[Asterisk-Users] How many FXS/FXO ports do I need?

[Andrew Kohlsmith]

On April 8, 2005 08:30 pm, Aaron O'Hara wrote:
> My understanding is that a standard residential/business phone line
> carries the signal over 2 wires.  Your 4-wire RJ11 wiring supports 2
> phone lines.  Given that each line takes 2 wires, and there are 8 wires
> in an FXO port, can I conceivably support 4 phone lines on one FXO port?

No; the FXO ports only have the middle two pins wired to anything.  One port = 
one line.

> On the phone/FXS side of things, can you also have multiple lines per
> FXS port?

See above; only the middle two are wired.  The TDM400P uses RJ45 because they 
can use the same backplate with the TE4xxP cards.  :-)

> If I want to hookup 5 phones to my residential phone service with 2
> lines, what # of FXO & FXS ports do I need?

if you want each of the 5 phones on their own 'extension' then you need 5 FXS 
ports.  Irrespective of that, you need two FXO ports.

I'd say get a TDM22P; that gives you two internal extensions (say 3 phones on 
one, and 2 on the other), and access to your two lines.

I'd recommend against the X101Ps; not only will you have double the interrupts 
of a single TDM that can handle twice the number of ports, but you will also 
have a poorer hybrid interface to the PSTN; the TDM4xxP's FXO modules can be 
tuned much better.

-A.