Calling Line Identifier
CMX602A
1.6.2 'Off-Hook' Operation
The CIDCW (Calling Identity on Call Waiting) system described in this section operates when the telephone set
is in use (off-hook) to display the number of a waiting caller without interrupting the current call.
Bellcore documents GR-30-CORE and SR-TSV-002476, BT specifications SIN227 and SIN242 Part 2 and ETS
300-659-2 all describe similar systems in which a successful CIDCW transaction 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.
A.
B.
C.
D.
E.
F.
Normal conversation with both near and far end voice present.
Central Office mutes far end voice, sends CAS and becomes silent.
CPE recognises CIDCW initiation and mutes near end voice and keypad.
CPE sends dtmf ACK to Central Office to signal its readiness to receive FSK data.
Central Office recognises ACK and sends FSK Caller ID data to CPE.
CIDCW transaction is complete. CPE unmutes near end voice and the Central Office
unmutes far end voice, returning to normal conversation.
Figure 9a : CIDCW Transaction from Near End CPE Perspective
The CAS signal 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 is very important because a “missed” signal causes Caller ID information to be lost and a 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 difficult to accomplish correctly.
Because the numbers of false responses (Talk-offs) and missed signals (Talk-downs) are related to the speech
levels at the CMX602A input, and because the level of near end speech from the local handset is normally
greater than that of far end speech coming from the Central Office, a further improvement in overall
performance can be obtained by taking the CMX602A’s audio input from the receive side of the telephone set
hybrid where this is possible.
The internal algorithms used by the CMX602A to drive the DET and IRQN outputs in Tone Alert Detect mode
have been optimised for the detection of off-hook CAS signals in the presence of speech when used according
to the following principles:
1.
If it is possible to mute the local speech from the microphone rapidly (within 0.5ms) without introducing
noise (i.e. where the CIDCW equipment is built into the telephone set) then this should be done
whenever the CMX602A is in Tone Detect mode and the DET output is high. Doing this will markedly
reduce the number of false responses generated by local (near end) speech. Note that the DET output
is not used for any other purpose in an off-hook application when the CMX602A is set to Tone Alert
Detect mode.
ã 1998 Consumer Microcircuits Limited
17
D/602A/4
CMLMICRO [ CML MICROCIRCUITS ]
