* Tracking Area Updates (TAU) become applicable after the UE has completed the Attach procedure and has moved into the EPS Mobility Management (EMM) REGISTERED state. They can be triggered in either RRC Idle mode or RRC Connected mode, but the UE must be in RRC Connected mode to actually complete the procedure
* The TAU procedure is used for:
o normal TAU due to mobility, i.e. when the UE enters a tracking area which is not included within the list of tracking areas with which the UE is registered
o periodic TAU after the T3412 timer expires. The value of T3412 is initialised during the Attach procedure within the Attach Accept message shown in Table 293. Periodic TAU confirm to the network that the UE is still present
o registering with the CS domain for non-EPS services when the UE is already attached for EPS services. This involves completing an IMSI attach as part of the TAU procedure
o registering with the EPS after an incoming inter-system change, e.g. reselecting the LTE network after being camped on the UMTS network
o re-registering with the EPS after a CS fallback connection has been completed
o MME load balancing. The UE initiates a TAU procedure if the eNode B releases its RRC connection using a cause value of 'Load Balancing TAU Required'
o updating the UE specific DRX cycle. UE are allowed to select their own DRX cycle and signal it to the MME. The selected DRX cycle can be specified within the Tracking Area Update Request message
o indicating that a UE has selected a Closed Subscriber Group (CSG) cell whose CSG Identity is not included within the UE's allowed CSG list
* The TAU procedure may involve a change of Serving Gateway and/or a change ofMME
* The signalling associated with the TAU procedure is illustrated in Figure 239. This example includes a change ofMME but maintains the same Serving Gateway. The example assumes that the UE starts in RRC Idle mode so the Tracking Area Update Request is sent within an RRC Connection Setup Complete message. Ifthe UE is already in RRC Connected mode when the TAU procedure is triggered, the Tracking Area Update Request is sent within an Uplink Information Transfer message
* The UE starts timer T3430 after sending the Tracking Area Update Request message. 3GPP has standardised a fixed value of 15 s for the T3430 timer. If T3430 expires prior to the UE receiving a response from the MME, then the current TAU attempt is aborted and the TAU attempt count is incremented by 1. The UE starts timer T34ll if'the resultant attempt count is less than 5. 3GPP has standardised a fixed value of 10 s for the T3411 timer. The UE re-transmits the Tracking Area Update Request message once T3411 has expired
* If the TAU attempt count reaches 5, the UE starts timer T3402, and deletes its list of equivalent PLMN. 3GPP has standardised a default value of 12 minutes for the T3402 timer. The network can signal a different value for T3402 within the Attach Accept and Tracking Area Update Accept messages. A value signalled by the network is only applicable within the set of assigned Tracking Areas.The default value is re-adopted if the UE receives an Attach Accept or Tracking Area Update Accept message which does not specify a T3402 value. The TAU procedure can be re-initiated once T3402 expires
* The first 3 information elements within the Tracking Area Update Request message represent the message header. The ProtocolDiscriminator indicates that the subsequent message is an EPS mobility management message. Its value is specified within 3GPP TS 24.007. The Security Header Type indicates whether or not the NAS message is integrity protected and ciphered. The Tracking Area Update Request Message Identity indicates that the message itself is a Tracking Area Update Request
* The main content of the Tracking Area Update Request message starts with the EPS Update Type which can be configured with values of 'TA Updating', 'Combined TAJLA Updating', 'Combined TAJLA Updating with IMSI Attach' and 'Periodic Updating'.The 'TA Updating' value is used if the UE is only registered with the EPS, i.e. only registered for PS services. The 'Combined TAJLA Updating' value is used if the UE is registered for both EPS and non-EPS services, i.e. both CS and PS services. The 'Combined TA/LA Updating with IMSI Attach' value is used if the UE is registered for EPS services but wishes to also register for non-EPS services. The 'Periodic Updating' value is used when the T3412 timer expires
* The NAS Key Set Identifier (KSI) is used to identify the security parameters for authentication, integrity protection and ciphering. Security is not activated at this point in time when sending the Tracking Area Update Request message within an RRC Connection Setup Complete message so the KSI information element is not relevant
* The Old Globally Unique Temporary UE Identity (GUTI) information element provides the network with an identity for the UE. The GUTI can be derived from a previous P-TMSI and the associated RAI if they are available
* Once the eN ode B receives the RRC Connection Setup Complete message, the Tracking Area Update Request message is extracted and placed within an S 1 Application Protocol (Sl-AP) Initial UE Message. The content of this message is presented in Table 287 within section 26.1. The eN ode B includes information regarding the new Tracking Area Identity (TAl) and Cell Global Identity (CGI). The S-TMSI can also be included. The eNode B proceeds to forward the Initial UE Message to the new MME (this example assumes the TAU procedure involves a change ofMME)
* The new MME uses the old GUTI information from within the Tracking Area Update Request to derive the address of the old MME. The new MME then sends a Context Request message to the old MME. This message belongs to the GTP control plane protocol (GTPv2-C) and is specified within 3GPP TS 29.274. It is used to request user information from the old MME.
