6 User data and signalling procedures between RL and BSSGP SAPs

3GPP48.018Base Station System (BSS) - Serving GPRS Support Node (SGSN)BSS GPRS protocol (BSSGP)General Packet Radio Service (GPRS)Release 17TS

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

6.1 Downlink UNITDATA procedure

On the downlink, a DL-UNITDATA PDU shall contain information elements to be used by the RLC/MAC function and an LLC-PDU. There shall be only one LLC-PDU per DL-UNITDATA PDU. The LLC-PDU shall always be the last information element in the DL-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.

An SGSN provides the BSSGP with a current TLLI, identifying the MS. If an SGSN provides a second TLLI, indicating that an MS has recently changed its TLLI, this shall be considered as the "old" TLLI. A BSS uses the "old" TLLI to locate an MS’s existing context. Subsequent uplink data transfers for this MS shall reference the current TLLI, and not the old TLLI.

The SGSN shall include the IMSI in the PDU. As an exception, the SGSN may omit the IMSI in the PDU if the mobile station identified by the TLLI is in MM non-DRX mode period (i.e. during a GMM procedure for GPRS attach or routing area updating defined in 3GPP TS 24.008) and the SGSN does not have a valid IMSI.

The SGSN may include the Service UTRAN CCO (Cell Change Order) information element in the PDU (relevant if the network initiated cell change order to UTRAN, network initiated cell change order to E-UTRAN, PS handover to UTRAN or PS handover to E-UTRAN procedures are used ). If this information element is received in multiple PDUs (either DL-UNITDATA PDU(s), CREATE-BSS-PFC PDU(s) or PS-HANDOVER-REQUEST PDU(s)), the information element contained in the last received PDU shall take precedence.

If the SGSN has valid DRX Parameters for a TLLI, then the SGSN shall include them in the PDU. Nevertheless, the SGSN can omit the DRX Parameters if the MS identified with the TLLI is in MM non-DRX mode period to speed up the transmission of the LLC-PDU on the radio interface. The SGSN shall not send a DL-UNITDATA PDU without the DRX Parameters IE if the MS identified with the TLLI is not in MM non-DRX mode period.

An exception case is when the SGSN has uplink and downlink Coverage Class information (previously received from the BSS in the UL-UNITDATA PDU) or eDRX information (negotiated during NAS signalling) or both available for a given MS in which case it shall proceed as follows when sending a DL-UNITDATA PDU for that MS:

– If uplink and downlink Coverage Class information is available for the MS then the SGSN shall include the Coverage Class information in the DL-UNITDATA. If a negotiated eDRX value is available then it shall also be included. The SGSN considers the eDRX to be negotiated upon accepting the eDRX cycle value requested by a MS during NAS signalling. If a negotiated eDRX value is not available but DRX information is available then it shall be included. If the target cell supports EC-GSM-IoT the corresponding LLC-PDU is delivered as follows:

– If the MS is in idle mode a downlink packet resource assignment is sent on the EC-AGCH of its EC_CCCH_GROUP (see 3GPP TS 45.002 [32]) using the indicated downlink Coverage Class information and the lowest eDRX cycle and the LLC PDU is sent thereon.

– If the MS is in packet transfer mode the LLC PDU is delivered using either an existing downlink EC TBF or using a new downlink EC TBF established by sending a downlink packet resource assignment on the EC-PACCH (see 3GPP TS 44.060 [22]) using the downlink Coverage Class information the BSS is currently using for that MS.

– If Coverage Class information is not available but a negotiated eDRX value is available for the MS then the SGSN shall include the negotiated eDRX value in the DL-UNITDATA PDU. If the target cell supports eDRX the corresponding LLC-PDU is delivered as follows:

– If the MS is in idle mode a downlink packet resource assignment is sent on the AGCH of its CCCH_GROUP (see 3GPP TS 45.002 [32]) using the lowest eDRX cycle and the LLC PDU is sent thereon.

– If the MS is in packet transfer mode the LLC PDU is delivered using either an existing downlink TBF or using a new downlink TBF established by sending a downlink packet resource assignment on the PACCH (see 3GPP TS 44.060 [22]).

– The SGSN shall apply these rules also when sending a DL-UNITDATA PDU to a MS for which the GMM procedure for GPRS attach or routing area updating (defined in 3GPP TS 24.008) has not yet completed.

