Calling Line Identifier with VMWI
CMX612
1.6.3 VMWI Operation
VMWI is an indicator for a group of services that is offered by telephone companies to their customers.
For example, it allows voice messages to be stored for later retrieval by the subscribing customer (Voice-
Mail Notification). Messages may be entered into a mailbox by any of the following methods:
1. A call placed to the customer’s line is unanswered after a certain number of rings and is then
forwarded to the voice-mail system (forward on do not answer).
2. A call placed to the customer’s line receives a busy signal and is then forwarded to the voice-mail
system (forward on busy).
3. A message is forwarded to the customer’s voice-mail box by a party directly through the voice-mail
system, without the customer’s line ever being called.
In each case, the presence of messages waiting in the customer’s voice-mail box is indicated to the
customer by generation of a VMWI signal. In Bellcore systems, the identification of a VMWI signal can
be performed by detecting either a Stuttered Dial Tone (off-hook) or an FSK signal (on-hook).
SDT Mode
When there is a message waiting in a customer’s voice-mail box, the telephone company’s Central Office
(CO) switch may apply a special dial tone to the customer’s line when it is taken off-hook. The special
dial tone is called Stuttered Dial Tone and is generally characterised as follows:
1. Steady dial tone frequencies (350Hz + 440Hz).
2. Steady dial tone amplitude up to -12dBV per tone applied from the Central Office (CO) switch to the
line.
3. A cadenced signal of 100ms on, 100ms off, repeated between 3 and 10 times and then steady dial
tone.
According to the US-FCC Alameda order, the Customer Premises Equipment (CPE) will make a
Stuttered Dial Tone check either:
1. When the subscriber takes the phone off-hook to make a call or
2. within 4 minutes of an unanswered call or
3. within 30 seconds of a completed call.
The telephone is taken off-hook to make a Stuttered Dial Tone check. The CPE then puts the CMX612
from ‘Zero-Power’ mode into Dial Tone Detect mode. If both dial tones are detected, then the DET output
will be set high and the IRQN output will be set low.
The DET output may then be polled every 40 - 80ms to check if it has been cleared. The DET output will
only be cleared if one or both tones are removed. If this occurs within 100ms, a counter may be
incremented in the CPE (external to the CMX612) and the IRQN output should then be cleared. This may
be done by taking the CMX612 out of Dial Tone Detect mode into ‘Zero-Power’ mode and back into Dial
Tone Detect mode.
On receiving another interrupt, the polling routine described above should be repeated. If the counter
reaches an appropriate value (e.g. 10) within an appropriate time (e.g. 2.3 seconds) then Stuttered Dial
Tone has been detected and a visual message indicator will then be lit by the CPE.
Other algorithms to detect Stuttered Dial Tone (e.g. continuous polling of the DET output once an
interrupt has been received) are also possible.
ã 2002 Consumer Microcircuits Limited
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D/612/3
CMLMICRO [ CML MICROCIRCUITS ]
