10base-T & POTS in same Cat-5 cable?
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Posted by DaveC on April 27, 2005, 7:16 pm
Please log in for more thread options 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. Thanks, -- Please, no "Go Google this" replies. I wouldn't ask a question here if I hadn't done that already. DaveC me@privacy.net This is an invalid return address Please reply in the news group | ||||
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Posted by Floyd L. Davidson on April 27, 2005, 1:03 pm
Please log in for more thread options That is in fact what CAT5 was designed for! The higher twist (and generally every other characteristic of twisted pair designed for 10baseT frequencies) makes it a superb cable for VF frequencies. The most likely problem is that any kinks or other damage to the cable would allow cross-talk from ring current on the phone line to interfere with the 10baseT signal, but obviously that would be rare, intermittent, and of very short duration to the point of virtually never being even noticed (unless you have a couple teenagers using the phone line... :-) . The most serious problem would be wiring the connectors wrong. If they are connected just straight across, with pairs on 1 & 2, 3 & 4, and so on, the Ethernet segment will have a crossed pair using the tip from one pair and the ring from another. That will guaranteed result in the phone line, and perhaps other sources of noise, interfering with the Ethernet. On relatively typical short runs of CAT5 it will still work though, for 10baseT, and may not ever be discovered. The same cable will not work at all for 100Mbs Ethernet though, so if you move to a higher speed network that would immediately become critical. -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
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Posted by James Knott on April 27, 2005, 10:30 pm
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Floyd L. Davidson wrote: > The most likely problem is that any kinks or other damage to the
> cable would allow cross-talk from ring current on the phone line > to interfere with the 10baseT signal, but obviously that would > be rare, intermittent, and of very short duration to the point > of virtually never being even noticed (unless you have a couple > teenagers using the phone line... :-) . > Given the great difference in frequencies used, how is cross-talk an issue. Ringing is generally 20 Hz and voice is less than 4 KHz, whereas ethernet is up around 5 - 10 MHz (for 10 Mb ethernet). | ||||
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Posted by Floyd L. Davidson on April 27, 2005, 9:23 pm
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>Floyd L. Davidson wrote:
> >> The most likely problem is that any kinks or other damage to the
>> cable would allow cross-talk from ring current on the phone line >> to interfere with the 10baseT signal, but obviously that would >> be rare, intermittent, and of very short duration to the point >> of virtually never being even noticed (unless you have a couple >> teenagers using the phone line... :-) . >> >
>Given the great difference in frequencies used, how is cross-talk an issue. >Ringing is generally 20 Hz and voice is less than 4 KHz, whereas ethernet >is up around 5 - 10 MHz (for 10 Mb ethernet). If the transition period of the eithernet signal happens at a time when the induced voltage from crosstalk is greater than the threshold for the ethernet receiver, the receiver will decode an incorrect value. The significance that frequency plays is just how often and for how long that effect lasts. Essentially a 20Hz ring voltage could, for example, wipe out 1/40th of a second worth of 10baseT data if, for example, the entire positive transition of the ring signal is able to block the eithernet receiver. Granted however that the sensitivity to a 20 Hz interfering signal is going to be significantly less than to a single frequency that falls inside the spectrum of the desired signal. But then the voltage level for the ring voltage is huge by comparison too... I misspoke about the spectrum of 10baseT ethernet too. You are right that it is 5 - 10 Mhz, not DC to 5 Mhz (which is what 10 Mbps would be using NRZ encoding). -- Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
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Posted by DaveC on April 27, 2005, 11:18 pm
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On Wed, 27 Apr 2005 11:03:15 -0700, Floyd L. Davidson wrote > If they are connected just straight across, with pairs on 1 & 2,
> 3 & 4, and so on, the Ethernet segment will have a crossed pair > using the tip from one pair and the ring from another. What you're saying is to be sure to correctly wire 1,2,3,6 as Ethernet pairs, and 4,5 as POTS, and all should go well. Yes? -- Please, no "Go Google this" replies. I wouldn't ask a question here if I hadn't done that already. DaveC me@privacy.net This is an invalid return address Please reply in the news group | ||||
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>being used for 10base-T? Wondering about cross-talk, etc., introducing noise
>between these two.