HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual page 399

In addition to wire and the demarc, the local loop for an ISDN connection
includes:
ISDN switch—At the public carrier's CO, the ISDN switch multiplexes and
de-multiplexes channels on the twisted pair wiring of the local loop. It
provides the physical and electrical termination for the ISDN line and then
forwards the data onto the public carrier's network.
Repeater—A repeater receives, amplifies, and retransmits the digital
signal so that the signal is always strong enough to be read. Because ISDN
lines use 2B1Q coding, which operates at a lower frequency range than
T1 or E1 encoding, repeaters are only required every 5.49 km (18,000 feet).
In contrast, T1 or E1 encoding requires a repeater approximately every
1.6 km (1 mile or 5,280 feet).
Network Interface Unit (NIU)—The NIU automatically maintains the
WAN connection and enables public carrier employees to perform simple
management tasks from a remote location. The NIU is usually located
outside the subscriber's premises so that public carrier employees can
always access it. (The NIU is commonly referred to as the "smart jack" in
North America.)
Network Termination (NT) 1—The NT1 provides the physical and electri-
cal termination for the ISDN line. It monitors the line, maintains timing,
and provides power to the ISDN line. In Europe and Asia, public carriers
supply the NT1. In North America, however, the subscriber provides the
NT1. In fact, many ISDN vendors are now building the NT1 directly into
ISDN equipment such as routers.
NT2—PRI ISDN also requires an NT2, which provides switching functions
and data concentration for managing traffic across multiple B channels.
In many regions, the NT1 and NT2 are combined into a single device,
which is called an NT12 (NT-one-two) or just NT.
Terminal equipment (TE) 1—TE1 devices are ISDN-ready devices and can
be connected directly to the NT1 or the NT2. TE1 devices include routers,
digital phones, and digital fax machines.
TE2—TE2 devices do not support ISDN and cannot connect directly to
an ISDN network. TE2 devices require a terminal adapter (TA) to convert
the analog signals produced by the TE2 device into digital signals that can
be transmitted over an ISDN connection. TE2 devices include analog
telephones and analog fax machines.
Terminal adapter (TA)—A TA allows you to connect a TE2 device to an
ISDN network.
Configuring Demand Routing for Primary ISDN Modules
Overview of ISDN Connections
8-7