Path: utzoo!attcan!uunet!cs.utexas.edu!uwm.edu!zaphod.mps.ohio-state.edu!usc!ucsd!ucbvax!hplabs!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet diagnostic tools
Message-ID: <2230093@hprnd.HP.COM>
Date: 26 Jun 90 17:03:05 GMT
References: <1990Jun21.134653.5298@hellgate.utah.edu>
Organization: HP Roseville Networks Division
Lines: 42

> 
> 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 for
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the parameter is quoted as a current."  "The collision detect
> threshold is a value built into every transceiver on the network and
> hence is subject to a manufacturing spread.  The DC component launched
> by a transceiver is also subject to a manufacturing spread and so
> there is some scope for conflict in which some transceivers see
> spurious collisions from certain stations but not other."
> 
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.)

If the transmitting MAU and the tester were at the middle of a
10 ohm segment, the MAU is transmitting into a 30 ohm load rather
than the 25 ohms it would see with no cable resistance.  This
introduces a measurement error of 20%.

If the transmitting MAU is at one end of a 10 ohm segment and
the tester is at the other end, the MAU is transmitting into
a 27 ohm load.  The tester is seeing 5/6 of the voltage produced
because of the resistive divider of the cable and terminating
resistor.  A 10% measurement error (in the other direction).

This ignores smaller sources of error such as the 1% resistor
tolerance, leakage current from other MAUs, input resistance of
other MAUs etc.

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.)

Pat Thaler