On improving voice quality under dynamic encoding algorithms in ATM networks

We consider a voice multiplexer which supports ATM packet voice traffic. This system model also applies to the multiplexing operation carried out at the output queue of each output queueing ATM switch. The voice multiplexer implements an encoding algorithm under which the voice quantization levels are dynamically adjusted according to the congestion status of the multiplexer. Using an embedded voice encoding scheme, a voice packet is encoded into lower and higher priority ATM cells. When high congestion conditions are observed, lower priority voice cells (which contain the least significant bits) are discarded. In previous studies, the voice quality under a dynamic encoding algorithm has been described by the long-term average voice encoding rate (AER_∞). Due to the strong correlation properties exhibited by the packet voice traffic, the voice degradation levels incurred under the employed dynamic voice encoding algorithms are not distributed uniformly among the voice sources. As a result, the encoded voice stream for a source can exhibit severe short-term voice quality degradation, while the long-term AER_∞ level remains satisfactory. To correct this situation, we introduce in this paper a "probabilistic" component in the cell-discarding process. A voice degradation indicator (VDI) is introduced to measure the short-term voice quality degradation experienced by a voice stream. Through numerical examples, we identify and compare the system performance under the deterministic and probabilistic algorithms. Under the same AER_∞ levels, a probabilistic algorithm is shown to yield better short-term voice quality behavior as well as to improve the mean cell waiting time performance.