6.3 Interactions Between SGSN and MSC/VLR

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.3.0 General

The interactions described in this clause shall be supported if the optional Gs interface is installed. All functionality of this clause applies for A/Gb mode and Iu mode unless stated differently.

NOTE: The functionality described in this clause operates independently of whether or not the MS supports E‑UTRAN access.

An association is created between SGSN and MSC/VLR to provide for interactions between SGSN and MSC/VLR. The association is created when the VLR stores the SGSN number and the SGSN stores the VLR number. The association is used for co-ordinating MSs that are both GPRS-attached and IMSI-attached.

The association supports the following actions:

– IMSI attach and detach via SGSN. This makes combined GPRS / IMSI attach and combined GPRS / IMSI detach possible, thus saving radio resources.

– Co-ordination of LA update and RA update, including periodic updates, thus saving radio resources. A combined RA / LA update is sent from the MS to the SGSN. The SGSN forwards the LA update to the VLR.

– Paging for a CS connection via the SGSN.

– Alert procedures for non-PS services.

– Identification procedure.

– MM Information procedure.

– CS and PS registration coordination in networks that support network sharing as defined in TS 23.251 [83] so that a UE is registered with the same core network operator in the CS and PS domain.

6.3.1 Administration of the SGSN – MSC/VLR Association

The SGSN ‑ MSC/VLR association is created at the following occasions:

– Combined GPRS / IMSI attach.

– GPRS attach when the MS is already IMSI-attached.

– Combined RA / LA update when the MS performs IMSI attach and is already GPRS-attached.

– Combined RA / LA update when an IMSI and GPRS-attached MS changes from an area of network operation mode II to an area of network operation mode I.

The association is initiated by the SGSN. The SGSN creates an association by sending a BSSAP+ message concerning a particular MS to the VLR. An SGSN that does not provide functionality for Intra Domain Connection of RAN Nodes to Multiple CN Nodes uses the RAI to determine the VLR number. An SGSN that provides functionality for Intra Domain Connection of RAN Nodes to Multiple CN Nodes uses the RAI and a hash value from the IMSI to determine the VLR number. During a CS connection, an MS in class‑B mode of operation (A/Gb mode) cannot perform GPRS attach nor routeing area updates, only MSs in class‑A mode of operation can perform these procedures. If a GPRS attach was made during a CS connection, the association shall be initiated by a combined RA / LA update after the CS connection has been released.

The association is updated on the following occasions:

– When an MS changes VLR.

– When an MS changes SGSN.

The association is not updated during a CS connection.

NOTE: When the Idle mode Signalling Reduction feature described in TS 23.401 [89] is active, the association is not impacted just because of mobility between GERAN/UTRAN and E‑UTRAN, unless CS Fallback is in use (see TS 23.272 [93]).

When the MS is in idle mode (see GSM 03.22 [7] and TS 23.122 [7b]), the association is updated with the combined RA / LA updates procedure.

In relation to a CS connection, the association is managed in the following way:

MS in class‑A or CS/PS mode of operation:

An MS in class‑A or CS/PS mode of operation makes RA updates but no combined RA / LA updates during the CS connection. If the MS changes SGSN, the SGSN (according to normal RA update procedures, see clause "Inter SGSN Routeing Area Update") updates the HLR and the GGSN, but not the VLR, about the new SGSN number.

If the MS changes MSC during the CS connection, the subscriber data still remains in the old VLR until the CS connection is released and a combined RA / LA update or LA update is made. The association is also not updated during the CS connection.

After the CS connection has been released, a combined RA / LA update is performed (if there has been a change of RA, or if a GPRS attach was performed and the new cell indicates network operation mode I), and the association is updated according to combined RA / LA update procedures, see clause "Combined RA / LA Update Procedure". If the new cell indicates network operation mode II, then the MS performs an LA update.

