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

[Jon Pounder]

Wilton Helm wrote:

one thing I missed mentioning about fxs devices - the linksys/sipura 
ones actually allow you to set line characteristics on the slic inside 
it. you can vary from the 600ohm default, and tweak gains a bit. Some 
mix of a capacitive line or different resistance may help. never tried 
myself but there are a ton of things you can play with.


> There are a lot of factors that impact this. First, CAT 5, while 
> usable is overkill. Cat 3 (otherwise known as I/O wire) works equally 
> well for voice grade lines. That being said, for that long a run, a 
> heavier gauge wire would be better. I believe telcos use 18 – 22 guage 
> (Cat 5 and Cat 3 are both 26 awg). This has less resistive loss.
>
> Most FXS or ATA devices use 24 volts or less for “battery”. That works 
> fine for short loops, but limits the range. A central office POTS port 
> normally uses 48 VDC which works well to several KM. If the customer 
> is at the end of a long run in a rural area, they use a “long line” 
> card which uses 75 volts. (In rural communities, they often place the 
> line cards in a roadside “remote terminal” and use statistically 
> multiplexed T1s to make it appear to the switch as a part of it.
>
> That addresses the DC characteristics, which can be reduced to ohms 
> law. A phone needs around 8 V @ .02 A. The wire resistance determine 
> the drop (E = IR) and the source voltage determines whether there will 
> be enough left. The A.C. characteristics are more complicated. The FXS 
> must do a 2 wire to 4 wire conversion, which involves matching the 
> impedance of the line. The FXS is generally designed for relatively 
> short lines, so might not be able to match either the resistance or 
> capacitance found in a long run. Heavier wire will minimize this. In 
> addition to that, the transmit side of the 2 wire to 4 wire circuit 
> must be able to drive the load it sees, and again it may not be 
> designed with a long run in mind. Finally, COs line cards have the 
> ability to adjust receive and transmit gain to compensate for sound 
> level losses in long lines. While this isn’t routinely done on simple 
> circuits, it is an option an FXS doesn’t generally have. In addition, 
> the more gain that is inserted, the harder it is to balance to 2 wire 
> to 4 wire circuit, and the more complex it has to be in order to 
> support this.
>
> Wilton
>
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