Foreign Exchange (FX) Signaling

Think of FX as an extension cord for the 2-wire analog interface between a switch and a telephone . When extending a line from a PBX, FX is also called an ‘ If this extension cord runs through a frame relay network, the voice interface functions must be supplied by CO\VFRAD at each end. The CO\VFRADmimic a phone at the switch and a switch at the phone. In other words:

1. the ‘customer premises equipment interface’ (CPE-I) at the customer end of the loop plant must be duplicated in the CO\VFRAD (FXO) on the CO/PBX side of the frame relay link;

2. the switch interface at the CO end of the loop plant must be duplicated in the CO\VFRAD (FXS) on the CPE side of the frame relay link.

At one end of the cord (the office end) the switch supplies battery, generates ringing voltage, and accepts dialing. The transmission equipment (the CO\VFRAD) at that end of the FX extension cord is ‘FXO’ for foreign exchange, office.’ At the other end of the transmission system, the phone accepts battery current and ringing voltage, and generates dialing. The transmission equipment there (CO\VFRAD) presents an ‘FXS’ interface (‘foreign exchange, subscriber’ or ‘station’) to face the station equipment.

The name describes what the port faces, not what it is (office or subscriber). To send a call to a phone over the frame relay network, the PBX rings an extension port as if the phone were attached locally. The local CO\VFRAD, emulating the phone, supports the FXO functions: it absorbs the ringing current, recognizes it as ringing, and sends a call request or start ringing message to the remote CO\VFRAD

When a call request message arrives from the FR network, the remote CO\VFRAD applies ringing voltage to the 2-wire voice interface. When the called phone goes ‘off hook’ the CO\VFRAD halts ringing and delivers loop current, returning an ‘off hook’ signal to the calling end. The voice path is established between the CO\VFRAD

When the far end’s ‘off hook’ message arrives at the originating (calling) CO\VFRAD, it goes ‘off hook’ itself. This action draws loop current from the PBX and stops ringing. The PBX then opens its talk path to the local CO\VFRAD, which has already set up a talk path to the remote CO\VFRAD. Now, audio energy received by the remote CO\VFRAD on the analog interface is treated as voice: digitized, compressed, packetized, encapsulated in frame relay, and transmitted via the FR network. At the local end the process is reversed to present analog speech to the PBX.

To originate a call at a remote extension, that phone goes off-hook, which draws loop current to tell the remote CO\VFRAD someone wants to place a call. The remote CO\VFRAD signals the local CO\VFRAD (at the switch) via a signaling frame, which uses the same channel ID as the voice path.

The local CO\VFRAD goes ‘off hook’ to draw loop current from the PBX. This causes the PBX to send dial tone, which is signaled to the remote CO\VFRAD. DTMF or pulse dialing from the remote station arrives as coded messages which are translated by the local CO\VFRAD into re-created DTMF tones or dial pulses. When the remote extension returns to *on hook’ the remote CO\VFRAD signals the local CO\VFRAD to go ‘on hook’ also, opening the loop to stop drawing current. This indicates to the PBX an ‘idle’ extension. The call is cleared.

A straight FX function is 2-wire analog at both ends, FXS to FXO. With signaling conversion in the CO\VFRAD an FX interface (FXS or FXO) may be mixed with an E&M or digital interface at the other end.

On some equipment the voice path is 4-wire. The two paths are transformer coupled so that each pair may be used as a signaling lead. The call processing procedures are the same as for a single loop, as described below, substituting the A and B leads for tip and ring. With a 4-wire interface, a signal in each direction feeds directly into an amplifier where necessary. A hybrid is not needed to split the signal—eliminating the prime source of echo. Lack of echo is one of the advantages of this interface type, particularly when the network is 4-wire also.

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