MS in class‑B mode of operation (A/Gb mode):

An MS in class‑B mode of operation does not make any RA updates during a CS connection. The SGSN number therefore remains the same during the CS connection and does not need to be updated in the VLR. If the MS changes MSC during the CS connection, the subscriber data still remains in the old VLR until the CS connection has been released and a combined RA / LA update or LA update is made. Therefore, the VLR number remains the same during the CS connection. After the CS connection has been released, the MS performs an RA update and an LA update if the RA has changed and the new cell indicates network operation mode II, or a combined RA / LA update if the RA has changed and the new cell indicates network operation mode I. The association is updated according to the combined RA / LA update procedures, see clauses "Inter SGSN Routeing Area Update" and "Combined RA / LA Update Procedure".

The SGSN – MSC/VLR association is removed at the following occasions:

– At IMSI detach.

– At GPRS detach.

– SGs association establishment, see TS 23.272 [93].

When the MSC/VLR receives an LA update via the A or Iu interface from an MS or establishes a SGs association with an MME for which a Gs association exists for that MS, the MSC/VLR shall remove the association without notifying the SGSN. When the SGSN receives a (non-combined) RA update from an MS for which an association exists, the SGSN shall remove the association without notifying the MSC/VLR. When the MSC/VLR receives a BSSAP+ MS Unreachable message from the SGSN indicating that PPF is cleared, the state of the association shall not be changed at the MSC/VLR.

6.3.2 Combined RA / LA Updating

When the MS is both IMSI and GPRS-attached, the LA and RA updating is done in a co-ordinated way to save radio resources if supported by the network operation mode. When the MS enters a new RA in network operation mode I, the MS sends a Routeing Area Update Request message to the SGSN, as described in clause "Combined RA / LA Update Procedure". The LA update is included in the RA update. The SGSN then forwards the LA update to the MSC/VLR. The MSC/VLR optionally returns a new VLR TMSI that is sent to the MS via the SGSN.

An MS in class‑A mode of operation involved in a CS connection makes only RA updates and no combined RA / LA updates to the SGSN.

An MS in CS/PS mode of operation involved in a CS connection makes only RA updates and no combined RA / LA updates to the SGSN.

An MS in class‑B mode of operation involved in a CS connection does not make any updates during the CS connection.

An MS in class‑C mode of operation never makes combined RA / LA updates. MSs in CS mode of operation and MSs in PS mode of operation never make combined RA / LA updates.

6.3.3 CS Paging (A/Gb mode)

When an MS is both IMSI and GPRS-attached in a network that operates in mode I, the MSC/VLR executes paging for circuit-switched services via the SGSN. If the MS is in STANDBY state, it is paged in the routeing area and in the null routeing area (see clause "Routeing Area Identity "). If the MS is in READY state, it is paged in the cell. A paging timer in the MSC supervises the paging procedure. The SGSN converts the MSC paging message into an SGSN paging message.

The CS Paging procedure is illustrated in figure 18. Each step is explained in the following list.

Figure 18: CS Paging Procedure in A/Gb mode

1) The SGSN receives a Page (IMSI, VLR TMSI, Channel Needed, Priority, Location Information) message from the MSC. Channel Needed is defined in TS 48.008 [18] and indicates to the MS which type of CS channel is needed to be requested in the response. VLR TMSI and Channel Needed are optional parameters. Priority is the circuit-switched paging priority parameter as defined in TS 48.008 [18].

2) The SGSN sends a BSSGP Paging Request (IMSI, TLLI, VLR TMSI, Area, Channel Needed, QoS) message to the BSS serving the MS. Area is derived from either the MS’s MM context in the SGSN or, if no such information is available, from the Location Information received from the MSC/VLR. Area indicates a single cell for a READY state MS or a routeing area for a STANDBY state MS. VLR TMSI and Channel Needed are included if received from the MSC. If Channel Needed was not received from the MSC, then a default Channel Needed parameter indicating circuit-switched paging is included by the SGSN. QoS indicates the priority of this Paging Request relative to other Paging Request messages buffered in the BSS. If the location area where the MS was last known to be located has an associated null routeing area, then the SGSN shall send an additional BSSGP Paging Request message to each BSS serving this null RA.

