LTE Tracking Area Update Procedure Call Flow
LTE Tracking Area Update is the mobility update procedure used when the UE is already registered and needs to refresh or change its tracking-area registration without performing a fresh attach.
It appears during mobility, periodic update timing, context recovery, or other cases where the network must update the UE registration state while keeping EPS service continuity.
Introduction
The LTE Tracking Area Update Procedure updates the registration and mobility context for a UE that is already known to the EPC. It often starts after idle mobility into a new tracking area or after a network timer forces a registration refresh.
The main nodes are the UE, eNB, and MME, with HSS and SGW involved when the update changes more than simple registration state.
What Is LTE Tracking Area Update in Simple Terms?
- What starts the procedure: The UE moves into a new tracking area, reaches a periodic update timer, or needs mobility-context recovery.
- What the UE and network want to achieve: Keep EPS registration valid while updating location and related mobility context.
- What success looks like: The UE sends Tracking Area Update Request and receives Tracking Area Update Accept.
- What failure means: The update is rejected, security restarts and fails, or the stored context is no longer usable and another procedure is needed.
Why this procedure matters
TAU is the main LTE mobility-registration procedure after attach. It explains how the UE stays reachable and registered while moving, camping idle, or recovering from stored-context changes.
Quick Fact Sheet
| Procedure name | LTE Tracking Area Update Procedure |
|---|---|
| Domain | EPS mobility update |
| Main trigger | Tracking area change, periodic registration timer, or recovery update |
| Start state | UE is EMM-REGISTERED with existing EPS context |
| End state | Tracking area and mobility context are refreshed and still registered |
| Main nodes | UE, eNB, MME, HSS |
| Main protocols | RRC, NAS, S1AP, Diameter |
| Main success outcome | UE remains registered with updated tracking-area context |
| Main failure outcome | TAU Reject, security failure, or forced return to attach/recovery behavior |
| Most important messages | Tracking Area Update Request, Security Mode Command, Tracking Area Update Accept, Tracking Area Update Complete |
| Main specs | TS 23.401, TS 24.301, TS 29.272, TS 36.300, TS 36.331 |
Preconditions
- The UE is already attached and has valid EPS registration state.
- The UE can access the serving LTE cell and deliver the NAS request.
- The MME can still interpret the stored identity and context used by the UE.
- The relevant security context is still valid enough for TAU continuation or can be refreshed.
Nodes and Interfaces
Nodes involved
| Node | Role in this procedure |
|---|---|
| UE | Starts the mobility update and sends TAU Request with the update type. |
| eNB | Provides the fresh signaling path and relays the NAS exchange. |
| MME | Validates and updates the stored mobility context. |
| HSS | Provides subscriber profile refresh when required. |
Interfaces used
| Interface | Endpoints | Role |
|---|---|---|
| LTE Uu | UE <-> eNB | Carries the access leg and TAU NAS container. |
| S1-MME | eNB <-> MME | Carries the TAU control exchange. |
| S6a | MME <-> HSS | Carries subscriber-profile handling when the update needs it. |
End-to-End Call Flow
UE eNB MME HSS
| | | |
|--RRC access---->| | |
|--RRC Setup Complete + TAU Request--------------->|
| |--Initial UE Message----------->|
| | |----ULR / profile update--------->|
| | |<---ULA / profile result----------|
|<--Security Mode Command-----------| |
|--Security Mode Complete---------->| |
| |<--TAU Accept--------------------|
|<--Downlink NAS Transport----------| |
|--TAU Complete-------------------->|---------------->|Major Phases
1. Mobility or timer trigger
The UE decides that the registration context must be updated.
2. TAU Request delivery
The UE sends the request with the update type and old context identifiers.
3. Context and security handling
The MME validates the stored context and may refresh security or subscriber profile data.
4. Accept and completion
The network returns the updated mobility result and the UE confirms completion.
Step-by-Step Breakdown
TAU trigger
Sender -> receiver: UE internal trigger
Message(s): No NAS message yet
Purpose: Decide that mobility context must be refreshed.
State or context change: UE prepares a TAU request instead of a fresh attach.
Note: The update type tells you whether this is normal TA change, periodic refresh, or recovery-related TAU.
