23.0093GPPHandover proceduresRelease 17TS
9.1 BSS/MSC and MS/MSC procedures in MSC‑A (functional unit 1)
The handover procedures in this functional unit consist of:
i) signalling between the MS and the MSC;
ii) signalling between the BSS and the MSC for access management.
9.2 Call control procedures MSC‑A (functional unit 2)
The call control procedures related to handover in MSC‑A can be divided into two functional entities:
– the first entity is the call control procedure as part of the normal interworking between the PSTN/ISDN and the PLMN; for an MS originating call MSC‑A is the originating exchange, for an MS terminating call MSC‑A is the destination exchange;
– the second entity is the call control procedure for the connection between MSC‑A and MSC‑B in case of a handover from MSC‑A to MSC‑B. For this call control procedure the following applies.
– the connection to MSC‑B is set up by procedures relevant to the signalling system used in the PSTN/ISDN to which MSC‑A is connected. The call is set up by using the MS Handover Number received from MSC‑B as part of the MAP procedure;
– the call set-up direction will always be from MSC‑A to MSC‑B, even when the call was originally established by the MS. Functional unit 2 (see figure 2) should therefore keep information on call set-up direction in order to be able to interpret correctly any clearing signals (see below);
– the unit should indicate the address complete condition to functional unit 3 and through-connect without awaiting the answer signal from MSC‑B. This applies also to signalling systems where address complete signals are not supported. In such cases an artificial address complete is established by functional unit 2.
– call clearing consists of two parts: after inter-MSC handover, clearing of the MS-BSS connection and clearing of the inter-MSC connection. 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‑A shall begin clearing the call to the network and queue the call release to the MS until the MS has resumed communication. This includes the case when MSC‑B and/or MSC‑B’ are involved;
– the MAP procedures are used to transfer information between MSC‑B and MSC‑A in order to maintain full call control within MSC‑A. MSC‑A determines, based on information received from MSC‑B, the appropriate signals (according to 3GPP TS 24.008 ) to be sent to the MS, and sends this information to MSC‑B;
– when MSC‑A clears the call to the MS it also clears the call control functions in MSC‑B and sends the MAP-SEND-END-SIGNAL response to release the MAP resources in MSC‑B. The clearing of the connection is by procedures relevant to the signalling system in the PSTN/ISDN to which MSC‑A is connected;
– when the Signalling System no 7 ISDN User Part is used, the normal symmetric release procedures apply on both the connection to the fixed network and to MSC‑B;
– when a signalling system is used without a symmetric release possibility, some notice should be given to the clear-forward and clear-back procedures;
– for MS terminating calls the following conditions apply on clear-forward and clear-back:
– when a clear-forward signal is received on interface B’ (see figure 1), MSC‑A clears the circuit to MSC‑B by normal clear-forward procedures;
– when a clear-back signal is received from MSC‑B, MSC‑A starts normal clear-back procedures towards the fixed network (interface B’) and sends the clear-forward signal on interface B” in order to clear the connection with MSC‑B.
NOTE 1: This case corresponds to a fault situation.
– for MS originated calls the following applies:
– when MSC‑A receives a clear-back signal from MSC‑B, this signal must be interpreted as indicating a clear-forward condition. MSC‑A then clears both the connection on interface B’ (see figure 1) and to MSC‑B by normal clear-forward procedures.
NOTE 2: This case corresponds to a fault situation.
– when MSC‑A receives a clear-back signal on interface B’, MSC‑A should distinguish between national and international connections:
– for international connections where the Q.118  supervision is done in the ISC, MSC‑A sends a clear-forward signal on both interface B’ to the fixed network and interface B” to MSC‑B;
– for national connections or for international connections where the Q.118  supervision is not done in the ISC, a timer is started according to national practice for clear-back supervision and MSC‑A proceeds as follows:
i) if a clear-back signal is received from MSC‑B, MSC‑A interprets this as indicating a clear-forward condition and proceeds by clearing the connections on interface B’ and to MSC‑B by normal clear-forward procedures;
ii) if the timer expires, MSC‑A proceeds by normal clear-forward of the connections on interface B’ and to MSC‑B.
9.3 Handover control procedures MSC‑A (functional unit 3)
The procedures of functional unit 3 are given in terms of SDL diagrams in figure 41. 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 procedures of functional unit 3 include:
i) initiation. The initiation condition is shown by the signal A-HANDOVER-REQUIRED.
