Integrated Services Digital Network (ISDN)

ISDN has long been an alternative to the slower modem WAN connections but at a higher cost. ISDN allows the transmission of voice and data over the same physical connection.

ISDN connections are considerably faster than regular modem connections. To access ISDN, a special phone line is required, and this line is usually paid for through a monthly subscription. You can expect these monthly costs to be significantly higher than those for traditional dial-up modem connections.

To establish an ISDN connection, you dial the number associated with the receiving computer, much as you do with a conventional phone call or modem dial-up connection. A conversation between the sending and receiving devices is then established. The connection is dropped when one end disconnects or hangs up. The line pickup of ISDN is very fast, allowing a connection to be established, or brought up, much more quickly than a conventional phone line.

ISDN has two defined interface standardsBasic Rate Interface (BRI) and Primary Rate Interface (PRI).


BRI ISDN uses three separate channelstwo bearer (B) channels of 64Kbps each and a delta (D) channel of 16Kbps. B channels can be divided into 4 D channels, which allows businesses to have 8 simultaneous Internet connections. The B channels carry the voice or data, and the D channels are used for signaling.

The two B channels can be used independently as 64Kbps carriers, or they can be combined to provide 128Kbps transfer speeds.


PRI is a form of ISDN that is generally carried over a T1 line and can provide transmission rates of up to 1.544Mbps. PRI is composed of 23 B channels, each providing 64Kbps for data/voice capacity, and one 64Kbps D channel, which is used for signaling. Table 1 compares BRI and PRI ISDN.

Table 1 BRI and PRI ISDN Comparison










Transmission carrier