ISDN BRI Function Groups and Reference Points

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and in some geographies, and more slowly by other people. Therefore, the ITU defined several different options as to what equipment might be used at the customer site while still ultimately using an ISDN line. By doing so, ISDN could grow and give customers many migration options.

The ISDN specifications identify the various functions that must be performed to support customer premises equipment (CPE). ISDN uses the term function group to refer to a set of functions that a piece of hardware or software must perform. Because the ITU wanted several options for the customer, it defined several different function groups. Because the function groups might be implemented by separate products, possibly even from different vendors, the ITU needed to explicitly define the interfaces between the devices that perform each function.

Therefore, ISDN uses the term reference point to refer to this interface between two function groups.

Many people get confused about the ISDN terms reference point and function group. One key reason for the confusion is that only some function groups—and therefore some reference points—are used in a single topology. Another reason is that if you just work with ISDN on routers, you do not typically need to think about some of the function groups and reference points. Also, most people who work with ISDN every day do not even use the terms function group and reference point. In an effort to clear up these two topics, consider the following inexact but more-familiar definitions of the two:

■Function group—A set of functions implemented by a device and software

■Reference point—The interface between two function groups, including cabling details

Most people understand concepts better if they can visualize or actually implement a network. A cabling diagram is helpful for examining the reference points and function groups. Figure 10-3 shows the cabling diagram for several examples.

Routers A and B represent typical cabling with a Cisco router, using ISDN, in North America. Router A is ordered with an ISDN BRI U interface; the U implies that it uses the U reference point, referring to the I.430 reference point for the interface between the customer premises and the telco in North America. No other device needs to be installed; the line supplied by the telco is simply plugged into the router’s BRI interface.

Router B uses a BRI card with an S/T interface, implying that it must be cabled to a function group NT1 device in North America. An NT1 function group device must be connected to the telco line through a U reference point in North America. When using a router BRI card with an S/T reference point, the router must be cabled to an external NT1, which in turn is plugged into the line from the telco (the U interface).

332 Chapter 10: ISDN and Dial-on-Demand Routing

Figure 10-3 ISDN Function Groups and Reference Points

Both Router A and Router B use an ISDN BRI card, but they have different reference points. The list price of the BRI-U card is typically several hundred dollars higher than a BRI-S/T card, because the BRI-U essentially contains the NT1 function group on the card, making an external NT1 unnecessary.

A router can connect to an ISDN service with a simple serial interface, as shown with Router C in Figure 10-3. Router C must implement an ISDN function group called TE2 (Terminal Equipment 2) and connect directly to a device called a terminal adapter using the R reference point.

Fun, isn’t it? The real goal of ISDN is to allow various migration paths. Imagine that you already own a router at a remote site, and the router does not support ISDN interfaces. You could buy an external ISDN TA and NT1, and use ISDN, for a lot less money than buying a new router with an ISDN BRI-U card. For another site, you might deploy a new router, so you buy a router with a BRI-U card, and you can avoid worrying about all the external devices. In either case, the ITU tried to provide options to the public and to the service providers selling ISDN to the public—with the unfortunate side effect of requiring CCNA candidates to memorize a lot of details!

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Table 10-5 summarizes the types shown in Figure 10-3. Tables 10-6 and 10-7 summarize the formal definitions.

Table 10-5 Function Groups and Reference Point Summary

Table 10-6 Definitions for the Function Groups Shown in Figure 10-3