System information

ISDN Services
Book Title
17-374
typically found in digital private branch exchanges (PBXs), that performs Layer 2 and 3 protocol
functions and concentration services. An NT1/2 device also exists; it is a single device that combines
the functions of an NT1 and an NT2.
A number of reference points are specified in ISDN. These reference points define logical interfaces
between functional groupings such as TAs and NT1s. ISDN reference points include the following:
• R—The reference point between non-ISDN equipment and a TA.
• S—The reference point between user terminals and the NT2.
• T—The reference point between NT1 and NT2 devices.
• U—The reference point between NT1 devices and line-termination equipment in the carrier
network. The U reference point is relevant only in North America, where the NT1 function is not
provided by the carrier network.
A sample ISDN configuration is shown in Figure 17-1. This figure shows three devices attached to
an ISDN switch at the central office. Two of these devices are ISDN compatible, so they can be
attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN
telephone) attaches through the R reference point to a TA. Any of these devices could also attach to
an NT1/2 device, which would replace both the NT1 and the NT2. And, although they are not shown,
similar user stations are attached to the far right ISDN switch.
Figure 17-1 A Sample ISDN Configuration
ISDN Services
The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI
B-channel service operates at 64 kbps and is meant to carry user data; BRI D-channel service
operates at 16 kbps and is meant to carry control and signaling information, although it can support
user data transmission under certain circumstances. The D-channel signaling protocol comprises
Layers 1 through 3 of the OSI reference model. BRI also provides for framing control and other
overhead, bringing its total bit rate to 192 kbps. The BRI physical layer specification is International
Telecommunication Union Telecommunication Standardization Sector (ITU-T; formerly the
Consultative Committee for International Telegraph and Telephone [CCITT]) I.430.
TE1 device
(computer)
NT2
S
NT1
T U
Switched
network
TE1 device
(ISDN
telephone)
NT2
S
NT1
T U
Packet
network
NT2
S
TA
R
NT1
T U
Private-line
network
ISDN
switch
ISDN
switch
TE2 device
(standard
telephone)