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When sizing communication links, especially costly satellite links, I am frequently asked to answer two questions.

  1. Given a carrier capable of x Mbps throughput, how many gigabytes could I transmit in one day?
  2. What is the minimum number of megabits per second (Mbps or Mbit/sec) necessary to transmit x gigabytes (GB) in one day?

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We use Quintech SRR2150 L-band switches at our teleports. These are simple devices for switching L-band inputs and outputs. The most common application for one is to switch inputs to a spectrum analyser. This allows me to use one spectrum analyser to monitor several inputs (multiple antennas, multiple polarisations on the same antenna, etc).

Quintech’s switches are pretty basic. They have a front control panel, an interactive shell accessible by serial or telnet, and a custom communications protocol over 9100/TCP. Quintech provide a basic Windows management application, but it is either rudimentary (version 1.0) or totally broken (version 2.06). Why not control it from the command line? Thankfully Quintech have fully documented their management protocol, and implementing it in Perl was a few hours’ work back in 2007.

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In the satellite world we frequently work with the decibel scale. Decibels (dB) are a way of expressing the relative difference in signal strength between two sources. It is a logarithmic scale, so increasing numbers represent an exponentional increase in power. In simple terms, they are a convenient way of using addition instead of multiplication.

There are plenty of good places to understand decibels. I want to focus on how this helps us answer a few important questions in carrier sizing.

  1. I have two carriers, P1 and P2. What is the difference in power (Y) between them? Commonly this question appears as “Customer has capacity P1 and wants P2. How much additional power does he need to support the new carrier?”
  2. I have carrier P1 of a certain size, and Y dB margin (additional power available). How large can I make my new carrier P2? Or if Y is negative, how small?

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Satellite engineers have a very poor showing on the Internet. I started writing about satellite stuff because of how difficult it has been for me to find reliable information. I’ve had to do a lot of learning by doing. Perhaps other satellite engineers enjoy their business being a black art, but like locksmiths and security alarm salesmen, they’re just waiting for the light of responsible disclosure to toss their world upside down.

And that brings me to a trend I have noticed in the satellite Internet business. There are two camps: radio guys who somehow configure a router, and IP guys who somehow align an antenna. Usually the IP guys do better in business than the radio guys. Perhaps this is because radio guys tend to be former military and unused to competition. Perhaps this is because they think all you have to do is deliver a carrier and everyone is happy. But the customer doesn’t care about Hertz, he cares about bits. And really, the space link is still a Layer 1 problem, and the harder work is above that.

On with revealing a little black magic.

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I frequently travel to the Middle East, which means I often find myself on the wrong end of a slow Internet link. Sometimes that is oversold undersea fibre, such as in Dubai. More often – because of my work – it is VSAT. I’m an engineer, which means I’m a curious monkey that takes everything apart just to understand how it works. Network topology is one of those things.

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