TAU Request
Sender -> receiver: UE -> MME
Message(s): Tracking Area Update Request
Purpose: Ask the network to keep the UE registered while refreshing mobility context.
State or context change: MME starts the mobility update transaction.
Note: Old GUTI and update type are the first high-value fields to inspect.
Security and profile continuation
Sender -> receiver: MME -> UE and MME -> HSS
Message(s): Security Mode and profile update when needed
Purpose: Make sure the stored context is still valid and protected.
State or context change: TAU either continues under valid protected context or fails into another branch.
Note: Not every TAU needs the same amount of profile refresh, so always validate the scenario first.
Accept and completion
Sender -> receiver: MME -> UE -> MME
Message(s): Tracking Area Update Accept, Tracking Area Update Complete
Purpose: Close the mobility update cleanly.
State or context change: UE remains registered with updated location context.
Note: Returned timers and TAI list often explain later UE behavior better than the accept result alone.
Important Messages
| Message | Protocol | Sender -> Receiver | Purpose in this procedure | What to inspect briefly |
|---|---|---|---|---|
| Tracking Area Update Request | NAS | UE -> MME | Starts the mobility update procedure. | Update type, old GUTI, active flag, and cause for the update. |
| Security Mode Command | NAS | MME -> UE | Refreshes protected NAS continuation when needed. | Whether security restart is expected for the scenario. |
| Tracking Area Update Accept | NAS | MME -> UE | Returns the updated registration and mobility result. | TAI list, GUTI reallocation, timer values, and final result. |
| Tracking Area Update Complete | NAS | UE -> MME | Confirms that the UE accepted the TAU result. | Whether it arrives promptly after accept. |
Important Parameters to Inspect
| Item | What it is | Where it appears | Why it matters | Common issues |
|---|---|---|---|---|
| Update type | The reason for the TAU procedure. | Tracking Area Update Request | Separates normal, periodic, and recovery TAU behavior. | Wrong scenario assumption. |
| Old GUTI | Stored temporary identity from the earlier registration. | Tracking Area Update Request | Lets the MME locate existing UE context quickly. | Stale identity, wrong MME context. |
| TAI list and timers | Returned mobility parameters after TAU succeeds. | Tracking Area Update Accept | Explain later idle mobility and retry behavior. | Unexpected timer values, wrong area list. |
Successful Completion
A successful TAU keeps the UE registered while updating tracking-area and related mobility context. The UE remains available for later paging, service request, and normal EPS operation.
Common Failures and Troubleshooting
TAU Request is sent but the network rejects it
Likely cause: Stored context is invalid, roaming or area policy does not allow continuation, or the update type is not acceptable.
Where to inspect: TAU Request, reject cause, old identity, timer context.
Relevant message(s): Tracking Area Update Request, Tracking Area Update Reject
Relevant interface(s): NAS, S1-MME
Likely next step: Use the reject cause to decide whether attach, retry, or subscriber correction is needed.
TAU Accept is sent but the UE does not complete
Likely cause: NAS completion was lost or the UE did not apply the returned context.
Where to inspect: TAU Accept, uplink NAS transport, timer behavior.
Relevant message(s): Tracking Area Update Accept, Tracking Area Update Complete
Relevant interface(s): LTE Uu, S1-MME
Likely next step: Confirm whether the UE received the accept and whether the final uplink message reached the MME.
What to Check in Logs and Traces
- Start with the TAU update type before analyzing the rest of the trace.
- Check the old identity and whether the MME resolved it cleanly.
- Inspect returned TAI list, timers, and any GUTI reallocation.
- Confirm the UE returned Tracking Area Update Complete.
Related Pages
Related sub-procedures
Related message reference pages
Related troubleshooting pages
Notes
TAU updates mobility state without rebuilding the full attach path. The most useful checks are the update type, old context identity, returned timers, and whether the UE stayed in a usable registered state afterward.
FAQ
What is LTE TAU used for?
It refreshes the UE tracking-area and mobility registration context while keeping EPS registration active.
How is TAU different from Attach?
Attach builds fresh EPS registration. TAU updates an already registered UE.
What should I inspect first?
Start with the TAU update type and old GUTI in the request.