ed 150 ms one-way delay tors, including (including endpoints) as the limit for "excellent" voice Delay quality. One-way delays in excess of 250 ms can cause Jitter "talk-over," which is when each person starts to talk Packet loss because the delay prevents them from realizing that the other person has already started talking. Packet mis-order Long WAN transports must be considered as a major Available bandwidth contributor to the network delay budget. Some WAN Packet prioritization service providers can lower delay in their network if it is Network design negotiated and recorded as part of the company's SLA. Endpoint audio characteristics (sound card, micro- phone, earpiece, etc.) Transcoding Jitter Silence suppression Because network congestion can be encountered at any Duplex time within a network, buffers can fill instantaneously. This instantaneous buffer utilization can lead to a differ- Codec selection ence in delay times between packets in the same voice Router and data-switch configuration stream. This difference, called jitter, is the variation be- WAN protocols tween when a packet is expected to arrive and when it QoS (Quality of Service)/CoS (Class of Service) policy actually is received. Encryption/Decryption Jitter can create audible voice-quality problems if the variation is greater than 20 ms (assuming an existing Delay 20 ms packet size). Symptoms of excessive jitter are very similar to symptoms of high delay, because in both cases Packet delay is the length of time it takes a packet to packets are discarded if the packet delay exceeds half the traverse the network. Each element of the network- jitter buffer size. switches, routers, distance traveled through the network, To compensate for network jitter, many vendors firewalls and jitter buffers (such as those built into H.323 implement a jitter buffer in their H.323 voice applica- audio applications like Microsoft NetMeeting)-adds to tions. The purpose of the jitter buffer is to hold incoming packet delay. Router delay depends not only on hardwar