LTE: Tracking Area Update Request

The TRACKING AREA UPDATING (TAU) procedure is always initiated by the UE and is used for (some of) the following purposes:

·         Normal TAU: When the UE enters a tracking area that is not in the list of tracking areas that the UE previously registered in the MME

·         Combined TAU: When a UE operating in 'CS/PS mode 1' or 'CS/PS mode 2' enters a tracking area that is not in the list of tracking areas that the UE previously registered in the MME

·         Periodic TAU: Upon the expiry of timer T3412, periodic TAU procedure is initiated to periodically notify the availability of the UE to the network

·         IMSI attach for non-EPS services when the UE is attached for EPS services. This procedure is used by a UE in 'CS/PS mode 1' or 'CS/PS mode 2' of operation

·         In various cases of inter-system change from “Iu mode to S1 mode” or from “A/Gb mode to S1 mode” or from “S101 mode to S1 mode”

·         MME load balancing: When the UE receives RRC CONNECTION RELEASE message with cause ‘load balancing TAU required', it initiates TAU procedure

·         To update certain UE specific parameters in the network (ex:- when the UE changes the “UE network capability information” or the “MS network capability information” or the UE specific DRX parameter etc...)

The UE initiates the TAU procedure by sending TRACKING AREA UPDATE REQUEST message. The IEs (some of them are explained below) in this message include EPS update type, NAS key set identifier, Old GUTI, GPRS ciphering key sequence number, Old P-TMSI signature, Additional GUTI, UE network capability, Last visited registered TAI, DRX parameter, UE radio capability information update needed, EPS bearer context status, MS network capability, Old location area identification, Supported Codecs, Additional update type, Voice domain preference and UE's usage setting, Device properties etc…

In the IE EPS update type the first 3 bits (LSBs) shall be used to indicate update type (ex:- ‘TA updating’ or ‘combined TA/LA updating’ or ‘combined TA/LA updating with IMSI attach’ or ‘periodic updating’). Additionally, if a UE has uplink user data pending when it initiates the TAU procedure or uplink signalling not related to the TAU procedure, it may also set an "active" flag (bit4) in this IE to indicate the request to establish the user plane to the network and to keep the NAS signalling connection after the completion of the TAU procedure

The IE Old GUTI shall be handled as follows:

·         UEs supporting A/Gb mode or Iu mode or both:

-      If TIN = "P-TMSI" and the UE holds a valid P-TMSI and RAI, the UE shall map the P-TMSI and RAI into the Old GUTI IE, and include Old GUTI type IE with GUTI type set to "mapped GUTI". Additionally, if the UE holds a valid GUTI, then it shall be indicated in the IE Additional GUTI

-      If TIN = “GUTI” or “RAT-related TMSI” and the UE holds a valid GUTI, the UE shall indicate the GUTI in the Old GUTI IE, and include Old GUTI type IE with GUTI type set to "native GUTI"

·         UEs not supporting A/Gb mode or Iu mode: The UE shall include a valid GUTI in the Old GUTI IE. In addition, the UE shall include Old GUTI type IE with GUTI type set to "native GUTI"

The IE Additional Update Type provides additional information about the type of request for a TAU procedure. Bit1 value 0 means that there is “no additional information” and 1 means that “SMS only”

Voice domain preference and UE's usage setting IE shall be included if the UE supports CS fallback and SMS over SGs, or if the UE is configured to support IMS voice, but does not support 1xCS fallback. This IE provides the network with the UE's usage setting and the voice domain preference for E-UTRAN. Refer to 3GPP TS 24.008, section 10.5.5.28 for detailed information

The IE Last Visited Registered TAI shall be included if the UE holds a valid last visited registered Tracking Area Identity (TAI)

The purpose of the Device properties IE is to indicate if the MS is configured for NAS signalling low priority. The network uses the Device properties IE for core-network congestion handling and for charging purposes. This IE can indicate “MS is not configured for NAS signalling low priority” or “MS is configured for NAS signalling low priority”

Reference: 3GPP TS 24.301 and 24.008

Example1: TRACKING AREA UPDATE REQUEST message (Normal TA updating)

Example2: TRACKING AREA UPDATE REQUEST message (Periodic TA updating)