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

[Wilton Helm]

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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