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VoIP Voice and Fax Signal Processing by Sivannarayana Nagireddi

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15.7 MODEM BASIC FUNCTIONS ON PSTN

A modem is used with a computer for data communication, such as with Internet connectivity. Computer or data terminal generates digital data bits, which are loaded into a modem box through either RS-232, the universal serial bus (USB), or peripheral component interconnect (PCI) interfaces. Inside the modem box or modem hardware, these bits are modulated to generate the samples and finally the analog signal on the telephone TIP-RING interface. The PSTN network works like a transparent line. At the destination, the modem server, which is usually referred to as the remote access server (RAS) modem, demodulates the analog signal to extract the bit pattern. The RAS modem interface with the IP network [ITU-T-V.150 (2003), Bingham (1988), URL (dialup)] for extending the data communication.

Many functions of a modem are similar to fax except for the scanner and printer operations. The calling modem generates a V.25 CT or V.8 CI signal. The receiving modem generates 2100 Hz tones with phase reversal as an indicator. The phase reversal happens once in 450 ± 25 milliseconds. This signal is passed through the compression codec with acceptable distortion, and this tone can be detected at the receiving side. Most modems use higher data rates compared with fax transmission. The modulation and demodulation modules are referred to as the “modem data pump”. The management of a modem call is done through modem high-level applications. Modem signals are made to appear ...

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