8.2.2.1 General

A single N1 NAS signalling connection is used for each access to which the UE is connected. The single N1 termination point is located in AMF. The single N1 NAS signalling connection is used for both Registration Management and Connection Management (RM/CM) and for SM-related messages and procedures for a UE.

The NAS protocol on N1 comprises a NAS-MM and a NAS-SM components.

There are multiple cases of protocols between the UE and a core network function (excluding the AMF) that need to be transported over N1 via NAS-MM protocol. Such cases include:

• Session Management Signalling.
• SMS.
• UE Policy.
• LCS.

RM/CM NAS messages in NAS-MM and other types of NAS messages (e.g. SM), as well as the corresponding procedures, are decoupled.

The NAS-MM supports generic capabilities:

• NAS procedures that terminate at the AMF. This includes:
• Handles Registration Management and Connection Management state machines and procedures with the UE, including NAS transport; the AMF supports following capabilities:
• Decide whether to accept the RM/CM part of N1 signalling during the RM/CM procedures without considering possibly combined other non NAS-MM messages (e.g. SM) in the same NAS signalling contents;
• Know if one NAS message should be routed to another NF (e.g. SMF), or locally processed with the NAS routing capabilities inside during the RM/CM procedures;
• Provide a secure NAS signalling connection (integrity protection, ciphering) between the UE and the AMF, including for the transport of payload;
• Provide access control if it applies;
• It is possible to transmit the other type of NAS message (e.g. NAS SM) together with an RM/CM NAS message by supporting NAS transport of different types of payload or messages that do not terminate at the AMF, i.e. NAS-SM, SMS, UE Policy and LCS between the UE and the AMF. This includes:
• Information about the Payload type;
• Additional Information for forwarding purposes
• The Payload (e.g. the SM message in the case of SM signalling);
• There is a Single NAS protocol that applies on both 3GPP and non-3GPP access. When an UE is served by a single AMF while the UE is connected over multiple (3GPP/Non 3GPP) accesses, there is a N1 NAS signalling connection per access.

Security of the NAS messages is provided based on the security context established between the UE and the AMF.

Figure 8.2.2.1-1 depicts NAS transport of SM signalling, SMS, UE Policy and LCS.