GTP-C (GPRS Tunneling Protocol, Control Plane)

GTP-C (GPRS Tunneling Protocol, Control Plane) is a protocol that is used in mobile telecommunications networks. It is primarily used for the communication between the GPRS (General Packet Radio Service) core network and the GPRS support nodes, including the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). GTP-C is responsible for establishing, maintaining, and releasing GPRS tunnels for the transmission of user data packets across the GPRS network.

GPRS is a packet-switched technology that is used for the transmission of data over mobile networks. The GPRS network is composed of a number of elements, including the mobile stations, base stations, and the core network. The core network is responsible for the routing and switching of data packets between the mobile stations and external networks, such as the internet.

In order for the core network to be able to provide GPRS services, it needs to establish a GPRS tunnel between the mobile station and the external network. A GPRS tunnel is a logical path that is established between the mobile station and the external network, through which data packets can be transmitted. The GPRS tunnel is established through the use of GTP-C.

GTP-C operates on the control plane of the GPRS network. The control plane is responsible for the signaling and control functions of the network, including the establishment, maintenance, and release of connections between the various network elements. GTP-C is used to establish and maintain the GPRS tunnels on the control plane.

GTP-C is designed to be a lightweight protocol that is optimized for the transmission of control plane messages over the GPRS network. It is based on the IP protocol and uses a tunneling mechanism to transport the GPRS control plane messages between the various network elements.

The GTP-C protocol consists of a number of message types that are used for the communication between the different network elements. The most important message types are as follows:

1. Create PDP Context Request (CPCR): This message is used by the SGSN to initiate the establishment of a GPRS tunnel with the GGSN. The message contains information about the mobile station and the external network that the GPRS tunnel is being established with.
2. Create PDP Context Response (CPCF): This message is sent by the GGSN in response to the CPCR message. It contains information about the GPRS tunnel that has been established, including the GPRS tunnel identifier (GTP-U tunnel ID) and the IP address of the GGSN.
3. Modify PDP Context Request (MPR): This message is used to modify an existing GPRS tunnel. It is sent by the SGSN to the GGSN and contains information about the changes that need to be made to the GPRS tunnel.
4. Modify PDP Context Response (MPCF): This message is sent by the GGSN in response to the MPR message. It contains information about the changes that have been made to the GPRS tunnel.
5. Delete PDP Context Request (DPCR): This message is used to release an existing GPRS tunnel. It is sent by the SGSN to the GGSN and contains information about the GPRS tunnel that needs to be released.
6. Delete PDP Context Response (DPCF): This message is sent by the GGSN in response to the DPCR message. It confirms that the GPRS tunnel has been released.

In addition to these message types, GTP-C also includes messages for error handling, status reporting, and congestion control.

GTP-C uses a number of identifiers to uniquely identify the various network elements and GPRS tunnels. The most important identifiers are as follows:

1. GPRS Tunnel Identifier (GTP-U tunnel ID): This is a 32-bit identifier that is used to uniquely identify a GPRS tunnel on the user plane. The GTP-U tunnel ID is assigned by the SGSN and is used by the GGSN to route data packets to the correct mobile station.
2. TEID (Tunnel Endpoint Identifier): This is a 32-bit identifier that is used to uniquely identify a GPRS tunnel on the control plane. The TEID is assigned by the SGSN and is used by the GGSN to route control plane messages to the correct SGSN.
3. IMSI (International Mobile Subscriber Identity): This is a unique identifier that is assigned to every mobile station in the network. The IMSI is used by the SGSN and GGSN to identify the mobile station and route data packets and control plane messages to the correct destination.
4. NSAPI (Network Service Access Point Identifier): This is a 5-bit identifier that is used to identify the data flow between the mobile station and the external network. The NSAPI is assigned by the SGSN and is used by the GGSN to route data packets to the correct destination.

The GTP-C protocol operates over UDP (User Datagram Protocol) and uses a tunneling mechanism to transport the control plane messages between the various network elements. When a GPRS tunnel is established, a tunnel endpoint is created at each end of the tunnel. The tunnel endpoints are responsible for encapsulating and decapsulating the GTP-C messages as they are transported over the network.

When a mobile station initiates a data session, the SGSN sends a Create PDP Context Request (CPCR) message to the GGSN. The CPCR message contains information about the mobile station, including the IMSI and NSAPI. The GGSN responds with a Create PDP Context Response (CPCF) message, which contains the GTP-U tunnel ID and the IP address of the GGSN.

Once the GPRS tunnel has been established, data packets can be transmitted between the mobile station and the external network. The data packets are encapsulated in GTP-U messages and transported over the GPRS tunnel. The GTP-U messages contain the GTP-U tunnel ID, which is used by the GGSN to route the data packets to the correct destination.

When the data session is complete, the SGSN sends a Delete PDP Context Request (DPCR) message to the GGSN to release the GPRS tunnel. The GGSN responds with a Delete PDP Context Response (DPCF) message, confirming that the GPRS tunnel has been released.

In summary, GTP-C is a protocol that is used for the establishment, maintenance, and release of GPRS tunnels in mobile telecommunications networks. It operates on the control plane of the network and uses a lightweight protocol optimized for the transmission of control plane messages. GTP-C is based on the IP protocol and uses a tunneling mechanism to transport control plane messages between the various network elements. The GTP-C protocol is essential for the operation of the GPRS network and enables the transmission of data packets between the mobile station and external networks.