N2 interface of 5g control plane protocol

N2 Interface protocol overview

N2 Interface is gNB and AMF The interface of , Agreement is NGAP,N2 Our transport layer is SCTP, The upper users are 5G-NAS(MM Mobility management 、SM Conversation management, etc ).
According to the official N2 Description of interface capability ,N2 The interface shall have the following capabilities ：
1、 Support the establishment of 、 Hold and release PDU The session NG-AN Side ( resources 、UE Context ) The ability of ;
2、 Support intra-RAT and inter-RAT(5G Inside 、5G and 4G Between ) Handoff ;
3、 transparent transmission UE and AMF Between NAS Signaling ;
4、 Resource reservation mechanism of user data message ;
5、 It can distinguish different users on the protocol stack , And finish UE Related signaling management .

chart 1 5G-AN and AMF Between the control planes

chart 2 5G-AN and SMF Between the control planes

N2 Interface process classification

According to whether and UE relevant , Divided into two types of processes ：

Depending on whether you need to respond ,N2 The process is divided into Class1 and Class2 Two types of ,Class1 Need to respond ,Class2 There is no need to respond .

common N2 Interface process

1、 stay UE Before I came ,gNB Want to be with AMF All in the pool group AMF establish N2 relation , What is used is NG Setup technological process . In this process ,gNB And all of them AMF establish SCTP coupling , And get the AMF Weight value and other parameters , For subsequent AMF choice .

The messages used are ：

• NG Setup Request
• NG Setup Response

2、 At this time, it can serve users .UE launch Registration process ,gNB coordination AMF establish UE The context of ( allow NSSAI、GUAMI、UE Safety capability 、 Mobility restriction list and other parameters ).

Messages used ：

• Initial UE Message
• UPLINK NAS TRANSPORT
• DOWNLINK NAS TRANSPORT
• Initial Context Setup Request
• Initial Context Setup Response

3、 here UE Want to surf the Internet , Launched PDU Session setup process .gNB According to the requirements of the network side , by UE Distribution and this PDU The session and Qos Flow Related resources ( Such as DRB).

The messages used are ：

• PDU Session Resource Setup Request
• PDU Session Resource Setup Response
• UPLINK NAS TRANSPORT
• DOWNLINK NAS TRANSPORT

PDU Some common problems after session establishment N2 technological process

UE In the process of surfing the Internet, if conditions or scenes change , Trigger the corresponding N2 technological process ：
1) Move to other gNB, Trigger N2 or Xn Switch process ;
The relevant news is ：Handover Required、Handover Requset、Handover Requset Ack、Handover Command、Handover NotifyUE Context Release Command、UE Context Release COMPLETE( Release old gNB User context )、Path Switch Request、Path Switch Request Ack
2) No internet access , Release PDU conversation , Trigger PDU Session resource release process ;
The relevant news is ：PDU Session Resource Release Command、PDU Session Resource Release Response
3) Downlink data arrives in idle state , Trigger Paging technological process ;
The relevant news is ：paging
4) The network side decides to adjust UE Of Qos Or other parameters , Trigger PDU Session resource modification ;
The relevant news is ：PDU Session Resource Modify Request、PDU Session Resource Modify Response
5) Other auxiliary and management (OAM) technological process
The relevant news is ：NG Reset、Overload Start、Trace Start、Error Indication etc.