An SGSN provides the BSSGP with MS specific information, enabling the RLC/MAC entity in a BSS to transmit an LLC-PDU to the MS in a user specific manner. The information made available to the radio interface includes:

– MS Radio Access Capability. This defines the radio capabilities of the ME. If there is valid MS Radio Access Capability information known by the SGSN for the associated MS, the SGSN shall include it in the DL‑UNITDATA PDU. Otherwise, MS Radio Access Capability shall not be present;

– Packet Flow Identifier. This identifies the packet flow context associated with the LLC PDU and is included by the SGSN if the packet flow context feature is negotiated. If the mobile station does not support the PFC feature or if the PFI is not known (e.g. the new SGSN did not get the PFI from the old SGSN during a RAU) then the SGSN shall use the pre-defined PFI to indicate best-effort QoS;

– QoS Profile. This defines the (peak) bit rate, the type of BSSGP’s SDU (signalling or data), the type of LLC frame (ACK, SACK, or not), the precedence class, and the transmission mode to be used when transmitting the LLC-PDU across the radio interface;

– PDU Lifetime. This defines the remaining time period that the PDU is considered as valid within the BSS. If the PDU is held for a period exceeding the "PDU Lifetime" time period, the PDU shall be locally discarded. The PDU Lifetime is set within the SGSN by the upper layers.

An SGSN provides the BSSGP with Enhanced Coverage restriction information which indicates to the BSS whether the use of enhanced coverage is restricted or not for a given MS, thereby enabling the BSS to decide the allowable coverage classes (uplink and/or downlink) for the MS while operating in packet transfer mode.

A BSS may incorporate the PDU Lifetime, the Precedence and the (peak) bit rate into its radio resource scheduler. If the PFI is present then the BSS may incorporate the information from the associated ABQP into its radio resource scheduler. The algorithm to do this is out of scope of the present document.

If the PFI is known in the BSS and does not correspond to a predefined PFI, then:

– the (peak) bit rate and the precedence class fields present in the QoS Profile IE shall be ignored by the BSS;

– if the Allocation/Retention Priority was provided at the time the corresponding PFC was created or last modified, then the ‘Priority’ IE, if present in the downlink UNITDATA PDU, is discarded.

Two types of BSSGP SDU are distinguished within the QoS Profile: layer 3 signalling and data. Layer 3 signalling may be transmitted over the Um interface with higher protection. If the MS has an RR connection to the network (see 3GPP TS 44.018), Layer 3 signalling may be transmitted over the Um interface on the main signalling link of the RR connection, provided that the LLC PDU meets length restrictions imposed by the BSS. In this case, the BSS shall include the LLC PDU contained in the BSSGP PDU in the correspondent Layer 3 Um interface message (see 3GPP TS 44.018).

The type of LLC frame indicates if the LLC frame type is an ACK or SACK command/response, or not (see 3GPP TS 44.064). An ACK or SACK command/response frame type may be transmitted over the Um interface with higher protection.

Two transmission modes across the radio interface are possible: acknowledged (using RLC/MAC ARQ functionality) and unacknowledged (using RLC/MAC unitdata functionality). These transmission modes do not apply when the MS has an RR connection to the network and BSS uses the main signalling link of the RR connection, in which case the acknowledged transmission mode is used.

If Priority is present, assuming it shall not be discarded according to the rule above, only the priority-level field shall be regarded. The management of priority levels is implementation dependent and under operator control. The preemption capability indicator, the queuing allowed indicator and preemption vulnerability indicator shall be ignored in this case.

In addition to constructing the DL-UNITDATA, the SGSN supplies the LSP, the BVCI, the NSEI, and for an IP sub-network the NS Change IP endpoint, associated with the MS to the lower layer network service, enabling network service routeing to the peer entity. These parameters are not transmitted as part of the BSSGP across the Gb-interface for the purpose of identifying the receiving endpoint (they are sent in the BSSGP Perform-Location-Request PDU to identify the serving cell of the target MS).

If the Gb-interface is supported using an IP sub-network, then the Resource Distribution function at the SGSN may transmit a BSSGP DL-UNITDATA PDU with an LLC-PDU Length Indicator set to 0. The BSS uses this DL‑UNITDATA to change the IP endpoint at the SGSN to which any future UL-UNITDATA for the TLLI (indicated in the DL-UNITDATA) is sent. The LLC-PDU with a Length Indicator set to 0 is not sent across the radio interface.

