Brief about ISDN Line Interface

Integrated services digital network (ISDN) is a type of telephone circuit that allows digital signals of data and voice to be sent using the standard telephone wire. It gives better reproduction of signals and speed of transmission. ISDN line interfaces allow routers and multi-channel port boards to be connected to the ISDN system or the central office. A set of protocols and codecs are defined that allow the encoding of digital voice and data so that they are understood by analog systems.

Types of ISDN line interfaces:

• Basic rate interface (BRI): These are used in TV and radio broadcasting centers and use the MPEG codec. They use up to two 64 kbps channels.
• Primary rate interface (PRI): These are used for PBX connections. They can have up to 23 channels.
• There are two types of channels: B (bearer) type used for data and information transmission and D (delta) channel used for signaling.

Selection parameters for ISDN line interfaces:

• Send/receive separation: To reduce leakage of signals from send and receive paths, extra circuits are attached so that signals from the send path do not mix with those from the receive path.
• Acoustic coupling reduction: In studios, when the speakers use a microphone, an acoustic path is formed between the sent and received paths. By using functions like reducing the dynamic gain and adaptive cancellation, this issue is reduced.
• DAC conversion: 16bit digital to analog converters are used for connecting the analog equipment to the digital.
• Sampling rate conversion: The studio and the ISDN equipment operate at different frequencies. A suitable step-up and down process is needed to get them both in sync.
• Auto gain control: This is used for both the send and receives signals to smooth out the variations and takes care of different types of phone instruments.
• Dynamic equalization: This feature balances the frequencies of different makes of equipment. The processor of three band dynamic equalization is used.
• Caller ducking: the host to ‘kill’ unwanted conversation from other callers.
• Caller ID verification: Allows caller ID information to be transmitted.

Other parameters to be considered are: memory per channel, voltage/current, tempratures, etc.

Applications:

ISDN line interfaces are used to transmit voice and data in studios, PBX, TELCO, radio broadcasting stations, conferencing between remote speakers, etc.

Author: Deepak Raina (Component Engr.)

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