* The new MME responds with a Context Response message which includes the requested UE information. The new MME finalises the 3-way handshake by sending a Context Acknowledge message to the old MME. This triggers the old MME to mark its gateway and Home Subscriber Server (HSS) information for the UE as invalid. The context itself is maintained and is not deleted at this stage
* The new MME sends a Modify Bearer Request message to the Serving Gateway. This message also belongs to the GTP control plane protocol (GTPv2-C). It informs the Serving Gateway that the MME associated with the existing PDN connection has changed. Additional Modify Bearer Request messages are sent if the UE has multiple PDN connections. The Serving Gateway acknowledges the new information using the Modify Bearer Response message
* The new MME then proceeds to send an Update Location Request message to the HSS. This message belongs to the Diameter protocol and is specified within 3GPP TS 29.272. The MME is connected to the HSS using the S6a interface. The Update Location Request identifies the UE by its IMSI and informs the HSS that the UE has moved to the new MME. The HSS contacts the old MME using the Cancel Location message which also uses the Diameter protocol. The old MME then deletes the context for the UE and responds with a Cancel Location Acknowledge. Finally, the HSS uses the Update Location Acknowledge message to confirm to the new MME that is has updated its records for the UE
* The new MME then progresses the TAU procedure by sending a Tracking Area Update Accept message to the UE. This message is transferred across the Sl interface using an SlAP: Downlink NAS Transport message. The content of the Tracking Area Update Accept message is shown in Table 303
* The Tracking Area Update Complete message is a simple acknowledgement which only includes header information, i.e. Protocol Discriminator, Security Header Type and Message Identity
* The eN ode B extracts the Tracking Area Update Complete from within the RRC message and packages it within an SlAP: Uplink NAS Transport message. This message is then sent to the new MME across the S 1 interface
* 3GPP References: TS 24.301, TS 36.413, TS 36.331
* The TAU procedure is used for:
o normal TAU due to mobility, i.e. when the UE enters a tracking area which is not included within the list of tracking areas with which the UE is registered
o periodic TAU after the T3412 timer expires. The value of T3412 is initialised during the Attach procedure within the Attach Accept message shown in Table 293. Periodic TAU confirm to the network that the UE is still present
o registering with the CS domain for non-EPS services when the UE is already attached for EPS services. This involves completing an IMSI attach as part of the TAU procedure
o registering with the EPS after an incoming inter-system change, e.g. reselecting the LTE network after being camped on the UMTS network
o re-registering with the EPS after a CS fallback connection has been completed
o MME load balancing. The UE initiates a TAU procedure if the eNode B releases its RRC connection using a cause value of 'Load Balancing TAU Required'
o updating the UE specific DRX cycle. UE are allowed to select their own DRX cycle and signal it to the MME. The selected DRX cycle can be specified within the Tracking Area Update Request message
o indicating that a UE has selected a Closed Subscriber Group (CSG) cell whose CSG Identity is not included within the UE's allowed CSG list
* The TAU procedure may involve a change of Serving Gateway and/or a change ofMME
* The signalling associated with the TAU procedure is illustrated in Figure 239. This example includes a change ofMME but maintains the same Serving Gateway. The example assumes that the UE starts in RRC Idle mode so the Tracking Area Update Request is sent within an RRC Connection Setup Complete message. Ifthe UE is already in RRC Connected mode when the TAU procedure is triggered, the Tracking Area Update Request is sent within an Uplink Information Transfer message
* The UE starts timer T3430 after sending the Tracking Area Update Request message. 3GPP has standardised a fixed value of 15 s for the T3430 timer. If T3430 expires prior to the UE receiving a response from the MME, then the current TAU attempt is aborted and the TAU attempt count is incremented by 1. The UE starts timer T34ll if'the resultant attempt count is less than 5. 3GPP has standardised a fixed value of 10 s for the T3411 timer. The UE re-transmits the Tracking Area Update Request message once T3411 has expired
* If the TAU attempt count reaches 5, the UE starts timer T3402, and deletes its list of equivalent PLMN. 3GPP has standardised a default value of 12 minutes for the T3402 timer. The network can signal a different value for T3402 within the Attach Accept and Tracking Area Update Accept messages. A value signalled by the network is only applicable within the set of assigned Tracking Areas.The default value is re-adopted if the UE receives an Attach Accept or Tracking Area Update Accept message which does not specify a T3402 value. The TAU procedure can be re-initiated once T3402 expires
* The first 3 information elements within the Tracking Area Update Request message represent the message header. The ProtocolDiscriminator indicates that the subsequent message is an EPS mobility management message. Its value is specified within 3GPP TS 24.007. The Security Header Type indicates whether or not the NAS message is integrity protected and ciphered. The Tracking Area Update Request Message Identity indicates that the message itself is a Tracking Area Update Request
