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FXS vs FXO | difference between FXS and FXO

This page on FXS vs FXO mentions difference between FXS and FXO. The full forms of FXS and FXO interfaces are Foreign eXchange Subscriber and Foreign eXchange Office respectively.

As we know, in analog telephone system connection between CO(Central Office) and Subscriber premises is two copper wires. These wires are twisted together to reduce EMI. Hence the connection is referred as "twisted pair". This system is often known as POTS i.e. Plain Old Telephone Service. Both the terms FXS and FXO are associated with analog POTS system. These are basically interfaces between various elements of analog telephony as shown in the figure.

FXS-Foreign eXchange Subscriber interface

This interface provides POTS service from the local central office of the telephone company. This must be conected to subscriber side. The terminating points may be telephones, fax machines or modems etc. These are basically subscriber equipments. The FXS interface basically points to the subscriber side.

The FXS interface delivers following services to the subscriber.
• Ring Voltage
• Dial Tone
• Battery Current


FXO-Foreign eXchange Office interface

This interface type gets the POTS service usually from CO of the PSTN. PSTN refers to Public Switched Telephone Network. This FXO interface point towards telco office as shown in the figure. The FXO interface ports provides on-hook as well as off-hook indication (i.e. loop closure) to local telco office.

This question on "what is the difference between FXS and FXO" is often asked during interview.

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