ISDN Demonstrator User’s Manual
40-400-00011, Rev. B
7
CHAPTER 2:
ISDN-BRI
Technology Overview
The Integrated Services Digital Network - Basic Rate Interface (ISDN-BRI) is
a technology that gives individuals affordable access to the world’s largest
digital network using the copper wire currently entering their home or office.
With ISDN-BRI, users may connect almost any form of personal
communications device to a single phone line and establish multiple,
simultaneous voice and data connections at speeds up to 128 thousand bits
per second.
ISDN is an extremely robust networking technology based on the Open
Systems Interconnection (OSI) model set forth in the International Standards
Organization specification, ISO84.
The OSI model specifies a layered design architecture. Layer 1, or the
physical layer, consists of two new interfaces, referred to as the
U and S/T
Interfaces
. Layer 2, or the data link layer, is a packet transport protocol
known as
Link Access Protocol - D Channel
(LAP-D), which was adapted
from ITU-T Recommendation X.25. Layer 3, or the network layer, is a flexible
switching and signaling language referred to as
ISDN Call Control
.
The ISDN Demonstrator is configured to support the first three layers of the
OSI model.
ISDN services are referred to as
Call Types
. Common call types are Circuit
Mode Voice (CMV), Circuit Mode Data (CMD), and Packet Mode Data
(PMD). Circuit mode call types support high-speed data and digitized voice,
while packet mode calls may be used for computer networking.
All ISDN-BRI user traffic travels over two 64Kbs
B Channels
and one 16Kbs
D Channel
. The B Channels are always used for circuit mode calls and the D
Channel is always used for transporting Call Control signaling packets
between the ISDN switch in the central office and the end-user
communications device. The B and D Channels may optionally transport user
Packet Mode Data.
Layer 1 - Physical Operation
Typical ISDN-BRI terminations and interfaces are shown in Chapter 5. Each
interface, which the telephone company terms a
reference point
, is designed
for a specific task.
U Reference Point
This interface connects the central office to the customer’s premises. It is a
160Kbs, two-wire, bidirectional transmission path that employs 2B1Q
encoding to endure long distances. It is designed to provide as much
bandwidth as possible over existing copper wire pairs. The U interface signal
carries two B channels, one D channel, overhead bits for framing and echo
