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From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re: Thinwire vs. Thickwire (Why 30 nodes only on thin?)
Message-ID: <2480003@hprnd.HP.COM>
Date: 13 Oct 89 18:15:53 GMT
References: <8909291306.AA06775@jvnca.csc.org>
Organization: HP Roseville Networks Division
Lines: 51

> / hprnd:comp.protocols.tcp-ip / kratz@bnrgate.bnr.ca (Geoff Kratz) / 11:11 am  Oct  1, 1989 /
> In article <8909291306.AA06775@jvnca.csc.org>, aggarwal@JVNCA.CSC.ORG (Vikas Aggarwal none) writes:
> > 
> > Just to collect one's views on Thinwire ethernet vs Thickwire ethernet,
> 
> What about Unshielded Twisted Pair (UTP)?  We are using this exclusively 
> now, and we have found a number of advantages:
> 
>     - extremely cheap (uses existing 4-wire in the building)
>     - MAUs cost much less than transceivers
>     - You can use the BIXX cross-connects to simplify moves of workstations
>       (a move requires moving jumper wires from one cross-connect to another)
> 
> The disadvantage, of course, is the distance.  UTP is only good to about 
> 400 meters.  Within a building, though, this is usually adequate.  Another 

The design goal for 10BASE-T is 100 m on 24 AWG UTP.  400 meters is 
excessive.
> thing is the tolerance on the clock crystals.  We found a number of 
> workstation manufacturers who's specs on the clock were less than adequate, 
> thus causing a lot of jams or CRC's.  However, for our site (1000+ 

The clock problem has to do with the effect of excessive clock skew on
repeaters, not with any characteristic of twisted pair vs coax (except that
with coax you may not have had to have a repeater between your nodes).  
Repeaters decode the incoming signal and reencode it with their internal
clock in order to remove jitter.  The clocks of the sending DTE and the
repeater may be slightly different, but the must be within 10 MHz +- 0.01%
to meet the spec.  A device with a fast clock can transmit a maximum length
packet in about 300 ns faster than a device with a slow clock.  To allow
for this skew, a repeater stores at least 3 bits in its FIFO before it
starts transmitting (so it won't run out of bits if it is fast) and 
has space for at least 3 more bits to accumulate (so it won't drop
bits if it is slow).  If clock frequency is out of spec then packets
can get corrupted.  

Since people switch from a small unrepeatered coax network to a twisted
pair network, they discover the problem when the repeater is inserted.

> workstations in 5 buildings with constant workstation moves), UTP gives us 
> more benefits in topology planning, installation and $$$.
> 
> And yes, we run at full ethernet (10 Mbps).
> 
> Anyone else out there using UTP for their ethernets?
> -- 
> Geoff Kratz         Bell-Northern Research, Ltd.    Ph: (613) 763-5784
> Internet Systems      P.O. Box 3511, Station C      FAX:(613) 763-3283
>                     Ottawa Ontario Canada K1Y 4H7
> BITNET: kratz@bnr.ca
> ----------