S1-c (Control-plane part of S1)

S1-C (S1-Control) is the control-plane part of the S1 interface in Long-Term Evolution (LTE) and 5G networks. It is responsible for the signaling and control functions between the base station (eNodeB in LTE or gNodeB in 5G) and the core network (Evolved Packet Core or EPC).

The S1 interface is a critical interface in LTE and 5G networks that connects the base station to the core network. It carries both user plane traffic (S1-U) and control plane signaling (S1-C). The S1-C interface is used for the exchange of control messages between the base station and the core network elements such as the Mobility Management Entity (MME) and the Serving Gateway (S-GW) in LTE, and the Access and Mobility Management Function (AMF) and the Session Management Function (SMF) in 5G.

The control-plane part of S1 (S1-C) primarily handles the following functions:

1. Connection Management: S1-C is responsible for establishing, maintaining, and releasing the signaling connections between the base station and the core network elements. It handles the setup and teardown of signaling tunnels, known as S1-AP (Application Protocol) signaling bearers. These signaling bearers carry control messages between the base station and the core network for various procedures such as attach, handover, paging, and release.
2. Mobility Management: S1-C facilitates mobility management functions by exchanging mobility-related messages between the base station and the core network. This includes procedures such as tracking area updates, handovers, and location updates. The base station informs the core network about the mobility status of the connected devices, allowing the network to make informed decisions for efficient handling of user sessions.
3. Session Management: S1-C is involved in session establishment, modification, and release procedures. When a device establishes a connection with the network (attach procedure), the S1-C interface is used to set up the necessary session context in the core network. S1-C carries session-related information such as Quality of Service (QoS) parameters, IP addresses, and security parameters between the base station and the core network.
4. Paging and Notification: S1-C enables the base station to request the core network to initiate paging procedures for a specific device. Paging is performed to notify a device about incoming calls or data, or to locate a device for handover purposes. S1-C carries the paging requests and responses between the base station and the core network.
5. Security: S1-C plays a crucial role in ensuring the security of the signaling and control messages exchanged between the base station and the core network. It handles the authentication and authorization procedures, secure key exchange, and encryption of sensitive signaling data.

Overall, the S1-C interface in LTE and 5G networks forms the backbone of control-plane communication between the base station and the core network. It enables the network to manage device mobility, establish and control user sessions, and ensure the security and integrity of the signaling messages exchanged between these network elements.