A contended VSAT service is a connection which you will be sharing with several other users/vessels. The amount of users/ships you share the link with is called the contention ratio. Most commercial systems offer contented services because they are less costly than their dedicated counterparts.
The advantage of sharing the connection is that the cost of the satellite data connection is going to be split over all the remotes which use the same satellite connection and this allows you to save money or achieve higher data throughputs than you would normally be able to afford if you were using the satellite connection just for yourself;
A shared connection does have a bit of overhead and needs more expensive equipment at the HUB/Teleport in comparison to dedicated connections but the individual cost can be significantly less. If you are not going to use your satellite connection to its maximum all the time it is wiser to choose for a contended service.
Common contention ratios
- 1:2 share your connection with maximum 1 other ship so your guaranteed minimum speed is 50% of the total
- 1:4 share with 3 other ships and have a guaranteed speed of 25% of the total
- 1:8 share with 7 other ships and have a guaranteed speed of 12.5% of the total
Contention ratios over 1:20 will rarely be available.
Contention ratio is in direct relationship with CIR and MIR.
MIR & CIR
Directly related to contention ratio are CIR (Committed Information Rate) which is the minimum guaranteed speed you will have and MIR (Maximum Information Rate) which is the maximum information rate you will be able to achieve.
In general you can use the following formula to calculate the individual values:
CIR x Contention ratio = MIR
Bear in mind that the contention ratio in your upload might not always be the same as in your download. In this case you will mostly find only CIR and MIR mentioned. The reason for different contention ratios in up- and download is that sometimes (often when using lower speeds) you want to guarantee a certain minimum speed in a certain direction. A typical example of this is when using VoIP telephone lines. VoIP will require a minimum bandwidth (e.g. 64 Kbps) in uplaod as well as download. If you have a high contention ratio then it coul dbe that the CIR on your upload doesn’t guarantee the 64Kbps. In this case you might want to have a higher (min. 64Kbps) upload CIR which you can achieve by having a lower contention ratio in the upload.