Understanding the network.pdf - Back to Home

• Network Termination type 2 (NT2)—This is an ISDN-compatible switch or
multiplexing device, or interface card on an ISDN-capable switch. It can
provide support for ISDN Layer 1, 2, and 3 functions along with maintenance
and circuit monitoring. The most common example of an NT2 device is a PBX.
This is also the PRI circuit termination point.
• Network Termination type 1 (NT1)—This is the ISDN equipment responsible
for the BRI circuit termination. The NT1 in the United States is a CPE device.
In Europe and Japan, the NT1 is provided by the telephone company. The
NT1 provides the same function as a smart jack in a TDM network
configuration. It provides the circuit termination, the two-pair-to-single-pair
conversion needed for PSTN transmission, and a testing interface used by
the telephone company for troubleshooting and monitoring.
• Line Termination (LT)—LT is the telephone company CO ISDN circuit
termination point. This is the port on the CO switch provisioned for your
ISDN circuit termination. It is through this interface that the ISDN adapter
negotiates its Terminal Endpoint Identifier (TEI). The TEI is a unique ID used
between the CO switch and the ISDN CPE for identification.
• Exchange Termination (ET)—This is also the CO switch. This interface
represents the switch trunk interface responsible for providing the
interconnection services between LT and the rest of the ISDN (PSTN)
network.
Now, let's look at the four ISDN demarcation reference points. Figure 5.12
illustrates these demarcation points in relation to the wire pair path and their
physical CPE device interfaces.
Figure 5.12. ISDN reference points and their physical
interface implementations.