E&M or FXO for PBX connection

[tato386]

Most of the example configurations that I see use either a E&M or FXO ports on the router to connect to the PBX/Key system. How do you know which to use? What is the difference between the two?

Thanks,

Diego

[robrhodes]

E&M is a trunk interface, either 2 or 4 wire, generally for switch-to-switch connections. E&M can support DID whereas FXO cannot. You're more likely to find E&M interfaces on a PBX than a key system unless your key system had analog DID lines.

An E&M i/f connects to another E&M - an FXO connects to an FXS.

http://www.cisco.com/en/US/tech/tk652/tk653/technologies_tech_note09186a00800a6210.shtml#Topic4

http://www.cisco.com/en/US/tech/tk652/tk653/tk754/tech_protocol_home.html

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a0080080afd.html#1133451

[tato386]

Is it correct to assume then that one would use E&M ports on a PBX or key system to connect to the PSTN? Can an FXO port connect to the PSTN in any way?

Thanks,

Diego

[robrhodes]

The most common interface will probably be an FXO. The issue is really cost. You may get more functionality from an E&M, but on a PBX you need the E&M trunk line card(s) rather than just a couple of station ports. The telcos, at least in my area, would rather not provision analog E&M. They're happy to sell you POTS (FXO on your side) or some flavor of T1.