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From: pat@hprnd.rose.hp.com (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: 10Base-T hubs
Message-ID: <2230148@hprnd.rose.hp.com>
Date: 11 Apr 91 18:01:27 GMT
References: <1991Apr03.004515.12021@rs6000.cmp.ilstu.edu>
Organization: HP Roseville Networks Division
Lines: 38

>   >
>   >- is there any advantage to "cascading" hubs (attaching the next one to a
>   >port in the previous one - you lose two ports) as opposed to putting both
>   >directly on a thin/thick backbone and using all ports for the UTP star?

>   Lose two ports? If you are connecting two "out" ports together, you will
>   have problems. To cascade, you need to connect an out port of the parent,
>   to the AUI port of the child, with a 10BaseT transceiver. Without the tree
>   heirarchy, the timing gets all crunched and one or both of the hubs will
>   stop working until the problem is corrected. 

There seems to be some confusion here.  Perhaps it's because 1BASE5
(1 mb/s twisted pair version of IEEE 802.3, usually called StarLAN)
had hubs it a special port to cascade to the next higher hub.  In
any 10 mb/s IEEE 802.3 repeater, any port can be connected to another
repeater port.  Repeater operation, timing, etc are the same for all
ports.

There is one thing you should check when connection repeaters together
by their twisted pair ports.  Pins 1 & 2 are the TX pair for DTEs
and pins 3 & 5 are the Rx pair for DTEs.  Inorder to allow wiring
straight through without crossing the wire pairs, repeaters normally 
include an internal crossover function.  That is, they receive on
pins 1 & 2 and transmit on pins 3 & 5.  When connecting repeaters
together, you need to get one repeater's transmitter connected
to the other's receiver.  You do this either by swapping the
wire pairs, by changing a switch in the repeater which swaps
the pairs internally (removes the crossover function), or by using
an external MAU connected to an AUI port.

The IEEE 802.3 standard allows up to 4 repeaters between any two
nodes in a collision domain (ie not seperated by bridges).  Some
vendors will support more repeaters in certain configurations and
there has been some discussion in the 802.3 Working Group of 
including more flexible configuration rules within a future 
addition to IEEE 802.3.

Pat Thaler