The ISDN function groups represent the devices in an ISDN environment such as terminals, terminal
adapters, network-termination devices and line-termination equipment. It is important to
understand the different function groups when you design and troubleshoot your ISDN network.
Figure 26.2 shows the function groups and their placement in an ISDN network.
FIGURE 2 6 . 2
ISDN function groups
PRIMARY-NET5 NET5 switch type for the U.K. and most of Europe
VN3 VN3 and VN4 switch types for France
PRIMARY-NTT Japanese ISDN PRI switches
PRIMARY-NI AT&T National ISDN switch type for the U.S.
TABLE 2 6 . 3
PRI Switch Types
(continued)
Switch Type Typically Used
S/T
S/T
R
S0
U
bri0
bri0
Native ISDN interface—int bri0
Nonnative ISDN interface—int serial 0
(EIA/TIA-232, V.35, X.21)
The following are definitions and examples of ISDN BRI function groups as they relate to
Figure 26.2:
Terminal equipment 1 (TE1)
A device that understands ISDN digital-signaling techniques.
Examples of TE1 devices are digital telephones, routers with ISDN interfaces, and digital facsimile
equipment. TE1 devices are 4-wire (2 pair) and need to be 2-wire (1 pair) to communicate
with an ISDN network. A TE1 will connect into a network termination type 1 (NT1) to connect
the 4-wire subscriber wiring to the 2-wire local loop facility.
Terminal equipment 2 (TE2)
Equipment that does not understand ISDN signaling standards.
Examples of TE2 devices are analog telephones, X.25 interfaces, and serial interfaces on a router.
A TE2 device needs to be converted to ISDN signaling, which is provided by a terminal adapter
(TA). After that, it still needs to be converted to a 2-wire network with an NT1 device.
Network termination type 1 (NT1)
This device is used to convert a 4-wire ISDN connection
to the 2-wire ISDN used by the local loop facility. This device is primarily used in the United
States, because European service providers retain ownership of this functionality.
Network termination type 2 (NT2)
This device is used to direct traffic from ISDN devices
(TEs) to an NT1. This is probably the most intelligent device in the ISDN network; it provides
switching and concentrating and can sometimes even be a private branch exchange (PBX).
Terminal adapter (TA)
This device enables a TE2 device to communicate with the telco’s network
by providing any necessary protocol and interface conversion. In essence, a TA adapts the
unipolar signal coming from a non-ISDN device into a bipolar signal used by the ISDN network.
Local termination (LT)
This is the same device as an NT1, but located at the provider’s site.
Exchange termination (ET)
The connection to the ISDN switch, typically an ISDN line card.
The ET forms the physical and logical boundary between the digital local loop and the carrier’s
switching office. It performs the same functions at the end office that the NT performs at the
customer’s premises. Both the LT and the ET together are typically referred to as the local
exchange (LE).