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CIR - Committed Information Rate | MIR - Maximum Information Rate | Contention Ratio | Oversubscription |
(Pronounced C - I - R  like "Sea Eye Are")
CIR or Committed Information Rate is a way of guaranteeing that, even though you share a bandwidth pool with many other users, you are assured that you will receive, at least a part of this, no matter how busy the link gets.

As we know, the cost of providing bandwidth to satellite users (driven by the development, launch, maintenance and ground segment of a satellite) is extremely high. The only way to make this affordable is to share the bandwidth among several users. 

Having a small portion of dedicated bandwidth becomes important when you are using the internet for something that has a critical minimum bandwidth, like a VoIP phone call that can require 30kbps, 20kbps, and sometimes more or less, depending on the type of call you are making. A free Skype call typically requires about 30-40 kbps, but a call through the network providers VoIP router could be considerably less. If the bandwidth is not available at the time of your phone call, you could have very disappointing results with the call dropping or parts of the conversation dropping out.

To guarantee that minimum bandwidth, costs money, as this is bandwidth that the satellite provider cannot sell to  other users. So expect to pay a premium for CIR.

To ensure quality voice calls, you only need to guarantee the bandwidth to support as many voice calls as your VoIP router will support, typically 10 kbps - 20 kbps per simultaneous call.

 
(Pronounced M - I - R   like "Em Eye Are")
BIR - Burst Information Rate, or MIR - Maximum Information Rate is the theoretical maximum to which your bandwidth can increase as bandwidth becomes available. This is the size of the complete data pipe that you are sharing with others, and the rate normally advertised by providers.

In practice, it is rare that you will ever actually achieve this rate, even when you are the only subscriber to the service. There are overheads and bottlenecks both on the vessel and on shore. Typically one would achieve bandwidths of between 50% and 90% of the advertized rate.

If this does not meet your requirements, you  will need to subscribe to  a higher level of service, or increase your CIR, both of which will cost more money.
 
Contention Ratio is the number of other subscribers on the same network competing for the same bandwidth.

Generally speaking, the contention ratio is the MIR (Maximum Information Rate) divided by the CIR (Committed Information Rate). So with a contention ratio of 4:1 on a 1 Mbps downlink, the 4 subscribers would each have 256kbps guaranteed, while they can each burst up to the full 1 Mbps as it is available and not in use by their peers.

A quality, marine shared network may have a contention ratio of 5:1 and this generally will provide good results. Theoretically this means that in the assigned MIR bandwidth that there are a total of 5 subscribers using the bandwidth simultaneously. Due to the sporadic nature that we use the internet, not everyone is downloading at the same time. Not everyone is even using the internet all the time, and when they do, it is in fits and starts with many idle times in between, like while you are reading the web page that you have downloaded. With voice calls, hardly any bandwidth is used while you are not speaking, between sentences, between words, and even between syllables. So there is plenty of free bandwidth for others to use.

Some providers offer a choice of an entry level package of 8 : 1 contention ratio which will work fine for most internet applications, and a premium package of 4:1 contention ratio which should provide excellent throughput near to the contracted rate. Most providers also advertise a 1:1 contention ratio, but this is a very expensive proposition and should only be considered if your applications demand the full bandwidth all of the time. It is often possible to temporarily bump up the bandwidth to 1:1 if you have that requirement for short periods of time and then drop back to a shared service during idle times.
Some of the low budget providers (perhaps not marine) have contention ratios of "many to one (20:1 or 40:1). Some terrestrial internet providers use contention rations of 50:1. Depending on your usage requirements, this is often fine for casual use and provides offshore VSAT service to vessels that would not normally have this in their budget.

There is talk that some providers will take license with the contention ratios and oversubscribe the network, where they put more subscribers than the defined contention ratio on the same bandwidth. They justify this by saying that the contention ratio is the average number of subscribers using the network at any given time, not the total number of subscribers assigned to that network. They count only the users that are actually online at that time. They monitor the usage and dynamically adjust the bandwidth to keep a satisfactory user experience for all. As long as this is done diligently this should not be an issue, and if the user experience is not effected, hopefully this economy will be reflected in reduced monthly fees to the subscribers.

The bottom line is the user experience and the monthly communication budget of the vessel. If you are not getting satisfactory results, you may need to increase your service agreement and pay for more MIR or CIR. If you have an absolute minimum bandwidth requirement (like VoIP), increase your CIR (or subscribe to a lower contention ratio). If you want more speed, then you will want to increase your MIR.