ISDN(INTEGRATED SERVICES DIGITAL NETWORK)

The Integrated Services Digital Network (ISDN) is a method used to bridge "the last mile" between the Central Office and the premise connection (home).
ISDN uses the existing wiring so no new cabling is required.
There are two basic services offered:
Basic Rate Interface (BRI) consists of 2B + D channels.
Which stands for 2 Bearer channels of 64 kbps each for data and one D channel of 16 kbps for handshaking and control.
Having a separate channel for handshaking and control is called "out of band" signalling. The 2B channels can be bonded together for a single data channel with a 128 kbps transfer rate.
Primary Rate Interface (PRI) consists of 23B + D channels.
Which stands for 23 Bearer channels of 64 kbps each for data and one D channel of 64 kbps for handshaking and control.
The Bearer channels can be bonded in any combination as required.
ISDN lines can be dedicated lines that are always up and connected or they can be dial on demand (DOD) lines.
When the line is required the connection is dialed up and made.
The connection time for an ISDN line is very quick, in the order of 0.5 second or so.
This can result in a substantial cost saving if used over long distance or paying by the minute.
The line charges are only for when data is being transferred and not when it is sitting idle.

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