ISDN Components and Reference Points

When ISDN is the choice of wide-area connection, the customer equipment may include terminal equipment (TE) (types 1 and 2), terminal adapters (TAs), and network termination (NT) equipment (types 1 and 2). A common two-wire facility forms the local access loop for ISDN. The line termination (LT) and exchange termination (ET) equipment are located at the edge of the service provider's network (see Figure 11-1).

Figure 11-1 ISDN Components and Reference Points

Figure 11-1 ISDN Components and Reference Points

The ISDN (TE) is classified into two types: TE2 and TE1. TE2 devices are non-ISDN terminals such as a standard telephone, a personal computer, or a router with no ISDN interface. TE1s are ISDN devices such as an ISDN telephone, or a router with a BRI interface. A TE2 device needs (to be connected to) an ISDN TA to get connected to an ISDN network. The TA may be an external device or a board that can be plugged into the TE2. If the TA is an external device, the TE2 connects to it with a standard serial cable/connection such as EIA/TIA-232-C (formerly RS-232-C), V.24, or V.35.

The TE1 (and TA) connects to NT1 using a four-wire cable (RJ48). You can also use a straight-through RJ45 cable for this purpose, knowing that only the inner four wires out of the eight wires of the RJ45 cable will be used. ISDN standards allow multiple terminals

(TE1s and TAs) to share an NT1 device (technically the S/T bus), one at a time, through a contention control mechanism provided by the NT2 device. The NT2 device naturally appears before the NT1 device. NT2 is typically found in digital private branch exchanges (PBXs). There are NT1/NT2 boxes also, which provide the functionality of both an NT1 and an NT2 device. The multiple TE1s (and/or TAs) connect to NT2 using a four-wire facility, and the NT2 device connects to the NT1 device with a four-wire cable as well. The NT1 device, however, connects to the LT equipment in the carrier network using the conventional two-wire (local loop) facility. In North America, it is the customer's responsibility to provide or pay for the NT1 device, while in Europe NT1 falls in the service provider's realm of responsibility.

As a result, the routers with ISDN BRI interface(s) that are produced for the North American market usually have the NT1 functionality built-in the BRI interface, whereas the routers produced for the European market don't. When the NT1 functionality is built into the BRI interface, this interface has a U label. On the other hand, if the NT1 functionality is not built into the interface, the interface has the S/T label. Table 11-1 provides four examples of 800 series Cisco routers, two of which (801 and 802) target the European market and the other two (803 and 804) target the North American market.

Table 11-1 Examples of800 Series Routers with S/T and U BRI Interfaces

Model	BRI Interface	Ethernet Interface	Comment
801	S/T	Single RJ45 10BaseT port	Target Europe
802	S/T	4 port internal 10BaseT hub	Target Europe
803	U	Single RJ45 10BaseT port	Target North America
804	U	4 port internal 10BaseT hub	Target North America

ISDN reference points define logical interfaces between functional groupings such as TEs and NTs. Refer to Table 11-2 for a list of ISDN reference points. Figure 11-1 also shows where the reference points fit on the ISDN network.

Table 11-2 ISDN Reference Points

Reference Point	Place
R	Between terminal equipment type 2 (TE2) and terminal adapter (TA).
S	Between terminal equipment type 1 (TE1) or terminal adapter (TA) and
	the network termination type 2 (NT2) device.
T	Between the network termination type 2 (NT2) and the network
	termination type 1 (NT1) device.
U	Between the network termination type 1 (NT1) and the service provider's
	(carrier's) local termination (LT) device.
V	Between the service provider's (carrier's) local termination (LT) device
	and their exchange termination (ET) device.

A Cisco router may have no BRI interfaces, in which case you will have to connect its serial interface to an ISDN TA. If the router has a BRI interface with the S/T label on it, it means that you must connect the BRI interface to an external NT (NT2, NT2/NT1, or NT1) device. If the BRI interface of a router has NT1 built in, then the interface is marked with the U label. If the BRI interface of a router has the NT1 built in, it can then be directly connected to the service provider's switch (using the two-wire facility). If you connect a BRI interface that has an NT1 built in (one that has the U label) to an external NT1 device, the router's BRI interface and/or the external NT1 box may get damaged severely.

Continue reading here: ISDN Layer

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