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
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
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.
|
Characteristic |
PRI |
BRI |
|---|---|---|
|
Speed |
1.544Mbps |
128Kbps |
|
Channels |
23B+D |
2B+D |
|
Transmission carrier |
T1 |
ISDN |