ISDN is a switched network that incorporates a digital (instead of analog) local loop connection to the central office over ordinary telephone copper wire or other media. As the worldwide telephone network trunks between switching centers are converted to digital, ISDN is being incorporated, allowing for end-to-end switched digital connections.

In general, there are three main goals for ISDN:

• Provide end-to-end digital connectivity.
• Support a wide range of services, both voice and non-voice.
• ISDN access through a small set of standard user-to-network interfaces.

There are two methods for accessing ISDN.

• The basic rate interface (BRI) is intended for residential subscribers and small offices.
• The primary rate interface (PRI) is intended for users with greater data-transfer capacity requirements, such as offices with digital phone switches.

A single BRI interface consists of one D channel operating at 16 Kbps and two B channels operating at 64 Kbps (or 56 Kbps in North America). The D channel is used for signaling (dialing the connection). The B channels are used for the actual data transfer. Since there are two channels, two independent communications can occur at the same time (e.g., allowing a phone and modem connection from your house). You may even use a "bonding" protocol to combine the two B channels to form a single 128 Kbps connection.

A PRI interface has 24 B channels, 23 for user data and one for signaling. This interface can also be "bound" to form larger channels.

Typically, B channels are found at remote sites (or homes), and are combined with other B channels (via Telco switches) and terminated as a PRI interface at the corporate office.