The diagram also includes queuing when there is no channel available. Calls for which handover has been initiated should be queued with priority higher than normal calls. They should have lower priority than emergency calls.
ii) handover of calls within the area of MSC‑A, i.e. handover case i). In this case MSC‑A controls the procedures on both the previous and the new radio channel, using signals A-HANDOVER-REQUEST and A-HANDOVER-COMMAND. The handover procedure is completed when A-HANDOVER-COMPLETE is received. If this signal is not received (expiry of timer T102), the radio path and the connection on interface B’ are released.
In the case of ongoing GSM voice group calls for subsequent users of the VGCS channel uplink the original connection shall always be maintained.
For handover devices with three-party capabilities the handover device is first set up so that all interfaces A’, A” and B’ are connected (illustrated by the signal ‘set up handover device’). This is done when the Handover Command is sent to the MS . The device is connected in its final position (i.e. A” to B’ for case ii)) (illustrated by the signal ‘connect handover device’) when A-HANDOVER-COMPLETE is received.
iii) handover to MSC‑B . This procedure is the one described in clauses 7.1 and 7.2. For handover devices with three-party capabilities the handover device is set-up when MSC‑A sends the Handover Command to the MS , i.e. the interfaces A’, B’ and B” are then connected. The device is connected in its final position (i.e. B’ to B”) when the successful procedure indication is received from functional unit 4.
iv) subsequent handover to MSC‑A . The procedure is described in clauses 7.3 and 7.4. When a handover to MSC‑A indication is received from functional unit 4, the handover device is set up so that interfaces B’, B” and A’ are connected (for handover devices with three-party capabilities). When A-HANDOVER-COMPLETE is received, the device is connected in its final position (i.e. B’ to A’).
If A-HANDOVER-COMPLETE is not received (expiry of timer T104), the handover device releases interface A’, B’ and B”.
v) subsequent handover to a third MSC (MSC‑B’) . The procedure is described in clauses 7.3 and 7.4. The handover device is set up in its initial position, (i.e. interconnection of interfaces B’, B” and B”’) when the connection to MSC‑B’ has been established. MSC‑B is informed via functional unit 4 that the connection has been established and that the procedure on the radio path can be initiated. The device is connected in its final position (i.e. B’ to B”’) when a successful procedure indication is received from functional unit 4. MSC‑B is informed that all procedures in MSC‑B can be terminated (illustrated by the MAP-SEND-END-SIGNAL response). The device returns to the state where B’ and B” are connected if the subsequent handover procedure fails.
Timers in MSC‑A.
The 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:
T101: this timer supervises the queuing time for a free channel. If T101 expires, a no channel indication is generated, a retry procedure could be applied as described in clause 6.1. T101 is set by O&M,
T102: this timer supervises the time for handover completion for handover between BSSs in MSC‑A. T102 is set by O&M,
T103: this timer supervises the time between issuing an A-HANDOVER-COMMAND from MSC‑A and receiving a successful procedure indication from MSC‑B. This timer also supervises the time between sending an A-HO-REQUEST-ACKNOWLEDGE to MSC‑B and receiving a successful procedure indication from MSC‑B’. If T103 expires, the handover procedure is terminated. T103 is set by O&M,
T104: this timer supervises the time between sending of an A-HO-REQUEST-ACKNOWLEDGE to MSC‑B and receiving the A-HANDOVER-COMPLETE from BSS-B on MSC‑A. If the timer expires, the new radio channel is released. T104 is set by O&M.
9.3A BSS Internal Handover with MSC Support control procedures
The "BSS Internal Handover with MSC Support" for AoIP is performed by the MSC that is currently serving the connected BSS (in the following just termed "serving MSC"), it may be either MSC-A, MSC-B, 3G_MSC-A or 3G_MSC-B.
The "BSS Internal Handover with MSC Support" control procedures in serving MSC include:
i) Handover enquiry. This procedure is only part of the MSC-initiated "BSS Internal Handover with MSC Support" described in clause 6.3.3. The MSC initiates the handover enquiry by sending an A-INTERNAL-HANDOVER-ENQUIRY message and starting timer T106.
