12 Frame Synchronization

29.0073GPPGeneral requirements on interworking between the Public Land Mobile Network (PLMN) and the Integrated Services Digital Network (ISDN) or Public Switched Telephone Network (PSTN)Release 17TS

Potentially two links are involved in the MSC/IWF regarding the need for frame synchronization, i.e. the link towards the UE and the link towards the fixed network. The links towards the UE are covered by 3GPP TS 48.020 and 48.060 for A/Gb mode and 3GPP TS 25.415 for Iu mode. For the link towards the fixed network, the appropriate sections of ITU-T Recommendation V-series modem, V.110, V.120 and PIAFS Recommendations apply.

12.1 Initial frame synchronization

12.1.1 Terminating side (towards the UE)

In Iu mode, the terminating side is not synchronous.

In A/Gb mode, for transparent/non‑transparent and interworking to the PSTN or ISDN the interface to the RAN is managed as follows. As soon as the outband signalling exchange indicates that the traffic channel is available the MSC/IWF will start sending frames with the frame contents set as indicated in subclause 9.2.3.4.1 towards the RAN. The MSC/IWF will seek to attain V.110 or A-TRAU frame synchronization on the incoming data from the RAN. V.110 synchronization will be considered to be completed in line with the procedures described in subclause 9.2.3.4.1.1. A-TRAU frame synchronization will be considered to be completed in line with the procedures described in subclause 9.2.3.4.1.2. The incoming data will only be considered valid once the frame synchronization procedure defined in subclause 9.2.3.4.1 is complete. For non-transparent interworking to the PSTN or ISDN, the procedures described in subclause 9.2.4.10.1 shall be followed.

12.1.2 Transit side (towards the fixed network)

12.1.2.1 Interworking to the PSTN

In the case of interworking to the PSTN the procedures for initial synchronization for the transparent services are covered in subclause 9.2.3.4.2 and the non-transparent services in subclause 9.2.4.10.2.

12.1.2.2 Interworking to the ISDN

In the case of interworking to the ISDN the procedures for initial synchronization for the transparent services are covered in subclause 10.2.3.4.2 and the non-transparent services in subclause 10.2.4.10.4.2.

12.2 Action on loss of frame synchronization

The IWF should attempt to recover synchronization as described in the following subsections. If the resynchronization attempt fails, the IWF may clear the call.

12.2.1 Loss on the transit side (towards the fixed network)

If loss of frame synchronization is detected from the fixed network in line with the procedures specified in the ITU‑T or PIAFS recommendation applicable to the type of interworking (V.110, V.120, PIAFS or V-series modem), then re‑synchronization is initiated in line with the procedures specified in that recommendation. No change of behaviour of the MSC/IWF on the RAN/MSC link is necessary.

12.2.2 Loss on the terminating side (towards the UE)

In Iu mode, the terminating side is not synchronous, so loss of synchronisation is not possible. For T services, frames may be lost or arrive irregularly, which handling is implementation dependent.

In A/Gb mode, if the MSC/IWF detects a loss of frame synchronisation on one or more substreams on the RAN/MSC link, the MSC/IWF initiates a re-synchronisation on the substreams in question as specified in the following.

The MSC/IWF shall detect a loss of V.110 frame synchronisation in line with the rules specified in ITU-T Recommendation V.110. The MSC/IWF shall detect a loss of A-TRAU frame synchronisation when an A-TRAU frame has been received with at least one error in the synchronisation pattern (ref 3GPP TS 48.020).

If loss of synchronization is detected on the RAN/MSC link then a re‑synchronization process should be initiated. However for this link to the RAN it is only necessary to search for the frame alignment pattern incoming from the RAN. In the case of A-TRAU the synchronisation shall take care of the multiframe alignment according to subclause 9.2.3.4.1.2 and the MSC/IWF shall set the control bit UFE (Uplink Frame Error, see 3GPP TS 48.020) in the next downlink A-TRAU frame to indicate the framing error to the RAN.

There shall be no action regarding the outgoing frame towards the RAN, other than to continue sending the rate adapted frames made up of the incoming data from the fixed network. During the re‑synchronization process data shall continue to be sent towards the fixed network via the modem or ISDN (V.110, V.120, or PIAFS) TA-function as if the frame synchronization were still available. The mapping of the status bits is unchanged during re-synchronization.

Once synchronization has been re‑attained the RLP will recover any possible loss of data on the RAN/MSC link in the case of non-transparent services. The indication of UFE will be stopped in the case of A-TRAU.