FXO
This quick little post was simply meant to clear up confusion that often surrounds the basics of FXO vs. FXS ports. Foreign Exchange Office or FXO interfaces are ports that receive the analog line. In most cases this usually consist of a single line phone or an group of lines. When a group of lines or rotary is provided by the Telco a voice gateway is required on Customer end to facilitate the group of inbound lines. Other FXO devices include Fax lines, Credit Card Terminals etc. They deliver the on-hook indication or loop closure.
FXS
Foreign Exchange Subscriber or FXS interfaces are the ports that actually deliver the analog line to the Customer primes. In other words it is the incoming line from the Local Telephone Company or Telco that delivers the dial tone, and battery current for the ring voltage usually -24 volts in most cases.
Voltage
- FXO Contact Closure or Loop Closure
- FXS Low end -24 Volts DC – High end -48 Volts DC
Loop Start vs. Ground Start
Ground start is a method of signaling from a terminal of a subscriber local loop to a Telephone exchange, where one side of a cable pair is temporarily grounded to request dial tone.
Loop Start signaling allows a User or the Telephone company to seize a line or trunk when a subscriber is initiating a call. It is primarily used on local loops rather than on trunks. The current can now flow through the telephone circuit. The CO or Central Office or FXS module detects the current and returns a dial tone
Common FXO devices
- VIC2-2FXO
- VIC2-4FXO
- Point Of Sale
- Credit Card Terminals
- Overhead Paging
Common FXS devices
- VIC3-2FXS / DID
- VIC3-4FXS / DID
- Night Bell or Night Ringer
- Fax Line
The following simple diagram represents a simple flow or relationship between the Telco and the Customer primes.
- Telco – – – [ FXS ] – – – > PSTN – – – [ FXO ] – – – > Office
- Telco < – – – [ FXS ] < – – – PSTN < – – – [ FXO ] < – – – Office
