Logical Division of a Physical Line

The core innovation of ISDN is its ability to logically divide a single physical line (like a copper telephone pair) into multiple digital channels. This allows for simultaneous communication and separates user data from the network's control signals. There are three main types of channels defined for narrowband ISDN: B, D, and H channels.

The B Channel (Bearer Channel) - The Workhorse

The B Channel is the primary channel for user information. It acts as a clear, digital pipe for transmitting whatever data the user needs to send.

• Purpose: Its main task is to carry the user's payload, known as a Bearer Service. This can be digitally encoded voice, computer data, fax images, or video streams.
• Bit Rate: The standard bit rate of a B Channel is $64 \text{ kbit/s}$. This rate was chosen because it's the required bandwidth to transmit high-quality digitized voice using Pulse Code Modulation (PCM).
• Switching Mode: It operates in circuit-switched mode. This means a dedicated, end-to-end connection is established for the duration of the call, guaranteeing a constant throughput and low latency, which is ideal for real-time communication like voice calls.
• Transparency: The network does not place any restrictions on the format of the data transmitted over a B channel, providing a clear "unrestricted" digital pipe.

The D Channel (Delta Channel) - The Control Center

The D Channel is primarily used for out-of-band signaling. It acts as the nervous system of the connection, carrying control messages between the user's equipment and the network to manage the B channels.

• Purpose: Its main function is to carry signaling information for setting up, managing, and tearing down calls on the B channels. This includes dialing digits, call progress tones, and caller ID information.
• Bit Rate: The D channel's bit rate depends on the access type:
  • For Basic Rate Interface (BRI): $16 \text{ kbit/s}$.
  • For Primary Rate Interface (PRI): $64 \text{ kbit/s}$.
• Switching Mode: Unlike the B channel, the D channel is packet-switched. This allows multiple control messages for different calls and terminals to be interleaved and sent efficiently.
• Secondary Functions: The D channel can also be used for low-speed user data transmission and for services like teleaction (e.g., remote telemetry or alarms).

H Channels (High-Speed Channels) - For Bandwidth-Intensive Needs

For applications that required more than a single $64 \text{ kbit/s}$ B channel, ISDN defined H channels, which are formed by aggregating multiple B channels into a single, higher-speed circuit-switched channel.

• H0 Channel: Aggregates 6 B channels to provide a total throughput of $384 \text{ kbit/s}$.
• H1 Channels:
  • H11: Based on the T1 hierarchy, it aggregates 24 B channels for a total throughput of $1536 \text{ kbit/s}$ (1.536 Mbps).
  • H12: Based on the E1 hierarchy, it aggregates 30 B channels for a total throughput of $1920 \text{ kbit/s}$ (1.920 Mbps).
• Higher H Channels (for Broadband ISDN): Standards also defined even higher-speed channels like H2, H3, and H4, intended for broadband applications, reaching speeds up to $~139 \text{ Mbit/s}$.
• Applications: H channels were intended for services such as high-quality audio, video conferencing, and high-speed data file transfers.