Path: utzoo!attcan!uunet!wuarchive!gem.mps.ohio-state.edu!apple!motcsd!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet max length problems
Message-ID: <2230025@hprnd.HP.COM>
Date: 27 Sep 89 22:53:14 GMT
References: <634@elan.elan.com>
Organization: HP Roseville Networks Division
Lines: 50

HP makes both 802.3 repeaters and bridges.  As others have stated, most
ThinLAN repeaters are multiported devices.  HP's ThinLAN repeater has
4 ThinLAN (10BASE2) ports and an AUI port.

Tim J. Peiffer wonders why his repeater didn't partition with an incorrect
terminator.

Repeaters will partition a port if they see collisions every time they 
transmit or if a collision persists for an excessive time.  It generally
takes over 30 consecutive collisions to cause a partition.  Partitioning
was designed to isolate an unterminated segment and prevent it from
taking down the whole network.  The repeater transmits to a partitioned
port but ignores packets and collisions from that port.

Each 802.3 MAU acts as a current source when it transmits.  The transmitted
signal is a combination of a DC offset and an AC signal.  The AC signal
carries the data and the DC offset current establishes a DC voltage on
the cable.  When two or more MAUs (transceivers) transmit at the same
time the sum of their currents causes a larger DC voltage.  (Actually the
offset is negative, but most people find it easier to think in terms
of magnitude.)  MAUs detect a collision by sensing that the DC voltage
has crossed the collision threshold.

> Why did our repeater not partition under a large impedance mismatch?  I expect
> that the repeater should have seen many collisions...  We have segmented our 
> net into several lines where the aggregate lengths are less than 600 ft.  In
> the problem we had, one of our people were installing some portions of a
> cluster and accidentally installed a 75 ohm at the end.
> 
> 3com Multiconnect
> +---+
> |   | +-->> 50 ohm terminator
> | o---|
> |   | +---> 250 ft >-> 3x25ft >-> 18x6ft between MacII >-> PC/AT -> 75 ohm
> +---+
>
The larger the resistance the larger the resultant voltage will be.  If the
resistance gets too large, a collision may be detected when only one MAU
is transmitting.  The usual causes of excessive resistance are an
unterminated cable, an incorrect terminator value, or an excessively
long cable (because of the series resistance of the cable).

In the case above where a 75 ohm terminator is placed at the far end
of the cable, a transmitter near the 75 ohm terminator sees a higher
voltage than the repeater's MAU will see when it transmits.  It is 
fairly likely that the repeater's MAU will not see any false collisions
even though the PC/AT might see a collision every time it transmits.
The repeater therefore will not partition.

Pat Thaler