In this tutorial, we will discuss the overview of LTE Tracking Area update and then discuss in details the end-to-end call flow signalling procedure.
Before we begin, let’s check what is a Tracking Area in LTE network.
LTE Tracking Area
The tracking area is similar to Location Area and Routing area in UMTS, which is basically a geographical combination of several base stations (eNodeBs as in LTE).
Each Tracking Area has two main identities:
- Tracking Area Code (TAC)
- Tracking Area Identity (TAI)
Tha Tracking Area Code identifies the tracking area within a particular network and if we combine the TAC with PLMN-ID then we will get the globally unique Tracking Area Identity.
TAI = PLMN-ID + TAC
The Tracking Area Code has a range of 0 to 65536.
Tracking Area Update
A UE triggers Tracking Area Update after the UE completes the Attach Procedure and moves into the EMM-REGISTERED state.
EMM: EPS Mobility Management
Also, note that the UE can trigger Tracking Area Update in RRC Idle state or RRC Connected state, but the procedure ends only when the UE is in RRC Connected State.
Reason For Tracking Area Update
A UE starts the Tracking Area Update procedure in following events:
- When a UE moves to a new Tracking Area which is not included in its list of Tracking Areas with which the UE is registered.
- When the timer T3412 expires the UE triggers the Tracking Area Update procedure. The value of T3412 is initialised in the Attach Accept message during the Attach procedure.
- Registering with the CS domain for non-EPS service when the UE is already attached for EPS services. This involves completing an IMSI attach as part or Tracking Area Update procedure.
- Registering for EPS service after an inter-system change. For example reselecting to LTE from UMTS.
- Re-registering to LTE after CS-fallback completed.
- MME load balancing. The UE initiates TAU procedure if the eNodeB releases the RRC connection with a cause ‘Load Balancing TAU Required’.
- When UE-specific DRX cycle is updated.
- Indicating that the UE has selected a Closed Subscriber Group (CSG) cell whose CSG Identity is not included in the UE’s allowed CSG list.
Tracking Area Update Call Flow
The following example shows Tracking Area Update procedure when UE changes the MME but maintains the Serving Gateway. Also, we assume that the UE starts in RRC Idle mode. So the Tracking Area Update Request message is sent within the RRC Connection Setup Complete message.
If the UE is already in RRC Connected state when TAU procedure is triggered, the Tracing Area Update Request is sent within an Uplink Information Transfer message.