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From: pat@hprnd.rose.hp.com (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet cabling/repeater specifications?
Message-ID: <2230136@hprnd.rose.hp.com>
Date: 5 Feb 91 19:56:20 GMT
References: <1991Jan23.050705.5029@tmsoft.uucp>
Organization: HP Roseville Networks Division
Lines: 30

The term half-repeater is really a misnomer.  A coax-to-coax repeater
has the same functionality and delay as a coax-to-fiber repeater.
There is nothing "half" about the coax-to-fiber repeater.  What is
different is the delays of the different MAU types and effect of
the media on delays, particularly delays in detecting start and
end of collision.  Collision detect occurs significantly faster
on link segments (Fiber Optic Inter-repeater Link (FOIRL) and
10BASE-T) than on coax segments (10BASE2 and 10BASE5).

So, there is sufficient delay budget for a maximum path between any
two stations on the network of (condensed from IEEE 802.3 Section 13):

  4 Repeaters
  5 Segments
     At most 3 of those segments may be coax
     The remainder may be link segments of up to 500 m each.

10BASE-T and FOIRL repeaters are often called "hubs" or "concentrators"
because of their position at the hub of star configured wiring.

You could have a backbone of 3 coax segments connected
by two repeaters and attach repeaters connecting 10BASE-T segments
to the backbone.  This configuration obeys the rules above.

Counting coax-to-link-segment repeaters to be half-repeaters breaks
down in this example.  You would count 2 repeaters plus 2 half-repeaters
equals 3 repeaters; one too many by the "half-repeater rules."  Yet
you would have a legal configuration.

Pat Thaler