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From: jerry@oliveb.UUCP
Newsgroups: comp.dcom.lans
Subject: Question on using thin ethernet
Message-ID: <740@oliveb.UUCP>
Date: Tue, 31-Mar-87 21:15:25 EST
Article-I.D.: oliveb.740
Posted: Tue Mar 31 21:15:25 1987
Date-Received: Fri, 3-Apr-87 06:36:53 EST
Organization: Olivetti ATC; Cupertino, Ca
Lines: 52
Keywords: thin ethernet multiport backbone

I am faced with the prospect of cabling our building for an Ethernet
LAN, mostly to connect PCs to our central computers.  The building is
small enough that cable distance is not a consideration.

My first proposal was to use a thick ethernet backbone running from
floor to floor and between the two wings.  I would then use a "Thinwire
Multiport Repeater" like the DEC DEMPR to cable out to the work
stations.  (The DEMPR has connections for 8 thinwire cables and a
tranceiver.)  A repeater would then connect the backbone to the computer
room ethernet, isolating it from cabling faults.
                                                                      
    Thin to WS                   Thin to WS             Thin to WS
    | | | | |                    | | | | |              | | | | |
     DEMPR                        DEMPR                  DEMPR
       |                           |                       |
       =====================================================
                Thick Backbone     |                                         
                                 REPEATER                                   
                                   |                                         
                   ==============================                            
                   Existing Computer Room Ethernet                     

It just struck me that a cheaper configuration is possible.  If I use
one of the DEMPR's thin segments to connect the repeaters to each other
then I save the cost of the repeater, thick cable, and 4 tranceivers.

     DEMPR                Thin Cable                       DEMPR    
   | | | | | |________________             ______________| | | | | |
   Thin to WS                 |           |               Thin to WS
                              | Thin to WS|    
                              | | | | | | |    
                                  DEMPR                                     
                                   |                                         
                    ==============================                            
                    Existing Computer Room Ethernet                     

The building distances are well within the 185 meter limit on thin
ethernet.  The DEMPR provides the same isolation that a repeater would
and counts as a repeater for purposes of delay.

One disadvantage I can see is that each DEMPR will only support 203
(29x7) WS instead of 232.  Big deal.  More significant is that there are
3 "repeaters" instead of 2 for WS to WS comunication.  (Unfortunately a
built in terminator prevents connecting the central DEMPR with a T
connector.)
	
Has anyone ever tried something like this?  Are there problems I havn't
thought of?

					Jerry Aguirre
					Systems Administration
					Olivetti ATC