ISDN Function Groups and Reference Points

Many people are 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. Cisco expects CCNAs to be familiar with all function groups and reference points. 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. However, for a good understanding of function groups and reference points, keep the following facts in mind:

• Not all reference points are used in any one topology; in fact, one or two might never be used in a particular part of the world.

• After the equipment is ordered and working, there is no need to think about function groups and reference points.

• The router configuration does not refer to reference points and function groups, so many people ignore these details.

A cabling diagram is helpful for examining the reference points and function groups. Figure 9-6 shows the cabling diagram for several examples.

Figure 9-6 ISDN Function Groups and Reference Points

Figure 9-6 ISDN Function Groups and Reference Points

Reference Point

Router A is ordered with an ISDN BRI U reference point, referring to the I.430 reference point defining the interface between the customer premises and the telco in North America. Router B is bought with an ISDN BRI 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; the S/T interface defines the connection to Router B. Router B is called a TE1 (Terminal Equipment 1) function group device. Non-ISDN equipment is called a TE2 (Terminal Equipment 2) device and is attached using the R reference point to a terminal adapter (TA) function group device. Alternatively, a TE1 can connect using an S reference point to an NT2 function group, as shown in case D of Figure 9-6.

Table 9-12 summarizes the types shown in Figure 9-6. Tables 9-13 and 9-14 summarize the formal definitions.

Table 9-12 Function Groups and Reference Point Summary


Function Group(s)

Connected to Which Reference Point(s)

Type of Interface Used in Router


TE1, NT1


ISDN card, U interface



S/T (combined S and T)

ISDN card, S/T interface




Serial interface—no ISDN hardware/software in router

D TE1 S Serial interface—no ISDN

hardware/software in router

Table 9-13 Definitions for the Function Groups Shown in Figure 9-6

Function Group

What the Acronym Stands For



Terminal Equipment 1

ISDN-capable four-wire cable. Understands signaling and 2B+D. Uses an S reference point.


Terminal Equipment 2

Equipment that does not understand ISDN protocols and specifications (no ISDN awareness). Uses an R reference point, typically an RS-232 or V.35 cable, to connect to a TA.


Terminal adapter

Equipment that uses R and S reference points. Can be thought of as the TE1 function group on behalf of a TE2.


Network Termination Type 1

CPE equipment in North America. Connects with a U reference point (two-wire) to the telco. Connects with T or S reference points to other customer premises equipment.


Network Termination Type 2

Equipment that uses a T reference point to the telco outside North America, or to an NT1 inside North America. Uses an S reference point to connect to other customer premises equipment.



A combined NT1 and NT2 in the same device. This is relatively common in North America.

Table 9-14 Definitions for the Reference Points Shown in Figure 9-6

Reference Point

Connection Between


TE2 and TA.


TE1 or TA and NT2.


NT2 and NT1.


NT1 and telco.


TE1 or TA, connected to an NT1, when no NT2 is used. Alternatively, the

connection from a TE1 or TA to a combined NT1/NT2.

Jargon definitely confuses the issue with home ISDN services. Figure 9-7 outlines the problem.

Jargon definitely confuses the issue with home ISDN services. Figure 9-7 outlines the problem.

Figure 9-7 Home ISDN User and Reference Points

Interface Card Isdn Bri

Popularly used, ISDN terminology for home-based consumers sometimes muddles the terminology from the ISDN specifications. The home user orders the service, and the telco offers to sell the user one of several "ISDN modems." What is actually received is a TA and NT1 in one device. A PC uses a serial port to connect to the TA, which uses reference point R. However, the terms reference point, TA, and NT1 are almost never used by providers—hence the confusion.

One other detail of the ISDN protocols that might be on the exam is the ISDN SBus. The ISDN SBus allows multiple devices to share the same BRI by sharing the S reference point. SBus is a great idea, but it has not been deployed extensively. SBus takes the S reference point and allows multiple TE1s to connect to the same NT1. This allows multiple TE1s to use the same BRI. If all the TE1s were data devices, instead of using an SBus, a better solution would be to place all TE1s on a LAN and use an ISDN-capable router. However, the SBus can be used to support ISDN phones, fax, video, and data TE1 devices. Figure 9-8 shows a basic SBus topology.

Figure 9-8 ISDN SBus

PC Phone


ISDN signaling

ISDN signaling can be created by TE1s and responded to by TE1s. However, because the BRI is shared among the TE1s, the SPID received in a call setup request no longer uniquely identifies the TE1. Therefore, a suffix called a subaddress is added to the SPID. Each TE1 on the SBus uses a different subaddress. The service provider connected to this NT1 and to any other NT1 from which calls are set up must support subaddressing before the user can use the SBus.

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    What are the different reference points in isdn?
    4 years ago
  • hana
    What are the functional group of ISDN?
    3 years ago
  • kristian
    What are the functional groups of an ISDN?
    3 years ago
  • felix
    Why weee need isdn reference point?
    3 years ago
  • tina
    What is a function group?
    2 years ago
  • jamila tesfalem
    What is the difference between functional groupings and reference point with reference to ISDN?
    1 year ago

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