Echo cancellation plays an important part in ensuring voice quality through the telecommunications network.
The Com.X product range incorporates state of the art, innovative, embedded on-DSP-chip echo cancellation that off-loads resources from the system server.
Echo cancellation can how-ever affect fax and modem signaling and data. It is therefore important to understand the impact of echo cancellation on fax and modem negotiation and configure devices correctly for fax use.
Echo cancellation is performed at the source device, where ‘source’ here indicates a network endpoint that introduces a signal into the PBX system. In the Com.X systems echo cancellation takes place on FXO, BRI and PRI ports/channels terminating traffic from the PSTN for routing in the system, as well as FXS ports originating calls via the PBX.
If not configured properly for fax support, faxes and modems may fail or higher baud rate operation might not be possible.
At these sources, if a fax start “CNG” tone is detected and the port / channels are correctly configured for fax, the echo cancelers are automatically disabled, and a clear channel is offered for fax service.
In the case of PRI and BRI channels, channels may serve both voice and fax traffic, and detection of the fax tone is required before echo cancellation can be switched off. This detection is configurable from the Hardware panel for BRI trunk groups and PRI ports.
Since fax devices are usually affixed to individual ports, and specific incoming DIDs reserved for fax use, in the case of FXS and FXO, ports need to be deliberately reserved for fax use by disabling echo cancellation. This is done for FXO ports by enabling the “Detect Fax” option, and for FXS ports by enabling the “Fax channel” option in the hardware panel, which disables echo cancellation and configures additional jitter buffering for the port.