Sign in Subscribe

AVCTP (Audio/Video Control Transport Protocol)

Last updated on 

Introduction:

The Audio/Video Control Transport Protocol (AVCTP) is a protocol designed to enable the control of audio and video streams. It is a subprotocol of the Audio/Video Control Protocol (AVCP) and is used to facilitate the transport of control information between devices. AVCTP is used in a variety of applications, such as streaming music or videos over Bluetooth, and is used to control the playback, volume, and other aspects of the media being streamed. In this article, we will discuss the basics of AVCTP, its architecture, and its functions.

Architecture:

AVCTP is a client-server protocol, where the client is typically the device sending the control commands and the server is the device receiving the commands and executing them. The protocol uses a simple request-response mechanism, where the client sends a command to the server, and the server responds with an appropriate response. The commands and responses are structured using a set of defined messages.

AVCTP operates on top of a transport layer protocol, such as Bluetooth L2CAP or TCP/IP. It uses a message-based communication model, where each message consists of a header and a payload. The header contains information about the message, such as the type of message and the length of the payload, while the payload contains the actual control information.

Functions:

The main functions of AVCTP are to enable the control of audio and video streams, and to facilitate the transport of control information between devices. The following are some of the key functions of AVCTP:

  1. Transport of Control Information: AVCTP is primarily used for transporting control information between devices. This includes commands for starting, stopping, pausing, and seeking audio or video streams, as well as commands for controlling volume and other playback settings.
  2. Message Exchange: AVCTP uses a simple request-response mechanism for message exchange. The client sends a command to the server, and the server responds with an appropriate response.
  3. Message Format: AVCTP uses a specific message format to structure the messages exchanged between devices. The message format includes a header and a payload, where the header contains information about the message and the payload contains the actual control information.
  4. Identification of Devices: AVCTP uses a unique identifier to identify the devices involved in the communication. This identifier is used to ensure that the control commands are sent to the correct device.
  5. Error Handling: AVCTP includes mechanisms for handling errors that may occur during the communication between devices. This includes error messages that can be sent by either the client or the server in response to a failed command.
  6. Security: AVCTP includes security mechanisms to ensure the integrity and confidentiality of the control information being transmitted between devices. This includes authentication and encryption mechanisms.

Message Types:

AVCTP uses a set of defined message types to structure the messages exchanged between devices. These message types are used to indicate the type of control information being sent and the direction of the message. The following are the message types defined by AVCTP:

  1. Command: A command message is sent by the client to the server to request a specific action. The payload of the command message contains the actual control information.
  2. Response: A response message is sent by the server in response to a command message. The payload of the response message contains the result of the command.
  3. General Reject: A general reject message is sent by the server to indicate that the command message cannot be executed.
  4. In-Session Command: An in-session command message is sent by the client to the server during an ongoing session. The payload of the in-session command message contains the actual control information.
  5. In-Session Response: An in-session response message is sent by the server in response to an in-session command message. The payload of the in-session response message contains the result of the command.
  6. Abort: An abort message is sent by either the client or the server to terminate the session.
  7. Fragmentation: A fragmentation message is used to split a large control message into smaller fragments for transmission.
  8. Start: A start message is sent by the client to initiate a new session.
  9. Suspend: A suspend message is sent by the client to pause an ongoing session.
  10. Abort Response: An abort response message is sent by the server in response to an abort message.
  11. Resume: A resume message is sent by the client to resume an ongoing session that was previously suspended.

Conclusion:

AVCTP is a crucial protocol for enabling the control of audio and video streams in a variety of applications. It provides a simple request-response mechanism for message exchange and uses a set of defined message types to structure the messages exchanged between devices. AVCTP is used in conjunction with other protocols, such as Bluetooth L2CAP or TCP/IP, to facilitate the transport of control information between devices. Its functions include the transport of control information, message exchange, message format, identification of devices, error handling, and security. Understanding the basics of AVCTP is essential for developers and engineers working on audio and video streaming applications.