37Mbps profile?

[#1]

I'm not being nasty here, just genuinely curious.
FTTC is touted as "Up to 40Mpbs".
My Plusnet profile says 37Mpbs, and indeed reading here, most get either 20Mpbs or 37Mpbs as their profiles.
So my pondering leads me to..... how can I ever expect to achieve 40Mbps, if Plusnet set my limit at 37?  And further, why was 37 chosen, and not say.... 40? 

[#2]

Because Up to is based on sync rate and not profile.

[#3]

According to the small print it is

Quote

† Fibre broadband speed is described as up to 40Mb because your actual download speeds will vary. We'll give you the best download speed available on your line up to 40Mb, dependant on your location, phone line, home wiring and time of day. We'll provide you with a speed estimate when you check availability of our products. Find out why speeds vary.

Except nobody ever will download anything at 40Mb.
The small print is a lie.

[#4]

I suppose my point is can't rather than won't.
By setting my profile at 37Mpbs, I can't  achieve 40Mbps regardless of what I sync at or any other magic I do at my end.
So, as Avatastic says, the small print isn't quite right either, as I will be capped at 37Mbps regardless of sync speed.
As an aside:  How can I even tell my sync speed now that the modem is inaccessible?

[#5]

The bulk of the difference is in the data that is also being transferred, but isn't useful to you as the end user. All those headers that make sure your data gets to you (and not to a Sony hacker), and gets there in the right order.
On ADSL, the overhead is even worse, as that includes ATM as one of the transport mechanism, which has a considerable overhead.
On ADSL, take an example of an MTU of 1458, which is the number of bytes of "TCP/IP Payload" in a packet. It is this data whose rate is throttled by the IP Profile.
However, on top of this set of data is overhead for:
- PPP: 2 bytes
- AAL5 trailer: 8 bytes
- ATM headers: 155 bytes
- ATM padding: 20 bytes
or about 13% overhead. The padding can vary from 0 to 40 bytes, so the overhead could be as high as 14%
On an 8Mbps line, with interleaving turned on (so FEC protection overheads reduces the actual volume to 7.6Mbps), the 13% overhead takes you down to 6.6Mbps of actual TCP/IP payload.  BT then choose an IP Profile of 6500, and slow you down a tad more. Then remember that "M" is 1,000*1,000, and Speedtesters reporting slower speeds can be reporting a 6.2Mbps where the "M" is 1,024*1,024.
Even then, that isn't useful data to you - as that data has still included the TCP and IP headers of 40 bytes, or another 3%. If you manage a perfect transfer of 6.5Mbps (in 1,000) to match the profile, you'd still only manage to FTP a file at 6.3Mbps (in 10^3 format) or 6.1Mbps (in 2^10 format). And that doesn't take account of the fact that there is some latency in sending packets back-and forth, and overhead of the FTP protocol itself.
More likely, you'd get a line IP speed of around 6.2Mbps, and actually transfer the file at about 5.7Mbps. Yet the ADSL link is still almost fully loaded at 8Mbps.
Now consider Fibre carrying PPPoE packets. The same MTU of 1458 has overhead of:
- PPP: 2 bytes
- PPPoE headers: 6 bytes
- Ethernet Headers: 18 bytes
or about 2%. Rather better...
On a 40Mbps line, the extra 2% takes away 0.8Mbps. BT chooses an IP Profile of 38717, so obviously restrict the speed a little more (I think, in the case of VDSL2, there is an additional channel being sent back-and-forth for management data)
And even with an IP Profile of 38717, it seems that most speed testers will give results of 37Mbps or so. If they're reporting an "M" in 2^10 format, that is about bang on equivalent.
Unfortunately, so many numbers are fed around, in a rather blase manner, that it is very hard to be sure that you are comparing apples with apples.

[#6]

On the aside: You can't, at least not with the BT modem.

[#7]

I should say that, while we're disputing how achievable any DSL "headline" speed is, I think PN did the right thing for ADSL2+. They called it "Up to 20Mbps", even when the technology headline speed is 24Mbps, which is how others sell it (such as Be). While technically more accurate, it also conceivably leaves PN at a marketing disadvantage for those customers who don't understand it is identical underlying technology, and that the name or headline speed doesn't matter.
The trick is to really understand that the "Up To XXX" name is only a marketing name for a generation of technology.

[#8]

I see your point entirely, and indeed agree with the chosen up to 20 used, as I'm sure precious few got even close to 24, overheads aside.
I see your point on the apples with apples comparison.  My understanding is PN need to have the profile set otherwise your line gets swamped with data and you get errors/drops... or at least that's what I think was said. 
So being positive, PN have set it to 37, because (38717 - the overheads) is pretty much 37.  Ie. without the profile, you'd get 37 anyway.
I do wonder if PN will need to reconsider their marketing and profiles though... on the basis that it appears a good many people are getting at or close to to the 40Mb limit of VDSL, then there will be far more people like me wondering why their 40 service is set at 37.

[#9]

I just thought of another explanation - which makes all my waffle last night superfluous. It was all correct, with lots of techno-babble, but too much...
First, what does a BT SpeedTest say your IP Profile is? You should get all 3 figures in there - a download speed, an IP Profile, and a sync speed.
Now onto the thing I'd forgotten, which is...
That Plusnet's speed of 37M (on the portal "high speed broadband" page) is no longer a reflection of the exact profile at BT, like it used to be for ADSL and ADSL2+ services. Instead, their setting of 37 lets everything flow at full speed.
Indeed, for a while during the FTTC and FTTP trials, they didn't even have this setting - and it was labelled as 20M - but both FTTC and FTTP would go as fast as they could.
So... I'm pretty sure that everyone on FTTC (and FTTP) has a speed setting of 37, irrespective of the actual profile value on BT.
All I have to do now is find some evidence down through the older messages on here....

[#10]

Hi
I've been reading this post with interest.
I'm on FTTP and my profile shows the current line speed as 20Mb but I am regularly exceeding 90Mb - oops, sorry about that! 
Cheers
Howard

[#11]

FTTC user here, profile shows as 37Mb as well

[#12]

Quote from: WWWombat

I just thought of another explanation - which makes all my waffle last night superfluous. It was all correct, with lots of techno-babble, but too much... First, what does a BT SpeedTest say your IP Profile is? You should get all 3 figures in there - a download speed, an IP Profile, and a sync speed.

FYI:  It says:
Your configured download throughput speed for this service is 38717 k
Your configured upload throughput speed for this service is 10000 k
These are also referred to as IP Profile.
No sync speed is given. 
Sounds like no limiting at P/N, and either some rounding went into BT marketing, or transport overheads knock it down a bit.  Best sustained transfer seen in real life was portal 2, in the early hours of the morning at 35.2Mbps, which ain't too bad in my eyes.

[#13]

That's strange. I just ran one here (still ADSL, but not much longer now) and it definitely shows all 3 values:
- Sync Speed 8128 Kbps down, 448 Kbps up.
- IP Profile 7150 kbps
- Download of 6436 Kbps
Does the FTTC result not include the sync speed? It is called "DSL Connection rate" on my screen.

[#14]

Nope, here's my FTTC grab, Sync missing:

[#15]

HI, I'm quite sure it's normal not get a connection rate result on fibre.