Core Network: Architecture and Signaling Procedures

The UE starts the deactivation by sending a NAS SM Deactivate PDP Context Request message (TS 24.008 clause 9.5.14) to the SGSN through the radio access network. When the UE sends this message, the UE enters the PDP-INACTIVE-PENDING state and starts timer T3390 (TS 24.008 clause 6.1.3.4.1), whose value is 8 seconds (TS 24.008 Table 11.2c). The message identifies the PDP context by its TI and carries an SM cause, for example #36 "regular deactivation", #37 "QoS not accepted", or #25 "LLC or SNDCP failure (A/Gb mode only)" (TS 24.008 clause 6.1.3.4.1). If the UE deactivates all active PDP contexts that share the same PDP address and APN, the UE includes the Tear down indicator IE; the UE then deactivates the other PDP contexts locally, without peer-to-peer signaling (TS 24.008 clause 6.1.3.4). The UE also includes the Tear down indicator IE when the UE deactivates the PDP context that the PDP Context Activation procedure created (TS 23.060 clause 9.2.4.1, step 1), and a UE that supports S1 mode always includes the IE when the UE deactivates the default PDP context (TS 24.008 clause 6.1.3.4). The message can also carry the PCO (TS 24.008 clause 9.5.14.2).

The SGSN checks the TI and sends a GTPv1-C Delete PDP Context Request message (TS 29.060 clause 7.3.5) to the GGSN over the Gn/Gp interface. The message tells the GGSN to release the control-plane and user-plane GTP tunnels of the PDP context that the NSAPI identifies. The GTP header carries the GGSN TEID for control-plane messages. The NSAPI is the only mandatory IE (TS 29.060 Table 11). The SGSN copies the Teardown Ind IE from the Deactivate PDP Context Request message if the UE included it, and sets the Teardown Ind to "1" when the PDP context is the last PDP context of the PDN connection; with the Teardown Ind set to "1", the GGSN tears down all PDP contexts that share the PDN connection. The SGSN copies the PCO transparently from the Deactivate PDP Context Request message, includes the User Location Information, which holds the CGI or the SAI of the UE, together with the ULI Timestamp, which records when the SGSN acquired the User Location Information, and includes the MS Time Zone if the MS Time Zone changed since the SGSN last reported it. If the SGSN sends the message because of a network problem, the SGSN includes the Cause IE, for example "Network Failure" (TS 29.060 clause 7.3.5).

When the GGSN receives the request, and the GGSN acts as the PCEF of dynamic PCC, the GGSN sends a Diameter Credit-Control-Request (CCR) message to the PCRF over the Gx reference point. The content of the message depends on whether other PDP contexts of the IP-CAN session stay active, because in GPRS an IP-CAN session ends only when the last PDP context of the IP-CAN session ends (TS 29.212 Annex A.3.5). If other PDP contexts stay active, the GGSN sends a CCR-U message, that is a CCR message with the CC-Request-Type AVP set to UPDATE_REQUEST (2), and reports the PCC rules of the PDP context as inactive in the Charging-Rule-Report AVP; when the PCRF performs the bearer binding, the GGSN also includes the Bearer-Identifier AVP and the Bearer-Operation AVP set to TERMINATION (TS 29.212 clause 4.5.6 and Annex A.3.4). If the PDP context is the last PDP context of the IP-CAN session, the GGSN sends a CCR-T message, that is a CCR message with the CC-Request-Type AVP set to TERMINATION_REQUEST (3) (TS 29.212 clause 4.5.7 and Annex A.3.5), which can carry the reason for the termination in the Termination-Cause AVP, for example DIAMETER_LOGOUT (1) (IETF RFC 6733 section 8.15). Both messages identify the Gx session by the Session-Id AVP and carry the CC-Request-Number AVP. If the GGSN and the PCRF support the RAN-NAS-Cause feature, the GGSN includes the location of the UE in the 3GPP-User-Location-Info AVP and the time zone of the UE in the 3GPP-MS-TimeZone AVP, if available (TS 29.212 clauses 4.5.6 and 4.5.7). The GGSN also delivers the User Location Information and the UE Time Zone to the PCRF when the PCRF requested them (TS 23.060 clause 9.2.4.1, step 3).

The PCRF acknowledges the request with a Diameter Credit-Control-Answer (CCA) message to the GGSN over the Gx reference point. The message carries the Result-Code AVP set to DIAMETER_SUCCESS (2001) (IETF RFC 6733 section 7.1.2), the CC-Request-Type AVP with the same value as in the request, and the CC-Request-Number AVP. In reply to a CCR-U message, the PCRF can make a new policy decision and, when the PCRF performs the bearer binding, move the PCC rules of the terminated PDP context to a remaining PDP context; the GGSN removes the PCC rules that the PCRF does not move (TS 29.212 Annex A.3.4). In reply to a CCR-T message, the PCRF only acknowledges the request, and the CCR-T and CCA-T exchange ends the Diameter Credit Control session of the IP-CAN session (TS 29.212 clause 4.5.7).

