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From: ted@blia.BLI.COM (Ted Marshall)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet 4-repeater limit
Summary: 4 repeaters ONLY IF using inter-repeater links (fiber)
Message-ID: <9850@blia.BLI.COM>
Date: 26 Oct 89 23:59:16 GMT
References: <2549@aecom.yu.edu>
Organization: Britton Lee, Los Gatos, CA
Lines: 50


The four repeater rule only applies if you are using fiber optic or similar
links between pairs of repeaters. I.e.

	Ethernet ------------------------------------------
				   |
			repeater   O
				   :
			fiber opt  :
				   O
				   |
		 ------------------------------------------
					|
					O
					:
					:
					O
					|
		 ------------------------------------------

These repeaters use to be called "half-repeaters". If you use normal
"local" repeaters to directly connect together two Ethernet (thick or
thin) segments, that counts as TWO out of the four allowed.

Thus, two nodes cannot be more than three coax segments (500M each for
thick) plus two fiber segments (1000M TOTAL) plus 6 transceiver cables
(50M each) = 2800M apart.

Like most of the standard configuration rules, you can break this one
if you are conservative on other rules or have a light network load.
if the segments are short, you may be able to add more repeaters.  You
cannot get the same effect of 1000M of fiber link with the same length
of coax because the signal propigates slower through the coax.

If you have five coax segments, 500M each, strung in sequence between 4
full repeaters, the problem that you will see is a higher incedence of
collisions because it takes longer for a signal to travel from one end
to the other. More importantly, you'll get collisions that were not
detected by one of the transmitting stations because it finished
transmitting before the colliding packet reached it. Thus, the packet
is not retransmitted by the data link layer.

The bottom line in this configuration is that it will work fine under a
light load, but as the traffic goes up, your performance will go to
hell!

-- 
Ted Marshall          ...!uunet!blia!ted   <or>   ted@blia.bli.com
ShareBase Corp., 14600 Winchester Blvd, Los Gatos, Ca 95030     (408)378-7000
The opinions expressed above are those of the poster and not his employer.