Path: utzoo!attcan!uunet!cs.utexas.edu!ut-emx!sirius.cc.utexas.edu!spurgeon
From: spurgeon@sirius.cc.utexas.edu (Charles Spurgeon)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet diagnostic tools
Message-ID: <32637@ut-emx.UUCP>
Date: 27 Jun 90 16:35:55 GMT
References: <1990Jun21.134653.5298@hellgate.utah.edu> <2230093@hprnd.HP.COM>
Sender: news@ut-emx.UUCP
Reply-To: spurgeon@sirius.cc.utexas.edu.UUCP (Charles Spurgeon)
Organization: Dept. Of Fiddly Bits
Lines: 69

In article <2230093@hprnd.HP.COM> pat@hprnd.HP.COM (Pat Thaler) writes:
>> 
>> The NQA also checks the coax for DC bias voltage, and will look at the
>> DC component of a packet "as recovered by a 2 usec (approx) low pass
>> filter.  This value is measured as a voltage but presented as a
>> current, since co-ax impedance is known.  The IEEE specification ...
>>          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>The DC impedance into which the node is transmitting is only
>_approximately_ known.  The terminating resistor at each end should
>be 50 ohms +/- 1%, but the series resistance of the cable can
>be from 0 to 10 ohms.  (Back to how long can the coax be and how
>many stations/connectors; series resistance and its affect on
>collision detect is one of the things which limit the length.)
> ...
>
>A tester on the cable can tell you whether the MAU is grossly
>out of spec, but it cannot conclusively determine whether it is
>in spec.  (The possible exception is if the tester was connected
>right next to the MAU and the tester measured the DC impedance to
>calibrate itself.)
>

Thanks for the information on coax transmission characteristics and
the problems of measuring same.  It's always interesting to hear more
about how the analog electronics really function while delivering the
"digital" bits between computers.  Having written a fair amount of
documentation I know that this sort of information can be complex and
hard to describe, but I always find your postings quite clear.

The amount of engineering that goes into making a high speed bus
connection between computers work reliably under nearly all
circumstances is really impressive.  I take my hat off to the IEEE
802.3 committee members who have worked so hard all these years to
make the Ethernet/802.3 system as reliable and trouble free as it is.

In a perverse way, I suppose it's a testimony to the efforts of these
engineers that people manage to get signals down "Ethernets" that
egregiously violate the specifications.  :-)

Here's some more specs from photocopies of the NQA manual that I made
while it was on evaluation:

The "implant" they use for sensing signals is supposed to be installed
in the center of the coax, although they concede that it can be
installed at the end of the cable "as a last resort."  The manual
verbiage at this point implies that the reason for this is to improve
the DTDR traces.

The NQA also has a regular transceiver connection to the coax, which
should not be any closer than 2.5 meters to the implant and no further
away than four times that distance.  The combination of a transceiver
and the implant gives the NQA the ability to both sense the signals
and identify the MAC address of the controller emitting the signals.
This is something I haven't seen in any other low level signal
analyzer approach.

In their specifications they cite:
"Packet Jitter +/- 3 ns accuracy.  1.5 ns resolution at the rear panel."
"Packet AC Component +/- 10% accuracy. 4% resolution at implant."
"Packet DC Component +/- 5% accuracy. 4% resolution at implant."
"Packet Bit Rate +/- 200Hz accuracy, 400-4KHz deviation from 10MHz."
"Packet Fall Time +/- 20% accuracy.  1.5 ns resolution."
"Coax Bias Voltage:  +/- 5ma accuracy.  Resolution 5mV."


Charles Spurgeon                     |                                  |
UTnet Network Information Center     | spurgeon@emx.utexas.edu          |
University of Texas at Austin        | ...!cs.utexas.edu!ut-emx!spurgeon|
-------------------------------------------------------------------------