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| Q. When connecting a BRI SMartNode to a PBX trunk line I have bit-slips and problems with fax connections. How to solve this synchronization problem? |
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| A. In installations where a PBX is connected to the PSTN and to a SmartNode ISDN VoIP Gateway at the same time, synchronization problems can occure. The problem exists beacuse the PBX expects a synchronous clock on all trunk lines. The SmartNode ISDN VoIP Gateway however can only deliver a synchronous clock if it is connected to a reference network/clock. If this is not the case the SmartNode clock and the PSTN clock will not be synchronous leading to bit slips between different trunk lines of the PBX. These slips do not cause problems with voice calls, however fax and modem calls are impaired.
- The only universally applicable solution to this problem is to have one SmartNode BRI (or PRI) port connected to a reference clock . This solution will work with every PBX. The reference clock may come from an internal S-Bus on the PBX or from a PSTN connection
- In the case of one BRI port used for Voice, Fax and Modem over IP, many SmartNode models provide an extra BRI port for this refclock connection. E.g. the SN4552 and SN4630 series.
- If more then 4 active BRI ports are required, the solution can be provided with the SN2400 and 1-4 IC-4BRV interface cards.
- With some PBXs a reconfiguration of the trunk ports is possible that allows to deliver the ref clock to the SmartNode over the trunk line (PBX port Layer3 Usr (TE) but Layer 2 clock-master). This requires a reconfiguration of the PBX which is not possible on all PBX systems.
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