McDewey

Quality of Service for Voice over IP VoIP QoS over Leased Lines (Using PPP) 33 QoSVoIP.mif Figure 9 Typical VoIP Network Environment For low speed WAN links that are not well-provisioned to serve voice traffic, problems such as delay, jitter, and loss become even more pronounced. In this particular network environment, the following factors can contribute to poor voice quality: • Large data packets sent before voice packets introduce long delays. • Variable-length data packets sent before voice packets make delays unpredictable, resulting in jitter. • Narrow bandwidth makes the 40-byte combined RTP, UDP, and IP header of a 20-byte VoIP packet especially wasteful. • Narrow bandwidth causes severe delay and loss because the link frequently is congested. • Many popular QoS techniques that serve data traffic very well, such as WFQ and RED, are ineffective for voice applications: – If you apply WFQ to both voice and data, as the number of data and voice application flows increases across the link, flow-based WFQ will allocate less and less bandwidth for each flow. Unlike the elastic data traffic that adapts to available bandwidth, voice quality becomes unacceptable after too many drops and too much delay. Cisco 3640 voice gateway Ethernet switch Ethernet switch Ethernet switch Ethernet switch Ethernet switch Cisco 7206 Ethernet switch Voice gateway Medium-Sized Branch Office Corporate HQ Cisco 2611 voice gateway Ethernet switch Small-Sized Branch Office PRI 64K PPP PRI PRI PRI 54741