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Quantum and Evolution Series Installation and Operating Handbook
8.12.7.3 Ethernet Point-to-Multipoint Brouting
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This is used where there is a shared outbound carrier from the hub to remotes, and the
return path is not over satellite (i.e. there is an indirect terrestrial return).
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It relies on the presence of another route for IP traffic to pass from remotes back to the
hub.
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It is typically used as an overlay IP delivery system to boost bandwidth out from the hub
to remote subnets.
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In this specific mode, a 'Bridge filtering' option is available at the remotes to discard all
data not destined for the subnet that the remote modem is on (this is not required if
VLAN tagging or router filtering is being used).
8.12.8 TCP Acceleration
TCP traffic, as opposed to UDP, requires acknowledgements to be returned to the sender
as part of the protocol flow control process. TCP was never intended for systems with long
delays (such as satellite). With no acceleration, TCP traffic over satellite would limit itself to
a few hundred kbit/s (depending on the PC TCP window size), regardless of the actual
traffic bandwidth available over satellite. Because the satellite delay is taken as evidence of
link congestion, TCP throttles back the amount of data it sends. Acceleration allows approx
90% utilisation of whatever the available traffic bandwidth is over satellite. Acceleration can
be used in point-to-point and point-to-multipoint bridging modes, as well as in routing mode.
When acceleration is used in bridging mode, all UDP packets are bridged. VPN packets,
although they use TCP, cannot be accelerated because acceleration relies on making
changes to the addresses in the original IP packet which is encrypted by the VPN as the
payload of a new IP packet (tunnel mode). Even in transport mode, where only the IP
packet payload is encrypted, authentication will detect when the modem alters any IP
address and reject the packet at the end point.
Acceleration works by the modem spoofing TCP acknowledgements back to the local
originating device as if they come from the remote end point, eliminating the satellite delay.
Since there is no significant delay, the originating PC assumes there is no congestion in the
link and will therefore ramp up the level of TCP output to fill the available bandwidth.
8-42
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