5 Procedures

03.543GPPDescription for the use of a Shared Inter Working Function (SIWF) in a GSM PLMNStage 2TS

5.1 General

A visited MSC uses its capabilities to direct data/fax calls to a SIWFS. This is applicable for both mobile originating and mobile terminating data calls. A loop method is used, where the data traffic is redirected from the visited MSC to the SIWFS, adapted, and then re-routed back to the visited MSC for routeing to the final destination.

Mobile originated calls can also use a non-loop method i.e. the calls can be routed directly from the SIWFS to the final destination.

Transit traffic through an SIWFS to another SIWFS is not allowed.

The call flows in chapter 5 are all examples based on ISUP/MAP signalling. The general principle by conveying all GSM-BC, converted ISDN BC (from GSM-BC), Call Direction (CD) and associated information like HLC and LLC (not showed in the call flows) by MAP procedures from the visited MSC to the SIWFS, result in that also less intelligent signalling protocol than ISUP can be used between the visited MSC and the SIWFS e. g. TUP. The general principle result in that ISUP (or TUP) is simple used for the handling of the required PCM channels between the visited MSC and the SIWFS.

5.2 Mobile Originated calls

This subclause describes the procedure used to establish a connection to the SIWFS for a Mobile Originated call.

Mobile originated calls shall follow the normal call set-up procedures. No additional manipulation at the MS is needed.

If the visited MSC decides to use an SIWF it uses its SIWFC to route the call towards an SIWFS. This includes:

– selection of an appropriate SIWFS.

– sending a request to the SIWFS.

– waiting for an acknowledgement from the SIWFS.

– routeing the call to the SIWFS and allocating an ‘a’ -circuit to this entity.

The SIWFS can be accessed after the negotiation of the GSM BC parameters and the allocation of radio resources in the visited MSC. The visited MSC decides the selection of a SIWFS after the ASSIGNMENT COMPLETE has been received. The selection of an appropriate SIWFS or MSC/IWF is based

– on the relevant parameters in the signalling messages (SETUP, CALL PROCEED and ASSIGNMENT COMPLETE),

– on status information from IWF and SIWFS (e.g. IWU busy or congested, IWU out of order, etc.) together with normal route supervision and

– on further criteria’s that can be determined by the operator (e.g. conditions for routeing).

The visited MSC allocate a temporary V-MSC address (V-MSC-Addr) similar to MSRN (E.164 number). This address will be used by the visited MSC to correlate the incoming IAM messages from SIWFS. The visited MSC sends a request to the selected SIWFS by means of a MAP_Provide_SIWFS_Number procedure. The MAP_Provide_SIWFS_Number procedure contains all information needed for the SIWFS to evaluate the requirement and to select and to seizure an appropriate IWU. In particular, these information are

– V-MSC-Addr/B-SubscrAddr: The address (an E.164 number) where the SIWFS has to route the call. In the case of the loop method it is the address of the visited MSC and in the case of the non-loop method it is the address of the B-subscriber.

– GSM-BC’: The GSM-BC negotiated with the MS.

– ISDN-BCn: The ISDN-BC mapped from the GSM-BC’. It is used by the SIWFS to route the call and to allocate an ‘n’-circuit (loop method) or a traffic channel towards the fixed network (non-loop method).

– CD: Call Direction to indicate the direction at call set-up (MOC/MTC)

– Chosen Channel: The Chosen Channel specifies the assigned radio channel.

The visited MSC waits for the acknowledgement MAP_Provide_SIWFS_Number_Ack.

The SIWFS assigns and returns a temporary SIWFS address (similar to MSRN), an E.164 number. This address will be used by the visited MSC to route the call, i.e. the IAM to the SIWFS and it will be used by the SIWFS to correlate the incoming IAM to the corresponding MAP message MAP_Provide_SIWFS_Number (i.e. Call Reference).

The SIWFS sends the acknowledgement MAP_Provide_SIWFS_Number_Ack back to the visited MSC.

If the acknowledgement is negative (possibly indicated via the MAP procedure), the visited MSC may release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

If the acknowledgement is positive, the visited MSC will set-up an ‘a’ circuit towards the SIWFS. For that it uses an appropriate ISDN-BCa and the transmission medium requirements set to „64 kbit/s unrestricted".

If no valid response is received in the ISUP-signalling the visited MSC may select an alternative route to the same SIWFS, release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

Normal ISUP signalling follows.

The SIWFS routes the call to the address that it has got from the visited MSC. In the case of the loop method it is the address of the visited MSC and in the case of the non-loop method it is the address of the B-subscriber. These addresses are conveyed as parameter in the message MAP_Provide_SIWFS_Number. So, the SIWFS routes the call to the visited MSC and allocates an ‘n’-circuit towards the visited MSC in the case of the loop method or it routes the call directly towards the fixed network. If the call routeing is not possible it releases the call by ISUP means.