3) The BSS translates the incoming BSSGP Paging Request message into one radio Paging Request message per cell. If a dedicated radio resource is assigned to the MS in a cell, then the BSS transmits one Paging Request (VLR TMSI or IMSI, Channel Needed) message on this radio resource, without stopping possibly ongoing data transfers for the MS. Otherwise, the BSS pages the MS with one Paging Request (VLR TMSI or IMSI, Channel Needed) message on the appropriate paging channel in each addressed cell. This is described in TS 43.064 [11].

4) Upon receipt of a Paging Request message for a circuit-switched service the MS may accept to respond to this request and shall follow the CS procedures for paging response (random access, immediate assignment, and paging response) as specified in TS 24.008 [13].

5) When received at the BSS, the Paging Response message is sent to the MSC, which shall stop the paging response timer.

6.3.3.1 Paging Co-ordination in A/Gb mode

The network may provide co-ordination of paging for circuit-switched and packet-switched services. Paging co-ordination means that the network sends paging messages for circuit-switched services on the same channel as used for packet-switched services, i.e. on the GPRS paging channel or on the GPRS traffic channel, and the MS needs only to monitor that channel. Three network operation modes are defined:

– Network operation mode I: the network sends a CS paging message for a GPRS-attached MS, either on the same channel as the GPRS paging channel (i.e. the packet paging channel or the CCCH paging channel), or on a GPRS traffic channel. This means that the MS needs only to monitor one paging channel, and that it receives CS paging messages on the packet data channel when it has been assigned a packet data channel.

– Network operation mode II: the network sends a CS paging message for a GPRS-attached MS on the CCCH paging channel, and this channel is also used for GPRS paging. This means that the MS needs only to monitor the CCCH paging channel, but that e.g. CS paging continues on this paging channel even if the MS has been assigned a packet data channel, unless BSS paging co-ordination as described in 8.1.6 is active.

– Network operation mode III: the network sends a CS paging message for a GPRS-attached MS on the CCCH paging channel, and sends a GPRS paging message on either the packet paging channel (if allocated in the cell) or on the CCCH paging channel. This means that an MS that wants to receive pages for both circuit-switched and packet-switched services shall monitor both paging channels in the cell, if the packet-paging channel is allocated. The core network performs no paging co-ordination. See, however, also 8.1.6 for description of paging co-ordination on BSS level.

Table 2: Paging Channel Configuration in different Network Operation Modes for A/Gb mode without BSS paging co-ordination

Mode	Circuit Paging Channel	GPRS Paging Channel	CN Paging co-ordination
	Packet Paging Channel (NOTE 1)	Packet Paging Channel (NOTE 1)
	CCCH Paging Channel	CCCH Paging Channel	Yes
	Packet Data Channel	Not Applicable
	CCCH Paging Channel	CCCH Paging Channel	No
	CCCH Paging Channel	Packet Paging Channel (NOTE 1)	No
	CCCH Paging Channel	CCCH Paging Channel
NOTE 1: Since Release 9 GERAN TS 44.060 [77] forbid deploying Packet Common Control Channels. This makes use of NMO-III and packet paging channel obsolete.

For MSs with an SGSN – MSC/VLR association, which is established via the GS interface, all MSC-originated paging of GPRS-attached MSs shall go via the SGSN, thus allowing network co-ordination of paging. Paging co-ordination shall be made by the SGSN based on the IMSI, and is provided independently of whether the MS is in STANDBY or in READY state. The network operates in mode I.

When no SGSN – MSC/VLR association exists, all MSC-originated paging of GPRS-attached MSs shall go via the A interface, and co-ordination of paging cannot be performed by the core network. The network shall then operate in mode II, meaning that the packet common control channel shall not be allocated in the cell.

The network operation mode (mode I or II) shall be indicated as system information to MSs. Additional system information can indicate that MSs configured to use the extended NMO I system information (see TS 24.368 [111]) shall use NMO I, regardless of what NMO is indicated by system information for other MSs. If this additional system information is absent, MSs configured to use the extended NMO I system information shall use the system information that represents the network operation mode for other MSs. From these indications, the MS determines which mode applies to it. That mode shall be used when using the procedures described in other clauses of this specification.

