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

Table 10-7 Definitions for the Reference Points Shown in Figure 10-3

Reference Point

What It Connects Between

 

 

R

TE2 and TA

 

 

S

TE1 or TA and NT2

 

 

T

NT2 and NT1

 

 

U

NT1 and the telco

 

 

S/T

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.

 

 

For the home PC user, marketing jargon can get in the way of learning the true ISDN terminology. Figure 10-4 outlines the problem.

Figure 10-4 Home ISDN User and Reference Points

ISDN

2 Wire

 

Modem

 

RS-232

Telco

(R)

(U)

TA

&

NT1

Popularly used ISDN terminology for home-based consumers sometimes muddles the terminology from the ISDN specifications. The home user orders the ISDN service, and the telco offers to sell the user one of several “ISDN modems.” What is actually received is a single device that includes a TA and an NT1. A PC uses a serial port to connect to the TA, which uses reference point R to connect to the PC and reference point U to connect to the

BRI line itself. However, the terms reference point, TA, and NT1 are almost never used by providers—hence the confusion with the official terminology covered here.

ISDN PRI Function Groups and Reference Points

The ITU planned for multiple implementation options with BRI because BRI would typically be installed when connecting to consumers. PRI was seen as a service for businesses, mainly because of the larger anticipated costs and a PRI’s larger number of B channels. So the ITU did not define function groups and reference points for ISDN PRI!