ISDN Channels (B, D, H)

Dividing the physical line into channels

ISDN divides a physical access interface into logical digital channels. Basic Rate Interface (BRI) provides two 64 kbit/s B channels and a 16 kbit/s D channel, while Primary Rate Interface (PRI) uses 64 kbit/s for both B and D channels. This arrangement allows user information and signaling to be carried at the same time. Narrowband ISDN specifies B and D channels together with higher-rate H channels.

B Channel for User Information

The B channel carries user information as part of a bearer service. Depending on the service, it can transport digitized voice, computer data, fax images, or video streams.

• 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 $64 \text{ kbit/s}$. For digitized voice, this corresponds to PCM sampling at 8 kHz with 8 bits per sample, which produces 64 kbit/s.
• Switching Mode: In a circuit-switched service, an end-to-end connection is established for the duration of the call. The reserved resources provide a constant bit rate and predictable delay, which suits real-time communication such as voice calls. A B channel can also carry a packet-mode service when the selected bearer service specifies it.
• Transparency: In the unrestricted bearer category, the network does not interpret the content of the bits carried by the B channel. This supports different user-data formats within the defined service parameters.

D Channel for Signaling

The D channel is primarily used for out-of-band signaling. It carries control messages between user equipment and the network, including Q.921/LAPD and Q.931 messages used to manage B-channel connections.

• 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 indicators, 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): $64 \text{ kbit/s}$.
• Frame Transport: For signaling, the D channel carries LAPD frames. Frames for different calls and terminals can be interleaved within the shared capacity. Packet-mode services can use a B, D, or H channel according to the specified access variant.
• Secondary Functions: In packet-mode services, the D channel can also carry low-speed user data and teleaction services such as remote telemetry and alarms.

H Channels for Higher Bit Rates

For applications requiring a higher bit rate than a single $64 \text{ kbit/s}$ B channel, ISDN defined H channels. These user channels support higher-rate services, including packet-mode services.

• H0 Channel: It provides a bit rate of $384 \text{ kbit/s}$, equivalent to six B channels.
• H1 Channels:
  • H11: Based on the T1 hierarchy, it provides $1536 \text{ kbit/s}$, equivalent to the capacity of 24 B channels (1.536 Mbps).
  • H12: Based on the E1 hierarchy, it provides $1920 \text{ kbit/s}$, equivalent to the capacity of 30 B channels (1.920 Mbps).
• Higher H Channels for Broadband ISDN: ITU-T Recommendation I.121 describes H2 and H4 for applications requiring greater capacity. H4 has a bit rate from 132 to 138.240 $\text{ Mbit/s}$.
• Applications: H channels were intended for services such as high-quality audio, video conferencing, and high-speed data file transfers.