Cisco High-Density Analog Voice and Fax Network Module
Feature Overview
FXO and FXS interfaces indicate on-hook or off-hook status and the seizure of telephone lines by one of two access signaling methods: loop start or ground start. The type of access signaling is determined by the type of service from the CO; standard home telephone lines use loop start, but business telephones can order ground start lines instead.
Loop-start is the more common of the access signaling techniques. When a handset is picked up (the telephone goes off-hook), this action closes the circuit that draws current from the telephone company CO and indicates a change in status, which signals the CO to provide dial tone. An incoming call is signaled from the CO to the handset by sending a signal in a standard on/off pattern, which causes the telephone to ring.
Loop-start has two disadvantages, however, that usually are not a problem on residential telephones but that become significant with the higher call volume experienced on business telephones. Loop-start signaling has no means of preventing two sides from seizing the same line simultaneously, a condition known as glare. Also, loop-start signaling does not provide switch-side disconnect supervision for FXO calls. The telephony switch (the connection in the PSTN, another PBX, or key system) expects the router’s FXO interface, which looks like a telephone to the switch, to hang up the calls it receives through its FXO port. However, this function is not built into the router for received calls; it only operates for calls originating from the FXO port.
Another access signaling method used by FXO and FXS interfaces to indicate on-hook or off-hook status to the CO is ground start signaling. It works by using ground and current detectors that allow the network to indicate off-hook or seizure of an incoming call independent of the ringing signal and allow for positive recognition of connects and disconnects. For this reason, ground-start signaling is typically used on trunk lines between PBXs and in businesses where call volume on loop start lines can result in glare. See the “Configuring Disconnect Supervision Commands” section on page 14 and “Configuring FXO Supervisory Disconnect Tone Commands” section on page 16 for voice port commands that configure additional recognition of disconnect signaling.
In most cases, the default voice port command values are sufficient to configure FXO and FXS voice ports.
Disconnect Supervision Commands
PBX and PSTN switches use several different methods to indicate that a call should be disconnected because one or both parties have hung up. The commands in this section are used to configure the router to recognize the type of signaling in use by the PBX or PSTN switch connected to the voice port. These methods include the following:
•Battery reversal disconnect
•Battery denial disconnect
•Supervisory tone disconnect (STD)
Battery reversal occurs when the connected switch changes the polarity of the line in indicate changes in call state (such as off-hook or, in this case, call disconnect). This is the signaling looked for when the battery reversal command is enabled on the voice port, which is the default configuration.
Battery denial (sometimes called power denial) occurs when the connected switch provides a short (approximately 600 ms) interruption of line power to indicate a change in call state. This is the signaling looked for when the supervisory disconnect command is enabled on the voice port, which is the default configuration.
Cisco IOS Release 12.2(2)XT and 12.2(8)T
