ATM overheads options: Difference between revisions

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The actual line carries data using a protocol called ADSL. This includes layers of data framing and control information which we do not have to consider. What matters is the line rate that is quoted. This is what your ADSL router will quote on its management interface as the sync rate and is what BT quote to us in line tests, etc. This is the headline line rate and it is this rate which can get up to 24Mb/s on ADSL2+ for download.

This rate is actually the ATM data rate - the rate at which the bits that make up the 53 byte ATM cells can carry. So 24Mb/s is 24,000,000 bits per second carrying ATM cells.

ATM cells are 53 bytes long. So at 24Mb/s it is possible to transfer 56,603 cells per second. Each cell contains 48 bytes of payload data and a 5 byte header. So in terms of transferring payload a 24Mb/s line can manage 21,735,849 bits/second.

On 20CN lines, BT also have a system whereby they rate limit traffic to one of a number of pre-set limits. This could mean as much as 1Mb/s less data throughput that you may expect (½Mb/s for rates under 8Mb/s, ¼Mb/s for rates under 2Mb/s).

So considering the throughput of TCP on a line syncing at 24Mb/s.

24,000,000b/s ATM rate

21,735,849b/s ATM payload for PPP

21,254,716b/s PPP rate assuming 1,502 byte PPP packets

21,226,415b/s IP rate assuming 1,500 byte IP packets

20,943,396b/s TCP payload rate assuming 1,460 MSS

There is also a low level 135K BRAS profile which is normally an indication of a serious problem with a line. For ADSL2+ there are BRAS rates of 9Mb/s to 21Mb/s in 1Mb/s steps.

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