NEC Sl2100 Networking Manual page 125

ISSUE 1.0
• Voice Activity Detection (VAD)
• Number of simultaneous calls
Layer 2 media is concerned with moving data across the physical links in the network. A few of the
most common layer 2 media types are Ethernet, PPP, and Frame Relay.
CODEC stands for Coder/Decoder and is the conversion of the TDM signal into an IP signal and vice
versa. A CODEC can also compress/decompress the voice payload to save on bandwidth.
Packet Size is the amount of audio in each PDU (protocol data unit) measured in milliseconds. The
larger the packet the less bandwidth used. This is because sending larger packets (more milliseconds
of voice) requires, overall, less packets to be sent. The downside of this practice is if a packet is
dropped/lost a larger piece of voice is missing from the conversation as the system waits the
additional delay for the next packet arrival.
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
IP Header
20 bytes
• VoIP packet with RTP header compression
Compressed
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.
Networking Manual
UDP Header RTP Header
8 Bytes 12 bytes
VOICE PAYLOAD
VOICE PAYLOAD
SL2100
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