Synchronization in Asterisk systems guarantees voice quality and integrity. When a system is not synchronized, clicks and other noises may interfere with the audio experience. This phenomenon occurs when PCM (digitized voice) packets are lost due to a slight difference in clock frequency among devices. Synchronization is critical to successful fax and modem communications. Click here for more information about fax synchronization.
To ensure system synchronization in your Astribank network, follow these simple rules:
· When FXO ports exist in a system, make sure that all the FXO ports are located in a single Astribank (see para. a below).
· When BRI or PRI ports and FXO ports exist in the same system, make sure that fax lines are routed through the BRI or the PRI ports (see para. b below).
· When using a third-party PRI, follow these instructions...
· When using a BRI connection, make sure that the PSTN is synchronizing the system continuously. The problem is that very frequently the BRI line is automatically deactivated after a pre-defined period of inactivity. In order to keep the BRI line continuously active, it must be configured for point-to-point mode (and not for point-to-multipoint). This configuration modification must be done by your local telephone company. Note that the BRI status LED should blink slowly and continuously (indicating a layer 2 connection).
The Astribank driver will choose the optimal synchronization source for the system. The synchronization source will be the first unit that exists in an installation, and according to the connection type, as per the following order of priority:
1. PRI
2. BRI
3. FXO
4. FXS
a) If more than 32 FXO ports are required, make sure that the fax and modem PSTN lines are connected to the Astribank unit that synchronizes the system. The SYNC LED on this unit will blink for about 250 milliseconds, and then pause for 750 milliseconds. The LED on the second (not synchronized) FXO unit blinks twice, pauses for 750 milliseconds and then blinks twice again, and so on, for every additional FXO Astribank in the network. Read more about the LED indicators in the Astribank User’s Manual.
b) BRI and PRI ports receive their clock from the PSTN. It is mandatory that they serve as the synchronization source for the Asterisk system. Due to technical limitations of the DAA (the component that serves as an interface between the analog PSTN line and the Astribank unit), FXO devices cannot synchronize themselves from an external clock source. Astribank has been specially designed to overcome this limitation. As a result, it provides good audio quality for voice calls.