Hence, the selection of the loop or non-loop method

– is transparent for the SIWFS and

– is controlled by the visited MSC by providing the appropriate address to the SIWFS.

In case the non loop method is used for dedicated PAD or dedicated Packet Access (BS4x or BS5x) calls, the call proceeds in accordance with GSM 09.05 or GSM 09.06. Acknowledgement is sent to the visited MSC. Acknowledgement and Answer for the call may have to be generated by the SIWFS.

There is one restriction:

– Dual services are only supported together with the loop method.

Normal ISUP signalling follows. When the SIWFS receives the ISUP Answer message it activates the IWU.

In the case of a dual service the SIWFS will be selected and activated either if the first GSM-BC does not indicate speech or if in speech position and an in-call modification is ordered by the MS.

5.2.1 Call flow MOC (Loop Method)

Figure 5: Call flow for MOC in the case of the loop method

5.2.2 Call flow for MOC (Non Loop Method)

Figure 6: Call flow for MOC in the case of the non-loop method

NOTE: In some PAD or PH interworking schemes (defined in GSM 09.05 and 09.06), the ISUP signals between SIWFS and ISDN are not used for Dedicated Services (Bearer Services 4x and 5x). In this case ACM and Answer have to be generated by SIWFS towards the visited MSC.

5.3 Mobile Terminated calls

This subclause describes the procedure used to establish a connection to the SIWFS for a Mobile Terminated call.

For MTC, the routeing is based either on associated information to call set-up (PSTN case) or contents in IAM. GMSC interrogates HLR for roaming number and HLR interrogate VLR due to the B-number. The VLR provides the MSRN in accordance with normal procedures. See GSM 09.07.

If the visited MSC decides to use an SIWF it uses its SIWFC to route the call towards an SIWFS. This includes:

– selection of an appropriate SIWFS;

– sending a request to the SIWFS;

– waiting for an acknowledgement from the SIWFS;

– routeing the call to the SIWFS and allocating an ‘n’ -circuit to this entity.

The SIWFS can be accessed after the negotiation of the GSM BC parameters and the allocation of radio resources in the visited MSC. The visited MSC decides the selection of a SIWFS after the ASSIGNMENT COMPLETE has been received. The usage of single or multinumbering scheme will not result in different handling of the Mobile Terminating call as the selection of the SIWFS only can be made after this procedure. The selection of an appropriate SIWFS or MSC/IWF is based

– on the relevant parameters in the signalling messages (IAM, CALL CONFIRMED and ASSIGNMENT COMPLETE);

– on status information from IWF and SIWFS together with normal route supervision.

The visited MSC allocate a temporary V-MSC address (V-MSC-Addr) similar to MSRN (E.164 number). This address will be used by the visited MSC to correlate the incomung IAM messages from SIWFS. The visited MSC sends a request to the selected SIWFS by means of a MAP_Provide_SIWFS_Number procedure. The MAP_Provide_SIWFS_Number procedure contains all information needed for the SIWFS to evaluate the requirement and to select and to seizure an appropriate IWU. In particular, these information are

– V-MSC-Addr: The address, an E.164 number, where the SIWFS has to route the call i.e. the address of the visited MSC.

– GSM-BC’: The GSM-BC negotiated with the MS.

– ISDN-BCn: The ISDN-BC received from the fixed network. The ISDN-BCa used to allocate an ‘a’-circuit to the visited MSC will be generated internally by using the ISDN-BCn.

– CD: Call Direction to indicate the direction at call set-up (MOC/MTC)

– Chosen Channel: The Chosen Channel specifies the assigned radio channel.

The visited MSC waits for the acknowledgement MAP_Provide_SIWFS_Number_Ack.

The SIWFS assigns and returns a temporary SIWFS address (similar to MSRN), an E.164 number. This address will be used by the visited MSC to route the call, i.e. the IAM to the SIWFS and it will be used by the SIWFS to correlate the incoming IAM to the corresponding MAP message – MAP_Provide_SIWFS_Number (i.e. Call Reference).

The SIWFS sends the acknowledgement MAP_Provide_SIWFS_Number_Ack back to the visited MSC.

If the acknowledgement is negative (possibly indicated via the MAP procedure), the visited MSC may release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

If the acknowledgement is positive, the visited MSC will set-up an ‘n’ circuit towards the SIWFS. For that it uses the ISDN-BCn received from the fixed network.

If no valid response is received in the ISUP-signalling the visited MSC may select an alternative route to the same SIWFS, release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

The SIWFS routes the call to the address that it has got from the visited MSC, i.e. the address of the visited MSC. This address is conveyed as parameter in the message MAP_Provide_SIWFS_Number. So, the SIWFS routes the call to the visited MSC and allocates an ‘a’-circuit towards the visited MSC. For that it uses an appropriate ISDN-BCa with the transmission medium requirements set to "64 kbit/s unrestricted". If the call routeing is not possible it releases the call by ISUP means.

