ISDN

Integrated Service Digital Network

ISDN (Integrated Service Digital Network) is a design for digital telephone/communications network. ISDN provide a wide range of services including voice, video, data etc. ISDN is a circuit switched telephone network even though it provides access to packet switched networks. The system allows simultaneous transmission of data and voice using end-to-end connectivity. Generally, ISDN has three layers that are respectively works in first three layers of the OSI model.  In ISDN, voice and data are carried by B channels (bearer channels) support a bandwidth of 64 Kbit/sec. The D channel (data channel) of the ISDN is used for signaling, it support 16 Kbit/sec. Two basic ISDN interfaces are Basic Rate Interface (BRI) and Primary Rate Interface (PRI).

Basic Rate Interface

BRI consists two B-channels and a D-channel (2*64+1*16=144 Kbit/sec). BRI services are mostly focused on individual users.

Primary Rate Interface

PRI provides greater capacity than BRI. BRI typically contain 23 B-channels and one 64 Kbit/sec D channel for a total of 1536 Kbit/sec (23*64+1*64=1536 Kbit/sec). In some regions especially in Europe, PRI consists 30 B channels plus one 64 Kb/sec D channel with total of 1984 Kb/s (30*64 + 1*64 = 1984 Kbit/sec).

H channels  are used aggregate multiple B channels. Commonly used H channels are:

H0 consists 6 B channels with total of 384 Kbit/sec (6*64= 384 Kbit/sec), H10 support 23 B channels with total of 1472 Kb/sec, H11 contains 24 B channels and H12 consist 30 B channels.

The CCITT defines the ISDN architecture to consist of four planes:

1. C-plane (Control Plane)
2. U-plane (User Plane)

3.      T-plane (Transport Plane)

4.      M-plane (Management Plane)

C-plane deals with User Network Interfaces and calls tearing processes.  U-plane manages user data carried by the bearer channels (B channels). 

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