Guaranteeing enterprise VoIP QoS with novel approach to DiffServ AF configuration

To satisfy the low delay, low jitter performance requirements of real-time traffic such as VoIP, DiffServ EF class using priority scheduling is normally recommended. This requires limits on admissible load, configured to meet the most stringent QoS in the real-time traffic mix. To handle multiple real-time service types with heterogeneous QoS, we propose a novel alternative: DiffServ AF classes with RED queue management. Recent queue theoretic advances have demonstrated RED's ability to control the delay distribution of inelastic traffic under congested conditions. This significantly reduces late delivery of packets at the cost of (probabilistically) dropping a greater proportion in the network. We investigated the load-quality trade-off in a DiffServ domain with both hop-based and link weight optimized OSPF routing. End-to-end delay, jitter and loss were measured for two AF classes carrying VoIP across all source-destination paths in order to compare the effects of tail-drop and RED queue management. We used the ITU-T E-model to express how these performance measures affect voice quality; results demonstrate that our novel AF configuration enables the network to carry more traffic for a given quality level, and to degrade more gracefully under severe congestion.