24-03-2012, 11:39 AM
dsp processor implementation of a modem for narrow bandwidth channel
DSP PROCESSOR IMPLEMENTATION OF A MODEM FOR NARROW BANDWIDTH CHANNEL - Copy.doc (Size: 1.4 MB / Downloads: 61)
INTRODUCTION
A typical communication system will input data, perform some form of processing and frequency translation, transmit the data, and perform the converse operations at the receiver. The transmission of data is done using Public Switched Telephone Network (PSTN). Traditionally, the PSTN has been viewed as an analog network. In reality, however, the PSTN as we presently know it has become an almost entirely digital network. In most of cases, the only part of the PSTN that has remained analog (and will likely remain so far many years to come) is the local loop, which represents relatively short connection from a home to central office.
DEVELOPMENT OF MODEMS
In the beginning there was a decided lack of any good way to communicate between small computers. The big guys, mainframes and such, could communicate but they were very expensive and the leased lines they used cost a fortune. The modem had been around for a long time, since the old Teletype days in fact, but it was not readily available to small computer users. The first small computer modems were built from diagrams in Popular Electronics and other related magazines.
DESCRIPTION OF MODEMS
What is Modem?
The Modem a contraction of the term modulator, demodulator, is a conversion device that facilities the transmission and reception of data over PSTN. The analog signal is converted into digital format and is passed through modulator and demodulator reconverts the digital format signal and channel output.
MODEM CONFIGURATIONS
There are two distinct classes of modem configuration:-
1. symmetric modem configuration
2. Asymmetric modem configuration.
Symmetric modem configuration:-
The simplest approach for design of modems is to treat the entire PSTN as a linear network. In such a setting, analog to digital and digital to analog conversions are needed whenever the modems send the signals to and receive the signals from PSTN.