• The digital pipe between the central office and the ISDN subscriber will be used to carry a number of communication channels.
• The capacity of the pipe and the number of channels carries may vary from user to user.

• Types of available channels in a transmission structure

•    B- channel :   64 kbps
•    D- channel :  16 (or) 64kbps
•    H- channel :  H0   - 384 kbps
H11 -  1536 kbps
H12 -  1920 kbps
B- Channel: - This is a user channel that can be used to carry digital data, PCM digital voice of 64 kbps.
Three kinds of connections can be set up over a B- channel
•    Circuit switched: - This is a switched digital service. The user places a call and a circuit switched connection is established with another network user.
•    Packet switched:- The user is connected to a packet switching node and data are exchanged    with other users via X.25
•    Semi permanent: - This is a connection to another user set up prior arrangement and not required a call establishment protocol. (Leased line)


D- Channel: -     This serves two main purposes
1)    It carries common channel signaling information to control circuit switched cells on associated with B- channels.
2)    This channel is used for packet switching connection low speed               (eg:100bps) telemetry at times when no signaling information is waiting.

H- Channel: -   This is provided for user information at higher bit rates like high speed trunk is a facsimile, video, high speed data, high quality audio.

The ISDN channel structure are classified as
•    Basic access   and
•    Primary access

•      Basic access: - It consists of two full duplex 64 kbps B- channels (2B) and a full duplex 16 kbps D- channel. The total bit rate is 144kbps [2B+D].
o    The basic services are used by individual users (residential subscribers) and very small offices.
o    It allows simultaneous use of voice and several data applications such as internet access, alarm service, facsimile and teletex.

•    Primary access: - This is intended for users with greater capacity requirement such as offices with a digital PBX (or) a LAN.

Primary service: - composition 
2.048Mbps: 30B channels at 64 kbps each 1D channel at 64 kbps
1.544Mbps: 23B channels at 64 kbps each 1D channel at 64 kbps
Primary interface may also be used to support H- channel

Primary rate interface H0 channel structures: - This supports multiple 384 kbps H0 channels. The structure is 3H0+D and 4H0 for the 1.544 Mbps interface and 5H0+D for the 2.048Mbps interface.

Primary rate interface H11 channel structure: - The H11 channel structure consists of 1536 kbps H11 channel. The H12 channel structure consists of 1920 kbps H12 channel and one D channel.

Primary rate interface structures for mixtures of B and H0 channels: - This consists of zero (or) one D- channel + any possible combination of B and H0 channels up to the capacity of the physical interface.
[E.g.: 3H0+5B+D (or) 3H0+6B for 1.544Mbps interface]