B Channel : Type of ISDN Channels

ISDN structure includes a central ISDN office. All the users are linked to this office through a digital pipe. This digital pipe may be of different capacities and may have different data transfer rates. These digital pipes between the customers and central office are organized into multiple channels of different size.

ISDN standard defines the following three channel types:

1. B Channel

2. D Channel

3. H Channel

1. B Channel

1. A Bearer Channel (B Channel) is defined at a rate of 64 kbps.
2. It is the basic user channel and can carry any type of digital information in full duplex mode as long as the required transmission rate does not exceed 64 kbps.
3. It can be used to carry digital data, digital voice and any other low data rate information.
4. Four kind of connections can be set up over a B channel:

(a) Circuit switched

The user places a call and a current switched connection is established with another network user.

(b) Packet switched

The user is connected to a packet switching mode and data are exchanged with other users via X.25.

(c) Frame mode

The user is connected to a frame relay mode and data are exchanged with other users via LAPF.

(d) Semi-permanent

This is a connection to another user set up by prior arrangement and not requiring-a call establishment protocol. It is equivalent to a leased line.

2. D Channel

• A data channel (D channel) carry control signal for bearer channels.

• This channel does not carry data and can be either 16 or 64 kbps, depending upon the user’s need.

• If we are using in-band signaling, the same cable carries data as well as control signals.

• If we are using out-of-band signaling (as in ISDN), then two different cables carry data and control information.

3. H-Channel

• A Hybrid channel (H-channel) is provided for user information at higher bit rates.

• There are three types of H-Channels depending on the data rates:

H0 with data rate of384 kbps

H11 with data rate of 1536 kbps

H12 with data rate of 1920 kbps

• Hybrid channels are used for high data rate applications such as video, teleconferencing.