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QMultiFlex-400™ Installation and Operating Handbook
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 perceived 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, 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.
8.7.8
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.
This section provides an overview of the traffic shaping feature,
including definitions of terms and worked examples. For a detailed
description of the actual traffic shaping menus, please see
>Service->General->Tx QoS
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->Service->General->Tx QoS.
Screen.
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