Integrated Services Digital Network (ISDN) is a service that allows telephone networks to carry
data, voice, and other digital traffic. There are two types of ISDN interfaces: Basic Rate Interface
(BRI) and Primary Rate Interface (PRI). BRI uses two B channels and one D channel. Each
of the two B channels operates at 64Kbps bidirectionally; the D channel operates at 16Kbps.
The B channels are used for transmitting and receiving data. The D channel is used for protocol
communications and signaling.
In contrast, PRI uses 23 B channels and 1 D channel. All 23 B channels are added to a rotary
group, as well. The D channel runs at the same line speed as the B channels—64Kbps. Because of the
D channel’s additional line speed, PRI has the equivalent line speed of a T-1 circuit (1.544Mbps). In
Europe, PRI offers 30 B channels and 1 D channel, making it the equivalent of an E-1 circuit.
Just as there are two types of ISDN interfaces, there are two terminal equipment types. Type 1
(TE1) is equipment that was built specifically for use on ISDN. Type 2 (TE2) is equipment that
was made before the ISDN specifications, and it requires a terminal adapter to actually interface
with ISDN. Terminal equipment, which is comparable to DTE as described in the “Frame Relay”
section earlier in this chapter, includes computers or routers.
In order for terminal equipment to work, it must be able to connect to a network termination.
There are three types of ISDN network terminations, known as NT devices. Type 1 (NT1) devices
are treated as customer premises equipment. Type 2 (NT2) devices are more intelligent devices
than NT1 and can perform concentration and switching functions. The last type is a combination
of Types 1 and 2. It is known as a Type 1/2 or NT1/2.
More information about troubleshooting ISDN is covered in Chapter 40, “Troubleshooting
ISDN.”