[asterisk-users] Maximum cable length for analog phone from FXS port

[Jon Pounder]

Miguel Molina wrote:
> randulo escribió:
>> On Tue, May 26, 2009 at 5:31 PM, Danny Nicholas <danny at debsinc.com> wrote:
>>   
>>> I run my analog telco over cat5, but that's in-house and definitely not 3km. That sounds really far for current loop stuff.
>>>     
>>
>> I was doing that too. I asked this same question a few years ago and
>> the answer was 100-200 meters. This is just a quick rule of thumb, but
>> it seems about right. 3km, I doubt that would work, but it depends, as
>> someone said, totally depending on ohm's law :)
>>   
> What I think about this is, the length of the copper cable between the 
> central office and home is usually several km, but definitely helped 
> by the central office circuitry (current source instead of voltage 
> source, that guarantees a minimum ringing voltage on the far end). 
> What I don't know is, a FXS port behaves the same as a central office, 
> electrically speaking? If that is so, you could extend your 3km of 
> cable without problems, but I think you can have some noise problems 
> depending on what places the cable has to go through.

from the ringing point of view, the CO ring generator is usually truly a 
sine wave and this propagates well through a cable. The cheap fxs ports 
are mostly square waves and lower voltages with limited current sourcing 
(check the REN numbers they are capable of ringing for a comparison if 
its listed) some of the cheap ones have trouble ringing a phone plugged 
in with a short line cord. So in addition to being frequency choked by 
the long run the square wave will get reduced in amplitude, and it may 
well have been marginal amplitude to begin with.

so depending what fxs hardware you have driving it and the load from the 
phone, results will range from works perfectly to does not work at all.




>
>> r
>>
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> Ing. Miguel Molina
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