Q-Flex Satellite Modem Installation and Operating Handbook
7-22
7.8.6 IP Interoperability
Over satellite, the modem uses HDLC to encapsulate the Ethernet frames or IP packets
(unless DVB-S2 is being used, in which case another form of encapsulation is used such
as MPE or ULE). If Cisco HDLC is selected on the modem then the received IP stream can
be forwarded out of a serial interface to a serial router that supports Cisco HDLC, which
can convert it back to IP.
In general different modem manufacturer’s IP, even when using HDLC, is not interoperable
as there is no standard defined in this area. If instead DVB-S2 is used, then generally IP is
interoperable between different manufacturer’s modems, so long as incompatible IP
features are not being used such as compression or acceleration.
7.8.7 IP Connectivity Modes
The modem software supports unidirectional and bidirectional point-to-point IP operation.
7.8.8 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.
Note that when TCP acceleration is used in both directions then the TCP
acknowledgements going in each direction compete with the data in each direction. Since
TCP will attempt to completely fill the data pipeline, there is a possibility that there will not
be sufficient bandwidth available for the acknowledgements, which could drastically reduce
the throughput level (typically in one direction). The Paradise software attempts to prioritise
TCP acknowledgements in this situation to prevent this from happening. However, a better
solution is to use the IP traffic shaping feature to guarantee sufficient bandwidth for the
acknowledgements.
