Bandwidth Calculations; Payload Calculation Voice - NEC Univerge SV9100 Manual

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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, 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

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

(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

Voice-Payload