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VoIP profile configuration task list 581
SmartWare Software Configuration Guide 47 • VoIP profile configuration
‘v150-vbd’ is selected, a re-invite will be sent even if the current voice coder is configured the same as the
modem bypass coder. Furthermore the re-invite contains a gpmd-attribute line with the value
‘vbd=yes;ecan=off ’ in the media description part. This attribute signals the remote device of the new media
transmission. If the command option ‘default’ is selected, the system behavior is the same as before.
SmartWare also supports the Cisco NSE standard which uses RFC2833 events for modem transmission over a
VoIP (SIP or H.323) network. Upon detecting a modem transmission, the called peer issues NSE Event 192.
NSE Event 192 indicates a Voice Band Data stream that forces the calling peer to deactivate Voice Activity
Detection and to reconfigure the de-jitter buffer for data reception. Afterwards it issues the NSE Event 193 to
trigger the calling peer to switch off Echo-Cancellation.
Configuring the dejitter buffer (advanced)
Packet networks always introduce a certain amount of jitter in the arrival of voice packets. To compensate for
the fluctuating network conditions, a dejitter buffer is integrated in the RTP processing engine. Typical voice
sources generate voice packets at a constant rate, the matching voice decompression algorithm also expects
incoming voice packets to arrive at a constant rate. However, the packet-by-packet delay inflicted by the net-
work may be different for each packet. As shown in figure 3, the result of the delays is that packets which are
sent equally spaced from the left-hand gateway arrive irregularly spaced at the right-hand gateway.
Figure 3. Jitter and dejitter buffer
The dejitter buffer delays incoming packets so it can present them to the decompression algorithm at fixed
intervals. It will also fix any out-of-order packets by looking at the sequence number in the RTP packets. Such
buffering has the effect of smoothing packet flow, and increasing the resiliency of the codec to packet loss,
delayed packets, and other transmission effects. The negative side of dejitter buffering is that it can add signifi-
cant delay. The dejitter buffer size is configurable and can be optimized for given network conditions, the size
is usually set to be a multiple of the expected packet arrival time in order to buffer an integral number of pack-
ets. It is not uncommon to see dejitter buffer settings around 80 ms for each direction.
Node
Node
Voice Packets
Voice
Decoder
Buffer
x x + dx
x
Node
Node
Network