If the GGSN sent an Accounting-Request (START) message for the PDP context, the GGSN sends a RADIUS Accounting-Request (STOP) message to the external AAA server over the Gi reference point (TS 29.061 clauses 16.3.1 and 16.4.4). The GGSN can send one Accounting-Request (STOP) message for each PDP context, primary or secondary, and the message carries the final usage counters of the PDP context, for example the Acct-Input-Octets, Acct-Output-Octets, and Acct-Session-Time attributes. When the PDP context is the last PDP context of the IP-CAN session, the GGSN includes the 3GPP Vendor-Specific sub-attribute 3GPP-Session-Stop-Indicator; the AAA server does not consider the IP-CAN session terminated until the AAA server receives this sub-attribute (TS 29.061 clause 16.2).

The GGSN removes the PDP context from its session tables, releases the bindings of the user-plane GTP-U TEIDs, and releases the dynamic PDP address if the PDP context is the last PDP context associated with this PDP address (TS 23.060 clause 9.2.4.1, step 3). The GGSN then returns a GTPv1-C Delete PDP Context Response message (TS 29.060 clause 7.3.6) to the SGSN over the Gn/Gp interface. If the GGSN sent an Accounting-Request (STOP) message, the GGSN sends the response at once, without waiting for the RADIUS Accounting-Response message (TS 29.061 clause 16.3.1). The GTP header carries the SGSN TEID for control-plane messages. The message carries the Cause, set to "Request accepted" (128), or to "Non-existent" (192) when the GGSN received the request for a PDP context that does not exist; in the second case, the GTP header carries the TEID value zero (TS 29.060 clause 7.3.6). The message can also carry the PCO (TS 29.060 Table 12). The User Location Information, the ULI Timestamp, and the MS Time Zone appear in this message only when the SGSN sends the response in the GGSN-initiated variant of the procedure (TS 29.060 clause 7.3.6).

If the GGSN sent an Accounting-Request (STOP) message, the external AAA server acknowledges the message with a RADIUS Accounting-Response message to the GGSN over the Gi reference point. If the AAA server assigned the IP address of the UE, the AAA server can make the address available for assignment again after the AAA server receives an Accounting-Request (STOP) message for every PDP context of the IP-CAN session (TS 29.061 clause 16.2).

When the SGSN receives the response, the SGSN removes the PDP context from its session database and sends a NAS SM Deactivate PDP Context Accept message (TS 24.008 clause 9.5.15) to the UE (TS 23.060 clause 9.2.4.1, step 4). The message carries the TI, and can carry the PCO, which the SGSN copies transparently from the Delete PDP Context Response message (TS 29.060 clause 7.3.6). When the UE receives this message, the UE stops timer T3390 (TS 24.008 clause 6.1.3.4.1) and enters the PDP-INACTIVE state for the PDP context that the TI identifies (TS 24.008 clause 6.1.2.1).

In Iu mode, the SGSN releases the access stratum resources of the PDP context with the RAB Assignment procedure, if a RAB exists for the PDP context (TS 23.060 clause 9.2.4.1, step 5). The SGSN sends a RANAP RAB Assignment Request message to the RNC over the IuPs interface, with the RAB to release in the RABs To Be Released List IE (TS 25.413 clause 8.2). For each RAB, the message carries the RAB ID, which contains the NSAPI of the PDP context (TS 25.413 clause 9.2.1.2), and the Cause (TS 25.413 clause 9.2.1.4), which is typically "Normal Release" (83) for a regular deactivation.

The RNC releases the radio bearers of the RAB and sends an RRC Radio Bearer Release message (TS 25.331 clause 10.2.30) to the UE over the Uu interface. The message tells the UE to release the RLC and PDCP entities of the radio bearers. The message carries the RRC transaction identifier and the RB information to release list, which holds the RB identity of each radio bearer to release, and can carry the RAB information to reconfigure list.

The UE releases the radio bearers and returns an RRC Radio Bearer Release Complete message (TS 25.331 clause 10.2.31) to the RNC over the Uu interface, to confirm that the UE released the radio resources. The message carries the RRC transaction identifier.

The RNC releases its local resources and returns a RANAP RAB Assignment Response message to the SGSN over the IuPs interface. The message carries the RAB ID of each released RAB in the RABs Released List IE, which confirms that the RNC released the transport resources of the PDP context (TS 25.413 clause 8.2).