NEC Univerge SV9100 Manual page 436

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SV9100 Networking Manual

VoIP packet with RTP header compression

Header 2 ~ 4 bytes

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

VOICE PAYLOAD