10 Detailed procedures in MSC‑B

23.0093GPPHandover proceduresRelease 17TS

10.1 BSS/MSC (MS/BSS) procedures MSC‑B (functional unit 1)

The handover procedures in this functional unit consist of:

i) signalling between the MS and the MSC;

ii) signalling between the BS and the MSC for access management.

Signals exchanged with functional unit 3 are indicated in clause 10.3.

10.2 Call control procedures MSC‑B (functional unit 2)

These procedures relate to the call control in MSC‑B of the "handover" connection with MSC‑A. For these procedures the following apply:

Call set-up:

– the connection is set up by MSC‑A. MSC‑B should provide, if possible, the following backward signals:

– signals indicating unsuccessful call set-up and, if possible, the cause of call failure;

– address complete signal;

– answer signal (see note).

NOTE: The answer signal is not related to answering by the MS and it has no meaning in the handover procedure between MSC‑A and MSC‑B. But after successful handover or successful subsequent channel assignment using a circuit connection between MSC‑A and MSC‑B this signal is needed for bringing the connection in the answered state in the intermediate PSTN/ISDN exchanges.

– there will be no indication that the call applies to a handover. This information has to be derived from the MS Handover Number received during call set-up in relation to the earlier MAP-PREPARE-HANDOVER request/MAP-PREPARE-HANDOVER response procedure between MSC‑A and MSC‑B.

Call clearing:

– call clearing consists of two parts after inter-MSC handover: clearing of the BSS-MS connection and clearing of the inter-MSC connection, this case is only applicable to calls successfully handed over. If a request to release the call is generated by the network while the MS is re-tuning from one BSS to another BSS, then MSC‑B shall begin clearing the call to the network and queue the call release to the MS until the MS has resumed communication;

– the MAP is used to transfer information between MSC‑A and MSC‑B in order to make it possible for MSC‑B to send the appropriate signals to the MS, specified in 3GPP TS 24.008 [10], and still leave the call control to MSC‑A. MSC‑A normally initiates release of the connection between MSC‑A and MSC‑B. Exceptionally MSC‑B is allowed to release the connection if no MAP-SEND-END-SIGNAL response is received, or if the Handover is to be aborted.

– when the Signalling System no 7 ISDN User Part is used, the normal symmetric release procedures apply. When a signalling system is used without a symmetric release possibility or a fault condition occurs, the following may apply:

– when MSC‑B receives a clear-forward signal from MSC‑A, it shall release the radio resources;

– in fault situation eg. machine malfunction or loss of the connection on interface A, MSC‑B may send a clear-back signal to MSC‑A.

10.3 Handover control procedures MSC‑B (functional unit 3)

The procedures of functional unit 3 are given in form of SDL diagrams in figure 42. To easily distinguish the interface concerned the messages received or sent from this unit are prefixed with either ‘MAP’ for a MAP message, ‘A’ for an A‑Interface message or ‘I’ for an ISDN/PSTN message. The procedure in functional unit 3 include:

i) handover from MSC‑A.

This case is initiated by MSC‑A, and includes allocation and establishment of the new radio channel. The procedure is outlined in clauses 7.1 and 7.2.

ii) intra-MSC handovers within the area controlled by MSC‑B.

This procedure is the same as that of i) in clause 9.3, except that the A-HANDOVER-REQUIRED is received by MSC‑B. After successful completion of the intra-MSC handover, MSC-B shall notify MSC-A by sending an A-HANDOVER-PERFORMED message.

iii) subsequent handover to another MSC (MSC‑A or MSC‑B’).

The initiation procedure is essentially the same as that of i) of clause 9.3. The Handover Command to the MS is now generated by MSC‑B after the A-HO-REQUEST-ACKNOWLEDGE is received from MSC‑A (via functional unit 4). The procedure is terminated in MSC‑B when MSC‑B receives a terminate procedure indication from functional unit 4.

Timers in MSC‑B.

The following procedures are supervised by timers in order to avoid a deadlock when responses are not received or the procedures fail.

The following timers are defined:

T201: this timer supervises the queuing time for a free channel. T201 is set by O&M;

T202: this timer supervises the time for handover completion for handover between BSSs in MSC‑B. If T202 expires, the radio path and the connection on interface B’ are released. T202 is set by O&M;

T204: this timer supervises the time between sending of address complete message to MSC‑A and receiving the A-HANDOVER-COMPLETE from BSS-B on MSC‑B. This timer also supervises the time between issuing the handover command to the MS and receiving the MAP-SEND-END-SIGNAL response from MSC‑A, for a subsequent handover. In the case of a handover without circuit connection between MSC‑A and MSC‑B this timer supervises the time between issuing the A-HO-REQUEST-ACKNOWLEDGE to the MSC‑A and receiving the A-HANDOVER-COMPLETE from BSS-B on MSC‑B. If the timer expires, then any new radio channel is released. T204 is set by O&M;

T210: this timer is used to supervise the time for establishing a circuit connection from MSC‑A to MSC‑B. When T210 expires, the allocated channel in MSC‑B is released. T210 is set by O&M. This timer is not started when MSC‑A explicitly indicates that no handover number is needed;

T211: this timer is used to control the time between requesting a subsequent handover (A-HO-REQUEST to the MSC‑A) and receiving the response from MSC‑A (A-REQUEST-ACKNOWLEDGE/A-HO-FAILURE). If T211 expires, the existing connection with the MS is maintained. T211 is set by O&M.

10.4 MAP procedures MSC‑B (functional unit 4)

The MAP procedures for handover are defined in 3GPP TS 29.002 [12]. They include:

– procedures for basic handover;

– procedures for subsequent handover;

– procedures for obtaining the handover number from the VLR.

These procedures are outlined in clause 7.

10.5 Interworking between Handover control procedures and MAP procedures in MSC‑B

The interworking between the Handover control procedures and the MAP procedures for handover is defined in 3GPP TS 29.010 [8]. It includes:

– interworking at basic handover completion;

– interworking at subsequent handover initiation.

This interworking is not described in the present document.

10.6 Compatibility with GSM Phase 1

If the MSC‑B accepts an Inter-MSC handover procedure according to Phase 2 MAP and BSSMAP protocols while using a Phase 1 BSSMAP protocol towards BSS-B, MSC‑B has to perform the protocol interworking.

The same holds if a Phase 1 MAP protocol is requested on the E-interface and MSC‑B uses a Phase 2 BSSMAP protocol towards BSS-B.