5.2 Information flow for retrieval of routeing information for an MT call

23.0183GPPBasic call handlingRelease 17Technical realizationTS

The information flow for retrieval of routeing information for an MT call is shown in figure 4. ISUP signalling between the originating exchange and GMSCB, and between GMSCB and VMSCB is shown by solid lines; signalling over the MAP interfaces between GMSCB and HLRB and between HLRB and VLRB, and over the B interface between VLRB and VMSCB is shown by chain lines; signalling over the Iu interface (for UMTS) or the A interface (for GSM) between VMSCB and BSSB is shown by dashed lines; and signalling over the radio interface between BSSB and MSB is shown by dotted lines.

NOTE 1: If pre-paging is used, paging is initiated after VLRB has accepted the PRN message. The paging procedure is described in clause 5.3.

NOTE 2: VMSCB starts the timer for the release of radio resources after it sends the Process Access Request message to VLRB. VMSCB releases the radio resource allocated for the MT call if the timer expires before the IAM is received, and when the MAP RELEASE_RESOURCES message is received from the GMSC.

NOTE 3: If an ISUP REL message is received at the GMSC between sending of SRI and receiving of SRI ack, the GMSC does not send IAM to the VMSC. Instead a MAP Release_Resources message may be sent to the VMSC.

Figure 4: Information flow for retrieval of routeing information for a basic mobile terminated call

When GMSCB receives an IAM, it analyses the called party address. If GMSCB can derive an HLR address from the B party address, it sends a request for routeing information (SRI) to HLRB. If GMSCB supports pre-paging (i.e. it is prepared to wait long enough for the SRI ack to allow pre-paging to be completed), it indicates this by an information element in the SRI message.

HLRB decides whether pre-paging is supported according to the following criteria:

– GMSCB has indicated that it supports pre-paging; and

– HLRB supports pre-paging (i.e. it is prepared to wait long enough for the PRN ack to allow pre-paging to be completed).

HLRB sends a request for a roaming number (PRN) to VLRB; if pre-paging is supported, it indicates this by an information element in the PRN message. If Paging Area function is supported in HLRB then HLRB sends the paging area if stored in HLR. VLRB returns the roaming number in the PRN ack, and HLRB relays the roaming number to GMSCB in the SRI ack. GMSCB constructs an IAM using the roaming number, and sends it to VMSCB.

If the GMSC performs domain selection through HLR interrogation and the HLR supports domain selection functionality, HLRB executes domain selection functionaility. The HLR shall:

– send PRN to VLRB as defined in this section , if the result of domain selection is to handle the call in CS domain; or

– reply with SRI ack without sending PRN to VLRB, if the result of domain selection is to transfer the call from CS domain to IMS domain.