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Understanding adsl speeds

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Traditional fixed 512kbps, 1Mb, 2Mb connections.

With traditional fixed speed adsl, the actual speed of your connection has little to do with the length of your line or internal wiring.
If your connection syncs at 2Mb (or 1Mb, 512k) then the speed you can get should be the maximum speed for that service.

It is a common misconception that if you have a long line, then your speeds will be slower. - Not true - either your connection can sync to the exchange at a particular speed... or it cant! *

*There is one exception to this and that is if you are on a long line and you have a low SNR margin. In this instance you may think you are getting slow speeds but what is actually happening is that the line is momentarily not able to send data packets and these are "dropped". If you have a router you should check and see if it records these errors. They will normally be displayed as CRC or HEC errors.

Rate Adaptive dsl (dslMAX, LLU, adsl 2+)

Rate adaptive dsl works differently, your maximum speed depends on various factors such as the length and condition of your line from the exchange. Generally speaking, the longer your line, the greater the attenuation. Signal to Noise Ratio plays a very important part in the maximum speed at which you can connect to the exchange. With rdsl your router will sync at the best speed it can allowing a "safe" margin of SNR. Therefore the better your SNR Margin the better your chance of getting the higher speeds.

With radsl your sync speed can change each time you logon. For more information see How dslMAX works.

Testing your speed

When checking your speed, if you are in the UK it is very important that you test using a UK based speedtester. Using a tester outside the UK will accrue additional time to the traverse the Internet and provide an inaccurate result.

The best place to go in the UK to check your speed is thinkbroadband as their speed tester is recognised as being fairly accurate by BT Wholesale and most UK ISPs.
On some occasion some test servers may be busy so if you get an unusual result, its worth while trying another tester. - List of broadband Speed testers.


Types of Speeds

- Line Speed.

The speed at which your connection is provided at, commonly 512kb, 1Mb or 2Mb.

- Connection (sync) Speed.

Your connection speed (576 / 1152 / 2272 kbps) is the rate at which your modem synchronises to the local exchange. Do not make the mistake of assuming that your connection speed is your actual speed.
This higher figure allows an extra margin for ATM overheads. (BTw use an ATM network on their backhaul.)

- True Speed.

The speed at which data travels including any overheads. Adsl-guide estimate that overheads add an additional 8% on top of your actual speed. The title true speed can be a bit misleading since we can never know for sure exactly how much overheads are added when traversing a network.

- Actual (Throughput) Speed.

The "Actual Speed" is the rate at which useful data is sent and received without any overheads. Therefore this figure is the one that users should use when quoting their line speed.

Other speed testers may quote a figure somewhere between the true and actual speed, which can vary according to each speed tester.

Understanding Overheads.

Every time we send data across a network, we also send additional information such as where the packet is from and where it is going - even how the data can be read at the other end when it reaches its destination. This additional information is known as overheads.

There are different types of overheads that may increase the size of the original data.
Simplified - You should think of overheads being the "wrapper" and the label around the "parcel" of data that you are sending.

The different types of overheads may be "wrapped" around other overheads increasing the size of the original packet.

The most common type of overheads are TCP/IP overheads. There are also many other types of overheads that your data may be wrapped in whilst traversing the internet such as UDP, IP, PPP, L2TP, ATM.

What speeds should I expect to see on my connection?

The following is a guide as to the theoretical max speeds you should get on the adsl-guide speed tester for each connection speed:-

If you are on dslMAX check the IP profile table on the bottom of the How dslmax works page.

Line Speed Sync Speed Theoretical Max Actual Speed My own speeds on each line speed.
512 kb 576 kbps 480 kbps 478kbps
1 Mb 1152 kbps 960 kbps 955kbps
2 Mb 2272 kbps 1920 kbps 1910kbps

Remember it is very rare that you will ever reach the max actual speed. My own connection is tweaked to the max, and the above speed tests were taken when my connection was rock steady and they never really varied much from these figures.
Recently though BTw have been "messing" at some exchanges and for a while I found that I was consistently getting speeds between 1940 up to 1991 kbps.
Twice in the past 3 months I've been moved to different RAS's both times these have had an effect on my speed and latency due to the routing.

What causes slow speed test results?

Various things can affect your speed tests:-
  ~ Congestion on your ISP's network
  ~ Contention at the local exchange
  ~ Your own network (other traffic such as virii or malware).

If you are getting consistently slow speeds then check out my slow speeds page.

Is there anything I can do to improve my Speeds?

Yes. You can tweak your connection so that the size of packets you send "best fit" BT's ATM network.
Info on how to tweak your connection.
If you are on a radsl high speed (dslMAX) connection then look at improving your SNR Margin.


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