For proper operation, the mode of operation should be the same in each cell of a routeing area.

Based on the system information provided by the network, the MS can then choose, according to its capabilities, whether it can attach to GPRS services, to non-GPRS services, or to both.

6.3.4 CS Paging (Iu mode)

When an MS is both IMSI- and GPRS-attached in a network that operates in mode I, the MSC/VLR executes paging for circuit-switched services via the SGSN.

In the MSC, a paging timer supervises the paging procedure.

The CS Paging procedure is illustrated in Figure 19. Each step is explained in the following list.

Figure 19: CS Paging Procedure in Iu mode

1) The SGSN receives a Page (IMSI, VLR TMSI, Location Information) message from the MSC. If VLR TMSI is omitted, the IMSI is used instead of the TMSI as a paging address at the radio interface. If location information is not included, the SGSN shall page the MS in all the cells served by the VLR and the SGSN, unless the SGSN has reliable information about the location of the MS.

NOTE 1: If the PS network support paging optimization and MS has Emergency Service in CS domain, CS paging procedure via SGSN may fail.

2) The 3G‑SGSN sends a RANAP Paging (IMSI, TMSI, Area, CN Domain Indicator) message to each RNS.
IMSI is needed by the RNS in order to calculate the MS paging group and to identify the paged MS. TMSI is included if received from the MSC. Area indicates the area in which the MS is paged, and is derived from either the MS’s MM context in the SGSN or, if no such information is available, from the Location Information received from the MSC/VLR. CN Domain Indicator indicates which domain (CS or PS) initiated the paging message, and in this case it must be set to "CS" by the SGSN. The list of CSG IDs for paging is included when the SGSN is configured to support paging optimisation described in clause 5.3.9. For paging optimisation, the CSG IDs of expired CSG subscriptions and valid CSG subscriptions are both included in the list.

NOTE 2: An expired CSG subscription indicates that the MS is not allowed service in the CSG. However, since the removal of the CSG from the MS is pending, it is possible the MS will camp on that CSG and therefore the MS is still paged for the CSG.

3) For more details on the radio resource part of the paging procedure, see clause "Paging Initiated by CN".

4) Upon receipt of a Paging Request message for a circuit-switched service, the MS responds to this request and returns the paging response as specified in TS 44.018 [85] in an RRC Initial Direct Transfer message as specified in TS 25.331 [52]. CN Domain Indicator is set to "CS" in the Initial Direct Transfer message.

5) When received at the RNS, the Paging Response message is sent in an RANAP Initial UE message to the MSC, which shall then stop the paging response timer.

6.3.4.1 Network Operation Modes for Iu mode

The network operation mode is used to indicate whether the Gs interface is installed or not. When the Gs interface is present, MSs initiate combined procedures.

Table 3-1: Network Operation Modes for Iu mode

Mode	Network configuration	Combined procedure by MT
	Gs interface is present	Yes
	Gs interface is not present	No

The network operation mode (mode I or II) shall be indicated as system information to the MSs. Additional system information can indicate that MSs configured to use the extended NMO I system information shall use NMO I, regardless of what NMO is indicated by system information for other MSs. If this additional system information is absent, MSs configured to use the extended NMO I system information shall use the system information that represents the network operation mode for other MSs. From these indications, the MS determines which mode applies to it. That mode shall be used when using the procedures described in other clauses of this specification.

For proper operation, the mode of operation should be the same in each cell of a routeing area.

Based on the system information provided by the network, the MS derives whether to initiate combined update procedures or separate update procedures.

NOTE: Network operation modes I and II for Iu mode correspond to modes I and II for A/Gb mode, respectively.

