258
VoIP Profile Configuration
Figure 26-5: Jitter and Dejitter Buffer
Since the receiving decompression algorithm (voice decoder) requires fixed spacing between the
packets, the typical solution is to implement a dejitter buffer within the gateway. The dejitter buffer
deliberately delays incoming packets in order to present them to the decompression algorithm at
fixed spacing. It will also fix any out-of-order errors by looking at the sequence number in the RTP
frames.
Such a buffer has the effect of smoothing the packet flow, increasing the resiliency of the codec to
packet loss, delayed packets and other transmission effects. However, the downside of the dejitter
buffer is that it can add significant delay. The dejitter buffer size is configurable and can be
optimized for given network conditions. The dejitter buffer size is usually set to be an integral
multiple of the expected packet inter-arrival time in order to buffer an integral number of packets. It
is not uncommon to see dejitter buffer settings approaching 80 ms for each direction.
SmartWare offers two operation modes for the dejitter buffer, adaptive and fixed, as illustrated in
Figure 26-6.
Software Configuration Guide, Revision 1.03