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Technology Behind These Services
Technology-wise, the input of a telephone call is fed to a computer via a modem (see Figure 3). Essentially, a voice modem can only digitize the voice. A data modem is capable of handling plain data in some prespecified protocols, such as Z-modem.
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Figure 3 A modem.
A data FAX modem can receive facsimiles along with FTPs because it has firmware for FAX format. It has the FAX command set to interpret the codes for encrypting the document and decrypting the same on the receiving end. Hamming codes are commonly used for retrieving the message.
The voice modem has the command set to interpret an incoming phone call, as well as detect the DTMF frequency. The hard-wired, signal-processing algorithms inside the voice modem chips are the core of whole lot of features, such as Interactive Voice Response, Call Detection, Automated Call Attendant and Distribution. A user can even, check his voice mail locally or from a remote PSTN/cellular mobile telephone as long as he has the authorization.
How Voice Mail Works
The system at the receiving end operates in the following steps:
The system receives the ring.
The voice modem goes auto off-hook after a preprogrammed number of rings
Detects the DTMF (Dual Tone Multifrequency) key from the remote end to open the appropriate voice mailboxes.
Informs the PC (computer) to play the message corresponding to the DTMF key.