6.3.4a CS Paging (in case Selective RA Update)

When an MS is both IMSI- and GPRS-attached in a network that operates in mode I, and the MSC/VLR executes paging for circuit-switched services via the SGSN that support Selective RA Update Procedure, if the MS is in STANDBY or PMM-IDLE state, the SGSN shall cause the page to be sent in all cells in the routeing area where the MS is located. This can include both A/Gb mode and Iu mode cells (see clause "Selective RA Update").

The CS Paging procedure in A/Gb mode is illustrated in figure 18 and the CS Paging procedure in Iu mode is illustrated in figure 19.

6.3.5 Non-GPRS Alert

The MSC/VLR may request an SGSN to report activity from a specific MS. In this case, the MSC/VLR shall send a BSSAP+ Alert Request (IMSI) message to the SGSN where the MS is currently GPRS-attached.

Upon reception of the Alert Request (IMSI) message, the SGSN shall set NGAF. If NGAF is set for an MS, the SGSN shall inform the MSC/VLR when the next activity from that MS (and the MS is both IMSI- and GPRS-attached) is detected, and shall clear NGAF.

If the activity detected by the SGSN leads to a procedure towards the MSC/VLR, the SGSN shall just follow this procedure. If the activity detected by the SGSN does not lead to any procedure towards the MSC/VLR, the SGSN shall send an MS Activity Indication (IMSI) message towards the MSC/VLR.

6.3.6 MS Information Procedure

When the MS is marked at the VLR as both IMSI- and GPRS-attached, the VLR may perform the MS Information procedure via the SGSN. If the information requested by the VLR in the MS Information procedure is known by the SGSN, then the SGSN shall return this information to the VLR without interrogating the MS.

If the information requested is MS identity information (e.g. IMEI) that is not known by the SGSN but is known by the MS, then the SGSN shall interrogate the MS in a similar manner to that described in clause "Identity Check Procedures".

In A/Gb mode, if the information requested is MS location information, then this indicates a request for Cell Global Identity and Cell Identity Age. In Iu mode, if the information requested is MS location information, then this indicates a request for Service Area Identity and Service Area Identity Age, and in this case if an Iu connection for the MS exists, then the SGSN shall use the Location Reporting procedure (see clause "Location Reporting Procedure") in order to retrieve the Service Area Identity.

The MS Information procedure is illustrated in Figure 20. Procedure steps are explained in the following list.

Figure 20: MS Information Procedure

1) The MSC/VLR sends an MS Information Request (IMSI, Information Type) message to the SGSN. Information Type indicates the information that the MSC/VLR is requesting for that IMSI.

2) If the information requested is not known by the SGSN but should be known by the MS, then the SGSN interrogates the MS in a similar manner to that described in the clause "Identity Check Procedures". The SGSN sends an Identity Request (Identity Type) message to the MS.

3) The MS responds with an Identity Response (Mobile Identity) message to the SGSN.

4) In Iu mode, if an Iu connection for the MS exists, then the SGSN shall use the Location Reporting procedure to retrieve the Service Area Identity. If the BSS/RNS cannot determine the current Service Area of the MS, it indicates in the Location Report message that the request could not be fulfilled and may report the Last Known Service Area with an indication of how long has past since the MS was known to be in the indicated Service Area.

5) The SGSN sends an MS Information Response (IMSI, Information) message to the MSC/VLR. Information contains the information requested by the MSC/VLR.
If an Iu connection for MS exist and RAN node cannot determine current Service Area and Last Known Service Area is not reported, the SGSN shall include in the MS Information Response message the last successfully received Service Area Identity with time elapsed since it was saved by SGSN.

6.3.7 MM Information Procedure

When the MS is marked at the VLR as both IMSI- and GPRS-attached, the VLR may perform the MM Information procedure via the SGSN. The MM Information procedure is typically used to inform the MS about such things as the network name and the local time zone of the mobile.

The MM Information procedure is illustrated in Figure 21.

Figure 21: MM Information Procedure

1) The SGSN receives an MM Information (IMSI, Information) message from the MSC/VLR. Information is the information that the MSC/VLR is sending to the MS.

2) The SGSN sends an MM Information (Information) message to the MS including the information received by the MSC/VLR.