Timing in the system is very important to ensure successful fax / modem communication. The iTA device (e.g. the internal BRI / PRI enabled iTA MPC card in the Com.X1 case or the BRI / PRI iTA in the Com.X2 case) connected to the telco interface should have the highest timing priority (line N), and other iTAs connected should derive timing from this device (system). Please see section 188.8.131.52 for detail on configuring the timing source on an iTA device.
In scenarios where multiple PRI links are configured to the same telco, setting the first PRI as PRI Line 1 is recommended (i.e. deriving clocking from the telco line) and setting all other PRIs to System (i.e. sharing the clocking of the first PRI in the transmit direction and honoring the telco’s clocking on each PRI in the receive direction.
In gateway scenarios, where one or more PRIs face the telco and one or more PRIs face a 3rd party device, the telco-facing PRIs should be configured as above, and the PRIs facing the 3rd party device should be configured to use System timing. In this scenario the “transmit clocking” setting should be enabled the relevant PRI ports in the GUI hardware panel.
This indicates to the Com.X that the receive direction clocking (from the 3rd party device) should not be honored (as in the telco case), but that the Com.X should propagate its system-derived timing to the 3rd party device. In such configurations the 3rd party device should be configured as a clock slave.
This additional configuration is not required for BRI ports, as the NT/TE mode setting on BRI pre-selects the correct timing driver on the gateway leg facing the 3rd party device. BRI timing configuration follows the same Line Priority and System configuration pattern as described above for PRI.
If an iTA is connected to a switch located between the iTA and the Com.X host server, network packet delays could occur which could also affect timing. If this is the case, from the GUI, Monitor – Telephony – dahdi statistics should show slips and skips increasing on the span to the iTA. The Com.X can be instructed to buffer in the receive direction from the iTA in order to alleviate such network delays. This is done by enabling the “Network slip tolerant” setting on the GUI under Setup – Global Settings.
Please also refer to the Com.X troubleshooting guide for more information on fax and modem troubleshooting.