limited choices - retrocabling FREE cabling and networking Helpdesk
 Question by steve lieb posted 18 Jun 2002
 limited choices - retrocabling
I hope you can help.
We are trying to bring a 19th century hope into the 21st century, and this is proving a challenge because of VERY limited options for running cable behind the walls.
We have a room that we hope to establish as a "computer room". However, to bring in any network connectivity (DSL line or ethernet line from the DSL splitter) we have to drop the cable along a vent pipe to the basement. The run is at least 5m along this pipe, probably closer to 7m.
1) will sending the Cat5e cable along this (iron) vent pipe cause any interference?
2) there ARE power lines also dropped in this space - are they going to cause noticeable interference? They do not run to anything with a heavy draw - only bedrooms, etc. - although one room might have a window-mount AC unit. Ultimately, if this room needs more power, we may have to pull that additional line in this same space AGAIN. This would be powering 3 PC's and stuff (printers, hubs, etc.). Would this make it even worse?
3) presuming your answer to #2 is that there will be significant interference, is there anything we can do to mitigate it? Will a 1.5 mbps signal be degraded in a 100BaseT cable? Will the interference be in lost (corrupted) packets or just a degraded connection?

Sorry for all the questions, but this really has me concerned, and I don't see a way around it.
 Answer by Dmitri Abaimov posted 18 Jun 2002
Dear Steve,

I don't think the 7 m. run along the pipe with the power cables will add much to the interference picture in your cable. The signals that are anticipated to run over the CAT5E cable(s) are in the balanced form, meaning that only the voltage difference between the conductors matters, not the voltage to the ground (pipe in your case). Of course the ground does affect the signal, changing the impedance and causing the reflection losses, but this effect becomes visible if we are talking about millimeters. If you run your cable just a centimeter or two away from the ground conductor, the affect is so little that no application will see it.

Power cables are also not a problem on the 7 m. run. This is considering, of course, there will be no industrial-type heavy loads connected to these cables. As long as the cables are on 20 A breaker or less and the loads are regular household or office equipment, you should be fine.

And, lastly, before the DSL signal gets to your home, it's already so much distorted and garbled that the last 7 meters are just not going to change anything.

Happy browsing!

Dmitri Abaimov, RCDD

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