Whispers & Screams
And Other Things

An Introduction to layer 4 handling of RT traffic on satellite networks.

slideSatellite telecommunications is, by its very nature, prone to long propagation delays and higher error rates which can impair the performance of the TCP protocol and most specifically the use of TCP to transport real time applications. At Apogee Internet, the use of satellite broadband services to enable the use of such services is a core component of the services delivered. As such therefore, it is important to understand these effects and how they impact the efficiency of the TCP exchange and the consequent streaming video delivery.

In this regard, we have examined the field using a framework of techniques which can serve to maximise the usability of these channels and in some cases to simply ensure they are usable in the first place. There are various implementations of TCP that can be used which enhance protocol performance by means of adjusting the role of acknowledgements or delaying them.

Most existing solutions do not live up to the requirements of today’s real time applications which at best results in inefficient utilisation of bandwidth and in extreme cases can affect the transponder in use quite dramatically.

Satellite systems have evolved through the delivery of television services to the point where nowadays, they have an integrated part to play in any national broadband IP delivery strategy. With their ubiquitous reach and ability to broadcast, todays core communications satellites enable the delivery of time sensitive information over macrogeographical areas. These systems however do have their drawbacks such as bandwidth asymmetry. Also, due to the inherent propagation delays involved in transmission across such vast distances, these networks always have a high Bandwidth Delay Product (BDP) and can certainly be described as Elephant Networks (LFN’s).

These long transmission distances also result in low power channels which in turn bring about high relative Bit Error Rates which are always higher than terrestrial networks.

The mainstream layer 4 protocols in use today are not best placed to make efficient use of these conditions. TCP for example, built on the principles of Slow Start, Congestion Management and Additive Increase Multiplicative Decrease was designed for far more error free networks such as hard wired networks demonstrates that it is manifestly unsuitable for use in heterogeneous network environments such as satellite links.

TCP has three major shortfalls in these circumstances.

1                     Ineffective Bandwidth Utilisation

2                     Chatty Congestion Prevention Mechanisms

3                     Wasteful Windowing

In future posts, we shall go on to examine the implications of this shortcoming in the layer 4 mechanisms as well as ways to mitigate the undesirable effects.

Evening Slant
The North wind doth blow


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Tuesday, 21 May 2019
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