Equipment

It's important to understand just what type of equipment you need and why. Because there are two major types of ISDN, BRI and PRI, it makes sense that you'll have different equipment needs. The point where responsibility for the data or voice connection passes from the telco to you is known as the demarcation point. Depending on where you are, this point might be outside the building or inside the building.

T1 and E1 connections are actually pretty simple. You or the telco need to run a cable from the demarcation point to the next device. You can purchase a channel service unit/data service unit (CSU/DSU), run the cable to there, take care of the channeling, run a cable to the router, and connect it to a serial port. Or you can buy an appropriate T1 or E1 interface with integrated CSU and run the cable straight to the router.

BRI gets a bit more complicated because of backward compatibility with analog devices and who controls what segments of the network connection. To allow for worldwide regulations and policies, ISDN got a little complicated on the BRI side.

Geographical Concerns

Different places have their own way of doing things and that holds true for WAN access. In the U.S., the telco is responsible for getting the connection to your property and that's about it. Once they attach it to the demarcation point and test it, they're done. Other countries vary, with some being responsible for the connection all the way up to the interior wall.

It's important to remember that ownership and control don't have a place in ISDN standards. The implementation is the same no matter who is responsible for the equipment.

Reference Points

A reference point defines where something happens. We normally aren't concerned with a cable or wire, but we get concerned with buying the wrong piece of expensive equipment. This section covers the reference points that define functions which occur from the demarcation point to the end device. The following letters don't mean anything specific in the standards, but Cisco sometimes refer to them with another name. These letters essentially serve as easy ways to identify parts of the network. Imagine if a networking engineer decided to name every patch cable from a PC to the wall with the letter "P," and you'll understand where this topic is going. Those names are included here. Figure 7.2 illustrates the reference points in action with detailed explanations of each point following the illustration.

Figure 7.2. Reference points.

The telco provides a pair of wires that terminates at the U (user) interface on an NT-1 device. The NT-1 converts the single pair of wires into a double pair, each pair used for either transmit or receive. The four-wire interface is called an S/T.

Because regions differ, some subscribers are responsible for the equipment from the U interface, and others are responsible from the S/T interface and never see the U. Some equipment comes with a built-in NT-1 and can have a two-wire U plugged in directly. The device does the two-wire-to-four-wire conversion itself.

The S/T interface is a four-wire interface that supports up to seven devices on its bus. Technically, the S and T interfaces are separate, but an integrated NT-2 device performs the conversion. Nowadays, the S, T, and NT-2 can be considered a single device. Just be sure to understand that they are different for test purposes.

Several types of devices can attach to the bus, including phones, faxes, and analog conversion devices. Any device that speaks ISDN natively, such as an ISDN telephone, is known as a terminal endpoint 1 (TE-1). These devices get plugged into the S/T bus. But because an analog device can't effectively be plugged in, it needs a converter, a terminal adapter (TA). The TA converts S/T into an R (rate), which can then be used to connect to an analog telephone, fax, or other modem, a terminal endpoint 2 (TE-2).

There are other interfaces and equipment, such as the V interface, but because they aren't a part of the flow from the telco to you, you won't be tested on them.