Battery and Ringing
on Cisco FXS Ports

FXO/FXS port definition:

- FXS (Foreign Exchange Subscriber) supplies voltage and tones to the subscriber
- FXO (Foreign Exchange Office) is the subscriber, and supplies signalling requests

An FXS port connects to the Subscriber: it provides Dial-tone = extension port
An FXO port connects to the Central Office: it provides on/off-hook indication = trunk port
FXS receives a call, FXO initiates a call

Idle Battery Voltage:

On Premise Services (ONS): -24Vdc/36Vdc
Off Premise Services (OPS): -48Vdc

Ring Voltage:

Between 80 and 100Vrms, typically around 90Vrms @ 20Hz (US) or 25Hz (EU).
Note: An FXO port should be able to detect a ring signal as low as 40Vrms.

Resolving Line Seizure Problems:

Problems with call answering and initiation by automated telephony devices, eg: fax machines, may be resolved by increasing the idle battery voltage. The default is ONS, though OPS may be necessary with long cable lengths, or where a devices threshold is higher.
Use the following command on the FXS voice port to increase idle battery voltage from -24 volts to -48 volts:

Router(config-voiceport)#idle-voltage {low | high}

Note: This option is not available on the VG248, VIC-2FXS, and WS-x6624 FXS interfaces.

Resolving Ringing Problems:

Sometimes, anti-tinkle circuits are used to stop ringer chirp or tinkling during dialing. It might be necessary to adjust the FXS voice port ring frequency to match the connected devices filter frequency. Configure the ring frequency with the following command:

Router(config-voiceport)#ring frequency ?
25 ring frequency 25 Hertz                     for 17xx, 26xx, 36xx, and 37xx platforms
50 ring frequency 50 Hertz

Router(config-voiceport)#ring frequency ? 
20 ring frequency 20 Hertz                     for the 3810 platform 
30 ring frequency 30 Hertz

Router(config-voiceport)#ring frequency ? 
20 ring frequency 20 Hertz                     for the IAD2400 platform
25 ring frequency 25 Hertz 
30 ring frequency 30 Hertz 
50 ring frequency 50 Hertz

Another way to prevent ringer tinkle when dialing is by setting a small DC offset voltage. An increase in offset voltage can often overcome any ringer tinkling. Configure the DC offset voltage on IAD24xx routers with the following command:

Router(config-voiceport)#ring dc-offset ? 
10-volts Ring DC offset 10 volts 
20-volts Ring DC offset 20 volts 
24-volts Ring DC offset 24 volts