GCP (Gateway control protocol)

Gateway Control Protocol (GCP) is a signaling protocol used in Voice over IP (VoIP) networks to control and manage media gateways. Media gateways serve as the interface between traditional Public Switched Telephone Network (PSTN) and Voice over IP (VoIP) networks. GCP is used to establish, modify, and terminate voice calls across these networks.

GCP is an application layer protocol that runs on top of the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). It is defined in RFC 3525 and is part of the H.248 protocol family. GCP is designed to be extensible and can support different types of media gateways.

The main functions of GCP are call control, media control, and endpoint control. Call control is used to establish and terminate calls between endpoints. Media control is used to manage media streams and resources such as codecs, bandwidth, and quality of service (QoS). Endpoint control is used to manage the media gateway and its interfaces.

GCP messages are divided into two categories: requests and responses. Requests are initiated by a control point and are used to request an action from the media gateway. Responses are generated by the media gateway and provide information about the status of the requested action.

GCP messages consist of a message header and message body. The message header contains information about the message such as the message type, message length, and transaction identifier. The message body contains the data specific to the message type.

GCP supports a wide range of call control features such as call setup, call teardown, call transfer, call hold, and call park. Call setup involves establishing a call between two endpoints. Call teardown involves terminating a call between two endpoints. Call transfer involves transferring an active call from one endpoint to another. Call hold involves placing an active call on hold. Call park involves placing an active call in a virtual location where it can be retrieved by another endpoint.

Media control features supported by GCP include codec negotiation, bandwidth allocation, and QoS management. Codec negotiation involves negotiating the type of codec to be used for the media stream. Bandwidth allocation involves allocating the required bandwidth for the media stream. QoS management involves managing the QoS parameters such as delay, jitter, and packet loss.

Endpoint control features supported by GCP include endpoint discovery, endpoint registration, and endpoint management. Endpoint discovery involves discovering the available endpoints in the network. Endpoint registration involves registering the endpoints with the media gateway. Endpoint management involves managing the endpoints such as enabling or disabling them.

GCP uses a centralized architecture where the media gateway is controlled by a central controller called the Media Gateway Controller (MGC). The MGC is responsible for managing the media gateway and its interfaces. The MGC communicates with the media gateway using GCP messages. The MGC can be a standalone device or can be integrated with other network elements such as softswitches, application servers, and Session Border Controllers (SBCs).

GCP is designed to be extensible and can support different types of media gateways. GCP defines a set of base packages that provide a common set of call control and media control features. GCP also defines a set of profile packages that provide additional features specific to a particular type of media gateway.

In summary, GCP is a signaling protocol used in VoIP networks to control and manage media gateways. It is designed to be extensible and can support different types of media gateways. GCP provides call control, media control, and endpoint control features. GCP uses a centralized architecture where the media gateway is controlled by a central controller called the Media Gateway Controller (MGC).