N1 and S1

In the context of 5G networking, N1 and S1 modes refer to different ways a user equipment (UE) can access the network, with specific implications for the kind of network architecture the UE is interfacing with.

N1 Mode: In N1 mode, the UE has access to the 5G core network (5GC) via the 5G access network. This mode is indicative of a successful 5GC attach, which means the UE is connected to the 5G network and is able to utilize its services. For further details on N1 mode operation, check out this note.

S1 Mode: When a UE is capable of S1 mode, it implies that the UE has a single usage setting that applies to both the 5G system (5GS) and the evolved packet system (EPS). In this mode, the UE is recognized as having a successful EPS attach. This mode is important for ensuring backward compatibility with the LTE network and for situations where the UE might be moving between areas that have different generations of network technology

In short, N1 is the Standalone mode with 5G core giving ideal performance and features, while S1 is the Non-Standalone mode reusing 4G core to simplify early 5G introductions.

N1 mode refers to Standalone mode with direct connection between 5G RAN and 5G Core. S1 mode refers to Non-Standalone with connection between 5G RAN and 4G EPC core.
In N1 mode, the NG interface is used between RAN and 5GC. In S1 mode, existing 4G S1 interface is reused between RAN and EPC.
N1 mode allows full 5G core network features like network slicing, advanced QoS, unified authentication, etc. S1 mode has limitations of 4G core.
The 5G core network functions like AMF, SMF, UPF are utilized in N1 mode. In S1 mode, 4G elements like MME, SGW, PGW are still used.
N1 mode simplifies the architecture with a common core for different access types. S1 mode retains the overlapping 4G and 5G cores.
N1 mode requires deploying the new 5G core first. S1 mode allows introducing 5G NR with existing 4G core.
Performance like throughput, latency is superior in N1 mode compared to when anchored via 4G core in S1 mode.
Initial 5G deployments are using S1 mode for faster rollout leveraging existing 4G infrastructure. N1 mode allows full 5G capabilities.
In S1 mode, the 5G gNB connect to the 4G EPC core network using the existing S1 interface.
To support this interworking between 5G RAN and 4G Core, the NG interface between gNodeB and MME is referred to as N26.
So N26 is essentially the reference point for the signaling between 5G gNodeB and 4G MME in S1 mode.
The NG-C and NG-U interfaces are also used in S1 mode for control and user plane between gNodeB and the 4G EPC respectively.

Followings are some of the quotes directly copied from 3GPP.

24.501 states :

If the UE included S1 mode supported indication in the REGISTRATION REQUEST message, the AMF supporting

interworking with EPS shall set the IWK N26 bit to either:

a) "interworking without N26 interface not supported" if the AMF supports N26 interface; or

b) "interworking without N26 interface supported" if the AMF does not support N26 interface

in the 5GS network feature support IE in the REGISTRATION ACCEPT message.

The UE supporting S1 mode shall operate in the mode for interworking with EPS as follows:

a) if the IWK N26 bit in the 5GS network feature support IE is set to "interworking without N26 interface not

supported", the UE shall operate in single-registration mode;

b) if the IWK N26 bit in the 5GS network feature support IE is set to "interworking without N26 interface

supported" and the UE supports dual-registration mode, the UE may operate in dual-registration mode; or

NOTE 7: The registration mode used by the UE is implementation dependent.

c) if the IWK N26 bit in the 5GS network feature support IE is set to "interworking without N26 interface

supported" and the UE only supports single-registration mode, the UE shall operate in single-registration mode.

If the UE is registered in S1 mode and operating in dual-registration mode, the PLMN that the UE chooses to

according to 23.122

If the UE indicates "mobility registration updating" in the 5GS registration type IE and the UE supports S1 mode, the

UE shall:

- set the S1 mode bit to "S1 mode supported" in the 5GMM capability IE of the REGISTRATION REQUEST

message;

- include the S1 UE network capability IE in the REGISTRATION REQUEST message; and

- if the UE supports sending an ATTACH REQUEST message containing a PDN CONNECTIVITY REQUEST

message with request type set to "handover" to transfer a PDU session from N1 mode to S1 mode, set the HO

attach bit to "attach request message containing PDN connectivity request with request type set to handover to

transfer PDU session from N1 mode to S1 mode supported" in the 5GMM capability IE of the REGISTRATION

REQUEST message.

A UE supporting S1 mode shall include the IE "S1 UE network capability", unless the UE performs a periodic registration updating procedure.

Reference