Newsgroups: alt.engineering.electrical,comp.sys.ibm.pc.hardware.networking,sci.electronics.basics,sci.electronics.components,sci.electronics.misc
Subject: Re: twisted pair and coaxial cable speeds
References: <[email protected]>
"by"  writes:

> You know there're two kinds of network cables: twisted-pair and coaxial. As
> far as I can tell, the computer network coaxial cable is the same kind of
> coaxial used for cable TVs, and twisted-pair are basicly telephone wires.

The basic contruction on coaxial cables used in cable TVs etc.
The mechanical measures and insulating materials of those cables
are different so they have different impedances. TV systems
use 75 ohm cable and coaxial ethernet uses 50 ohm cable. For
more details on cable impedances take a look at
http://www.hut.fi/Misc/Electronics/docs/wiring/cable_impedance.html

Twisted pair wiring used in computer networks is basically quite
similar to telephone cable, although the contruction in computer
cables is more controlled. Generally the computer cables are much
tigter twisted (pick up less interference) and have a standardized
charateristic impedance of 100 ohms. The telephone cables can vary
much more widely and can have almost any impedance from 100 ohm 
to 300 ohm on typical cables or even up to 600 ohms on open wiring
on telephone poles.

> And ever since school days, I thought (and still think) those coaxial cables
> has a bandwidth of 1GHz, and I don't suppose those telephone wires can go up
> to 1GHz.

Those ciaxial cables have bandwidths to even many GHz on good cables.
Norma cable TV network cables (at least good ones) cna go to
1 GHz and the cabling used in satellite reception systems go to
2 Ghz.

Best twisted pair cables used for computer networking can typically
go up to few hundred megahertz.

> Intuitively, if you put these two kind cables side by side, you can tell
> coaxial is more solid, better-shielded, and should be capable of higher
> bandwidth.

Technically yes. 

> So I was quite surprised when I run across some documentation that said
> networks using coaxial cables can only go up to 10Mbps, while those using
> twisted-pair can go up to 100Mbps. 

The market pressure is on the wisted pair wiring and that's where the
majority of the products come out early. There are high speed networking
possibilities for coaxial cable designed, but the real products
on that side are rare because of lack of demans on that side.
Because lack of demand, competition and because of very small volumes
on production the prices of those are higher than more
complicated twisted pair systems which go to high speed.

The markets have advanced so that twisted pair and fiber have
taken the networking markets. Coaxial cable is mainly still
in used in radio systems, cable TV networks, video systems,
some digital audio applications and laboratory equipments where
it has some real edge over the twisted pair and fiber. And
even those fields are changing (fiber and twisted pair
are even pushign to those fields because of the advancements
made on those at networking side which has cut costs on those).
I would expect the coaxial cable to have a bright future on
high frequency radio applications where some considerable
power or low level signals is needed to be transferred.
 
> I use networked computers daily, and most
> of computer networks I've seen used twisted-pair cables, and I always
> assumed that's due to the cost and ease of handling.

The reason to use twisted pair wiring is the cost (installability
and mainteance), reliabity and and flexibility of wiring (same wiring
can be used for many other signals). Twisted pair wiring is the
standardized cable type installed for modern computer networks in offices
and other new buildings.

> Have I been wrong all along ? What am I missing here ?

Purely technically coaxial cable is better menium. In real world applications
where the total cost, reliabity and ease of use have quite much more
effect that the technical excellence the twisted pair wiring has won.
The reason that it has won is that it is good enough for today's
applications and cheap. For high speed networking using twisted pair wiring
makes signal transmission harder to design (twisted pair wiring is
not as well shielded as coaxial cable), but cost overhead on network card
is justified on saving on the wiring side.
 
-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/