Tech Talk

A VSAT is a Very Small Aperture Terminal. If a satellite terminal has a dish bigger then 3.8 metres it is called an Earth Station rather then a VSAT. There are two components to a VSAT; an ODU which stands for Outdoor Unit and an IDU which stands for Indoor Unit. The ODU part of a VSAT is the dish, the BUC and the LNB. The IDU part of a VSAT is the satellite router or modem. How well your VSAT performs is a matter of how big your dish is and how much power your BUC has. The bigger the better for both!

We have seen that the new technology uses a satellite “router” rather then the old modem technology and how satellite routers feature QOS or Quality Of Service and why QOS is so important. If you want to do video conferencing or Skype calls over your VSAT, you need QOS. Certainly a satellite modem with enough bandwidth on the account will do Skype but the difference in quality between a modem and a router has to be heard to be believed! The key difference between an outdated satellite Modem and a new technology satellite Router is that Skype calls over a “router” need much less bandwidth than a satellite modem. This directly translates to less cost as you need less bandwidth with a router.

You will often be quoted speeds of 128k/64k or 256k/128k or even 512k/128k. What does this mean? Are you REALLY getting those speeds? The key thing to understand here is that those speeds quoted are BURSTABLE speeds! You do sometimes hit your speed quoted but most of the time you will get significantly slower speeds. The real question you need to ask is what is the REAL contention ratio? Contention ratio is a simple formula that tells you how many VSAT’s are sharing the same channel you have subscribed to. An excellent contention ratio is 4:1, meaning only 4 VSAT’s share the amount of bandwidth paid for but this is very expensive. A very good contention ratio is 10:1, but again is expensive. The most common contention ratio is around 20 or 25:1. This means 25 VSAT’s are sharing the amount of bandwidth you have paid for. Going back to our example above, a 128/64k connection on a 25:1 contention ratio means that if all those 25 VSAT’s were downloading a file off the Internet at the same time, each VSAT would have 128k of available bandwidth divided by 25 users to get a real speed of around ~ 5.12Kbps ~ ! NOT GOOD !! However if there are only you and two other VSAT’s downloading at the same time then you would divide that 128k of bandwidth by the 3 VSAT’s that are downloading, you and the two others, thus you would divide that 128k by 3 to get a real speed of 42.6Kbps. The term burstable means that your VSAT can “burst” up to the speed you have paid for but only when no other VSAT is downloading. This can often happen late at night or early in the morning, but once we get to around 10 am and further into the day you will generally see your speeds drop away – and if you’re with a provider who is not honest about the real contention ratio you can often see your speed all but disappear !

Now this is where it gets tricky. How do you determine if your service provider is telling the truth about the real contention ratio? If you are using a satellite “router” made by iDirect it is often possible to get graphs and statistics from the “HUB” and you can quite clearly see how many VSAT’s are on your channel. There is REAL transparency when using satellite routers as opposed to satellite modems. With a satellite modem there is no real way to “see” how many VSAT’s are on the channel and you need to rely on the honesty and integrity of your supplier, which, as most of us know, can be difficult to find in this world !

The next thing to understand about contention ratio is how this figure is determined. It should be simple and straight forward, but this is often not the case. In its most simple form you would simply add up all the VSAT’s on a given channel with a given amount of bandwidth available and come to a straightforward figure. However there are many different ways to arrive at a contention ratio and most of these ways are a cloak of deception! One common method of cheating on contention ratio is to advise a ratio of 20:1 when in reality it is often 80:1! It is a clever play on words and what happens is this: they quote 20:1 in terms of how many VSAT’s are only ever accessing the bandwidth at the same time. What they try to get away with is that it has been statistically proven that only 4 VSAT’s will ever hit the bandwidth at the same instant at any given time, so therefore they sell the connection at a rate of only 20:1 meaning in their opinion only 20 users will hit the bandwidth at the same time. If we multiply this by the statistical average then we really have 80 users on the same channel!

Another area where end users get confused is the speed of 128k. What exactly is a speed of 128k? The speeds quoted for Internet connection speeds are always in Kbps. This stands for Kilobits per second. There are 8 bits in one Byte. So you simply divide your advertised speed of 128Kbps by 8 to arrive at the speed in KB, or KiloBytes per second. Note there is a difference between Kilo BITS and Kilo BYTES. So we can see that 128 Kbps is only 16KB, something the sellers of those 128Kbps connections really rather wished you took no notice of! A simple way to keep track of this fact is this: KiloBITS are referred to as small kb and KiloBYTES are referred to as big KB. All in all we can say you should be looking for more than a 128/64k VSAT connection, especially when used in conjunction with those rogue operators who continually add new users to the channel but rarely if ever increase the amount of bandwidth available to the users on that channel.

A simple test to find this out is to seek out a user of a small KU band VSAT using some of the various “Africa” services and ask them how they have seen speeds keep declining over time. Almost invariably they will tell you that when they first installed their VSAT a few years ago it was really good, but these days it just keeps getting slower and slower. This is a sure tell tale sign that the service provider is simply adding more and more users without adding more expensive bandwidth. It’s interesting to note that with the new iDirect series of satellite routers, this cannot happen! As always, buyer beware.

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