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From: smb@ulysses.UUCP (Steven Bellovin)
Newsgroups: net.lan
Subject: Re: Ethernet <=> Ethernet Link
Message-ID: <942@ulysses.UUCP>
Date: Sat, 4-Aug-84 11:34:13 EDT
Article-I.D.: ulysses.942
Posted: Sat Aug  4 11:34:13 1984
Date-Received: Sun, 5-Aug-84 00:22:50 EDT
References: <340@deepthot.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 60

Julian is almost right.  The Ethernet specification explicitly permits "a
maximum of 1000 meters of point-to-point link anywhere in the system".
Schematically, this is a single repeater with a *very* long mid-section.

The diagram below is an attempt to reproduce a picture in the 1.0
Ethernet specification.  The asterisks represent hosts, the periods are
drop cables, and the pairs of parentheses connected by lines are
repeaters.  Each of the coaxial cable segments can be 500 meters, thus
permitting an end-to-end distance of considerably greater than one
mile.  To be precise:  the absolute upper bound can be calculated by
adding together the maximum cable lengths, including the
controller-transceiver drop cable of 50 meters.  If everything shown
below was connected at the appropriate end of the cable, the distance
between hosts 1 and 2 could be 50 (drop cable from host 1) + 500 (cable
A) + 50 (drop cable to repeater) + 50 (drop cable from repeater) + 500
(coaxial segment C) + 50 (drop cable to point-to-point link) + 1000 +
50 (drop cable from point-to-point link) + 500 (coaxial segment D) + 50
(drop cable to host 2), or 2800 meters.


	A						B

   1 *..|						|..*
	|						|
	|..*					     *..|
	|						|
     *..|						|..*
	|...(-)		*		*	  (-)...|
	     .		.		.	   .
	     .		.		.	   .
	    ----------------------------------------- C
				.
				.
				(---------------|
						|
						|
						|
						| 1000 meters		D
						|
						|			|
						|			|..*
						|			|
						|-----------------).....|
									|
									|..*
									|
								   2 *..|


Now, I'm not claiming that you can use this to connect two buildings a mile
apart -- but you can do far better than 500 meters.  (Those who wish to
calculate the true worst-case round-trip time should read the spec.  It's
rather more complex than one would think, since there are different figures
for signal speed in assorted media (.77c for coax, .65c on the drop cables,
etc.), startup times for encoders and repeaters, signal strength considera-
tions, etc.  The final answer, according the 1.0 standard (my copy of the
2.0 standard is in my office) is 44.99 usec.


		--Steve Bellovin