as you lenghten the cable.

To give an example of how the 10BASE-T task force came up with the specs:
We had to decide on the tolerance for the transmitter's output voltage.
If the number was too tight, transceiver cost could be driven up.  If too
loose, we would be excessively limiting the length of cable we could drive
(because a transmitter at minimum signal level might be interfered with
by crosstalk from a transmitter at maximum signal level).   So an IC vendor
said a transmitter could meet a certain tolerance which was fairly tight.
Someone else asked how that could be met considering that the output 
voltage depended on the 5 V supply voltage.  There ensued a discussion
about whether machines could be expected to have 5 % or 10 % tolerance 
on their 5 V supplies.  It was decided allowance should be made for 10%
supplies since they were fairly common and the effect of that was added.
Then temperature was brought up and it turned out that the original voltage
mentioned was for 25 degrees C.  Also the original voltage was nominal,
not taking into account the process variations in the ICs.  The transmitter
spec of 2.2 to 2.8 V was agreed upon.

It is discussions like the one above which decide the numbers that go 
into the spec.  Often, an individual engineer doing the calculation does
not come up with all the factors.  For instance, one company came in
with a proposal they thought would go 150 to 175 meters.  Another came
in with a proposal that they thought would go 100 meters.  The two proposals
were the same, but the first company had not included all the necessary
factors in their calculations.

Using a small sample to determine how long you can make your cables is risky.
You are only going to see a fraction of the variability that you will see
in a full network operating over time.  Will it continue to work when
the temperature changes?  Are you aware that some cable types have significant
increase in attenuation as the temperature rises (especially PVC cables)?
Have you tested with cable/IC's/etc from different lots?  Within a cable,
crosstalk can easily vary by 9 dB depending on the pairs you use.

We aren't out to get you.  There is no conspiracy.  We just want it to
work.

Pat Thaler
Newsgroups: comp.dcom.lans.ethernet