From: barr@tramp.Colorado.EDU (BARR DOUG)
Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Repeaters
Date: 8 Apr 92 16:04:51 GMT
Article-I.D.: ucsu.1992Apr8.160451.11508
Organization: University of Colorado, Boulder

In article <1992Apr6.140534.1488@isunix.cdx.mot.com> Mehrtens_T@msm.cdx.mot.com writes:
>In article <1992Apr1.151835.8309@picker.com> SYSNET@engvax.picker.com (Leonard
>A. Visconti) writes:
>>We are beginning to use 10baseT hubs ........
>>
>>I've often heard of a 'three repeater rule'..........

It's the four repeater rule, providing 5 segments, 2 links must be interepeater
links (links going only repeater--------to-------repeater). All twisted pair
and fiber links are interepeater links. Non-interepeater links are possible
with coax.

>>Is the minimum packet size is based on the fact that the sending station 
>>must still be transmitting a packet when the farthest node is receiving that
>>packet (for collision detection)? If this is the case, I should be able to
>>calculate, based on cable and repeater propagation to come up with answers
>>to how many repeaters I can use in a given segment.
>>Leonard A. Visconti 	Picker International 
>>visconti@picker.com	(216)473-4801
>
>    Leonard, I set up Motorols Codex' ethernet network for our development
>community, and have seen many scenarios where cascading three repeaters will
>kill a network, even though we followed the IEEE specs.  Digital Equipment
>recommends only two (2!!!) repeaters between stations (with devices connected
>on all three segments).  I have set our development network up in this manner
>(2), and have avoided a lot of problems.  For example, you can connect a
>repeater to a thin network segment, and connect thr remainder of the repeaters
>off of this thin segment.  This will give you two repeaters between all of the
>stations.  Please do not, ever, cascade repeaters.  It is not a wise practice,
>especially if the top device croaks.

The purpose of the spec is to give maximum allowable limits. Four repeaters
are ok if 2 are interepeater links!!! We "cascade" repeaters all the time.
Connecting to thin wire may work for you, but we are using twisted pair
between phone closets and fiber between buildings, thin wire is what we are
trying to avoid because we can get our twisted pair from the phone closets,
thus avoiding running thin wire every which way. If we have to use non-twist-
ed pair, we install fiber. For us thinwire is not available nor desirable.
Others may find thin works just fine.
Typical installation for us:

     router=====fiber to another router 
       |
       |             (the twisted pair segments are < 100 meters) 
       |
       |- repeater1------twisted pair--------workstation A
       |     |  ... up  to 12 total workstations on twisted pair
 short |     |------twisted pair--------workstation B
 thin  |    
section|- repeater2------twisted pair--------workstation C
       |      | ...up to  11 total workstations (save 1 for repeater)
       |      |------twisted pair----repeater3---twisted pair--workstation D
       |                                |---- ...up to 12
       |
       |- repeater4======fiber========repeater5---twisted pair----workstation E
                                        |
                                        |---up to 12
Now, let's count repeaters:
Workstation E broadcasts to workstation D
	It's signal goes through repeater5 (that's 1)
                                 repeater4 (that's 2)
                                 repeater2 (that's 3)
                                 repeater3 (that's 4 and that's the max)

Workstation E broadcasts to workstation B
         It's signal goes through repeater 5, 4, 1 (that's 3)
           (note that it does not go through repeater2)

>    What usually kills a network is high collision rates.  Another case is
>where the delay from one station to another, over the cables, transceivers, and
>thru the repeaters, is longer than the ethernet preamble (8 bytes).  The
>packets collide late (somewhere in the destination address (and I've seen it
>collide in the data field!)), causing many small, fragmented, unintelligible
>packets.  If I was forced to install a non-DEC compliant network, I would
>compute the nano-second delay from station to station, including every
>component (AUI, Txcvrs, fiber, etc.), and see if it exceeded the preamble.  If
>it did, the law of physics would guarantee networking problems.

There are things to consider besides delay, and that is the fact that
the clocks in repeaters are not in perfect synchronization. Also jitter.
To analyze the 802.3 specs bit by bit is a complicated task. To do
it requires more knowledge than I currently have. However I do not have
to understand the entire bit budget to design a network using the rules.

>    Network designs must also depend on the traffic loading of the segment.  I
>have one network segment which has no repeaters, is 30 feet long, has one
>server, one bridge, and one router connected to it.  It regularly exceeds 70%
>loading and it averages a 5% collision rate over 24 hours!  Many times the
>colision rate is nearly 35%.  It litterally dies if I eliminate the bridge
>between the server and the 1 repeater.  As one can see, you can't be too
>careful in setting up ethernet networks.  You'll get the most volume out of
>your network with the smallest possible link between stations.
>
>  We have many 10Base-T devices (all Macintoshes) connected via Cabletron
>hardware, with great success.  Again, I have the router only "1-repeater" away
>from the users.
>
>""Insert disclaimer here""
>Tom Mehrtens <Mehrtens_T@msm.cdx.mot.com>
>Motorola Codex, 20 Cabot Blvd C1-30, Mansfield, MA USA 02048-1193
>Office: +1 617 821-7427
>
There are several reasons to install a router. 
	1. High traffic (the classic reason)
	2. Logical seperation of workgroups
	3. Security
	4. Reduce the propogation of broadcast storms
	5. Ability to see router stats from remote network management software
Newsgroups: comp.dcom.lans.ethernet