Information Technology Reference
In-Depth Information
ISDN Components
ISDN components include terminals, terminal adapters (TAs), network-termination devices,
line-termination equipment, and exchange-termination equipment. ISDN terminals come in two types.
Specialized ISDN terminals are referred to as terminal equipment type 1 (TE1). Non-ISDN terminals
such as DTE that predate the ISDN standards are referred to as terminal equipment type 2 (TE2). TE1s
connect to the ISDN network through a four-wire, twisted-pair digital link. TE2s connect to the ISDN
network through a terminal adapter. The ISDN TA can be either a standalone device or a board inside
the TE2. If the TE2 is implemented as a standalone device, it connects to the TA via a standard physical
layer interface. Examples include EIA/TIA-232-C (formerly RS-232-C), V.24, and V.35.
Beyond the TE1 and TE2 devices, the next connection point in the ISDN network is the network
termination type 1 (NT1) or network termination type 2 (NT2) device. These are network-termination
devices that connect the four-wire subscriber wiring to the conventional two-wire local loop. In North
America, the NT1 is a customer premises equipment (CPE) device. In most other parts of the world, the
NT1 is part of the network provided by the carrier. The NT2 is a more complicated device, 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 U.
A sample ISDN configuration is shown in Example 16-1. This example 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.
Example16-1 A Sample ISDN Configuration
2503B#
show running-config
Building configuration...
Current configuration:
!
version 11.1
service timestamps debug datetime msec
service udp-small-servers
service tcp-small-servers
!
hostname 2503B
!
!
username 2503A password 7 0822455D0A16
ip subnet-zero
