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 changes 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, the signals are taking turns on the channel.
Figure 1 displays a sample ISDN topology. The ISDN connection may require a terminal adapter (TA) which is a device used to connect ISDN Basic Rate Interface (BRI) connections to a router.
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 BRI 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 (Figure 2).
- Primary Rate Interface (PRI) – ISDN is also available for larger installations. In North America, PRI delivers 23 B channels with 64 kb/s and one D channel with 64 kb/s 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 (Figure 3).
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.