What is the difference between N1 interface and N2 interface in 5G?

Question

In every 5G diagram, they have the N1 interface directly connected to the UE. Is it really a direct connection? or is it still going through the radio? If so what is the difference from N2?

Answer 1

1. N1 : Between UE and AMF (The N1 interface is used by the UE for non-radio signaling mainly NAS layer signaling.)

2. N2: Between gNB and AMF

3. N1/N2 : between gNB and AMF (supporting UE and non-UE related services.)

4. and 3) both are valid between gNB and AMF.

Answer 2

RAN uses N2 (control plane) and N3 (user plane) to interface to the core and transparent N1 interface to the user devices. N1 is a transparent interface from 5G-UE to AMF of core(via NG-RAN).

N1 interface is used by UE for transmitting non radio signalling between UE and AMF (something like NAS) which includes information related to connection, mobility and session related messages to the AMF which handles cases related to connection, mobility messages, but forwards session management info to SMF.

N2 interface supports control plane signalling between RAN and 5G core covering scenarios related to UE context management, PDU session/resource management procedures. N2 uses SCTP (NGAP) between 5GCN and access network.

Answer 3

N1 is for the Non-Access Stratum (NAS) protocol between the core network and the handset, N2 is a network interface between the core network and the base station.

Answer 4

The N1 is a direct connection between the UE and the 5G core network (AMF). As any connection is transmitted over the air, and it´s therefore a radio connection at whichever frequency. The difference here is that the NAS messages are transmitted "transparently" through the radio protocol stack within the gNodeB (without being processed through the L1-L3 layer stack, where PDUs get headers/padding added), but it is still going "through" the radio, there is no real (as in physical) connection between UEs and 5G core (actually the 5G core can be located many miles away from the gNodeB). Hope it helps.