* The main content of the Tracking Area Update Request message starts with the EPS Update Type which can be configured with values of 'TA Updating', 'Combined TAJLA Updating', 'Combined TAJLA Updating with IMSI Attach' and 'Periodic Updating'.The 'TA Updating' value is used if the UE is only registered with the EPS, i.e. only registered for PS services. The 'Combined TAJLA Updating' value is used if the UE is registered for both EPS and non-EPS services, i.e. both CS and PS services. The 'Combined TA/LA Updating with IMSI Attach' value is used if the UE is registered for EPS services but wishes to also register for non-EPS services. The 'Periodic Updating' value is used when the T3412 timer expires
* The NAS Key Set Identifier (KSI) is used to identify the security parameters for authentication, integrity protection and ciphering. Security is not activated at this point in time when sending the Tracking Area Update Request message within an RRC Connection Setup Complete message so the KSI information element is not relevant
* The Old Globally Unique Temporary UE Identity (GUTI) information element provides the network with an identity for the UE. The GUTI can be derived from a previous P-TMSI and the associated RAI if they are available
* Once the eN ode B receives the RRC Connection Setup Complete message, the Tracking Area Update Request message is extracted and placed within an S 1 Application Protocol (Sl-AP) Initial UE Message. The content of this message is presented in Table 287 within section 26.1. The eN ode B includes information regarding the new Tracking Area Identity (TAl) and Cell Global Identity (CGI). The S-TMSI can also be included. The eNode B proceeds to forward the Initial UE Message to the new MME (this example assumes the TAU procedure involves a change ofMME)
* The new MME uses the old GUTI information from within the Tracking Area Update Request to derive the address of the old MME. The new MME then sends a Context Request message to the old MME. This message belongs to the GTP control plane protocol (GTPv2-C) and is specified within 3GPP TS 29.274. It is used to request user information from the old MME.
* The new MME responds with a Context Response message which includes the requested UE information. The new MME finalises the 3-way handshake by sending a Context Acknowledge message to the old MME. This triggers the old MME to mark its gateway and Home Subscriber Server (HSS) information for the UE as invalid. The context itself is maintained and is not deleted at this stage
* The new MME sends a Modify Bearer Request message to the Serving Gateway. This message also belongs to the GTP control plane protocol (GTPv2-C). It informs the Serving Gateway that the MME associated with the existing PDN connection has changed. Additional Modify Bearer Request messages are sent if the UE has multiple PDN connections. The Serving Gateway acknowledges the new information using the Modify Bearer Response message
* The new MME then proceeds to send an Update Location Request message to the HSS. This message belongs to the Diameter protocol and is specified within 3GPP TS 29.272. The MME is connected to the HSS using the S6a interface. The Update Location Request identifies the UE by its IMSI and informs the HSS that the UE has moved to the new MME. The HSS contacts the old MME using the Cancel Location message which also uses the Diameter protocol. The old MME then deletes the context for the UE and responds with a Cancel Location Acknowledge. Finally, the HSS uses the Update Location Acknowledge message to confirm to the new MME that is has updated its records for the UE
* The new MME then progresses the TAU procedure by sending a Tracking Area Update Accept message to the UE. This message is transferred across the Sl interface using an SlAP: Downlink NAS Transport message. The content of the Tracking Area Update Accept message is shown in Table 303
The first 3 information elements within the Tracking Area Update Accept message represent the message header. The Protocol Discriminator indicates that the subsequent message is an EPS mobility management message. Its value is specified within 3GPP TS 24.007. The Security Header Type indicates whether or not the NAS message is integrity protected and ciphered. The Tracking Area Update Accept Message Identity indicates that the message itself is an Tracking Area Update Accept
* The EPS Update Result can be allocated values of 'TA Updated', 'Combined TAILA Updated', 'TA Updated and ISR Activated' and 'Combined TA/LA updated and ISR Activated'. Idle mode Signalling Reduction (ISR) is applicable to E-UTRAN UE which support GERAN and/or UTRAN. It allows a reduction of the signalling between the UE and the network, as well as a reduction of the signalling within the network
* The MME starts timer T3450 after sending the Tracking Area Update Accept message. 3GPP has standardised a fixed value of 6 s for the T3450 timer. IfT3450 expires prior to the MME receiving a response from the UE, then the Tracking Area Update Accept message is re-sent. The Tracking Area Update Accept message can be re-sent four times. The overall TAU procedure is aborted if T3450 expires a fifth time
* The eN ode B extracts the Tracking Area Update Accept from within the SlAP message and packages it within an RRC: Downlink Information Transfer message. This message is then sent to the UE across the air-interface
* The UE completes the overall procedure by sending a Tracking Area Update Complete message to the MME. This message is transferred across the air-interface using an RRC: Uplink Information Transfer message. The content of the Tracking Area Update Complete message is shown in Table 304
* The Tracking Area Update Complete message is a simple acknowledgement which only includes header information, i.e. Protocol Discriminator, Security Header Type and Message Identity
* The eN ode B extracts the Tracking Area Update Complete from within the RRC message and packages it within an SlAP: Uplink NAS Transport message. This message is then sent to the new MME across the S 1 interface
* 3GPP References: TS 24.301, TS 36.413, TS 36.331
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