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A31003-H3150-S104-2-7620, 03/2009
HiPath 4000 V5, IP Solutions - HG 3500 & HG 3575, Service Documentation
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G.729AB with Voice Activity Detection.
IMPORTANT: Note that there is virtually no delay introduced in generating G.711
encoding from TDM voice packets (Algorithmic Delay = 0 ms).
For more detailed information on codecs please refer to
Standards".
3.5.2 Sampling Time
Voice needs to be digitized before it can be assembled into IP packets.
For telephony applications, voice is sampled every 125 μs at 8bits. It would not
make sense to package each individual 8 bit sample into an IP packet. Instead,
for G.711 encoding at least 240 8 bit samples are combined into an IP packet.
It takes 30 ms to acquire 240 voice samples.
The Sampling Time is defined as the time it takes to sample a specified number
of voice samples that are sent out in one IP packet.
It is obvious that the longer the sampling time, the smaller the IP overhead gets.
Thus, in order to minimize bandwidth, the sampling time will have to be increased.
However, increasing the sampling time will increase the overall delay!
IMPORTANT: There is always a tradeoff between minimizing the delay versus
minimizing the required bandwidth.
For IP line gateways the sampling time can be set between 10 ms and 90 ms
depending on codec type.
3.5.3 IP Overhead
The IP overhead includes the Real Time Protocol (RTP) header, User Datagram
Protocol (UDP) header and IP headers as well as the Ethernet framing and
additional octets for QOS Tagging. A typical IP overhead value for G.711 is 30%.
gateways_03_arch_sys_requ.fm
Architecture and System Requirements
Bandwidth Requirements
Section 2.3, "Codec
31
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