jerry@oliveb.UUCP (04/01/87)
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 ATCkwe@bu-cs.UUCP (04/03/87)
In article <740@oliveb.UUCP> jerry@oliveb.UUCP (Jerry F Aguirre) writes: >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 > Violating the two repeater rule is the problem with this configuration. Workstations on the top left DEMPR will be unable to communicate with workstations on the top right DEMPR because they will be passing through three repeaters. They will be able to talk to hosts on the thick cable, but not out through any other repeaters. I think this is a serious rule violation and something to be avoided. Why not put the two DEMPRs on the same segment on the third DEMPR? DEMPR DEMPR | | | | | |___________________________________________| | | | | | Thin to WS | _____ Thin to WS | Thin to WS| | | | | | | | | | DEMPR | | ============================= The computer room Ethernet must be connected to a leg of the cascaded DEMPR through a thick-thin adapter. To quote the DECconnect handbook, "Cascading DEMPR configurations can never be connected to a standard Ethernet LAN." The top two DEMPRs can be connected to thin-net via the DESTA thinEther transceiver. The cascaded DEMPR is local only. What's wrong with this? Not much. The thick segment is isolated from the thin stuff through the DEMPR repeater. However, if one of the top DEMPRs fails, it may jam the other DEMPR since they share a segment. Beware of daisy chaining segments through the walls. You open yourself up to maintenance nightmares trying to find the pulled plug. I would never do it again. Daisy chain outside the walls in lab areas only. -- -------------------------------------------- | Kent W. England | | Network & Systems Engineering Group | | Boston University | | Information Technology | | 111 Cummington Street | | Boston, MA 02215 | | (617) 353-2780 | | kwe@buit1.bu.edu internet | | itkwe@bostonu BITNET | --------------------------------------------