The handover enquiry phase is completed when an A-INTERNAL-HANDOVER-REQUIRED message is received from the BSS with cause code "response to an INTERNAL HANDOVER ENQUIRY message". If this message is not received (expiry of timer T106), or the BSS responds with an A‑HANDOVER-FAILURE message, or the BSS sends an A-INTERNAL-HANDOVER-REQUIRED message with another cause code, then the MSC terminates the MSC-initiated "BSS Internal Handover with MSC Support".
ii) Initiation. The initiation condition is given by reception of the A-INTERNAL-HANDOVER-REQUIRED message. This starts the Internal Handover Preparation phase for the serving MSC; the serving MSC starts timer T105. Calls for which Internal Handover Preparation has been initiated should be handled with priority higher than normal calls. They should have lower priority than emergency calls. During that phase the serving MSC considers the A-INTERNAL-HANDOVER-REQUIRED parameters, tries to allocate the necessary resources.
The Internal Handover Preparation phase for the serving MSC ends when the serving MSC sends the A-INTERNAL-HANDOVER-COMMAND message or an A-INTERNAL-HANDOVER-REQUIRED-REJECT message or when timer T105 expires.
If the serving MSC can not perform the "BSS Internal Handover with MSC Support", then it shall send an A-INTERNAL-HANDOVER-REQUIRED-REJECT Message to the BSS and shall release all potentially allocated resources as if no A-INTERNAL-HANDOVER-REQUIRED message was received.
If timer T105 expires before the serving MSC could send the A-INTERNAL HANDOVER-COMMAND message, then the serving MSC shall consider the Internal Handover Preparation phase as terminated without success and shall release any allocated resources for the Internal Handover such that the status returns as it was prior to receiving the A-INTERNAL-HANDOVER-REQUIRED message. No response shall be sent to the BSS after the expiry of timer T105.
ii) Execution. Serving MSC controls the "BSS Internal Handover with MSC Support" by sending the A-INTERNAL-HANDOVER-COMMAND message. The "BSS Internal Handover with MSC Support" is completed when the A-HANDOVER-COMPLETE message is received. If this signal is not received (expiry of timer T102), the radio path and all the connections and resources associated to that call shall be released.
For handover devices with three-party capabilities, the handover device is first set up so that all interfaces A’, A” and B’ are connected. This is perfomed before the A-INTERNAL-HANDOVER-COMMAND message is sent to the BSS. The handover device may be adjusted when the A-HANDOVER-DETECT message is received. The handover device is connected in its final position (i.e. A” to B’) when the A-HANDOVER-COMPLETE message is received.
Timers in serving MSC for Internal Handover Preparation
The procedures are supervised by timers in order to avoid a deadlock when responses are not received or the procedures fail. The following additional timers are defined:
T105: this timer supervises the Internal Handover Preparation procedure between BSS and serving MSC. T105 is set by O&M in relation to timer "T25" (3GPP TS 48.008 ). T105 defines the maximum time a serving MSC may take to respond to an "INTERNAL HANDOVER REQUIRED" message. Timer "T25" (3GPP TS 48.008 ) defines the minimum time the BSS will to wait before it can send a new or repeated (INTERNAL) HANDOVER REQUIRED message or an A-HANDOVER FAILURE. T105 shall be configured to be atleast one round trip delay shorter than the time configured for "T25" (3GPP TS 48.008 ) to minimise the risk of crossing messages.
T106: this timer supervises the time between sending of an A-INTERNAL-HANDOVER-ENQUIRY message to the BSS and receiving an A-INTERNAL-HANDOVER-REQUIRED or A‑HANDOVER-FAILURE message from the BSS. If T106 expires, the handover procedure is terminated. T106 is set by O&M and should be sufficiently long so that no late responses from BSS can be expected after its expiry.
9.4 MAP procedures in MSC‑A (functional unit 4)
The MAP procedures for handover are defined in 3GPP TS 29.002 . They include:
– procedures for basic handover;
– procedures for subsequent handover.
These procedures are as outlined in clause 7.
9.5 Interworking between Handover control procedures and MAP procedures in MSC‑A
The interworking between the Handover control procedures and the MAP procedures for handover is defined in 3GPP TS 29.010 . It includes:
– interworking at basic handover initiation;
– interworking at subsequent handover completion.
This interworking is not described in the present document.
9.6 Compatibility with GSM Phase 1
If the MSC‑A initiates an Inter-MSC handover procedure according to Phase 2 MAP and BSSMAP protocols while using a Phase 1 BSSMAP protocol towards BSS-A, MSC‑A has to perform the protocol interworking.
The same holds if a Phase 2 BSSMAP protocol is used between MSC‑A and BSS-A and the E‑interface supports only Phase 1 protocol.