V. 32 and V. 33 modem model and analysis

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Virginia Tech


Over the past ten years, modem technology has evolved to reliably handle 9.6 kbps on general switched telephone networks (GSTN's) and 14.4 kbps on special quality leased circuits largely due to the maturation of Trellis Code Modulation (TCM). A TCM modem can tolerate more than twice the noise power that a Quadrature Amplitude Modulation (QAM) modem can while achieving the same block error rates. The schemes proposed by the International Telegraph and Telephone Consultative Committee (CCITT) in recommendations V.32 and V.33 take advantage of what TCM has to offer for high-speed modem applications. Further studies into the areas of Trellis Shaping and Trellis Precoding potentially offer even higher gains, potentially approaching the theoretical limit for TCM applications on limited bandwidth channels.

The purpose of this project was to develop a malleable model that would simulate modem transmissions of V.32 and V.33 type standardized modems. The model was to allow the user control over the channel noise and the viterbi memory length so that the limits of noise tolerance and the viterbi decoder could be studied in reference to TCM. The model was also to be modular so further extensions could easily be developed to enhance the model's abilities. For ease of use, the model was made to operate on standard ASCII text messages so users could operate the model without needing to create binary files beforehand.

This report includes a section on the history of modem technologies and an introduction to TCM. It also contains a detailed description of the model that was developed to aide in the teaching and research of TCM. The model includes controls for the channel noise and viterbi memory length and allows the user to choose what file they would like to transmit, what standard they would like to follow, and what they would like the report file to be called. Upon running, the model generates the following statistics on the transmission simulation made:

• Viterbi Memory Length Used • # of Bits Transmitted • Noise Reduction Factor Used • # of Symbols Transmitted • # of Bits Received In Error • #ASCII Characters Sent • # of Symbols Received in Error • # of Demodulation Faults • # of ASCII Characters in Error • Data Rate Achieved • Mean X-Coordinate Error • Mean Y-Coordinate Error

These statistics are written in an easy-to-read format to the file that is specified by the user. Many detailed analyses could be completed by using the above statistics to study a particular element of TCM.

To demonstrate the type of analysis that could be completed, this report also includes analyses of "Noise Tolerance" and "Viterbi Memory Length". These analyses were performed using the software model to illustrate the value of such a modeling tool. The final section of this report includes a section on enhancements that could be made to the software model that would further widen the range of analyses that could be conducted by using it. The Appendix of this report includes the source code that was written to generate this model. The code is well documented so it can be easily modified to include whatever elements future students and researchers desire.