Integrated Services Digital Network
Integrated Services Digital Network (ISDN) is a circuit-switching technology that enables the local loop of a PSTN to carry digital signals, resulting in higher capacity switched connections. ISDN cahnges the internal connections of the PSTN from carrying analog signals to time-division multiplexed (TDM) digital signals. TDM allows two or more signals or bit streams to be transferred as subchannels in one communication channel. The signals appear to transfer simultaneously, but physically are taking turns on the channel. A data block of subchannel 1 is transmitted during timeslot 1, subchannel 2 during timeslot 2, and so on. One TDM frame consists of one timeslot per subchannel. TDM is described in more detail in Chapter 2, PPP.
ISDN turns the local loop into a TDM digital connection. This change enables the local loop to carry digital signals that result in higher capacity switched connections. The connection uses 64 kb/s bearer channels (B) for carrying voice or data and a signaling, delta channel (D) for call setup and other purposes.
There are two types of ISDN interfaces:
Basic Rate Interface (BRI)-ISDN is intended for the home and small enterprise and provides two 64 kb/s B channels and a 16 kb/s D channel. The BRI D channel is designed for control and often underused, because it has only two B channels to control. Therefore, some providers allow the D channel to carry data at low bit rates, such as X.25 connections at 9.6 kb/s.
Primary Rate Interface (PRI)-ISDN is also available for larger installations. PRI delivers 23 B channels with 64 kb/s and one D channel with 64 kb/s in North America, for a total bit rate of up to 1.544 Mb/s. This includes some additional overhead for synchronization. In Europe, Australia, and other parts of the world, ISDN PRI provides 30 B channels and one D channel, for a total bit rate of up to 2.048 Mb/s, including synchronization overhead. In North America, PRI corresponds to a T1 connection. The rate of international PRI corresponds to an E1 or J1 connection.
For small WANs, the BRI ISDN can provide an ideal connection mechanism. BRI has a call setup time that is less than a second, and the 64 kb/s B channel provides greater capacity than an analog modem link. If greater capacity is required, a second B channel can be activated to provide a total of 128 kb/s. Although inadequate for video, this permits several simultaneous voice conversations in addition to data traffic.
Another common application of ISDN is to provide additional capacity as needed on a leased line connection. The leased line is sized to carry average traffic loads while ISDN is added during peak demand periods. ISDN is also used as a backup if the leased line fails. ISDN tariffs are based on a per-B channel basis and are similar to those of analog voice connections.
With PRI ISDN, multiple B channels can be connected between two endpoints. This allows for videoconferencing and high-bandwidth data connections with no latency or jitter. However, multiple connections can be very expensive over long distances.
Note: Although ISDN is still an important technology for telephone service provider networks, it is declining in popularity as an Internet connection option with the introduction of high-speed DSL and other broadband services. The "Consumer and industry perspectives" section at http://en.wikipedia.org/wiki/ISDN provides a good discussion on ISDN worldwide trends.