CMX602BD4 MX-COM, Inc., CMX602BD4 Datasheet - Page 15

• Manufacturer Part Number: CMX602BD4
• Description: Calling line identifier plus call waiting
• Manufacturer: MX-COM, Inc.

Calling Line Identifier plus Call Waiting (Type II)
5.2 'Off-Hook' Operation
The CIDCW (Calling Identity on Call Waiting) system, as 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, British Telecom 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 near end CPE (Customer Premises Equipment - e.g. a telephone) and the
Central Office as indicated in Figure 14.
A. Normal conversation with both near and far voice present.
B. Central Office mutes far end voice, emits CAS, and becomes silent.
C. CPE recognizes CIDCW initiation and mutes near end voice and keypad.
D. CPE emits DTMF ACK to Central Office to signal its readiness to receive Caller ID data stream.
E. Central Office recognizes ACK and emits FSK Data stream of Caller ID data which is received and
F. CIDCW transaction is complete. CPE unmutes near end voice and Central Office unmutes far end voice
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 number of false responses (Talk-offs) and missed signals (Talk-downs) are related to the speech
levels at the CMX602B input, 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 CMX602B's audio input from the receive side of the telephone set hybrid where this is
possible.
The internal algorithms used by the CMX602B to drive the DET and IRQ outputs in Tone Alert Detect mode
have been optimized 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
2. When the IRQ output goes low in Tone Alert Detect mode, this indicates that a CAS has been detected.
3. The CMX602B's DET output should be monitored for a period of 50ms after changing to FSK Receive
4800 Bethania Station Road, Winston-Salem, NC 27105-1201, USA
2000 MX-COM, Inc
decoded by CPE.
returning to normal conversation with both near and far voice present.
noise (i.e. where the CIDCW equipment is built into the telephone set), then this muting should be done
whenever the CMX602B is in Tone Alert 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 CMX602B is set to
Tone Alert Detect mode.
The local handset and keypad should then be muted as required by the Bellcore specification and the
CMX602B switched to FSK Receive mode to be ready to receive the FSK data. Doing this will also clear
the IRQ output.
mode and before sending the ACK signal. The transaction should be abandoned if the DET output goes
high during this time, which would be the case if a false CAS detect had been caused by far end speech.
Signals originating
at near end CPE
Signals originating
from far end CPE
and Central Office
Figure 14 CIDCW Transaction From Near End CPE Perspective
www.mxcom.com Tel: [phone] 744 5050 Fax: [phone]
far voice
near voice
A
CAS
B
C
15
All trademarks and service marks are held by their respective companies.
ACK
D
FSK data
E
CMX602B PRELIMINARY INFORMATION
near voice
far voice
Doc. # 20480204.002
F