2. Concept of Periodic TAU

A UE, while in Connected state, has an established end-to-end EPS bearer connecting the UE to the network (P-GW). The network (MME) keeps track of which cell the UE is located in. So, if there is any traffic destined to the UE, the network can deliver it immediately.

However, if a UE enters Idle state, the signaling connection and bearers (E-RAB bearer) between the UE and the network (MME) are all released. Then, the network (MME) loses track of the UE’s location. The network should always be aware of the current location of all UEs, whether in Active or Idle state, in order to deliver traffic to UEs that are in Idle state. So, those in Idle states should report their current location, i.e. in which TA (Tracking Area) they are located, to the network (MME) periodically even when there is no data to deliver. A TA is a group of cells, and managed by MME. The location of UEs in Idle state is recognized at a TA level.

To this end, MME provides a UE with a TAI list and TAU timer (T3412) as included in a Attach Accept message when the UE initially attaches the network. Using them, the UE performs a TAU procedure upon expiration of the TAU timer.

When MME receives TA information from a UE, it updates the UE’s current location information (TA, cell) to keep the latest information. In case there is traffic destined to the UE while it is in Idle state, the MME informs the UE about the new traffic by sending a Paging Message to the cells in the TA that has been reported by the UE as its current location¹.

Figure 1 shows an example of a TAU procedure performed by a UE in Idle state. The UE (UE1) has attached Cell 2 in eNB1 upon its initial attach, and has been allocated a TAI list (e.g. TAI={TAI1, TAI2}) and TAU timer (e.g. T3412=60 mins) through the Attach Accept message sent from the MME. Later, after transiting to Idle state, it travels from ❶ → ❷ → ❸ → ❹. For the purposes of this example, we assume i) the UE travels between only the TAs that are in the TAI list initially allocated through the Attach Accept message, ii) the MME the UE reports its TA information to is the one the UE’s context is stored in, and iii) both the UE and the MME have kept the NAS security context (K_NASint, K_NASenc, etc.) valid.

Figure 1. Concept of Periodic TAU 

After attaching and transiting to Idle state in Cell 2, UE1 wakes up and establishes an ECM signaling connection with the MME when the TAU timer expires at T1. Next, it sends the MME a TAU Request (TAI=TAI1, Last Visited TAI=TAI1) message that includes the TAI of its current cell, and the last visited TAI (the one reported through the last TAU request) (❶). Then, UE1 returns to Idle state when it receives a TAU Accept message. After receiving the TAU Request message from the Idle UE, the MME checks whether the UE’s last TAI (TAI of Last TAU) has been changed or not, and updates it with the latest information if it has been changed. If there is traffic heading to UE1, and thus the S-GW notifies the MME of the DL traffic, the MME finds out which TA the UE was in last time based on the TAI of Last TAU information, and sends a Paging Message to the cells in the TA (see our LTE technical document, “LTE EMM Procedure 4. Service Request” [2] for detailed information). Later, if UE1 moves to Cell 4, and the TAU timer expires (❷), UE1 sends the MME a TAU Request (TAI=TAI1, Last Visited TAI=TAI1) message as it is still in TA1. If UE1 moves to Cell 11, and the timer expires (❸), it sends the MME a TAU Request (TAI=TAI2, Last Visited TAI=TAI1) message, including TAI2 instead of TAI1, as now it belongs to TA2. Then the MME accordingly updates UE1’s TAI of Last TAU with TAI2.

Figure 2 shows the connections established, and the states of UE and MME in the user and control planes before and after the periodic TAU procedure.

Figure 2. Connections and States before/after TAU

A UE’s states before, during and after a periodic TAU procedure are as follows:

(i) Before the procedure, the UE stays in EMM-Registered, ECM-Idle and RRC-Idle state, and any resources allocated by E-UTRAN, i.e. the E-RAB (except for the uplink S1 bearer) and ECM signaling connection between the UE and the MME, are kept released.

(ii) During the procedure, the UE stays in EMM-Registered, ECM-Connected and RRC-Connected state. The periodic TAU procedure is different from procedures for initial attach or service request in that no E-RAB (between UE and MME) is set up, and only the signaling connection (ECM signaling connection) for delivering periodic TAU-related NAS messages is set up during this procedure.

(iii) After the procedure, the ECM signaling connection set between the two entities is released, releasing the E-UTRAN resources. Then, the UE returns to EMM-Registered, ECM-Idle and RRC-Idle state again.

Figure 3 is another illustration that shows the state transition of a UE before/after a periodic TAU procedure. Upon the expiration of T3412, the UE which has been in Idle state sends the MME a TAU Request message to report its current TA and last TA (then it transits to Connected state). Then the MME returns a TAU Accept message to the UE after updating the UE’s location information (TA). As a result, the signaling connection between the two entities is released, and the UE transits back to Idle state.

Figure 3. State Transition of a User Performing Periodic TAU