Content for TS 23.009 Word version: 18.0.0

1… 4… 4.3… 6… 6.2… 6.2.3 6.3… 7… 7.2… 8… 8.1.3… 8.2… 8.2.2… 8.2.4 8.3… 8.3.2… 9… 10… 11… 12… 12.8… 12.8.2 12.8.3 13… 14… 15…

7.2 Basic handover procedure not requiring the establishment of a circuit connection between MSC-A and MSC-B 7.3 Procedure for subsequent handover requiring a circuit connection 7.4 Procedure for subsequent handover not requiring a circuit connection
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7.2 Basic handover procedure not requiring the establishment of a circuit connection between MSC-A and MSC-B p. 45

The basic handover procedures to be used when no circuit connection is required by MSC-A are similar to those described in subclause 7.1 for circuit switched calls. The main differences to the procedures described in subclause 7.1 relate to the establishment of circuits between the network entities and the Handover Number allocation.

In the case of ongoing GSM voice group calls the circuit connections are already established therefore the procedures described in this clause are also applicable. When applied to ongoing voice group calls the clearing of resources on BSS-A shall not be used if the resources are still be used on the down link. Consequently the A-CLEAR-COMMAND message shall not be sent, but an HANDOVER-SUCCEEDED message shall be sent.

In the case of basic handover, MSC-A shall specify to MSC-B that no Handover Number is required in the MAP-PREPARE-HANDOVER request (see TS 29.002). As for the basic handover using a circuit connection, the A-HO-REQUEST is transmitted at the same time. Any subsequent Handover Number allocation procedure will not be invoked until the completion of the basic handover procedure (see clause: Subsequent Channel Assignment using a circuit connection). MSC-B shall then perform the radio resources allocation as described in subclause 7.1. The MAP PREPARE-HANDOVER response shall be returned to MSC-A including either the response of the radio resources allocation request received from BSS-B (A-HO-REQUEST-ACKNOWLEDGE/A-HO-FAILURE with possible extra BSSMAP information. These extra information are amended by MSC-B due to the possible interworking between the BSSMAP protocol carried on the E-interface and the BSSMAP protocol used on the A-interface) or potentially the A-QUEUING-INDICATION . The basic handover procedure will continue as described in subclause 7.1 except that no circuit connection will be established towards MSC-B.

The relevant case for the basic handover without circuit connection is shown in Figure 13. As can be seen the major differences to the equivalent Figure 12 is the omission of any circuit establishment messaging and the omission of handover number allocation signalling.

Copy of original 3GPP image for 3GPP TS 23.009, Fig. 13: Basic Handover Procedure without circuit connection

Figure 13: Basic Handover Procedure without circuit connection
(⇒ copy of original 3GPP image)

7.3 Procedure for subsequent handover requiring a circuit connection p. 46

After the call has been handed over to MSC-B, if the MS leaves the area of MSC-B during the same call, subsequent handover is necessary in order to continue the connection.

The following cases apply:

the MS moves back to the area of MSC-A;
the MS moves into the area of a third MSC (MSC-B').

In both cases the call is switched in MSC-A; the circuit between MSC-A and MSC-B shall be released after a successful subsequent handover has been performed.

7.3.1 Description of subsequent handover procedure i): MSC-B to MSC-A p. 47

The procedure for successful handover from MSC-B back to MSC-A is shown in Figure 14.

Copy of original 3GPP image for 3GPP TS 23.009, Fig. 14: Subsequent handover procedure i):successful handover from MSC-B to MSC-A using a circuit connection

Figure 14: Subsequent handover procedure i):successful handover from MSC-B to MSC-A using a circuit connection
(⇒ copy of original 3GPP image)

The procedure is as follows.

MSC-B sends the MAP-PREPARE-SUBSEQUENT-HANDOVER request to MSC-A indicating the new MSC number(MSC-A number), indicating also the identity of the cell where the call has to be handed over and including a complete A-HO-REQUEST message. (NOTE: MSC-B shall not send further MAP-PREPARE-SUBSEQUENT-HANDOVER requests while a handover attempt is pending or before any timeouts). Since MSC-A is the call controlling MSC, this MSC needs no Handover Number for routing purposes; MSC-A can immediately initiate the search for a free radio channel.

When a radio channel can be assigned, MSC-A shall return in the MAP-PREPARE-SUBSEQUENT-HANDOVER response the complete A-HO-REQUEST-ACKNOWLEDGE message received from the BSS-B and possible extra BSSMAP information, amended by MSC-A due to the possible interworking between the BSSMAP protocol carried on the E-interface and the BSSMAP protocol used on the A-interface. If the traffic channel allocation is queued by BSS-B, the A-QUEUING-INDICATION may optionaly be sent back to MSC-B. The further traffic channel allocation result (A-HO-REQUEST-ACK or A-HO-FAILURE) will be transferred to MSC-B using the MAP-FORWARD-ACCESS-SIGNALLING request.If a radio channel cannot be assigned or if a fault is detected on the target cell identity, or the target cell identity in the A-HO-REQUEST is not consistent with the target MSC number, the MAP-PREPARE-SUBSEQUENT-HANDOVER response containing an A-HO-FAILURE message shall be given to MSC-B, in addition MSC-B shall maintain the connection with the MS.

