NGAP- Next-Generation Application Protocol over N2 Interface

NGAP Services 

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.