ISDN Protocols

ISDN signaling protocols

For ISDN, signaling on the D channel uses a protocol stack. It covers the physical, data-link, and network functions of the user-network interface. These are functional interface layers, and their correspondence to the OSI model is approximate. ITU-T Recommendations I.430, I.431, Q.921, and Q.931 specify the interface and its signaling procedures.

ISDN layered architecture for the D channel

At the user-network interface, ISDN signaling is organized into three protocol layers, each with a defined scope.

• Layer 1: The Physical Layer (Standards: I.430/I.431)

  This layer defines transmission of bits over the ISDN user-network interface. I.430 covers basic rate access and I.431 covers primary rate access. A basic access commonly has two B channels and one D channel, while a primary access has a larger number of B channels and one D channel. The specifications include electrical characteristics, line coding, frame structure, and connector wiring at the defined reference points.

• Layer 2: The Data Link Layer (Standard: Q.921 / LAPD)

  This layer provides reliable transfer over one signaling link between terminal equipment and the network side. Q.921 defines LAPD procedures, derived from HDLC, for framing, error detection, retransmission, flow control, link addressing, and multiple logical links. The link layer carries Layer 3 signaling between the terminal and the local exchange.

• Layer 3: The Network Layer (Standard: Q.931 / DSS1)

  This layer establishes, maintains, and releases circuit-switched calls and controls supplementary procedures. Q.931 defines the signaling messages exchanged by the terminal and the network, including messages for call setup, alerting, connection, and release.

Layer 2: Q.921 (LAPD)

The protocol for Layer 2 is known as LAPD (Link Access Procedure on the D-channel), standardized in ITU-T Q.921. Its main functions are:

• Framing: Encapsulates Layer 3 messages into structured units called frames, with address and control fields.
• Error Detection and Retransmission: The Frame Check Sequence detects corrupted frames. LAPD procedures can request retransmission when a frame fails the check, and sequence numbers support ordered delivery.
• Flow Control: Ensures the sender doesn't transmit data faster than the receiver can process it, using the link procedures defined by Q.921.
• Multiplexing: Allows multiple logical connections, for example from different terminals on a bus, to share the single D channel through link identifiers.

Layer 3: Q.931 (DSS1)

The protocol for Layer 3 is DSS1 (Digital Subscriber Signalling System No. 1), specified by ITU-T Q.931 for basic call control. It defines messages exchanged between the terminal and the network throughout the call. The messages describe the call state and carry information such as the called number and requested bearer capability.

Example ISDN call flow

1. Call Setup: You pick up your ISDN phone and dial a number. Your phone sends a SETUP message over the D channel. This message contains the number you dialed and the type of service you want (e.g., a 64 kbps voice call).
2. Network acknowledgment: The network receives the SETUP message and sends back CALL PROCEEDING, indicating that it is processing the call.
3. Ringing the Destination: The network signals the destination phone. The destination phone starts ringing and sends back an ALERTING message. The network forwards this to your phone, and you hear the ringback tone.
4. Connection: The person at the other end answers. Their phone sends a CONNECT message. The network receives it, establishes the circuit-switched B channel between the terminals, and sends CONNECT to the calling phone. The calling terminal confirms with CONNECT ACK.
5. Conversation: The connection is now active. Your voice data flows over one of the B channels, while the D channel remains available for additional signaling, such as call-waiting notifications.
6. Call teardown: When you hang up, your phone sends a DISCONNECT message. The network releases the B channel and sends RELEASE to the terminals. The terminals respond with RELEASE COMPLETE until both ends are released.