If the procedure in MSC-A is successful then MSC-B can request the MS to retune to the new BSS-B on MSC-A. This is illustrated in Figure 14 by the A-HO-COMMAND message. The operation is successfully completed when MSC-A receives the A-HO-COMPLETE message.

After handover MSC-A shall release the circuit to MSC-B.

MSC-A must also terminate the MAP procedure for the basic handover between MSC-A and MSC-B by sending an appropriate MAP message. MSC-B will clear the resources in BSS-A when the MAP-SEND-END-SIGNAL response is received.

7.3.2 Description of the subsequent handover procedure ii): MSC-B to MSC-B' p. 48

The procedure for successful handover from MSC-B to MSC-B' is shown in Figure 15.

The procedure consists of two parts:

a subsequent handover from MSC-B back to MSC-A as described in subclause 7.3.1 (the same procedures apply if MSC-A is replaced by 3G_MSC-A); and
a basic handover from MSC-A to MSC-B' as described in subclause 7.1.

MSC-B sends the MAP-PREPARE-SUBSEQUENT-HANDOVER request to MSC-A indicating a new MSC number (which is the identity of MSC-B'), indicating also the target cell identity and including a complete A-HO-REQUEST, MSC-A then starts a basic handover procedure towards MSC-B'.

If MSC-A supports A interface over IP, then for speech calls MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request towards MSC-B'. For a detailed description of the handling of this codec list by MSC-A and MSC-B' see TS 23.153.

When MSC-A receives the ACM from MSC-B', MSC-A informs MSC-B that MSC-B' has successfully allocated the radio resources on BSS-B' side by sending the MAP-PREPARE-SUBSEQUENT-HANDOVER response containing the complete A-HO-REQUEST-ACKNOWLEDGE received from BSS-B' and possible extra BSSMAP information, amended by MSC-A due to the possible interworking between the BSSMAP protocol carried on the E-interface between MSC-A and MSC-B' and the BSSMAP protocol carried on the E-interface between MSC-A and MSC-B. Now MSC-B can start the procedure on the radio path.

For MSC-A the handover is completed when it has received the MAP-SEND-END-SIGNAL REQUEST from MSC-B' containing the A-HO-COMPLETE received from the BSS-B'. The circuit between MSC-A and MSC-B is released. MSC-A also sends the MAP-SEND-END-SIGNAL response to MSC-B in order to terminate the original MAP dialogue between MSC-A and MSC-B. MSC-B releases the radio resources when it receives this message.

If the traffic channel allocation is queued by the BSS-B', the A-QUEUING-INDICATION may optionally be sent back to MSC-B. If no radio channel can be allocated by MSC-B' or no circuit between MSC-A and MSC-B' can be established or a fault is detected on the target cell identity or the target cell identity in the A-HO-REQUEST is not consistent with the target MSC number, MSC-A informs MSC-B by using the A-HO-FAILURE message included in the MAP-PREPARE-SUBSEQUENT-HANDOVER response. MSC-B shall maintain the existing connection with the MS.

When the subsequent handover is completed, MSC-B' is considered as MSC-B. Any further inter-MSC handover is handled as described above for a subsequent handover.

Copy of original 3GPP image for 3GPP TS 23.009, Fig. 15: Subsequent handover procedure ii): Successful handover from MSC-B to MSC-B'requiring a circuit connection

Figure 15: Subsequent handover procedure ii): Successful handover from MSC-B to MSC-B'requiring a circuit connection
(⇒ copy of original 3GPP image)

7.4 Procedure for subsequent handover not requiring a circuit connection p. 49

As for the subsequent handover with a circuit connection, the same two cases of subsequent handover apply:

the MS moves back to the area of MSC-A;
the MS moves into the area of a third MSC (MSC-B').

7.4.1 Description of the subsequent handover procedure without circuit connection i): MSC-B to MSC-A p. 50

The procedure for successful handover from MSC-B back to MSC-A without circuit connection is shown in Figure 16. The only difference with the Figure 14, is that no circuit release is needed between MSC-A and MSC-B.

Copy of original 3GPP image for 3GPP TS 23.009, Fig. 16: Subsequent handover procedure i): Successful handover from MSC-B to MSC-A not requiring a circuit connection

Figure 16: Subsequent handover procedure i): Successful handover from MSC-B to MSC-A not requiring a circuit connection
(⇒ copy of original 3GPP image)

7.4.2 Description of the subsequent handover procedure without circuit connection ii): MSC-B to MSC-B' p. 50

The procedure for successful handover from MSC-B to MSC-B' is shown in Figure 17.

The procedure consists of two parts:

a subsequent handover from MSC-B back to MSC-A as described in subclause 7.4.1 (the same procedures apply if MSC-A is replaced by 3G_MSC-A); and
a basic handover from MSC-A to MSC-B' as described in subclause 7.2.

The only difference to the equivalent Figure 15 is the omission of the circuit and handover number allocation signallings.

Copy of original 3GPP image for 3GPP TS 23.009, Fig. 17: Subsequent handover procedure ii): Successful handover from MSC-B to MSC-B' without circuit connection

Figure 17: Subsequent handover procedure ii): Successful handover from MSC-B to MSC-B' without circuit connection
(⇒ copy of original 3GPP image)