Delta Channel

What Does Delta Channel Mean?

A delta channel (D channel) is a signaling channel in an integrated services digital network (ISDN). D channels take care of call set up, control and maintenance. The basic ISDN channel holds two bearer channels (B channels) and one D channel. These contain data that relates to signaling errors, framing and other management signals. The speed of a D channel is 16 Kbps for basic rate interface and 64 Kbps for primary rate interface.


The D channel’s technical capabilities include information on the terminal equipment that is originating and receiving calls. This includes required signaling type, and the terminal’s ability to handle special services and features.

D channels carry signaling between a customer’s terminal device and a carrier’s end-switching office. Signaling information with end-to-end significance travels between the carrier switching office on the carrier’s common channel-signaling network and the destination terminal through the receiving user’s D channel.

Techopedia Explains Delta Channel

D channels operate based on a well-defined pair of layered protocols. D channel Layer 2 protocol is Q.921 for DSS1 signaling and is called linked access procedures, D channel (LAPD). It resides at the data link layer. The Q.931 protocol operates at upper layers – Layer 3 and above.

LAPD protocol operates between the terminal equipment and network termination over the D channel of the ISDN interface. The fields within LAPD include address, control, command/response bit, information and frame check sequence.

Q.931 D channel signaling protocol performs integration, carrying signaling information about the nature of the ISDN service required for specific calls between the end user terminal equipment and the ISDN carrier’s end office. The protocol conveys information such as service information, terminal capabilities, handshaking, etc. Service information includes information on the nature of the service requested for calls such as D channel packet-switched data, B channel packet-switched data, circuit-switched data, video and fax.


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Margaret Rouse

Margaret is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical business audience. Over the past twenty years, her IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.