In the case where localised service area is supported the SGSN may inform the BSS as to which LSA identities that the mobile has preferences by sending the LSA INFORMATION element. The BSS stores this information and uses it e.g. for network controlled cell re-selection when determining specific cell selection parameters for the mobile. The algorithm for determining specific cell selection parameters for the mobile is not defined further in the present document. The SGSN may inform the BSS about the contents of SPID in the DL-UNITDATA PDU. In this case the SPID is stored in the BSS.

When the “Service Identification for improved Radio Utilization for GERAN” (SIRUG) feature (see clause 5.3.5.3 of TS 23.060 [7]) is supported, and the SGSN receives SCI information in the GTP-U header, the SGSN inserts the SCI IE and the GGSN/P-GW location IE in the associated DL-UNITDATA PDU(s). The BSS uses the Home PLMN ID contained within the IMSI and the GGSN/P-GW location information to determine whether or not it can handle the SCI information for that GGSN/P-GW. If it cannot, the BSS discards the SCI information and shall treat the rest of the DL-UNITDATA normally.

In abnormal cases, if the SCI IE is received but the GGSN/P-GW location IE is missing, then the BSS shall assume that the GGSN/P-GW is located in the VPLMN; and, if the GGSN/P-GW location IE is received but the SCI IE is missing, then the BSS shall ignore the GGSN/P-GW location IE.

Specific handling related to MOCN and GWCN configurations of network sharing, is described in sub-clauses 6.6 and 6.7.

When the SGSN is using a GWCN configuration, and the BSSGP DL-UNITDATA PDU contains a local TLLI, then the BSC shall use the registered Operator specific NRI value to identify the corresponding serving operator.

Specific handling releted to Dedicated Core Networks as well as MS assisted Dedicated Core Network selection is described in sub-clause 6.6.

An exception case is when the SGSN determines that there is data to be delivered to a MS for which the MPM Timer is running (see sub-clause 8b.2.1). In this case it shall not attempt delivery of that data until the MPM Timer expires or is stopped.

6.1.1 Abnormal conditions

The following actions are defined in periods of congestion.

To satisfy the maximum number of service requests, the BSS may redistribute MSs among cells (i.e. network controlled cell reselection is initiated). If this occurs, the BSS may inform the SGSN through the RADIO-STATUS PDU (Radio Cause value: cell reselection ordered). The BSS shall update any internal references that indicate the location of the MS. The BSS may attempt to internally re-route queued LLC frames to an MS that has been moved to a new cell. If this functionality is not supported, or if it is not possible to internally re-route LLC frames, the LLC frame shall be discarded.

It is the responsibility of the higher layer protocols in the SGSN to cope with discarded LLC frames.

6.2 Uplink UNITDATA procedure

On the uplink, a UL-UNITDATA PDU shall contain information elements derived from the RLC/MAC function (except when GTTP is used in the Um interface, see 3GPP TS 44.018), meaningful to higher-layer protocols in an SGSN, and an LLC-PDU. There shall be only one LLC-PDU per UL‑UNITDATA PDU. The LLC-PDU shall always be the last information element in the UL-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.

The BSS shall provide the TLLI associated to the MS to the SGSN.

The BSS shall provide a BVCI and an NSEI indicating the PTP functional entity (i.e. the cell) upon which the LLC‑PDU was received. The SGSN shall obtain the BVCI, the NSEI, and in the case of an IP sub-network may obtain the LSP and the NS Change IP endpoint, from the underlying network service; the BVCI and the NSEI are not visible in the UL-UNITDATA PDU.

The BSS provides the SGSN with the QoS Profile used in the LLC-PDU transmission from the mobile station across the radio interface.

– QoS Profile. This reports the (peak) bit rate, the precedence used at radio access and the transmission mode used across the radio path. The type of the BSSGP SDU, layer 3 signalling or data, and the type of LLC frame, SACK, ACK, or not, are not meaningful on the uplink and shall be ignored.

– Packet Flow Identifier. This identifies the packet flow context that is obtained from the mobile. If the mobile station does not provide a PFI then the BSS shall use the pre-defined PFI to indicate best-effort QoS.

In order to support location based services, the BSS shall include the cell identifier of the cell upon which the LLC‑PDU was received.

In the case where localised service area is supported, the BSS shall include the LSA identities of the cell upon which the LLC-SDU was received. The BSS may exclude LSA identities that are not included in the LSA INFORMATION element.

