Q-Flex Satellite Modem Installation and Operating Handbook
7-31
7.8.11 Header Compression
The modem supports RObust Header Compression (ROHC) of:
•
IP + UDP headers
•
IP + UDP + RTP headers
•
IP + TCP headers
•
All Ethernet headers
The 40 bytes of IP, UDP and RTP headers are typically compressed to between one to
three bytes.
Ethernet header compression is also supported. The 14 bytes of Ethernet frame (the
Ethernet CRC is not sent over satellite even when compression is off) are typically
reduced to one byte.
Example: G.729 packet with 20ms (20 bytes) of payload:
•
Original size = 40 byte header (IP+UDP+RTP) + 14 bytes Et 20 bytes
p 5 bytes HDLC
= 79 bytes
•
Following compression = 1 byte header (IP+UDP+RTP) + 1 byte Ethernet +
20 bytes p 5 bytes HDLC
= 27 bytes (best case)
This represents more than a 60% bandwidth reduction.
There is support for ‘per route’ configuration control, where you can specify up to 32
source or destination IP addresses – only packets with one of these addresses get
directed through the compressor.
Header compression runs up to 29000 packets per second one-way, 22000 packets per
second two-way on the IP Traffic card.
7.8.12 VLAN Operation
A VLAN allows virtual networks to be created as opposed to being limited to purely
physical segments. Benefits include creating a restricted broadcast domain so not
everyone sees all traffic, plus added security.
Transparent VLAN operation is supported in bridging mode.