Newsgroups: comp.dcom.telecom.tech,alt.dcom.telecom,comp.dcom.cabling,comp.dcom.lans.misc
Subject: Re: 10base-T & POTS in same Cat-5 cable?
References: <[email protected]>  <[email protected]>
[email protected] (Floyd L. Davidson) writes:

> James Knott  wrote:
> >DaveC wrote:
> >
> >> Is it acceptable to use 1 pair in a Cat-5 cable for POTS when 2 pair are
> >> being used for 10base-T? Wondering about cross-talk, etc., introducing
> >> noise between these two.
> >
> >Yes, it's entirely acceptable.  When twisted pair ethernet was created, it
> >was intended to be run on existing cat 3 telephone cables, along side phone
> >lines.
> 
> I'm not going to look that up to be certain, but I don't think
> it is correct.  T1 was certainly designed for telephone lines,

Yes. 

> but I don't think 10baseT was.  Look at the difference in the
> twist on CAT5 compared to lesser grade twisted pair!

10BASE-T was designed to operate on early Cat-3 UTP wiring. 
At the time 10Base-T was standardized there was not such 
thing as CAT5 wiring! At the time the 10Base-T system was 
started to be deployed on larger scale (somewhere in the 
beginnignof 1990's) there was no such thign as CAT5 
(maybe some cable vendors have dreamed on this and worked 
on standard, but nothign on market). At that time all that 
was available was CAT3 and CAT4 wiring. In know installations 
made at that time that used CAT4 for telephone outlets and CAT4 
for data. 

> >Also, the frequencies used on telephones are nowhere near those
> >used by ethernet, so interference is unlikely.  I've even seen
> >installations, where two 100 Mb ethernet circuits share one cable.
> 
> The frequency has little to do with whether it will interfere.
> The voltage level is what makes a difference.  

Frequency has a lot to quite much on the interferences. 
Most communications systems are designed to be frequency 
sensitive, so that they effectively receive the signals 
only on the range they operate at, the signals at other 
frequencies gets very much attenuated before they get 
into the receiving electronics. This means that signal 
outside normal operating range needs to be considerably 
higher in amplitude (compared to signal at operating frequency 
range) to cause harmdul interfence.

> Besides, 10baseT does *not* run at 10Mbps... 
> that is merely the highest bit rate.  

10Base-T pushes bits to the line at fixed 10Mbps rate. 
It either sends but out at that rate, or not (is silence 
when not transmitting). 

> The actual frequency spectrum runs from DC on up through
> at least 15 Mhz (three times the 5 Mhz fundamental for 10 Mbps).
> Which is to say that VF circuits and 10baseT do in fact share
> the same frequency range for the entire VF range.

10Base-T does not use / need the DC to 15 MHz frequency range. 
10Base-T does not use DC signals to anything. All the 
10Base-T Ethernet equipment are transformer coupled to the 
line, which means that none of then are capable of sending 
out DC to the line and they can't receive signal DC level. 
All the data sent to line is manchester coded, free of any 
DC component. This means that the frequency range of 10Base-T 
Ethernet does not start form DC. The practical frequency 
range of signal you cna find on line definately lies 
between few hundred kHz to 30 MHz, where the most 
signal energy lies between 5 MHz and 10 MHz frequency range.


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/