Download Speed Mystery

My downstream sync, as reported by Zen, is 62289 kbps.
My downstream BRAS profile, as reported by Zen, is 60293 kbps.

Most times, when I perform a wget on the 100Mb TBB test file, speeds fluctuate (as you would expect) but never exceed precisely 6.89 MB/s = 57797509 b/s = 57.80 Mb/s.

Indeed, a speed test just now ( http://www.speedtest.net/my-result/3636439021 ) matches this number (57.78).

So, why is this figure of 57.80 significantly lower than the BRAS profile of 60.29 ? Nearly a whole 2.5 Mb/s slower than what I believe it should be? Or is there some other traffic overhead at work which I've missed? (I thought it might be TCP but that does not account to where 2.5 Mb/s of speed has vanished to). Or are Zen applying their own limiting to a few % below the BRAS profile?

If it was me I probably wouldn't get hung up about 2.5Mbps at those speeds, but of course you have a valid question. Perhaps Zen do rate limit a few % below the BRAS profile.

The trouble is, I'd be amazed if you get an answer out of Zen, they seem totally secretive nowadays about their whole network. I think they a little bit paranoid that by disclosing details about their broadband network it may somehow affect their reputation/PR with their larger (say large business and enterprise) customers etc.

4% overhead seems reasonable.
https://learningnetwork.cisco.com/thread/43446

Hi,

Geordish shared something, but here's my own contribution which may help explain it:

http://packetpushers.net/tcp-over-ip-bandwidth-overh...

...a smaller MTU value would increase the percentage required for overheads too.

And to confirm - we don'have any limit imposed below BT's BRAS profile.

regards,
Phil.

Thanks Phil, and Geordish!

I wouldn't pay too much attention to it either - for reference I just checked my details, I was disconnected overnight so have just resynced - according to the line rate data, up/down is 6264/34913kbit/Sec, however I know via the modem that I'm synced at 5729.12/866.5KB/Sec, and I have been downloading/uploading at a higher rate than the line rate data says.

My upload speed has tanked from 1.1MB/Sec originally to 866KB/Sec (was <800KB/Sec yesterday!) making me think of downgrading the line, but thats another story.

Yeah, I'm not really fussed about the 2.5Mb/s - just trying to understand the reason for it, which I believe has now been answered in the other replies.

These small overheads due to network stacks have been there all along just that now we have faster connections it is easier to see them.

Hint: If someone posts a Gigabit connection speed test at over 950 Mbps a good chance the figures are wrong.