Re: OT, proper phone wire question?
On Tue, Nov 18, 2003 at 03:36:57PM -0500, Derrick 'dman' Hudson wrote:
> On Mon, Nov 17, 2003 at 07:59:08PM -0500, lee wrote:
> | Sorry to get off topic but I need to re-wire my DSL connection at home here
> | and need to buy some phone wire and am curious if there is a good/better/best
> | type of wire I should purchase..will be a direct run to my modem going about
> | 100' or so from my box..
>
> Typically phones use Cat 0 (or is Cat 1 the lowest?) cable. That
> cable has 0 twists in it.
This is commonly known as "station wire", and is used a lot less
these days in telco work primarily due to cross-talk problems
on multiple line systems.
>
> 10BaseT (10Mbs) ethernet requires at least Cat 3, which has 3-5 twists
> per yard (or something like that).
This is frequently used for phone pre-wires.
> 100BaseT (100Mbs) ethernet requires at least Cat 5, which has 3-5
> twists per foot (or something like that).
>
> Gigabit ethernet requires at least Cat 6 or 7, which has 3-5 twists
> per inch.
>
> (all of these are "UTP" - Unshielded Twisted Pair - type cables)
>
>
> Why are twists so important and why do the faster technologies need
> more twists in the cable? As an electric signal varies along a pair
> of wires, a magnetic field around the wires is created and then
> collapses. This field will induce a current in another wire that
> passes through the field. That induced current is "noise", because it
> intereferes with the electric current (signal) that is intentially
> being sent over that wire. Twisting the wires reduces the size of the
> generated magnetic field, and also reduces the cable's ability to pick
> up a signal from a nearby magnetic field.
This is what causes the crosstalk between the pairs in station wire.
> For voice-grade telephone, no twists are needed most of the time. For
> better signalling and less noise, get more twists. Using Cat 0 or Cat
> 3 should be fine, but the Cat 3 will cost more money. You *may* have
> noise problems with Cat 0 which will hurt your ability to connect,
> maintain a connection, and will limit transfer rates. Basically "more
> is better" when it comes to twists, but also "more costs more". It's
> up to you to find the desired return-on-investment for cable quality.
I just use cat-5 for everything, there is not enough cost savings with
cat-3 to justify having the extra boxes of wire. I don't use station
wire for any runs more than a just a few feet.
dt
--
Dave Thayer | WARNING: Persons denying the existence of
Denver, Colorado USA | robots may be robots themselves.
dave@thayer-boyle.com |
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