Burstable service

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We sell two variations of internet access service - uncontended and burstable.


The uncontended service has a guaranteed throughput between you and us. We have ample capacity to the internet for you to be able to use your link at the full speed both ways 24 hours a day if you wish. Please do bear in mind that the internet itself is, of course, a contended service and so congestion within the internet, at remote sites and load on remote services, can all cause things to be slower than the link you have.

Burstable service

The burstable service guarantees one level of service as uncontended to us as above, but allows bursting to a higher level. This bursting capacity is not guaranteed. The link between you and us over BT is configured in the same way. Also, we only budget for your use at the guaranteed level.

However, we understand that normal internet access is bursty and a 2M burstable to 10M service is ideal for typical internet use. On average the usage will be below the 2M, but you may wish to download up upload information at much higher rates from time to time.

Apart from any congestion on the link over BT, which is relatively unlikely, we also have a shaping system in place which means that continuous use above the guaranteed rate will gradually reduce your line rate. It will never go below the guaranteed rate. It also takes some time to change (around an hour, but we may adjust this over time). Once usage lowers the rate in increased again. This means if you try and run a 2M burstable to 10M service at 10M constantly you will find that over several hours the line rate has gone down to 2M only.

In practice, unless you start running constant bit torrents, we do not expect anyone to trigger this limiting as typical internet usage does average much lower than peak usage.


Both services operate with no usage limits and no extra costs for high usage. This is one of the main reasons the burstable service has the shaping in place so as to ensure that our costs are not higher than expected for the service. With either service you can safely use it flat out with no risk of any extra bills.

Smart shaping

Whether on the uncontended service or the burstable service, we shape the traffic to the line rate. We do this because we can shape the traffic in a way that is slightly smarter than BT. We currently deploy a simple system whereby small packets are sent in preference to large packets when a link is full. This has the main effect of making voice over IP work even when downloading at full speed on your link. Whilst we do not guarantee any tagging or priority for VoIP on the line (and neither do BT) this has proved to be very effective. The alternative for VoIP is a separate etherflow, but this adds to the cost considerably.

If you need this type of shaping between your offices on etherflows then we can supply similar equipment as a router/firewall on your network.

The shaping is set to allow for the overhead of Ethernet frames, and so is set slightly lower than the defined (Ethernet) speed that you have selected on an Etherflow, as our shaping system works at an IP level.

Some technical details on burst limiting

If you buy a burstable service from us, we make use of the FireBrick long term shaper options to manage your line. The exact details can be found on the FireBrick web site, and may change over time as the product is enhanced and we upgrade routers. The exact method and parameters we use are not defined in the product description so that we can adjust them if necessary to manage traffic sensibly. However, we aim to allow bursting for an hour at least, at the full burstable line rate, before applying any limits. We also ensure the limits are never below the committed rate you are buying. The following is based on how the management is currently done, but may be subject to subtle changes or even per customer adjustment. None of this applies if buying an uncontended service (i.e. non bursting).

Bursting itself is defined as transferring more data in a period than the chosen rate. This is measured on a per second basis initially. Once one second is a "busrt", i.e. more than the amount you can transfer at the chosen rate for that second, then the burst is worked out on an ongoing basis. I.e. at the end of the next second you are still bursting if the data in those two seconds is more than two seconds at the chosen rate. This means if you have a 2M-10M line, and use 10M for 1 minute, and then nothing, you will be considered to be bursting for 5 minutes, at which point the data you transferred in those 5 minutes finally reaches the same as the data you could transfer at 2Mb/s.

Having defined when you are bursting, and for how long, the logic works on allowing a minimum burst time. We normally set this to an hour, so you can be bursting for an hour before any action is taken.

Having reached the burst time limit, the line rate is reduced. We normally do this over a period of a further hour to reduce from the maximum rate down to your commit rate, and then stop at that rate until you stop bursting.

If, at any point, you stop bursting, i.e. usage catches up with the committed rate since you started bursting, we put the limit back to the maximum rate.