Figure1: Typical analog phone with Message Waiting Indicator (MWI)
A Message Waiting Indicator (MWI) is a light on an analog telephone set that indicates a voice message has been recorded and is waiting for the recipient at that extension.
Since signaling options for analog lines are limited, the PBX uses high voltage pulses over the analog telephone line to activate a small light bulb. In the past, telephone sets used a small neon bulb for this purpose. The neon bulb was activated by high voltage and consumed very little power. This made it an ideal message waiting indicator.
In addition to the MWI, most PBX appliances send a scattered dial tone when a message is waiting.
Asterisk PBX MWI Mechanisms
For Telephones with Integrated MWI
If your telephone sets support MWI functionality, enabling it is simply
a process of changing two parameters, one to enable VM in the GUI configuration (refer to the relevant Asterisk distribution documentation for instructions) and
the other in the Astribank driver.
MWI support is disabled by default in the Astribank FXS driver. This is because when a mailbox contains VM messages, Asterisk periodically sends FSK encoded message to the channel. This means that the FXS driver must always analyze the PCM coming from the Asterisk even when the FXS extension is in the on-hook state.
To enable MWI, the vmwineon parameter must be defined for xpd_fxs driver. To do this, add the following line to the /etc/modprobe.d/xpp file:
options xpd_fxs vmwineon=1
After that, restart Asterisk and Zaptel/DAHDI.
For Telephones without Integrated MWI
If your telephone sets are not equipped with integrated MWI functionality it is still possible to implement it using an external LED. Modern MWI mechanisms are based on a low-power Light Emitting Diode (LED) with a blocking component (like a Zener diode) that blocks line voltage which is lower than the required voltage (usually 80 to 100 Volts). See the schematic for this scenario in Figure 2, below:
Figure 2: This circuit shows how to activate MWI functionality for telephone sets that are not equipped with an integrated MWI.
To add the MWI functionality to your existing analog telephone set, simply create the above reference circuit and connect it in parallel to the telephone line.
Then, modify the vmwineon parameter, which must be defined for xpd_fxs
driver. To do this, add the following line to the /etc/modprobe.d/xpp
file:
options xpd_fxs vmwineon=1
Make sure the VM function is enabled in the GUI configuration (refer to the relevant Asterisk distribution documentation for instructions).
Popular Implementations
MWI is popular in hotels (click here to see our hotel implementation white paper), offices, and most public telephone networks.
Ease of implementation and accuracy of the MWI technology is illustrated below. The regularly oscillating signal voltage indicates the expected, clean wave shape of the electrical signal.
Figure 3: Scope probes are x10. Line voltage is 50 volts, jumping to 90 volts every 250 milliseconds.
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