Emulation study of speech communications over ATM networks.

Abstract

Speech communications over ATM networks is one of the important issues in the broadband ISDN. Although CCITT Study Group XIII has already proposed the Draft Recommendation I.121 on speech communications over broadband ISDN, there are many open issued to be further studied before the effective deployment of speech communications over broadband ISDN can take place. In this thesis, several issues on speech transmission over ATM network have been studied, such as packetization delay, network queuing delay, digital speech encoding algorithms and PVR algorithm. A packetized speech emulation device is proposed to provide the capability of subjective speech transmission quality evaluation over ATM network or other kind of packetized network. The boundary of speech transmission quality degradation that human hearing can tolerate against information loss rate, delay fluctuation and encoding mechanism are found through emulation. The echo effect in ATM network, which is a primary issue in speech communications is also discussed in the thesis. In particular, the emphasis is given to the specification, design and implementation of ATM speech emulator which consists of two subsystems: ATM Network Simulator (ATMNS) and Speech Transmission Emulator (STE). ATMNS has been specified according to the results of ATM network performance analysis, and STE is based on ATM specifications recommended by CCITT. A prototype of the emulator has been implemented on a personal computer and DSP5600 development system with a special designed audio interface to interconnect phone sets to the DSP5600 AD input. The software had been written in "C" and DSP assembly language. Subjective evaluation are conducted in terms of the following factors: Cell discarding rate, different network queuing delay and fluctuation to different PVR algorithms. These factors are basic issues which may affect ATM speech transmission quality in an ATM network. Finally, test results under different network conditions are given.