Shipra Agrawal P. P. S. Narayan Jeyashankher Ramamirtham Rajeev Rastogi Mark Smith Ken Swanson Marina Thottan VoIP service quality monitoring using active and passive probes. 2006 conf/comsware/2006 COMSWARE db/conf/comsware/comsware2006.html#AgrawalNRRSST06