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Recently I came to talk with a network guy about ADSL. He said that an ADSL of 2 Mbit/s means 2 Mbit/s from my modem to the ISP, not 2 Mbit/s from the modem to the internet. That means the speed of the line actually depends on how well the ISP can let me go to the internet.

Is that true? Is that the kind of speed that being advertised?

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migrated from serverfault.com Jan 30 '10 at 8:52

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That's the only speed it could ever be as the internet is not a single entity... –  Oskar Duveborn Jan 29 '10 at 14:40

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up vote 13 down vote accepted

Short answer: yes.

Suppose an ISP has 100 megabits to the internet. They might have 100 customers, each with 2 Mbps each. They are assuming that all 100 customers will not use that 2 Mbps at once. They may even use devices to "shape" the traffic to make it so everyone gets a fair share if everyone tries to use their max speed at once.

Also, 2 Mbps is one direction. ADSL has two speeds, the upload and download speeds.

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+1 Very concise answer –  Dave M Jan 29 '10 at 14:03
    
+ 1 for mentioning the contention ratio. Hugely important. The lower the better. –  dunxd Jan 29 '10 at 14:04
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You can delete "normally" in your last sentence. The "A" in ADSL mean "asymmetric", which means that the bandwidth in upload and download are always different by definition. –  splattne Jan 30 '10 at 8:50
    
Sadly, in the UK at least, a more realistic scenario would be an ISP sharing a 1Gb/s link across 100,000 users. Yes , you share your 2Mb/s slice of the pipe with 50 other users (on average) assuming a typical 50:1 contention ratio, or 40,000 users if you have a premium 20:1 contention ratio. –  CJM Sep 22 '10 at 13:12

They are not saying that you will get to enjoy 2Mbps of bandwidth between your computer and your favorite website. The speed that is advertised is the speed between you and your ISP. There are dozens of variables that will control/limit your speed to "the Internet".

Some variables:

  1. Route of traffic to remote site
  2. Response of Remote server
  3. In many cases amount of users online at the same time.

You will probably never get a full 2Mbps communication between yourself and servers you request information from. The ISP is stating their upper limit when they sell bandwidth like this. No matter what technology you go with (ADLS, Broadband, Wireless) you will always be sold on the possibility of the upper limit. Good luck getting close to that.

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One exception -- Frame Relay. In that case, there's often a CIR (Committed Information Rate), which is the guaranteed bandwidth that you'll get. Of course, even with that, you might have some protocol overhead, so some methods of testing could show slightly lower than the CIR. –  Joe H. Jan 29 '10 at 14:06
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Actually, in thinking about it -- another exception -- paying more to get a SLA (Service Level Agreement) with the ISP, where they'll guarantee a minimum level of service. Of course, this will significantly raise the cost, as it's normally only available in business class lines. –  Joe H. Jan 29 '10 at 14:08
    
@Joe - that's a good point. Hadn't considered that since I assumed Phoung wouldn't have access to it but I'm really glad you pointed it out. –  Patrick Jan 29 '10 at 14:31
    
@Joe - that still only guarantees you that bandwidth on hardware that the ISP controls. For example, they don't control Level3 and most likely don't support the remote site(s) you are getting the data in question from... –  Scott Lundberg Jan 29 '10 at 21:33

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