The IPprofile has an inbuilt allowance to cover the overheads.
Although the IPprofile states 7150, this relates to what BT call the bRAS ATM bit rate profile, which is what I believe they actually set on the RAS.
Theres a list on the ATM bit rate profiles that relate to each IPprofile
here.
For 7150 the bras profile is actually 7320, and I think this is supposed to cover an allowance TCP/IP overheads, with whatever else being ATM overheads.
As mentioned the IPprofile is merely an indication of the
maximum throughput speed you will see depending on your actually sync speed, and makes an allowance for (some?) the overheads.
Similar reason as to why when on 2Mb fixed syncing at 2272, throughput would only be around 1800-1900 kbps.
.. or sky's "2Mb" accounts only sync at 2000, yet throughput is normally around 1600 kbps.
I believe BTw do this so people dont say something like "Im syncing at 8128, so why is the highest that I get only <7Mbps on speedtesters?"
>> Are you saying that the 7150Kbps figure is reserved for payload only, and the TCP overheads have 'additional' bandwidth available?
Sort of... but there are also ATM overheads, which I believe has the largest overhead of all.
If your line is tweaked properly and theres no contention then it is possible to see throughput of around 7.1 Mbps.
Its been a while since Ive been on max.. but screen caps below showing throughput when on a 7150 profile. As you can see I was downloading at up to 903 KB/sec (7224 kbps)
On a quiet night I could get some speedtesters recording 7 - 7.09 Mbps. Most normally though on the TBB speedtester it would show around 6850 kbps on a good day.
I'm pretty certain that TBB used to say they estimated something like 10% for TCP/IP overheads.. if this is the case then (7150 - 715 = 6435), if the exchange was ok.. then I certainly many times saw speeds more than IPprofile minus overheads.
From RAS to the exchange there will be lots of different overheads going on aside from ATM & TCP/IP
If someone is really bored they could probably have a go at working out the overheads for each of the protocols.. but theres so many variables that could effect it.
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PS - I started to type this at about 2.30 ish, but then I had visitors so its been stuck on my screen since, waiting to hit the post button.
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