Issue 2.0
SV9100 Networking Manual
8-45
VoIP packet with RTP header compressio
n
Voice Activity Detection
(VAD) is suppression of silence packets from being sent
across the network. In a VoIP network all conversations are packetized and sent,
including silence. On an average a typical conversation contain anywhere from
35% ~ 45% silence. This can be interrupted as 35% ~ 45% transmission of VoIP
packets, as having no audio, using valuable bandwidth. With the VAD option
enabled, the transmitting of packets stops after a threshold is met determining
silence. The receiving side then injects comfort noise into the call so it does not
appear the call has dropped.
Bandwidth Calculations
The first step in calculating the bandwidth of a call is determining how many bytes
the voice payload is going to use. The amount is directly affected by the CODEC
and packet size. Below are the supported default CODEC speeds for SIP
Multiline telephones.
G.711 = 64000bps
G.722 = 64000bps
G.729 = 8000bps
Payload Calculation Voice
(Packet size * CODEC bandwidth) / 8 = Voice Payload in Bytes
Example of G.711 with a 20ms packet size
(.020 * 64000) /8 = 160 Bytes
Example of G.729 with a 30ms packet size
(.030 * 8000) /8 = 30 Bytes
Now that you have the voice payload in bytes you can calculate the overall
bandwidth including the layer 2 media. Below are some of the common layer 2
media types and their overhead.
Ethernet = 18 Bytes
802.1Q/P Ethernet = up to 32 bytes
PPP = 9 Bytes
Frame Relay = 6 Bytes
Multilink Protocol = 6 Bytes
Compressed
Header 2 ~ 4 bytes
VOICE PAYLOAD
Содержание Univerge SV9100
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