ND SatCom SKYWAN IDU 7000, Руководство пользователя

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122 Network Design and Engineering Guide 2010-10-26
Data Networking
SkyWAN
®
Internet Protocol Features
4.2.9 Robust Header Compression
For IP flows mapped to a Platinum Dynamic aggregate the Sky WAN
®
IDU may perform Robust
Header Compression (RoHC) according to the RFC 3095. Sky WAN
®
applies unidirectional
mode without feedback. Especially for VoIP connections header compression may save a sub-
stantial part of the bandwidth.
Up to 128 flows can be compressed and decompressed at every station. A flow may only be
compressed if the following conditions are met:
- A Platinum Dynamic aggregate with enabled header compression feature is defined for the
IP flow.
- The IP flow is a IP/UDP/RTP packet flow transporting one of the following video/audio co-
decs are specified in the RTP header:
Table 4-4 Codecs supported for RoHC
Note that the data rate defined in the Platinum Dynamic aggregate must be the required rate
for an uncompressed flow. After compression has set in, the part of the data rate which is not
any more required due to compression will be released. This capacity reduction works only for
the audio codecs, not for video codecs.
For data rate requirement for the above mentioned voice codecs with and without header com-
pression please refer to table 2-1 in chapter 2.2 ’Data and Voice Networking Overview’.
As an example, a typical VoIP connection is assumed using G.729 voice codec packets with
an RTP payload size of 20 Byte. The IP/UDP/RTP header adds another 40 Byte to the packet.
That means an IP data rate of 24 kbps is needed to transport a voice call with a codec rate of
8 kbps. Header compression reduces this bandwidth by exploiting the fact that within an IP flow
generated by one voice call most of the header information is static, i.e. the header parameters
are identical in all packets of the flow. Mapping these parameters to a ’Call ID’ at the compres-
sor station and replacing the Call ID by the original header information at the decompressor
station allows the reduction of the header size on the satellite link from 40 Byte to 3-4 Byte.
Codec Type Sampling Rate [Hz] RTP Payload Type
G.711 Audio 8000 0/8
G.722 Audio 8000 9
G.723 Audio 8000 4
G.728 Audio 8000 15
G.729 Audio 8000 18
H.261 Video 90,000 31
H.263 Video 90,000 34