LTE Access and Registration Procedures
Learn the main LTE access and registration procedures, including Attach, Service Request, Tracking Area Update, Paging, and Detach, and see how a UE enters EPS service, restores connectivity, maintains reachability, and exits the network cleanly.
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Entry Procedures
Restore and Maintain Service
Reachability and Exit
What This Covers
These procedures sit at the boundary between radio access and EPC control. On the access side, the UE and eNB create or restore signaling connectivity. On the core side, NAS signaling with the MME handles mobility registration, reachability, and service restoration.
This is also where the main LTE state transitions become visible in practice. Attach moves the UE into registered EPS service, service request brings an already registered UE back from idle toward active signaling, and TAU keeps the UE reachable while it moves.
When these flows break, the impact is usually immediate and visible: attach failure blocks service entry, service request failure blocks return from idle, TAU issues break reachability and paging behavior, and detach or paging issues leave stale or inconsistent state behind.
Initial Access and EPS Entry
These procedures are used when the UE first enters LTE service and starts EPS registration.
- LTE Attach Procedure — UE enters EPS service and establishes initial registration and bearer context.
- LTE Combined EPS / IMSI Attach Procedure — Combined attach for EPS and legacy mobility domains.
- LTE Emergency Attach Procedure — Emergency-oriented LTE access flow.
Service Restoration and Idle Return
These procedures are used when the UE is already registered but needs to restore active connectivity or respond to network-initiated activity.
- LTE Service Request Procedure — Restores UE service and user-plane availability after idle state.
- LTE Extended Service Request Procedure — Used for service recovery in cases such as CS fallback-related continuation.
Mobility Registration Maintenance
These procedures keep the UE tracking area registration aligned with the network while the UE remains in LTE service.
- LTE Tracking Area Update Procedure — Updates the UE's tracking area registration and mobility context.
- LTE Periodic Tracking Area Update — Periodic registration refresh driven by network policy and timers.
Exit Procedures
These procedures remove EPS registration and cleanly end LTE service state.
- LTE Detach Procedure — Releases EPS registration and ends LTE service context.