If EC-GSM-IoT is supported by the BSS and the SGSN, then the BSS shall provide in the UL-UNITDATA PDU:

– The uplink and downlink Coverage Class if the downlink Coverage Class is reported by the MS during system access see 3GPP TS 44.018 [25].

– The exception report flag if the UL-UNITDATA PDU contains an LLC PDU sent by the MS using an uplink EC TBF established in response to an EC PACKET CHANNEL REQUEST message indicating high priority (i.e. an exception report), see 3GPP TS 44.018 [25] and 3GPP TS 44.060 [22].

– The exception report flag if the UL-UNITDATA PDU contains an LLC PDU sent by the MS using an uplink EC TBF established in response to an EC PACKET DOWNLINK ACK/NACK message including channel request and indicating high priority (i.e. an exception report), see 3GPP TS 44.018 [25] and 3GPP TS 44.060 [22].

In addition to constructing the UL-UNITDATA, the BSS supplies the LSP, the NSEI, the BVCI, and for an IP sub-network the NS Change IP endpoint, associated with the MS to the lower layer network service, enabling network service routeing to the peer entity. These parameters are not transmitted as part of the BSSGP across the Gb-interface. If the Gb-interface is supported using an IP sub-network, then the Resource Distribution function at the BSS may transmit a BSSGP UL-UNITDATA PDU with an LLC-PDU Length Indicator set to 0. The SGSN uses this UL-UNITDATA to change the IP endpoint at the BSS to which any future DL-UNITDATA for the TLLI (indicated in the UL‑UNITDATA) is sent.

Specific handling related to the use of network sharing in a cell is described in sub-clauses 6.6 and 6.7 for a mobile station non supporting network sharing. In the case of a mobile station supporting network sharing the BSS shall include the selected PLMN ID within the UL-UNIDATA PDU when a foreign TLLI or a random TLLI is included in the same UL-UNIDATA PDU, as described within sub-clause 10.2.2; both MS support and selected PLMN ID shall be derived by the BSS from information within the RLC data block as described within 3GPP TS 44.060 [22].

Specific handling related to Dedicated Core Network as well as MS assisted Dedicated Core Network selection is described in sub-clause 6.6.

6.2.1 Abnormal conditions

None specified.

6.3 RA-CAPABILITY procedure

The SGSN stores an MS’s current radio access capability (which may be changed by higher layer mobility management procedures). An MS’s current radio access capability, and the TLLI identifying the MS, are conveyed to a BSS in a RA‑CAPABILITY PDU. The received MS’s radio access capability, if valid, shall then replace any radio access capability previously associated with the MS.

6.3.1 Abnormal conditions

If the BSS receives an unknown Access Technology Type in the MS Radio Access Capability field, it shall ignore the fields associated with that Access Technology type.

If the BSS receives unknown fields within a known Access Technology Type in the MS Radio Access Capability field, it shall ignore the unknown fields.

6.4 Downlink MBMS-UNITDATA procedure

On the downlink, a DL-MBMS-UNITDATA PDU shall contain information elements to be used by an LLC-PDU. There shall be only one LLC-PDU per DL-MBMS-UNITDATA PDU. The LLC-PDU shall always be the last information element in the DL-MBMS-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.

An SGSN provides the BSSGP with a current TMGI, if available, and MBMS Session Identity, identifying the MBMS Service Context.

The information made available to the radio interface includes:

A BSS may incorporate the PDU Lifetime into its radio resource scheduler.

In addition to constructing the DL-MBMS-UNITDATA, the SGSN supplies the BVCI and the NSEI to the BSS.

6.5 Uplink MBMS-UNITDATA procedure

On the uplink, a UL-MBMS-UNITDATA PDU shall contain an LLC-PDU. There shall be only one LLC-PDU per UL‑MBMS-UNITDATA PDU. The LLC-PDU shall always be the last information element in the UL-MBMS-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.

The BSS shall provide the TMGI and, if present in the MBMS Service Context, the MBMS Session Identity to the SGSN in order to identify the MBMS session.

The BSS shall provide a BVCI and an NSEI indicating the PTM functional entity upon which the LLC‑PDU was received. The SGSN shall obtain the BVCI, the NSEI, and in the case of an IP sub-network may obtain the LSP and the NS Change IP endpoint, from the underlying network service; the BVCI and the NSEI are not visible in the UL-MBMS-UNITDATA PDU.

