SHAPING THE FUTURE OF SATELLITE COMMUNICATIONS
9.6 QoS
9.6.1 Introduction
There are two aspects in handling QoS:
• The handling of several QoS classes: different QOS classes exist and are
handled with a different priority (e.g. real-time traffic, and non-real-time traffic).
Voice traffic for example, is treated as high priority;
• The handling of data flows: when different end-users make use of the same
bandwidth pipe, one user may not consume the full bandwidth and fairness
must be implemented.
9.6.2 QoS classes
A modulator (or modem) with Ethernet interface can be a congestion point in an IP
network, for example when traffic is carried from a 100 Mbit or 1000 Mbit network
to a relatively low speed satellite network.
When the satellite link has a low throughput, it becomes very important to
differentiate amongst different QOS classes.
When we go from a high-speed network to a low-speed network, we need to
implement buffering. This buffering is needed to avoid that the majority of a traffic
burst is discarded.
TCP typically requires large buffers. It makes use of a window mechanism defining
the maximum of outstanding non-acknowledged data. Moreover, for satellite
communication, the window sizes are often raised to a higher value (to overcome
the large satellite delay). This large window size causes relatively large bursts. It is
recommended to buffer at least 100 Kbytes on a satellite modulator. Suppose that
you have a low speed satellite throughput (e.g. 1 Mbps), and you buffer 100 Kbytes
on the modulator, then you can buffer up to (100*1024*8 / 1000000) = 819 ms of
data!
For voice traffic, typically high priority traffic, we require small buffers: buffering
voice packets, causes delay in the conversation. Roundtrip delays higher than 250
ms are noticed by end users (ITU-T G.114 recommends a maximum of a 150 ms
one-way delay). For satellite communication, we will experience an even longer
delay, because the satellite segment itself causes already a roundtrip delay of
about 500 ms.
If we put regular TCP traffic (that requires 819 ms of buffering) and voice traffic
(that requires minimal buffering) in the same queue, we won't be able to satisfy
TCP users and voice users.
Real-time traffic is stored in a small queue, and forwarded with high priority. Non-
real-time traffic (often TCP traffic) must be stored in a large queue, and forwarded
with lower priority.
In the Elevation equipment, rules can be implemented, defining which Ethernet
packets are directed to which queue.
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