NEC Univerge SV9100, Manual

Page: 210 / 766

Issue 2.0
SV9100 Networking Manual 5-15

VOIP packet with RTP header compression
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, having no audio,
use 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.
1.5.4 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 CCISoIP.

G.711 = 64000 bps

G.722 = 64000 bps

G.729 = 8000 bps

G.726 = 32000 bps

G.723 = 5400 bps
1.5.5 Payload Calculation Voice
(Packet size * Codec bandwidth) / 8 = Voice Payload in Bytes
Example of G.711 with a 20 ms packet size
(.020 * 64000) /8 = 160 Bytes
Example of G.729 with a 30 ms packet size
(.030 * 8000) /8 = 30 Bytes
Now that you have the voice payload in bytes you can now 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