In addition to constructing the UL-MBMS-UNITDATA, the BSS supplies the LSP, the NSEI, the BVCI, and for an IP sub-network the NS Change IP endpoint, associated with the MBMS session to the lower layer network service, enabling network service routeing to the peer entity. These parameters are not transmitted as part of the BSSGP across the Gb-interface. If the Gb-interface is supported using an IP sub-network, then the resource distribution function at the BSS may transmit a BSSGP UL-MBMS-UNITDATA PDU in order to change the IP endpoint at the BSS to which any future DL-MBMS-UNITDATA for the MBMS session (indicated with TMGI and, if available, MBMS Session Identity in the UL‑MBMS-UNITDATA) shall be sent from the SGSN.

NOTE: In this version of the specification, the procedure is used for resource distribution only meaning that the LLC PDU length indicator shall always be set to zero.

6.6 Rerouting procedure in case of MOCN configuration for network sharing, Dedicated Core Networks or MS assisted Dedicated Core Network selection

6.6.1 General

This procedure shall be supported by a BSS if it supports the MOCN configuration (see [43]), if it supports Dedicated Core Networks or if is supports MS assisted Dedicated Core Network selection. The rerouting procedure in a GWCN configuration is described in sub-clause 6.7.

In the MOCN configuration the radio access part of the network is shared. There may be more than one Gb-Interface towards the PS domain of different CN operators from the BSS.

Rerouting procedure is a mechanism used as part of the assignment of CN operator in shared networks with MOCN configuration or GWCN configuration (see sub-clause 6.7) when an MS not supporting network sharing performs initial attach/registration. In this case BSS may not know towards which SGSN to route the initial MS request message and the latter may be rerouted to another SGSN by BSS.

More precisely, in case of MOCN configuration, the selection of SGSN in BSS is based on the NRI (valid or invalid) or by random selection. In case where the SGSN cannot be deduced from the NRI and a GPRS attach or routing area updating initial layer 3 message (defined in [11]) shall be transferred in UL-UNITDATA message towards a SGSN, BSS shall choose a SGSN and initiate a rerouting procedure.

To trigger a rerouting procedure in MOCN configuration, the BSS adds the Redirect Attempt Flag IE to the UL-UNITDATA message, in order to indicate that the SGSN shall respond by including either Redirection Indication IE or Redirection Completed IE in DL-UNITDATA message.

For Dedicated Core Networks (DCNs) multiple DCNs may be deployed within a PLMN where each DCN may be dedicated to serve specific type(s) of subscribers.

– For DCN the Rerouting procedure is used as part of the selection of core network node to serve the MS when it has successfully transmitted an initial LLC PDU (i.e. Foreign or Random TLLI is used) to a BSS that supports DCN. More precisely, the procedure provides the BSS with the Null-NRI/SGSN group ID which the BSS uses to select a SGSN towards which the initial uplink LLC PDU shall be routed.

– To trigger a rerouting procedure in the case where a BSS cannot determine an appropriate DCN for forwarding an uplink LLC PDU, the BSS selects an available SGSN and adds the Redirect Attempt Flag IE to the UL-UNITDATA message used to forward the uplink LLC PDU to that SGSN.

– The inclusion of the Redirect Attempt Flag IE serves as indication to the SGSN that, after determining where to re-route the LLC PDU, it shall respond to the BSS by including either Redirection Indication IE or Redirection Completed IE in the DL-UNITDATA message it sends in response to the UL-UNITDATA message.

For MS assisted Dedicated Core Network selection the Rerouting procedure is used for selection of a core network node to serve the MS when it supports MS assisted Dedicated Core Network selection and has successfully transmitted an initial LLC PDU to a BSS that supports MS assisted Dedicated Core Network selection:

– The BSS selects a DCN for the selected PLMN based on DCN-ID information received from the MS and configuration information (i.e. information that allows the BSS to interpret DCN-ID values), see 3GPP TS 44.060 [22].

– The inclusion of the Redirect Attempt Flag IE in the UL-UNITDATA message serves as indication to the SGSN that, after determining if it is the correct DCN or not, it shall respond to the BSS by including either a Redirection Indication IE or a Redirection Completed IE in the DL-UNITDATA message it sends in response to the UL-UNITDATA message.

