11 Interworking between A/Gb mode MSC and Iu mode MSC

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

11.0 Signalling issues

11.0.1 Loss of BC Information during Handover from A/Gb mode to UTRAN Iu mode

In the case of inter-MSC handover from A/Gb mode to UTRAN Iu mode, the serving A/Gb mode MSC/VLR sends a MAP message Prepare Handover carrying the BSSMAP message Handover Request. This message includes the parameter Channel Type, indicating whether radio resources are to be allocated for speech or data (parameter ‘Speech or data indicator’) and, among other data, the type of data service (transparent/non transparent) and the user rates (both included in the parameter ‘Data rate and transparency indicator’).

As no other bearer capability related parameters are received, it is not possible to distinguish between any other services than ‘speech’, ‘data transparent’ and ‘data non-transparent’.

The mapping into QoS radio access parameters would be done as described in 3GPP TS 27.001[86], annex B, subclause B.1.13., limited to the services ‘speech’, ‘data, non-transparent’ and ‘data, transparent’.

11.0.2 Handover from UTRAN Iu mode to A/Gb mode

In case a UTRAN Iu mode call is set up in the CN, the BC IE parameters are mapped into QoS RAB parameters at call setup.

If the CN has to perform a handover towards A/Gb mode, the non-anchor MSC needs to perform an assignment based on GSM traffic channel parameters.

In case of handover from UTRAN Iu mode to A/Gb mode, the anchor MSC maps the BC IE parameters into A/Gb mode traffic channel parameters. This requires that the BC IE is coded according to A/Gb mode protocol requirements, i.e. all those parameters ignored in UTRAN Iu mode should nevertheless be correctly specified by the UE in order to perform a handover to A/Gb mode.

11.0.3 Loss of BC Information during Handover from A/Gb mode to GERAN Iu mode

Subclause 11.0.1 applies also to handover from A/Gb mode to GERAN Iu mode.

Additionally, the serving A/Gb mode MSC/VLR will include the parameter GERAN Classmark in the MAP message Prepare Handover, if this parameter is available. The GERAN Classmark, which indicates the capabilities of the BSS in the target cell (e.g. allowed channel codings and maximum number of traffic channels), shall be taken into account by the target MSC when it performs the mapping into QoS radio access parameters.

11.0.4 Handover from GERAN Iu mode to A/Gb mode

Subclause 11.0.2 applies also to handover from GERAN Iu mode to A/Gb mode.

NOTE: The protocol requirements for the coding of the BC IE according to GERAN Iu mode are the same as for A/Gb mode, i.e. all those parameters needed in order to perform a handover to A/Gb mode are available.

11.0.5 Handover from UTRAN Iu mode to GERAN Iu mode

The serving UTRAN Iu mode MSC/VLR will send a MAP message Prepare Handover carrying the RANAP message Relocation Request. When setting the QoS RAB parameters in the RANAP message Relocation Request, the serving UTRAN Iu mode MSC/VLR shall take into account:

– the GERAN Classmark of the target cell, if this parameter is available;

– the allowed channel codings and the maximum number of traffic channels from the BC IE, if the serving MSC is the anchor MSC; and

– the allowed radio interface rates (included in the parameter Channel Type), if the serving MSC is not the anchor MSC.

This requires that the BC IE is coded according to GERAN Iu mode protocol requirements, i.e. all those parameters ignored in UTRAN Iu mode should nevertheless be correctly specified by the UE in order to perform a handover to GERAN Iu mode. Furthermore, it requires that the anchor MSC maps the BC IE parameters into A/Gb mode traffic channel parameters and includes the parameter Channel Type in the MAP message Prepare Handover also for basic handover to UTRAN Iu mode.

11.0.6 Handover from GERAN Iu mode to UTRAN Iu mode

The serving GERAN Iu mode MSC/VLR will send a MAP message Prepare Handover carrying the RANAP message Relocation Request. When setting the QoS RAB parameters in the RANAP message Relocation Request, the serving GERAN Iu mode MSC/VLR shall take the mode of the target cell into account. (See 3GPP TS 27.001[86], annex B, subclause B.1.13. For non-transparent services, some of the RAB Subflow Combination bit rates are supported in GERAN Iu mode, but not in UTRAN Iu mode.)

11.1 Handover to A/Gb mode MSC

After a handover from an Iu mode MSC or an A/Gb mode MSC to an A/Gb mode MSC the user plane between the anchor MSC and the visited MSC shall comply to:

– the standard A-interface protocols if both MSCs are connected via a TDM interface, i.e.:

– A-TRAU or modified V.110 frames as defined in 3GPP TS 44.021 [27] and 3GPP TS 48.020 [28];

– up to four 16kbit/s substreams are multiplexed in one 64kbit/s channel (Split/Combine function and Multiplexing function as defined in 3GPP TS 44.021 [27] and 3GPP TS 48.020 [28]).

– the Nb UP protocol if the anchor MSC or MGW and the visited MSC or MGW are connected via an ATM transport interface or IP transport interface in a BICC based CS CN. The NbUP shall be configured in support mode, the data is transported in a 64 kbit/s bit stream, formatted in SDUs of 40 octets and transmitted every 5 ms, in accordance with Annex P of ITU-T Recommendation I.366.2 [81]. PDU type 0 is used, i.e., payload CRC is applied. This is needed for the framing to be handled the same for all transports but the Frame Quality Classification control shall be ignored (3GUP property Delivery Of Erroneous SDUs = yes) and therefore interim nodes shall only pass on the CRC. The data is encoded between MSC-B/MGW-B (non-Anchor) and MSC-A/MGW-A (Anchor) as for the TDM case. Furthermore, Clause 11a.1.3 is applicable.

