Information Technology Reference
In-Depth Information
local, active, and associated with serial 0. Statistics about frames sent and received on the PVC
are displayed, and you can see that the PVC was created 4 weeks and 3 days ago and that it last
changed status 2 weeks and 5 days ago.
ISDN
ISDN services are offered by telephone carriers that provide digital telephony and data-transport
services. ISDN links transport voice, video, and data traffic. For the CCIE test, it is important
to understand the architecture, interfaces, and protocols in ISDN.
ISDN Architecture
ISDN specifications define component types and reference points for its architecture. The ISDN
components are as follows:
•
TA (Terminal Adapter)
—Connects non-ISDN equipment to the ISDN network.
•
TE1 (Terminal Equipment type 1)
—Any terminal equipment, such as a router or
telephone, with an ISDN BRI. This device has a four-wire interface and is ISDN
compatible.
•
TE2 (Terminal Equipment type 2)
—Any terminal or router that is not ISDN
compatible. It connects to the TA through a EIA/TIA-232 or V.35 interface. The TA
connects the TE2 device to the ISDN network. Its input is the physical EIA/TIA-232
interface, and the output is the ISDN BRI.
•
NT1 (Network Termination type 1)
—A network termination device responsible for the
termination of the ISDN transmission facility at the customer premises. The NT1 connects
the 4-wire ISDN interface to the 2-wire phone line. In North America, the customer
provides this device. In the rest of the world, the carrier usually provides this device.
•
NT2 (Network Termination type 2)
—Provides switching and concentration of ISDN
lines between customer premise equipment, such as digital PBXs. The NT2 provides
Layer-2 and Layer-3 protocol and concentration functions. NT2 can be combined with
an NT1.
•
LE (Local Exchange)
—The carrier's ISDN central office (CO).
The ISDN architecture defines four reference points that are shown in Figure 5-10. The U (user)
reference point is located between the NT1 device and the local exchange. The ISDN local loop
in North America is a U reference point, which is a single pair of wires. The T (terminal) reference
point is the two pair of wires located between the NT1 and NT2 equipment, if necessary. The
S (system) reference point is located between user terminals (TE1 or TA) with BRI interfaces
and the NT2. Usually, the S and T points are combined as S/T where the NT1 and NT2 are
combined into a single device, sometimes referred to as NT12. And finally, the R (rate) reference
point is located between the TE2 (non-ISDN terminals) and the TA.
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