Normal ISUP signalling follows.

Note that the signals ALERT and CONNECT are independant of the ISUP signalling between V-MSC and SIWFS. In the case that these signals are received from the MS before IAM is received from SIWFS, the V-MSC withholds the signals until IAM from SIWFS is received before sending ANSWER to SIWFS.

When the SIWFS receives the ISUP Answer message it activates the IWU.

In the case of a dual service, the SIWFS will be selected and activated either if the first GSM-BC does not indicate speech or if in speech position and an in-call modification is ordered by the MS.

5.3.1 Call flow for MTC (Loop Method)

Figure 7: Call flow for MTC in the case of the loop method

5.3.2 Call Flow for MTC (Non-Loop Method)

The non-loop method is not defined for mobile terminated calls.

5.4 In Call Modification (ICM)

This subclause describes the procedure used to handle In Call Modification (ICM). ICM is applicable for Alternate Speech/fax (TS61), Alternate Speech/data (BS61) and Speech followed by data (BS81). In general the loop between the visited MSC and the SIWFS is only established when a dual service is in data/fax mode.

In the case when no resources are available at an ICM, the call will proceed in its existing mode, e.g. if no IWF resources are idle when going from speech to fax, the call will proceed in speech mode.

5.4.1 Call in Data/Fax Mode

The visited MSC that receives a MODIFY (GSM-BC, RCD) for a dual service in data/fax mode and accepts the ICM request, results in that the SIWFS connection is by-passed i.e.

– the loop to SIWFS is released;

– the MAP Dialogue is closed.

This procedure is independent whether the call originally has been established as MOC or MTC. The parameter RCD (Reverse Call Direction) is normally transformed according to figure 8. For release it may be omitted.

When receiving the next ICM (return-to-data ICM) the visited MSC re-connect the SIWFS connection in accordance with subclause 5.4.2. This SIWFS connection may not be the same as the previous one.

5.4.2 Call in Speech Mode

The visited MSC that receives a MODIFY (GSM-BC, RCD) for a dual service in Speech mode and accepts the ICM request, and which decides to use an SIWF, results in that a loop to the SIWFS is established. The procedure is similar to Mobile Originated Call (Subclause 5.2) and Mobile Terminated Call (Subclause 5.3). This includes:

– selection of an appropriate SIWFS;

– sending a request to the SIWFS;

– waiting for an acknowledgement from the SIWFS;

– routeing the call to the SIWFS and allocating an ‘a’ or ‘n’ -circuit to this entity.

The visited MSC decides the selection of a SIWFS after the ASSIGNMENT COMPLETE has been received. The selection of an appropriate SIWFS or MSC/IWF is based

– on the relevant parameters in the signalling messages (MODIFY and ASSIGNMENT COMPLETE);

– on status information from IWF and SIWFS together with normal route supervision;

– on further criteria’s that can be determined by the operator (e.g. conditions for routeing).

The visited MSC allocate a temporary V-MSC address (V-MSC-Addr) similar to MSRN (E.164 number). This address will be used by the visited MSC to correlate the incoming IAM messages from SIWFS. The visited MSC sends a request to the selected SIWFS by means of a MAP_Provide_SIWFS_Number procedure. The MAP_Provide_SIWFS_Number procedure contains all information needed for the SIWFS to evaluate the requirement and to select and to seizure an appropriate IWU. In particular, these information are

– V-MSC-Addr: The address, an E.164 number, where the SIWFS has to route the call i.e. the address of the visited MSC;

– GSM-BC: The GSM-BC in the MODIFY MESSAGE;

– ISDN-BC: The ISDN-BC mapped from the GSM-BC (MOC) or received from the fixed network (MTC);

– CDm: Call Direction to indicate the direction at call set-up (MOC/MTC) for the loop. It is derived from the original call set-up direction (CDo) and from the value of the reverese data Call Direction (RCD) parameter of the MODIFY message (see figure 8).

– Chosen Channel: The Chosen Channel specifies the assigned radio channel.

The visited MSC waits for the acknowledgement MAP_Provide_SIWFS_Number_Ack.

The SIWFS assigns and returns a temporary SIWFS address (similar to MSRN), an E.164 number. This address will be used by the visited MSC to route the call, i.e. the IAM to the SIWFS and it will be used by the SIWFS to correlate the incoming IAM to the corresponding MAP message MAP_Provide_SIWFS_Number (i.e. Call Reference).

The SIWFS sends the acknowledgement MAP_Provide_SIWFS_Number_Ack back to the visited MSC.

