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A computer connects to an ISDN line via an ISDN Terminal Adapter (often incorrectly referred to as an ISDN modem). An ISDN Terminal Adapter is not a modem because it does not convert a digital signal to an analog signal; ISDN signals are digital.

An ISDN line has two types of channels. The data is carried on special Bearer channels, or B channels, each of which can carry 64Kbps of data. A typical ISDN line has two B channels. One channel can be used for a voice call while the other is being used for data transmissions, and this occurs on one pair of copper wires. The second type of channel is used for call setup and link management and is known as the signal, or D channel (also referred to as the Delta channel). This channel has only 16Kbps of bandwidth.

To maximize throughput, the two Bearer channels are often combined into one data connection for a total bandwidth of 128Kbps. This is known as bonding or inverse multiplexing. This still leaves the Delta channel free for signaling purposes. In rare cases, you may see user data, such as e-mail, on the D line. This was introduced as an additional feature of ISDN, but it hasn’t caught on.

The main advantages of ISDN are:

■    It has a fast connection.

■    It offers higher bandwidth than POTS. Bonding yields 128Kb bandwidth.

■    There is no conversion from digital to analog.

However, ISDN does have a few disadvantages:

   It’s more expensive than POTS.

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