6.6.2 Reroute Indication

If the UL-UNITDATA message does not contain the CS Registered Operator IE and the SGSN cannot serve the request and reroute is possible (error causes are related to subscription options – defined in [11]), the reject Layer 3 Information LLC-PDU (e.g. GPRS attach Reject) and a Redirection Indication IE containing a Reroute Reject Cause shall be included in the DL-UNITDATA message for the downlink direction.

When the IMSI, or in case the BSS supports CS/PS coordination enhancements, the CS Registered Operator IE or the Selected Operator IE, is not included in the UL-UNITDATA message and CS/PS domain registration coordination is required (see [43]), the Initial LLC-PDU and a Redirection Indication IE containing the Reroute Reject Cause set to “CS/PS domain registration coordination required” shall be included in the DL-UNITDATA message for the downlink direction. CS/PS domain registration coordination may not be performed for the operator’s own subscribers by the SGSN.

In addition the DL-UNITDATA message shall contain:

– The Initial LLC-PDU received from the MS;

– The IMSI, if available;

– The Unconfirmed send state variable, if available.

If the SGSN supports CS/PS coordination enhancements, the DL-UNITDATA message shall also contain:

– The Old Routing Area Identification as indicated by the MS in the Initial LLC-PDU or,

– The Attach Indicator in case the Initial LLC-PDU contains a GPRS attach (see [11]).

In a MOCN configuration, if the BSS does not support CS/PS coordination enhancements and the Redirection Indication IE in the DL-UNITDATA message is received from a SGSN which is not the last attempted, then the BSS shall re-initiate the procedure towards another CN operator when possible (or possibly to the same CN in case when CS/PS domain registration coordination is required), with the following additional information in the UL-UNITDATA message:

– The Initial LLC-PDU as LLC-PDU;

– The Redirect Attempt Flag IE;

– The IMSI, if received from one of previously attempted CN operators;

– The Unconfirmed send state variable, if received from previously attempted CN operator.

If the BSS supports CS/PS coordination enhancements and the Redirection Indication IE in the DL-UNITDATA message is received from a SGSN which is not the last attempted, it shall proceed as follows:

– if the DL-UNITDATA message contains the Old Routing Area Identification and the BSS from this information (and from BSS internal configuration) is able to uniquely identify one of the CN operators in the shared network, then the BSS shall re-initiate the procedure towards the SGSN of the identified CN operator with the Selected Operator, Initial LLC-PDU, Redirect Attempt Flag, IMSI and Unconfirmed send state variable (if received from previously attempted CN operator) included in the UL-UNITDATA message;

– if the DL-UNITDATA message contains the Old Routing Area Identification and the BSS from this information (and from BSS internal configuration) cannot identify one of the CN operators in the shared network, then the BSS shall initiate the MS Registration Enquiry procedure in the CS domain (see [14]);

– if the DL-UNITDATA message contains the Attach Indicator then the BSS shall initiate the MS Registration Enquiry procedure in the CS domain (see [14]).

NOTE: The configuration by means the BSS identifies a CN operator from the received Old Routing Area Identification is implementation dependent and under operator control.

If, as a result of the MS Registration Enquiry procedure in the CS domain, the MS is found to be registered with one of the shared CN operators, then the BSS shall re-initiate the procedure towards the SGSN of the same CN operator with the CS Registered Operator, Initial LLC-PDU, Redirect Attempt Flag, IMSI and Unconfirmed send state variable (if received from previously attempted CN operator) included in the UL-UNITDATA message.

If the MS is not found to be registered in the CS domain with any of the shared CN operators, then the BSS shall perform CS/PS domain registration coordination based on the received IMSI and re-initiate the procedure towards the SGSN of the identified CN operator with the Selected Operator, Initial LLC-PDU, Redirect Attempt Flag, IMSI and Unconfirmed send state variable (if received from previously attempted CN operator) included in the UL-UNITDATA message.

Upon reception of the downlink Redirection Indication IE, the BSS shall store as part of the Rerouting Function the associated Reroute Reject Cause and LLC-PDU related to this SGSN.

In case the Reroute Reject Cause is set to "CS/PS domain registration coordination required" and the BSS does not support CS/PS coordination enhancements, then the BSS shall perform CS/PS domain registration coordination based on the received IMSI. In this case the Reroute Reject Cause value and the associated LLC-PDU shall not be stored.

