Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!lll-crg!nike!ucbcad!ucbvax!amdcad!phil
From: phil@amdcad.UUCP (Phil Ngai)
Newsgroups: net.lan
Subject: Re: two different flavors of fiber optic repeaters?
Message-ID: <12936@amdcad.UUCP>
Date: Mon, 8-Sep-86 01:00:34 EDT
Article-I.D.: amdcad.12936
Posted: Mon Sep  8 01:00:34 1986
Date-Received: Mon, 8-Sep-86 06:04:16 EDT
References: <2770@islenet.UUCP> <12896@amdcad.UUCP> <2778@islenet.UUCP>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Distribution: na
Organization: AMD, Sunnyvale, California
Lines: 54

In article <2778@islenet.UUCP> bob@islenet.UUCP (Bob Cunningham) writes:
>Here's the optic fiber repeaters I've been trying to compare specs on:
>
>DEC's DEREP-RA, costing $2k+.
>
>noticeable, useful differences include different types of LED indicators,
>automatic partitioning features...and lengths of warranties.

DEC doesn't seem to push their automatic partitioning as much as I
think is justified.

The goal is to keep your network up in the face of equipment failures.
For example, suppose one of the repeater's transceiver taps loosens up
and the transceiver starts reporting collisions all the time. Without
automatic partitioning your network is down, as the repeater will jam
every packet it hears.

The DEREP counts consecutive collisions. If this count reaches 64 it
concludes something is wrong and segments the colliding side. This
means data is transmitted to the bad side but data from the bad side
is not repeated to the good side. Also collisions on the bad side are
not enforced on the good side.

When a packet is transmitted to the bad side without a collision the
segmented state is exited and normal operation resumes.

While it is better to have some of your network up than none of your
network up, it is even better to have the whole network up. This can
be done at the cost of doubling your investment in equipment. A
repeater and its associated set of transceivers are installed in
parallel with the primary repeater.  This second repeater is in
standby mode. (switch selectable) The standby repeater is normally
passive, merely monitoring both coax segments.  If data is not
repeated for an entire slot time, it enters the active mode and
repeats packets.

Once the primary repeater is restored, it will start repeating
packets.  The standby repeater which has been active will then collide
with the primary repeater. The standby repeater counts up to 56
consecutive collisions and exits the active mode. As the primary
repeater does not segment until 64 consecutive collisions, it remains
in the active mode. Incurring 56 collisions is relatively time
consuming but only happens when a broken primary repeater is put back
into service.

I think this feature is a nice one. I wish DEC wouldn't be so shy
about telling customers about it.

-- 
 Rain follows the plow.

 Phil Ngai +1 408 749 5720
 UUCP: {ucbvax,decwrl,ihnp4,allegra}!amdcad!phil
 ARPA: amdcad!phil@decwrl.dec.com