UMTS - NAS Signaling Protocol


The NAS signaling protocol is genuinely a 3GPP protocol and developed for 3GPP only, hence, nowhere else than in 3GPP systems this protocol could be found.

  • UE for mobility and session management.
  • MME for EPC and legacy network nodes (SGSN for GPRS and MSC for CS domain).
  • Messages exchanged.

NAS Signaling Protocol for EPS Mobility Management

NAS signaling EPS mobility management procedures are listed in below table. The "X" in the column "C" indicates whether a variant on the combination with the CS NAS signaling protocol stack is on matching (the case for a configuration with CSFB allowed, under the purported combined).

Only cases of success were presented (if an error occurred using appropriate rejection messages, mainly through the network); the explanation does not include all possible cases. The dialogues messages are usually kept by timers against loss of messages; e.g. timer T3410 is used to supervise the procedure and attach expires after 15 seconds. So if no response (acceptance or rejection) from the network, a retry procedure is started. Counters are used to limit retries. Mobility EPS Management network operates seven timers and timers UE 14.

Procedure Messages Explanation
GUTI reallocation GUTI reallocation command ← Used to allocate a temporary identifier to the UE.
GUTI reallocation Complete →
Authentication Authentication req. ← Used to authenticate the UE.
Authentication resp. →
Security mode control Security mode command ← Used to negotiate between UE and MME.
Security mode command →
Identification Identity req. ← Used to determine a UE’s identity.
Identity resp. →
EMM information EMM information ← Used to transfer support information to UE.
EMM Status EMM status ← or → Used for error reporting.
Attach Attach req. ← Used to register the UE with the network and allocation of the resources.
Attach accept ←
Attach Complete →
Detach Detach req. → Used to remove a UE’s registration with the network.
Detach accept ← or →
Tracking area updating Tracking area update req. → Used for idle mode mobility.
Tracking area update accept ←
Service Request Service req. → Used when the UE is in idle mode.
Paging Request to lower layer for paging Used when UE is in idle mode and downlink traffic arrives.
Service req.
Transport of NAS message UL/DL NAS transport ← / → Used for SMS transport in encapsulated form.

NAS Signaling Protocol for EPS Session Management

The second block of the NAS signaling functionality is related to the handling of the session. Four insider network and four UE has initiated the procedures that exist in ESM. Following table lists the messages with the corresponding flows for the success (again, negative cases are created using appropriate reject messages). Some of them are grafted on EMM NAS messages, where a container is provided (for example a REQUEST PDN connectivity is packed on the attach request message). Also two simple messages have been defined for general information exchange.

Procedure Messages Explanation
Default EPS bearer context activation Activate default EPS bearer context req. ← Establishes a default EPS bearer context between UE and EPC.
Activate default EPS bearer context req. →
Dedicated EPS bearer context activation bearer context req.← context with QoS and TFT between UE and EPC.
Activate dedicated EPS bearer context req.→
EPS bearer context Modification Modification EPS bearer context req. ← Modifies an existing EPS bearer context for QoS and TFT between UE and EPC.
Modification EPS bearer context accept. ←
EPS bearer context deactivation Deactivation EPS bearer context req. ← Deactivate an EPS bearer context.
Deactivation EPS bearer context accept. ←
UE requested PDN connectivity PDN connectivity req. → Requests setup of a default bearer to a PDN.
PDN connectivity accept ←
UE requested PDN disconnect PDN disconnect req. → Disconnect UE from one PDN.
Deactivation EPS bearer context req. ←
UE requested bearer resources allocation Bearer resource allocation. → Used to request bearer resources.
Modify EPS bearer context req. ←
UE requested bearer resource modification Bearer resource modification req. → Used to modify.
Modify EPS bearer context req. ←
ESM information request ESM information request ← Used to request protocol configuration option.
ESM information response →
ESM status ESM status ← or → Informs to other NAS signaling peer of errors.