For Dedicated Core Networks or for MS assisted Dedicated Core Network selection, if the SGSN decides to reroute the handling of an initial uplink LLC PDU to another CN node the DL-UNITDATA message shall also contain:

– The Initial LLC-PDU received from the MS;

– The IMSI, if received unencrypted from the MS;

– The Null-NRI/SGSN group ID;

– Additional P-TMSI, if available;

– UE Usage Type.

If the BSS supports Dedicated Core Networks or if the BSS supports MS assisted Core Network selection and the Redirection Indication IE in the received DL-UNITDATA message indicates a new SGSN (i.e. different from the previously used SGSN), it shall select and route the initial uplink LLC PDU to the new SGSN. The selection is based on the Null-NRI/SGSN Group ID and possibly also based on the Additional P-TMSI received in the DL-UNITDATA message. If no valid SGSN can be identified within the set of valid nodes the BSS selects either the default DCN or an SGSN based on operator configuration see 3GPP TS 23.401. The UL-UNITDATA message sent to the selected SGSN shall contain:

– The Initial LLC-PDU received from the MS;

– The IMSI, if available;

– The Null-NRI/SGSN group ID;

– UE Usage Type.

The presence of Null-NRI/SGSN Group ID in the UL-UNITDATA message indicates to the SGSN that the message is a rerouted message and that the SGSN shall not further reroute the initial LLC-PDU (see sub-clause 10.2.2). The UE Usage Type shall be used by the SGSN to select a GGSN.

If the SGSN cannot serve the request, and if the Null-NRI/SGSN Group ID was provided in the UL-UNITDATA message carrying the initial LLC-PDU, the SGSN shall include the response LLC PDU (e.g. containing the GPRS attach Reject) and a Redirection Indication IE containing a Reroute Reject Cause (error causes are related to subscription options – defined in [11]) in the DL-UNITDATA message to the BSS.

In case all attempted CN operators have replied with a Redirection Indication IE, the BSS shall select the most appropriate Layer 3 Information received from the attempted CN nodes based on the stored information as part of the Rerouting procedure and send it back to the MS (see [11).

6.6.3 Reroute complete

If the SGSN can serve the request, the Redirection Completed IE with outcome value set to "MS is accepted" or "MS is already registered" and Layer 3 Information LLC-PDU (e.g. GPRS Attach Accept) shall be included in the DL-UNITDATA message for the downlink direction.

Upon reception of the UL-UNITDATA message containing the CS Registered Operator IE, the SGSN supporting CS/PS coordination enhancements shall conclude the Rerouting procedure and include the Redirection Completed IE in the DL-UNITDATA message.

Upon reception of the downlink Redirection Completed IE, the BSS shall send back the included LLC-PDU to the MS and terminate the Rerouting procedure.

6.6.4 Abnormal Conditions

If the SGSN cannot serve the request and rerouting is not possible, the Redirection Completed IE with outcome value set to "MS is not accepted" and Layer 3 Information LLC-PDU (e.g. GPRS Attach Reject) shall be included in the DL-UNITDATA message for the downlink direction.

If, as a result of the analysis of the DL-UNITDATA message, the MS Registration Enquiry procedure is initiated in the CS domain and the MSC does not support CS/PS coordination enhancements, then the BSS shall behave as if the MS is not registered with the CN operator of that MSC.

6.7 Rerouting procedure in case of GWCN configuration for network sharing

6.7.1 General

This procedure shall be supported by a BSS and an SGSN if and only if both nodes support CS/PS coordination enhancements in a GWCN configuration (see 3GPP TS 23.251 [43]). The rerouting procedure in a MOCN configuration is described in sub-clause 6.6.

In the GWCN configuration the radio access part of the network as well as the core network nodes (MSC and SGSN) are shared between the different CN operators.

Rerouting procedure is a mechanism used as part of the assignment of a CN operator in a shared network when a MS not supporting network sharing performs initial attach/registration. Upon reception of the initial MS request message, the BSS may not know which CN operator to select and thus which CN operator to forward the initial MS request message to. If the initial MS request is not accepted by the CN operator selected by the BSS, a reroute of the initial MS request message to another CN operator within the shared network may be needed.

More precisely, the selection of a CN operator in the BSS is based on the NRI (valid or invalid) or by random selection. In case the CN operator cannot be deduced from the NRI and a GPRS attach or routing area updating initial layer 3 message (defined in [11]) shall be transferred in the UL-UNITDATA message, the BSS shall select a CN operator and initiate the rerouting procedure.

