5 Optimal routeing for basic mobile-to-mobile calls: message flows

03.793GPPStage 2Support of Optimal Routeing phase 1TS

It is a network operator option whether to implement optimal routeing for basic mobile-to-mobile calls.

This clause does not consider the handling of calls to a fixed network B subscriber.

The message flow for an optimally routed call from one mobile subscriber to another mobile subscriber is shown in figure 3. For simplicity of description, it is assumed that forwarding of calls from the B subscriber is not required. Solid lines indicate circuit-associated signalling; dashed lines indicate connectionless signalling.

Figure 3: Message flow for optimal routeing of basic mobile-to-mobile call

5.1 Successful outcome

When VMSCA receives a Setup message from the MS, it sends a request for information to handle the outgoing call to VLRA, according to the procedures described in GSM 03.18 [6]. If VLRA determines that the MS is allowed service, it returns a positive acknowledgement. When VMSCA receives the acknowledgement, it constructs an IAM using the B subscriber address and sends it to GMSCA.

If GMSCA recognises the B subscriber address as belonging to a GSM PLMN (decision OR1:Y), it checks the identity of HPLMNB. If GMSCA is in a different PLMN from HLRB, it then sends a request for routeing information (SRI(B)) to HLRB; this request contains an indication that it is an optimal routeing enquiry for information to route a basic call. If HLRB is prepared to accept an optimal routeing enquiry from GMSCA (decision OR2:Y), it checks whether at least one of the three conditions:

– the GMSC is in the same country as VMSCB;

– the HLR is in the same country as VMSCB;

– the GMSC is in the same PLMN as the HLR.

is met. If it is (decision OR3:Y), HLRB sends a request for a roaming number (PRN) to VLRB; the request contains an indication that it is for an optimally routed call. If VLRB supports optimal routeing (decision OR4:Y), it returns the roaming number in the PRN ack, and HLRB relays the roaming number in the SRI ack to GMSCA. GMSCA constructs an ISUP IAM using the roaming number, and sends it to VMSCB, which processes the incoming IAM according to the procedures described in GSM 03.18 [6].

5.2 Unsuccessful outcome

Error situations which lead to failure of the call, rather than non-optimal routeing, are not described in this subclause.

5.2.1 B subscriber address not recognised as belonging to a GSM PLMN

If GMSCA does not support optimal routeing for basic mobile-to-mobile calls, or does not recognise the B subscriber address as belonging to a GSM PLMN (decision OR1:N), it constructs an IAM using the B subscriber address and sends it to GMSCB in HPLMNB. GMSCB analyses the address received in the IAM, and sends a request for routeing information (SRI(B)) to HLRB; this request contains an indication that it is not an optimal routeing enquiry. Because GMSCB is in the same PLMN as HLRB, it will always be able to derive an HLR address. HLRB sends a request for a roaming number (PRN) to VLRB. VLRB returns the roaming number in the PRN ack, and HLRB relays the roaming number in the SRI ack to GMSCB. GMSCB constructs an ISUP IAM using the roaming number, and sends it to VMSCB, which processes the incoming IAM according to the procedures described in GSM 03.18 [6].

5.2.2 HLRB or VLRB does not support optimal routeing

If HLRB is not prepared to accept an optimal routeing enquiry from GMSCA , because:

– it does not support optimal routeing for basic mobile-to-mobile calls, or

– because there is no agreement for optimal routeing for basic mobile-to-mobile calls between the operators of GMSCA and HLRB, or

– because optimal routeing of basic mobile-to-mobile calls to the specific B subscriber is not allowed

(decision OR2:N), it returns an SRI negative response (shown in figure 3 as ‘SRI error’). This causes GMSCA to construct an IAM using the B subscriber address and send it to GMSCB, as described in subclause 5.2.1.

If VLRB does not support optimal routeing (decision OR4:N), it returns a PRN negative response (shown in figure 3 as ‘PRN error’). This causes HLRB to return an SRI negative response (shown in figure 3 as ‘SRI error’), which in turn causes GMSCA to construct an IAM using the B subscriber address and send it to GMSCB, as described in subclause 5.2.1.

5.2.3 OR charging requirements contravened

If HLRB determines that the call cannot be routed directly from GMSCA to VMSCB without contravening the charging requirements for optimal routeing given in subclause 9.1 (decision OR3:N) it sends a request for subscriber information (PSI) to VLRB. VLRB sends a response indicating whether the B subscriber is detached or in some other state. If the B subscriber is not detached, HLRB sends an SRI negative response (shown in figure 3 as ‘SRI error’) to GMSCA, which constructs an IAM using the B subscriber address and sends it to GMSCB, as described in subclause 5.2.1.