Introduction

Recommendations E.180 to E.184 deal with tones and other indications to users of the telephone service. This Recommendation covers the related but perhaps different requirements of the ISDN.

A person's ability to learn, distinguish between, and remember different tones representing abstract conditions is limited (to about four to six tones). Users are frequently confused by unfamiliar tones encountered through travel or international communication. Section 2 of this Recommendation advises against the use of new tones in order to avoid introducing user difficulties.

Where indications originate from a network (as proposed to a distant terminal), two situations may exist between the origin of the indications and the user of an ISDN terminal:

a) ISDN signalling is used throughout

b) In-band signalling is used in some parts of the connection.

Considering that information is sent to an ISDN terminal on a D-channel, and that the terminal may transform it into perceptible form, choices exist as to¹⁾ the most appropriate way of passing indications on to the user (e.g., by means of a display or by tones).

¹⁾ The desirability of translating various ISDN indications, the situations governing such choices, and the relationships of these to terminal capability (particularly display capability), are subjects for further study.