To trigger a rerouting procedure in a GWCN configuration, the BSS includes the Redirect Attempt Flag IE in the UL-UNITDATA message. By including the Redirect Attempt Flag IE in the UL-UNITDATA message, the SGSN supporting GWCN shall respond by including either the Redirection Indication IE or the Redirection Completed IE in the DL-UNITDATA message.

6.7.2 Reroute indication

If the UL-UNITDATA message does not contain the CS Registered Operator IE and the CN operator selected by the BSS cannot serve the MS and rerouting is possible (error causes are related to subscription options – defined in [11]), the reject Layer 3 Information (e.g. GPRS Attach Reject) and a Redirection Indication IE containing a Reroute Reject Cause shall be included in the DL-UNITDATA message for the downlink direction.

When neither the CS Registered Operator IE nor the Selected Operator IE is included in the UL-UNITDATA message and CS/PS domain registration coordination is required (see 3GPP TS 23.251 [43]), the Initial LLC-PDU and a Redirection Indication IE containing the Reroute Reject Cause set to “CS/PS domain registration coordination required” shall be included in the DL-UNITDATA message for the downlink direction. CS/PS domain registration coordination may not be performed by the SGSN for the operator’s own subscribers.

In addition the DL-UNITDATA message shall contain:

– The Initial LLC-PDU received from the MS;

– The IMSI, if available;

– The Unconfirmed send state variable, if available.

– The Old Routing Area Identification as indicated by the MS in the Initial LLC-PDU or,

– The Attach Indicator in case the Initial LLC-PDU contains a GPRS attach (see 3GPP TS 24.008 [11]).

If the DL-UNITDATA message containing the Redirection Indication IE is not related to the last attempted CN operator, then the BSS shall proceed as follows:

– if the DL-UNITDATA message contains the Old Routing Area Identification and the BSS from this information (and from BSS internal configuration) is able to uniquely identify one of the CN operators in the shared network, then the BSS shall re-initiate the procedure towards the identified CN operator with the Selected Operator, Initial LLC-PDU, Redirect Attempt Flag, IMSI and Unconfirmed send state variable (if received from previously attempted CN operator) included in the UL-UNITDATA message;

– if the DL-UNITDATA message contains the Attach Indicator then the BSS shall initiate the MS Registration Enquiry procedure in the CS domain (see 3GPP TS 48.008 [14]).

NOTE: The configuration by means the BSS identifies a CN operator from the received Old Routing Area Identification is implementation dependent and under operator control.

If, as a result of the MS Registration Enquiry procedure in the CS domain, the MS is found to be registered with one of the shared CN operators, then the BSS shall re-initiate the procedure towards the same CN operator with the CS Registered Operator, Initial LLC-PDU, Redirect Attempt Flag, IMSI and Unconfirmed send state variable (if received from previously attempted CN operator) included in the UL-UNITDATA message.

Upon reception of the Redirection Indication IE, the BSS shall store as part of the Rerouting Function the associated Reroute Reject Cause and LLC-PDU related to this CN operator.

In case all attempted CN operators have replied with a Redirection Indication IE, the BSS shall select the most appropriate Layer 3 Information received from the attempted CN operators based on the stored information as part of the Rerouting procedure and send it back to the MS (see 3GPP TS 24.008 [11]).

6.7.3 Reroute complete

If the selected CN operator accepts the initial request from the MS, then the SGSN shall respond to the BSS with the Redirection Completed IE, with the Outcome value field set to "MS is accepted" or "MS is already registered", and the Layer 3 Information LLC-PDU (e.g. GPRS Attach Accept) included in the DL-UNITDATA message.

Upon reception of the UL-UNITDATA message containing the CS Registered Operator IE, the SGSN shall conclude the Rerouting procedure and include the Redirection Completed IE in the DL-UNITDATA message.

Upon reception of the DL-UNITDATA message with the Redirection Completed IE included in the message, the BSS shall send the LLC-PDU to the MS and terminate the Rerouting procedure.

6.7.4 Abnormal Conditions

If the selected CN operator cannot serve the initial request from the MS and rerouting to another CN operator is not possible, then the SGSN shall send the DL-UNITDATA message to the BSS with the Outcome value field set to "MS is not accepted" within the Redirection Completed IE, and the Layer 3 Information LLC-PDU (e.g. GPRS Attach Reject) included in the message.