ISDN is a synchronous network, meaning that all endpoints on the network need to synchronize on the same clock signal. Typically, one endpoint acts as the clock MASTER, generating the clock signal, and the other endpoints act as clock SLAVE, synchronizing on the clock signal received from the MASTER.
By default, a NT type endpoint acts as clock MASTER, and a TE type endpoint acts as clock SLAVE. This default behaviour can be changed by using the web interface of the Mediatrix units. The clock mode of the ISDN endpoints can be set to either clock MASTER or clock SLAVE.
The following is a description of the endpoints behaviour with regards to clock signal synchronization:
On Mediatrix units with multiple ISDN endpoints, it is possible to select which endpoint should be used as the clock reference. When an endpoint is designated as clock reference, the other endpoints on the Mediatrix unit use this endpoint s clock as a synchronization source for generating their own clock.
In the following example, the clock signal generated by the ISDN PBX is received on the Mediatrix 4404 #1 s TE Slave endpoint and taken as a reference for this unit. Therefore the clock signal generated by this unit s NT Master endpoint is synchronized on this signal. The Mediatrix 4404 #2 s TE Slave endpoint receives this signal and uses it as a reference for this unit, meaning that the clock signal generated by this unit s NT Master endpoint towards the ISDN phone is synchronized on this signal, therefore on the PBX s signal.
The Mediatrix unit selects the interface that is effectively used as clock reference in the following way:
There can be only one active clock synchronization source (clock reference) per unit for the Mediatrix 4400 series, and per slot for the Mediatrix 3000 series.
An ISDN network will be reliable when all endpoint clocks are synchronized correctly. When endpoints are not synchronized, they each run on their own internally-generated clock signals (free-running). The following are acceptable clock signal deviations from the reference specified clock frequency in ppm (parts-per-million).
|ITU-T I.430||Basic Rate (BRI)||100 ppm|
|ITU-T I.431||Primary Rate (PRI)||50 ppm|
Mediatrix units are tested and certified against these specifications, and are guaranteed to follow the specified clock signal accuracy.
When endpoints are not synchronized, the clock signals are obviously not running at the exact same frequency, because of normal hardware deviations. The clock signals therefore gradually drift away from each other and a periodical frame slip happens
With respect to the specified accepted deviations, the worst case frame slip rates are
calculated as follows:
Worst case difference between endpoints
Frame slip rate
Basic Rate (BRI)
2E-4 (one slip every 5000 frames)
Primary Rate (PRI)
1E-4 (one slip every 10000 frames)
It is important to configure each endpoint appropriately so the clock signal is synchronized correctly with the rest of the network. The following table shows the different endpoint clock mode combinations and the associated behaviour.
|Endpoint 1 \ Endpoint 2||Master||Slave|
When using Mediatrix units to connect different ISDN networks through IP, each ISDN network runs on its own clock because there is no way to share the clock signal between the networks. Therefore, VoIP calls between different ISDN networks always experience periodical frame slips, which result in periodical packet losses, or measured bit error rates.
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