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Keeping satellite data running under fire

Posted on 5 Dec 2011 at 13:08

Paul Ockenden looks at the optimisation software that keeps satellite data feeds running smoothly

Regular readers will know that I’ve been testing both a satellite phone and a BGAN (Broadband Global Area Network) data terminal supplied by the nice folks at Inmarsat.

It’s strange, but the more I use this “data anywhere” technology, the more I start to spot potential uses for it.

While watching the now-famous live pictures from Alex Crawford and her Sky News team as they drove into Tripoli, I thought to myself: “I bet they’re using a BGAN terminal for the feed”.

Sure enough, a few days later reports emerged about how this incredible live broadcast was achieved using a MacBook Pro plugged into the cigarette lighter socket of the truck, and a BGAN terminal balanced on the bonnet.

It was quite some achievement to maintain a satellite lock from a moving vehicle, with guns going off all around you and bullets raining down from the sky.

It was quite some achievement to maintain a satellite lock from a moving vehicle, with guns going off all around you and bullets raining down from the sky

With a BGAN terminal you get an on-screen signal-strength meter and, perhaps more usefully in this context, many terminals include an “alignment tone” that increases in pitch as the antenna is pointed in the right direction. Despite such visual and audio clues, it’s still amazing to keep the link up from a moving truck.

The biggest problems you’ll find when using satellite data systems are limited throughput, high latency caused by the trip into space and back, and cost.

Software support

All three factors can be mitigated, to some degree, with appropriate software. A mixture of compression and packet management can bump up the throughput, while at the same time keeping the cost down, and fiddling with the underlying TCP data stream can provide some worthwhile latency reduction.

There’s a range of BGAN optimisation software available, some of it eye-wateringly expensive, but if you hunt around on Inmarsat’s website you’ll find its own offering, which is free.

I was intrigued by what it does. As some of you’ll probably know, a typical TCP link will start off slowly and then gradually ramp up the speed to find the optimum rate.

This is often referred to as a “slow start mechanism”; it’s increasing the packet size each time an acknowledgement (or ACK) is received for the previous packet, a mechanism designed to arrive at the optimum speed for any given data connection.

Across normal wired or terrestrial wireless links this works fine, because the packet turnaround is rapid. On a satellite data link, however, the description “slow start” is appropriate since it takes a (relatively) long time for each packet to be acknowledged.

To make matters worse, if a packet isn’t ACKed – or, worse still, gets NAKed (negatively acknowledged, meaning the packet became corrupted en-route) – the protocol will drop back to smaller packets and the slow-start mechanism will gradually start to increase the packet size all over again.

Slow start

BGAN optimisation software such as Inmarsat’s TCP Accelerator removes the slow-start mechanism from the uplink by providing a false, locally generated ACK and hence bringing the packet size up to maximum fairly rapidly, then managing the packets sent over the data link, always filling the TCP window.

This trick, in combination with data compression, can make a huge difference, although as with any compression it does depend on the content – some things compress well, others don’t.

Also, its major benefit is on data being sent rather than received. Having said that, looking at a typical webmail system such as Gmail involves plenty of traffic between the browser and Google’s servers, since the page is built from lots of small files and uses techniques such as Ajax for behind-the-scenes data updates. I found Gmail more responsive with TCP accelerator.

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