Anomalies Due to Delay and Loss in AAL2 Packet Voice Systems: Performance Models and Methods of Mitigation

(PREVIOUS TITLE: USE OF SEQUENCE NUMBERS AND MITIGATION OF ANOMALIES DUE TO DELAY AND LOSS IN PACKET VOICE SYSTEM) A study has been performed to investigate the packet delay variation and packet loss probabilities to be expected in typical applications of Access and Trunking using AAL2 and consequences these may have, such as the need for sequence numbers in voice packet headers. Simulations have been run for the case of 32 kb/s ADPCM with silence elimination across a VCC whose bandwidth is 1/4, 1/2, or 1 times T1. The results show the number of calls that can be admitted before the 99th percentile queuing delays exceed 20ms. This is thought to be the delay that can be budgeted for access, given a one-way end-to-end delay budget of 150 ms for conversational voice. When the packet delay variation is significant, as it seems to be in this situation, sequence numbers can help a receiver to detect and to recover from anomalies - lose, early, or late packets and make fewer reconstruction errors than if sequence numbers were absent of ignored. Three circumstances are identified where packet slips can occur with ripple effects. An analysis of these cases leads to lower bounds on the sequence number modulus. From this it can be deduced how many bits of the voice packet header should be devoted to sequence numbering.