Q-Lite Satellite Modem Installation and Operating Handbook
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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.
7.8.10 Traffic Shaping
Traffic shaping provides control over the management of data within the modem.
Specifically it provides a guaranteed quality of service for defined IP data streams. It is
aligned with all of the major quality of service schemes and can be used to extend
terrestrial services over satellite to create fully provisioned end-to-end services, thereby
providing direct support for the implementation of customer service level agreements. Key
time-based performance metrics are gathered continuously by the modem and can be
extracted in order to be assimilated into customer quality of service reports.
A satellite modem in general normally acts as a transparent pipe for data, so the data that
is finally received at the destination on the terrestrial network at the far end of the link is
identical to that which has been supplied to the local modem for transmission over satellite.
This is not necessarily true for IP data. Being packet based, IP naturally supports
multiplexing of different data streams. These streams often have different inherent priority
levels and competing demands for bandwidth. What is transmitted over satellite often
requires careful management, both in terms of what is actually sent (versus what is filtered
out) and in relation to the order in which packets from different streams are sent (i.e. the
relative priority levels of packets from different streams and the effect this has on packet
jitter).
Traffic shaping essentially controls these two key aspects of traffic management, namely,
access to satellite bandwidth and the level of delay and jitter that is experienced.
The Paradise traffic shaping feature is controlled via the modem web user interface under
Edit->Unit->Advanced->QoS
.
7.8.10.1 Guaranteed Bandwidth
The allocation of bandwidth to a classified data stream is controlled via the
Edit->Unit-
>Advanced->QoS
tab of the web user interface.
The
Commited Information Rate
(CIR)
is the guaranteed bandwidth, in bits per second
(bps) (or as a percentage) that will be allocated to the specified data stream. This is the
