Impact of IP Chanel Parameters on the Final Quality of the Transferred Voice

Abstract

The subject of this dissertation is the measurement of quality of voice transmission over IP networks. Development in digitization, encoding and transmission of the speech is very fast, as well as the development of network elements and increasing transmission capacity and network reliability. It also puts increased demands on the development and implementation of methods for measuring the quality of voice transmission. Objective algorithms and subjective test methods must respond to new kinds of faults and disturbances. The problem is that the methodology of subjective and partly objective tests is based on ITU-T Recommendation P.800 of 1993 and was designed for use in conventional telephone networks. With the current low rate and irregular occurrence of errors, the used samples are too short. In the first part of this thesis, the protocols and codecs which are most often used for speech transmission are described. The following describes the methods and algorithms currently used to measure the transmission quality. In this thesis is mainly used algorithm POLQA then PESQ and 3SQM. In the second part of this thesis, the effect of various IP network parameters on the voice quality is verified in the series of experiments. Tested parameters are delay, delay variation (jitter), packet loss, QoS and various codecs. The influence of the length of the sample on the measuring results using an objective algorithm is also evaluated. The effects of static delay on the quality of the conversation and the effects of stress evaluators on the results of these tests are investigated using subjective tests. In the second part of this work, a methodology for subjective tests using long samples is suggested. The subject of the investigation is in addition to the length of the sample as well the method of collection and analysis of data from the evaluators. The investigated methods are: a classic one with evaluation at the end, equidistant with ratings at regular intervals and random with ratings anytime during the sample. These methods differ in the final evaluation of the sample and its uncertainty. Thanks to the use of longer samples, it is possible to reduce the number of repetitions and the subjective tests are simplified while maintaining the required accuracy. Experimental results showed the influence of jitter, packet loss, the codec used and methods of QoS on the transmission quality. Influence of static delay on the quality of the conversation was also confirmed. Differences in results are noticeable in comparison of objective algorithms. It is caused by the the progressive development of algorithms.