Design and implementation of a variable-speed data modem

The popularity of the Internet and the World Wide Web has created a dramatic increase in demand for residential Internet access to adaptive speed offered by voice-band modems. This thesis investigates methods of providing higher speed network access over the telephone channels. Users with short lines can have higher bit rate access to the network, while users placing farther away are not excluded from access, but -have reduced bit rate access to the same line. Three adaptive rate algorithms are introduced in the work. The first uses usual serial transmission mode, while the transmission of the other two systems based on packet format. The digital data is transmitted over the telephone channel through the use of different modulated signals and different modulation rates. For a given telephone channel, the rate adaptation algorithm selects the highest possible bit rate with an acceptable error rate. An acceptable transmission rate is assumed when the bit error rate (BER) is about 10~4[28]. The Rate adaptation algorithm performs at system start up by sending of test, data across the line at various bit rates starting at the highest rate and measuring the data errors at each rate. It was found through simulations that the rate adaptation algorithms provide data rates from 0.8 to 12.8 kbps covering a range of SNR (8- 17) dB. A hardware design of the adaptive rate algorithm up to 9.6 kb/s with synchronous mode is implemented using 80C52 Microcontroller that programmed to control, the modem
speed with different modulation techniques and baud rates.