Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!usc!apple!motcsd!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.HP.COM (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: Shielded vs. Unshielded
Message-ID: <2230012@hprnd.HP.COM>
Date: 27 Jun 89 18:13:19 GMT
References: <4015@quest.UUCP>
Organization: HP Roseville Networks Division
Lines: 28

There are a number of types of twisted pair cable.  In unshielded cables
the primary variable is wire size.  The 10BASE-T draft is designed to 
give performance comparable to coax media (in terms of BER) when run
over 100 m of 24 AWG UTP.  For 26 AWG UTP the distance will be shorter.

10BASE-T defines the media in terms of its end-to-end characteristics.
These characteristics are attenuation, impedance, jitter, delay, and 
crosstalk.  More than one type of twisted-pair media can meet the 
requirements for these characteristics for some distance. There should
not be significant performance variation for different types when
the requirements are met.
 
If when you say shielded twisted-pair you mean IBM Type 1 and 2, there
is one problem.  It has a characteristic impedance of 150 ohms.  UTP
is noninally 100 ohms at 10MHz.  Some manufacturers recommend using 
a balun to provide the correct impedance match when using this cable.
The IBM STP has lower attenuation than normal 24 AWG UTP and can achieve
longer distances than 100m once the proper match is provided.  The
superior characteristics and greater bulk of the IBM cables are not
entirely due to shielding.  The wire gauge is larger and the dielectric
is thicker.

There is also twisted pair which has an overall shield but no individual
shields between pairs.  This provides protection against external noise
sources.  Shielded 100 ohm cables are available.  Some have lower
attenuation than 24 AWG UTP and some don't.