Why Protocols? The Language of Digital Communication

Moving from analog to digital communication requires more than just sending bits; it requires a structured language for devices to understand each other. This is the role of protocols. For ISDN, which integrates various services, this language is organized into a protocol stack. This stack, defined by the ITU-T, is a simplified, practical implementation of the first three layers of the famous seven-layer OSI model, and it primarily governs the D Channel.

The ISDN Layered Architecture (D Channel)

The ISDN reference model divides the complex task of signaling into three distinct layers, each with a specific responsibility.

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

  This is the hardware layer. Its job is to transmit the raw bits of the D channel across the physical medium (the copper wire). It defines all the electrical and physical specifications, such as voltage levels, line coding, and the pinouts for the connectors used at the various reference points.

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

  This layer ensures that data is transmitted reliably across a single link (specifically, between the user's terminal equipment and the local exchange). Think of it as a meticulous postman ensuring every single letter arrives at the next post office intact, in order, and without errors.

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

  This is the "brain" of the operation. Layer 3 is responsible for establishing, maintaining, and tearing down the network connection (the call). It uses the reliable link provided by Layer 2 to exchange high-level signaling messages, like "I want to call this number" or "Hang up the call".

Layer 2 Deep Dive: 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 packets into structured units called frames, with headers and trailers for control.
• Error Detection and Correction: It adds a checksum (Frame Check Sequence) to each frame. If the receiver calculates a different checksum, it knows the frame is corrupted and can request a retransmission.
• Flow Control: Ensures the sender doesn't transmit data faster than the receiver can process it.
• Multiplexing: Allows multiple logical connections (e.g., from different terminals on a bus) to share the single D channel.

Layer 3 Deep Dive: Q.931 (DSS1)

The protocol for Layer 3 is DSS1 (Digital Subscriber Signalling System No. 1), standardized in ITU-T Q.931. This is the core call control protocol. It consists of a set of well-defined messages exchanged between the user's terminal and the network to manage the entire call lifecycle.

Example of a Simple 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 a CALL PROCEEDING message, indicating it is working on connecting your 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 this, establishes the circuit-switched B channel between you, and sends a CONNECT message to your phone.
5. Conversation: The connection is now active. Your voice data flows over one of the B channels, while the D channel remains free for any further signaling (like call waiting notifications).
6. Call Teardown: When you hang up, your phone sends a DISCONNECT message. The network tears down the B channel connection and sends a RELEASE message to both ends. Both terminals respond with RELEASE COMPLETE, and the call is over.