Calling Line Identifier

FX602

© 1998 Consumer Microcircuits Limited

15

D/602/7

Figure 8c : Bellcore System signals (without ring)

1.6.2

'Off-Hook' Operation

Introduction

CIDCW (Calling Identity on Call Waiting) is a telephone service which identifies a waiting caller without

interrupting your current call. It eliminates the ‘blind spot’ in traditional Call Waiting by giving a telephone user

the informed choice of whether or not to take the incoming call.

To support CIDCW, the circuits of Caller ID compatible telephone equipment and adjunct ‘boxes’ must detect a

subtle CPE Alert Signal (CAS), injected into phone conversations. The CAS is transmitted by the Central Office

to initiate a CIDCW transaction and consists of a 80ms burst of simultaneous 2130Hz and 2750Hz tones.

CAS detection accuracy is very important because both missed and false signal detection is evident and

annoying to telephone users. Missed signal detection causes Caller ID information to be lost, false signal

detection produces a disruptive tone which is heard by the far end caller. Because the CAS signals must be

detected in the presence of conversations which both mask and masquerade as the tone signals, this function

is very difficult to accomplish correctly.

This application note describes the use of the FX602 for accurate CAS detection, providing better CIDCW

performance than other solutions. The FX602's <1µA ring detect supply current, 3 to 5 volt supply range, and

16 pin package also offer significant advantages in battery life and final product size.

Overview

A successful CIDCW transaction as described in Bellcore SR-TSV-002476 consists of a sequence of actions

between the CPE (Customer Premises Equipment - e.g. a telephone) and the Central Office as indicated in

Figure 9a.