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	GUTI reallocation Complete →	Used to allocate a temporary identifier to the UE.
Authentication	Authentication req. ←	Used to authenticate the UE.
Authentication	Authentication resp. →	Used to authenticate the UE.
Security mode control	Security mode command ←	Used to negotiate between UE and MME.
Security mode control	Security mode command →	Used to negotiate between UE and MME.
Identification	Identity req. ←	Used to determine a UE’s identity.
Identification	Identity resp. →	Used to determine a UE’s identity.
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	Detach accept ← or →	Used to remove a UE’s registration with the network.
Tracking area updating	Tracking area update req. →	Used for idle mode mobility.
Tracking area updating	Tracking area update accept ←	Used for idle mode mobility.
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.
Paging	Service req.	Used when UE is in idle mode and downlink traffic arrives.
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.
Default EPS bearer context activation	Activate default EPS bearer context req. →
Dedicated EPS bearer context activation	bearer context req.←	context with QoS and TFT between UE and EPC.
Dedicated EPS bearer context activation	Activate dedicated EPS bearer context req.→	context with QoS and TFT between UE and EPC.
EPS bearer context Modification	Modification EPS bearer context req. ←	Modifies an existing EPS bearer context for QoS and TFT between UE and EPC.
EPS bearer context Modification	Modification EPS bearer context accept. ←
EPS bearer context deactivation	Deactivation EPS bearer context req. ←	Deactivate an EPS bearer context.
EPS bearer context deactivation	Deactivation EPS bearer context accept. ←	Deactivate an EPS bearer context.
UE requested PDN connectivity	PDN connectivity req. →	Requests setup of a default bearer to a PDN.
UE requested PDN connectivity	PDN connectivity accept ←	Requests setup of a default bearer to a PDN.
UE requested PDN disconnect	PDN disconnect req. →	Disconnect UE from one PDN.
UE requested PDN disconnect	Deactivation EPS bearer context req. ←	Disconnect UE from one PDN.
UE requested bearer resources allocation	Bearer resource allocation. →	Used to request bearer resources.
UE requested bearer resources allocation	Modify EPS bearer context req. ←	Used to request bearer resources.
UE requested bearer resource modification	Bearer resource modification req. →	Used to modify.
UE requested bearer resource modification	Modify EPS bearer context req. ←	Used to modify.
ESM information request	ESM information request ←	Used to request protocol configuration option.
ESM information request	ESM information response →	Used to request protocol configuration option.
ESM status	ESM status ← or →	Informs to other NAS signaling peer of errors.