Page 32-2
“U”, “S/T” , and “R” Interfaces
The ISDN specification defines a limited set of user-to-network interfaces, including reference
points for the BRI access method. The following are the main BRI reference points:
U Interface. The U interface is a two-wire (single pair) interface that supports full-duplex data
transfer from the phone switch. Only a single device can be connected to a U interface. This
device is called a Network Termination 1 (NT1) which converts the U interface to the S/T
interface (described below). The U interface is used in North America. Elsewhere in the
world, telephone companies supply the NT1 service, allowing customers the use of S/T inter-
faces.
S/T Interface. The S/T interface is a four-wire, bus interface on which multiple (up to eight)
ISDN access devices can be attached to gain shared access to ISDN’s data channels. The S/T
interface is the most commonly-used interface in Europe.
R Interface. The R interface is a general reference point at which non-ISDN devices can gain
access to an ISDN network through a device called a Terminal Adapter (TA). A Terminal
Adapter typically converts various standard interfaces, such as RS232 and V.35, to the S/T bus.
The “B,” “D,” and “H” Channels
ISDN supports three types of data channels: the “B” channel, the “D” channel and the “H”
channel. The line encoding and framing structure for each type of channel varies among the
U, S/T, and R interfaces and for different access methods. A brief description of the three
channels follows:
B Channel. The B channel is used for the transfer of information, which can be any type of
data that the endpoints agree on, such as digitized voice, digitized video or packet data. The
B channel operates at 64 kbps on both BRI and PRI interfaces, but is commonly rate-adapted
to 56 kbps in North America to accommodate switching system limitations. A single BRI inter-
face consists of one D channel operating at 16 kbps and two B channels operating at 64 kbps
(or 56 kbps in North America).
D Channel. The D channel operates at 16 kbps on BRI (64 kbps on PRI) and is used for carry-
ing common-channel signaling. The D channel is used both to establish and maintain circuit-
switched calls on the B channels. The D channel can also be used to carry low-speed packet-
switched data (the Omni Switch/Router does not support such usage).
H Channel. The H channel, supported only on PRI interfaces, is used to transfer information at
higher bit rates by aggregating B channels. The four implementations of the H channel are:
H0 (384 kbps, 6 B channels), H10 (1472 kbps, 23 B channels), H11 (1536 kbps, 24 B chan-
nels), and H12 (1920 kbps, 30 B channels). The use of the H channel is not supported by the
Omni Switch/Router because this channel requires a PRI interface.
To Next Page
Loading...