If the acknowledgement is negative (possibly indicated via the MAP procedure), the visited MSC may release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

If the acknowledgement is positive, the visited MSC will set-up an ‘a’- circuit (if CDm = MOC) or an ‘n’- circuit (if CDm = MTC) towards the SIWFS. The ISDN-BC1or ISDN-BC2 used for allocating the ‘n’-circuit indicates a "UDI" connection, the transmission medium requirement for the ‘a’-circuit is set to "64 kbit/s unrestricted".

If no valid response is received in the ISUP-signalling the visited MSC may select an alternative route to the same SIWFS, release the call or it may select an alternative SIWFS. The number of routeing alternatives is an implementation issue and not a part of the present document.

Normal ISUP signalling follows. The IAM to SIWFS contains ISDN-BC1, which is determined in accordance with figure 9.

The SIWFS routes the call to the address that it has got from the visited MSC i.e. the address of the visited MSC. This address is conveyed as parameter in the message MAP_Provide_SIWFS_Number. So, the SIWFS routes the call to the visited MSC and allocates an ‘a’-circuit or an ‘n’ circuit towards the visited MSC. The ‘a’- and ‘n’-circuit depends on the value CDm in the MAP_Provide_SIWFS_Number_Req. If the call routeing is not possible it releases the call by ISUP means.

Normal ISUP signalling follows. The IAM to the visited MSC contains ISDN-BC2, which is determined in accordance with figure 9. When the SIWFS receives the ISUP Answer message it activates the IWU.

CDo

Modify RCD

BC1

BC2

CDm

MOC

Reverse

n

a

MTC

MOC

a

n

MOC

MTC

Reverse

a

n

MOC

MTC

n

a

MTC

Figure 8: Selection of the ISDN-BC1, ISDN-BC2 and CDm. These are dependant on the original Call Direction (CDo) and Reverse Call Direction Information Element (RCD-IE) of the MODIFY message

5.4.3 Call Flow for ICM Procedure

Figure 9: Call flow for In Call Modification procedure (change from speech to data/fax)

5.5 Modifications in a connection

There are BSSAP messages and procedures that are relevant for the MSC/IWF and SIWFS, that are not mapped to any ISUP message. These are messages for modification on channel mode and messages for resource up- and downgrading. The MAP procedure MAP_SIWF_Signalling is used to convey these messages between the visited MSC and the SIWFS.

5.5.1 Channel Mode Modify (CMM)

Channel Mode Modify (CMM) concerns TS61 and TS62. The purpose of CMM is to adjust the radio channel bit rate to match the message speed negotiated end-to-end between the fax machines. The CMM procedure is initiated by the FA/IWF.

The procedures for CMM follow the procedures described in GSM 03.45.

Figure 10: Call flow for Channel Mode Modify procedure

5.5.2 User initiated resource up- and down grading

During a Non Transparent Multislot configuration the user may request the network to change the current maximum number of traffic channels and air interface user rate parameters. This procedure is described in GSM 03.34.

When the visited MSC receives the changed configuration after ASSIGNMENT COMPLETE the IWU of the SIWFS is adjusted accordingly by means of MAP_SIWF_ Signalling procedure.

Figure 11: Call flow for user initiated resource up- and downgrading. MODIFYCOMPLETE may be sent either as shown in figure or immediately after MODIFY

5.5.3 Network Resource up- and downgrading

During a call using a multislot configuration the Number of assigned traffic channels and/or the air interface user rate and/or the channel type may be modified. Also for a call using a singleslot configuration the assigned channel mode can change (e.g. from TCH/F to TCH/H or vice versa).

When the visited MSC receives the changed configuration e. g. the message HANDOVER PERFORMED the IWU of the SIWFS is adjusted accordingly.

Figure 12: Call flow for network Resource Up- and Downgrading

5.6 Release

RELEASE follows the ISUP procedures. RELEASE may be initiated by either V-MSC or SIWFS by sending an ISUP REL message. The call is released and the MAP link between V-MSC and SIWFS is closed (MAP-CLOSE).

5.7 Negotiation of parameters

The MSC is responsible for the negotiation and setting of parameters in accordance with GSM 07.01. The protocol on the K Interface supports the submission of all needed Bearer Capabilities (GSM and ISDN) in accordance with GSM 09.07.

5.8 Inter MSC handover

In case of an inter MSC handover, the originally MSC i.e. the anchor MSC maintains the connection to and control of the SIWFS resources. Thus it is to be noted that in such cases, three MSCs are involved at the serving side of the call and the route from the MS is in the sequence MSCb-MSCa-SIWFS-MSCa- Fixed network (in the loop method case).

5.9 Reset

RESET follows the ISUP procedures. RESET may be initiated by either V-MSC or SIWFS by sending an ISUP RSC message. The circuit is reseted and possible MAP link between the V-MSC and the SIWFS is closed (MAP-CLOSE).