NGAP is a Next-Generation Application Protocol that is found on the N2 reference point between the gNB and the AMF (Access and Mobility management function). It is to support both UE and non-UE associated services. This includes operations such as configuration updates, UE context transfer, PDU session resource management, and also support for mobility procedures. It is also used to convey uplink and downlink NAS (Non-Access Stratum) messages as a payload, as well as support CM (Connected Mode) idle and CM connected operations such as paging and UE context release.
NGAP consists of Elementary Procedures (EPs). An elementary procedure is a unit of interaction between the NG-RAN node and the AMF. these Elementary procedures are defined separately and are intended to be used to build up complete sequences in a flexible manner. The services provided by the NGAP are divided into UE associated and non-UE associated. The UE associated services are related to one UE and the non-UE associated services are related to the NG interface.
The NGAP is transferred over SCTP (Stream Control Transmission Protocol). NGAP protocol provides transport function between UE and AMF by offering NAS signaling transport. The NAS protocol provides mobility management and session management between the UE and the AMF.
NGAP provides the signaling service between the NG-RAN node and the AMF that is required to fulfill the NGAP functions. NGAP services are divided into two groups:
- Non-UE-Associated services: They are related to the whole NG interface instance between the NG-RAN node and AMF utilizing a non-UE-associated signaling connection.
- UE associated services: they are related to one UE. NGAP functions that provide these services are associated with a UE-associated signaling connection that is maintained for the UE.
NG Interface General Principles
The general principles of the interface are:
- The NG interface is Open
- This interface supports the exchange of signaling information between the NG-RAN and 5GC.
- NG is a point-to-point interface between the NG-RAN node and a 5GC node.
- A point-to-point logical interface is feasible even in the absence of a physical direct connection between the NG-RAN and 5GC
- The NG interface supports control plane and user plane separation
- The NG interface separates the Radio Network layer and transport Network layer
- The NG interface is future proof to fulfill differently new requirements and support new services and new functions
- The NG interface is decoupled with the possible NG-RAN deployment variants.
- The NG Application Protocol supports modular procedure designs and uses a syntax allowing optimized encoding/decoding efficiently.
NG Interface functions
- Paging function: It supports the sending of paging requests to the NG-RAN nodes involved in the paging area e.g the NG-RAN nodes of the TA(s) the UE is registered.
- UE Context Management function: It allows the AMF to establish, modify or release a UE context in the AMF and the NG-RAN node e.g to support user individual signaling on NG.
- Mobility management function: the mobility function for UE’s in ECM-CONNECTED includes the intra-system handover function to support mobility within NG-RAN and the inter-system handover function to support mobility from/to EPS system.
- PDU Session management function: it is responsible for establishing, modifying, and releasing the involved PDU sessions NG-RAN resources for user data transport once a UE context is available in the NG-RAN node.
- NAS transport function: it provides means to transport or reroute a NAS message for a specific UE over the NG interface.
- NAS Node selection function: the interconnection of NG-RAN nodes to multiple AMFs is supported in the 5GS Architecture. A NAS node selection function is located in the NG-RAN node to determine the AMF association of the UE, based on the UE’s temporary identifier, which was assigned to the UE by the AMF.
- NG Interface Management function: the NG-interface management functions provide: means to ensure a defined start of NG-interface, the means to handle different versions of application part implementations, and protocol errors (error indication).
- AMF Management function: the AMF management function supports AMF planned removal and AMF auto-recovery.
- AMF Load balancing function: the NG interface supports the indication by the AMF of its relative capacity to the NG-RAN node in order to achieve load-balanced AMFs within the pool area.
- Location reporting function: this function enables the AMF to request the NG-RAN node to report the UE’s current location.
- AMF re-location function: this function allows to re-direct an initial connection request issued by an NG-RAN node from an initial AMF towards a target AMF selected by 5GC.