ISDN Clock Mode
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.
- In transmission, all endpoints, regardless of their type, send a clock signal along with the data they send.
- In reception, all endpoints, regardless of endpoint type, use the clock they receive from the other end to synchronize the received data.
- In transmission, a clock SLAVE adjusts the clock it sends, based on the clock received from the other end in reception.
- In transmission, a clock MASTER sends an absolute clock signal that does not depend on the clock received from the other end.
Setting the Clock Mode
- Go to ISDN/Basic Rate Interface.
- In the Select Endpoint dropdown menu, select the endpoint you want to configure.
- In the Interface Configuration table, set the Clock Mode.
ISDN Clock Reference
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.
Selecting the Source of the Clock Reference
- Go to System/Hardware.
- From the Clock Reference Configuration table, select from the Suggestion list, several clock reference sources.
- Click Apply.
Current Clock Reference
- Only a clock SLAVE interface can be used as clock reference.
- If there is a configured preferred clock reference interface, this interface becomes the clock reference as soon as it is UP (and running in SLAVE mode).
- If the preferred clock reference interface is DOWN or if there is no configured preferred clock reference interface, the first clock SLAVE interface to become UP becomes the clock reference.
- If no clock SLAVE interface is UP, there is no clock reference and the unit uses its own internally-generated clock signal.
There can be only one active clock synchronization source (clock reference) per unit for the Mediatrix 4400 series.
Mediatrix Unit Clock Synchronization
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
|Interface||Acceptable deviation||Worst case difference between endpoints||Frame slip rate|
|Basic Rate (BRI)||
for each endpoint on a link
|200 ppm||2E-4 (one slip every 5000 frames)|
|Primary Rate (PRI)||
for each endpoint on a link
|100 ppm||1E-4 (one slip every 10000 frames)|
Direct ISDN Connection Between Endpoints
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|
IP Connection Between ISDN Networks
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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Several types of documents were created to clearly present the information you are looking for. Our documentation includes:
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- Reference guide: This exhaustive document has been created for advanced users. It includes a description of all the parameters used by all the services of the Mediatrix units. You will find, for example, scripts to configure a specific parameter, notification messages sent by a service, or an action description used to create Rulesets. This document includes reference information such as a dictionary, and it does not include any step-by-step procedures.
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