User manual

Basic Installation and Setup 2-

B- and D-channels

BRI typically contains three channels. These channels are

created using complex signaling techniques.

Usually BRI is made up of two 64 kbps B (bearer) -channels and

one 16 kbps D (delta) -channel. The B-channels carry data or

voice traffic. The D-channel is used for call control: the setting

up and tearing down of calls. See Figure 2 below.

Figure 2. ISDN BRI—Three Channels over One Pair of Wires

Inside the NETServer Plus I-modem

Each NETServer Plus 8-port I-modem contains four separate

ISDN terminal adapters. While these terminal adapters don’t

really look like the stack of Courier I-modems in the figure on

the next page, they do operate as if they were Courier desktop I-

modems plugged into a computing device (in this case, the

NETServer routing engine) via a pair of serial cables.

Unlike an analog modem, each I-modem must be able to process

a BRI ISDN phone line, containing two separate data channels.

An I-modem maps these B-channels to its internal serial

interfaces which are in turn connected to ports on the

NETServer’s routing engine.

To provide the unique emulation of analog modems, I-modems

will respond to AT commands received from either serial

interface. Keep in mind that each pair of B-channels/serial lines

is really serviced by only one device. Certain AT commands will

affect both serial interfaces simultaneously. See a later chapter in

this User Manual and the NETServer 8/16 Plus Reference Guide for

more details on I-Modem operation and commands.