– the CLEARMODE payload type, see IETF RFC 4040 [92], within RTP/IP if SIP-I over Nc is deployed in the CN. The data is transported in a 64 kbit/s bit stream. The data is encoded between MSC-B/MGW-B (non-Anchor) and MSC-A/MGW-A (Anchor) as for the TDM case. Furthermore, Clause 11a.1.3 is applicable.

11.2 Handover from A/Gb mode MSC to UTRAN Iu mode MSC

After a handover from an A/Gb mode MSC to an UTRAN Iu mode MSC the user plane between the anchor MSC and the visited MSC shall comply to:

– the A-TRAU’ protocol if both MSCs are connected via a TDM interface except for FNUR = 32 kbit/s (ITC = UDI), FNUR = 56 kbit/s (ITC=RDI) and FNUR = 64 kbit/s (ITC=UDI). For these exceptions a plain 64 kbit/s channel is used between the MSCs.

– the Nb UP protocol if the anchor MSC or MGW and the visited MSC or MGW are connected via an ATM transport interface or IP transport interface ina BICC based CS CN. The NbUP shall be configured in support mode, the data is transported in a 64 kbit/s bit stream, formatted in SDUs of 40 octets and transmitted every 5 ms, in accordance with Annex P of ITU-T Recommendation I.366.2 [81]. PDU type 0 is used, i.e., payload CRC is applied. This is needed for the framing to be handled the same for all transports but the Frame Quality Classification control shall be ignored (3GUP property Delivery Of Erroneous SDUs = yes) and therefore interim nodes shall only pass on the CRC. The data is encoded between MSC-B/MGW-B (non-Anchor) and MSC-A/MGW-A (Anchor) as for the TDM case (A-TRAU’ protocol or plain 64kbits/s). Furthermore, Clause 11a.1.3 is applicable.

11.3 Handover from A/Gb mode MSC to GERAN Iu mode MSC

11.3.1 User plane for transparent services

After a handover from a GERAN A/Gb mode MSC to a GERAN Iu mode MSC the user plane for transparent services between the anchor MSC and the visited MSC shall comply to:

– the A-TRAU’ protocol except for FNUR = 32 kbit/s (ITC = UDI), FNUR = 56 kbit/s (ITC=RDI) and FNUR = 64 kbit/s (ITC = UDI). For these exceptions a plain 64 kbit/s channel is used between the MSCs. The rate adaptation between 64 kbit/s and 32 kbit/s is based on ITU-T Recommendation I.460.

11.3.2 User plane for non-transparent services

After a handover from a GERAN A/Gb mode MSC to a GERAN Iu mode MSC the user plane for transparent services between the anchor MSC and the visited MSC shall comply to:

– the A-TRAU’’ protocol as defined below for the RAB subflows with 12 kbit/s, 24 kbit/s, 36 kbit/s and 48 kbit/s and the A-TRAU’ protocol for the RAB subflows with 14,4 kbit/s, 28,8 kbit/s, 43,2 kbit/s and 57,6 kbit/s, if both MSCs are connected via a TDM interface.

11.4 Handover within Iu mode PLMNs

After a handover from an Iu mode MSC to a UTRAN Iu mode MSC the user plane between the anchor MSC or MGW and the visited MSC or MGW shall comply to:

– the A-TRAU’ protocol if both MSCs are connected via a TDM interface except for the transparent case FNUR = 32 kbit/s (ITC = UDI or RDI), FNUR = 56 kbit/s (ITC=RDI) and FNUR = 64 kbit/s (ITC=UDI). For these exceptions a plain 64 kbit/s channel is used between the MSCs. The rate adaptation between 64 kbit/s and 32 kbit/s is based on ITU-T Recommendation I.460 [2].

After a handover from an Iu mode MSC to a GERAN Iu mode MSC the user plane between the anchor MSC or MGW and the visited MSC or MGW shall comply to

– the A-TRAU’ or A-TRAU” protocol or a plain 64 kbit/s channel if both MSC are connected via a TDM interface. The A-TRAU’ protocol shall be used for transparent services except for the transparent cases FNUR = 32 kbit/s (ITC = UDI), FNUR = 56 kbit/s (ITC=RDI) and FNUR = 64 kbit/s (ITC=UDI). For these exceptions a plain 64 kbit/s channel is used between the MSCs. The rate adaptation between 64kbit/s and 32kbit/s is based on ITU-T Recommendation I.460. For non-transparent services, the A-TRAU’’ protocol shall be used for the RAB subflows with 12 kbit/s, 24 kbit/s, 36 kbit/s and 48 kbit/s and the A-TRAU’ protocol shall be used for the RAB subflows with 14,4 kbit/s, 28,8 kbit/s, 43,2 kbit/s and 57,6 kbit/s.

11.5 Handover for 56kbit/s

The FNUR = 56 kbit/s in transparent mode can be supported in A/Gb mode by two configurations:

1. without IWF with the following channel codings

– 2*TCH/F32.0

– 5*TCH/F9.6

2. with IWF with the following channel coding

– 4*TCH/F14.4

The FNUR = 56 kbit/s in transparent mode is supported in Iu mode by a configuration without IWF only. Therefore handover for 56kbit/s in transparent mode between Iu mode MSC and A/Gb mode can be supported only for configurations without IWF.

Note: Handover between configurations with and without IWF are also not supported within A/Gb mode.