Experimental results on the impact of cell delay variation on speech quality in ATM networks

The concept of voice and telephony over ATM (VTOA) has been widely studied. Although ATM benefits voice communications in many aspects, its packet-switched nature impairs the speech quality. We evaluate the speech quality with a focus on the impact of the cell delay variation (CDV). We build an experimental system to do the evaluations. We also define a new parameter, named cell delay variation accumulation length (CDVAL), to measure the impact of CDV on the speech quality. We observe that the speech quality mainly depends on the value of the CDVAL, but does not strongly depend on the other items, including the distribution of cell end-to-end delay, the position and even the number of the intermittence caused by the CDV. We find that there exists an acceptable threshold of the CDVAL. According to our experiments, to keep the speech in “good” quality (MOS≅3.5), the CDVAL value must be less than 3 ms without any speech cell loss; such a threshold drops to 2 ms if 5% of the speech cells are lost. The results indicate that a buffer is necessary for CDV compensation at the far end devices in VTOA. The results are also very helpful for the proper design of the network architecture and control algorithms, and is especially important for the design of the CDV compensation schemes, when ATM networks are used to provide voice and telephony services