NEC SL1100, Networking Manual

Page: 175 / 292

NEC SL1100 Issue 6.0
Networking Manual 9 - 41
RTP Header Compression compacts the RTP header from 40 bytes in size
to 2 ~ 4 Bytes in size. RTP header compression is used only on low speed
links. Regularly on every voice packet there is an IP/UDP/RTP header that is
40 bytes in length. Compressing this header, down to 2 ~ 4 bytes, can save a
considerable amount of bandwidth. The following is an example of a VoIP
packet without RTP header compression and one of a packet with RTP
header compression.
Notice that the overall packet size, when using RTP header compression, is
considerably smaller.

VoIP packet without RTP header compression

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, 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
IP Header
20 bytes
UDP Header
8 Bytes
RTP Header
12 bytes
VOICE PAYLOAD
Compressed
Header 2 ~ 4 bytes
VOICE PAYLOAD