Calling Line Identifier with VMWI
CMX612
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 10a.
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 10a : CIDCW Transaction from Near End CPE Perspective
The CAS signal is transmitted by the Central Office to initiate a CIDCW transaction and consists of an
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 CMX612 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 CMX612’s audio input from the receive side of the
telephone set hybrid where this is possible.
The internal algorithms used by the CMX612 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 CMX612 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
CMX612 is set to Tone Alert Detect mode.
ã 2002 Consumer Microcircuits Limited
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CMLMICRO [ CML MICROCIRCUITS ]
