Kitz Forum
Broadband Related => Broadband Technology => Topic started by: Tirion on November 17, 2009, 11:55:45 AM
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So, I'm sat here reading through BT's IPStream Max Handbook (because I'm a geek), and I'm struggling to understand why the max IP profile is 7150Kbps for an 8128Kbps sync speed. Just checked my own line on BTW, and sure enough it's in sync at 8128Kbps and on the 7150Kbps profile.
Does anyone know the reason/technicalities as to why BTW don't have any profiles over 7150Kbps?
It's baffling me ;)
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It's baffling me
I'm a broadband/network engineer for a UK LLU provider
Not much hope for me then. :lol:
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The connection speed is the raw data speed capability. The IP profile is a net figure, after allowing for TCP/IP and ATM and other overheads. Read more about it here. (http://www.kitz.co.uk/adsl/IPprofile.htm)
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The connection speed is the raw data speed capability. The IP profile is a net figure, after allowing for TCP/IP and ATM and other overheads. Read more about it here. (http://www.kitz.co.uk/adsl/IPprofile.htm)
I think you possibly misread my question there ;)
What I'm asking, is why is the maximum profile only 7150Kbps, a full 978K lower than the sync speed?
Are you saying that the 7150Kbps figure is reserved for payload only, and the TCP overheads have 'additional' bandwidth available?
Forgive me if I'm being dim, it wouldn't be the first time :-[
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I don't think I misread your question. The IP profile is the level of actual practical download speed which is achievable. If all other factors are in your favour, with an IP profile of 7150 kbps you will be able to download real data at 7150 kbps. The overheads are in the difference between 7150 and 8128.
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In practice I don't think many would say thay actually achieve a throughput rate, as measured by e.g. ThinkBroadBand, as high as their IP profile.
Kind regards,
Walter
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>>> TCP overheads have 'additional' bandwidth available?
The 'spare' is reserved for TCP & ATM overheads.
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In practice I don't think many would say thay actually achieve a throughput rate, as measured by e.g. ThinkBroadBand, as high as their IP profile.
Kind regards,
Walter
It does happen, but I suppose that it happens more rarely with higher IP profiles. My current IP profile is 4500 kbps, and when connecting to fast servers I sometimes see download speeds of 550 kB/sec, which is very close to the theoretical maximum.
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Hi
@Tirion
This link below has the full list of profiles V connection speed and some explanation.
http://www.kitz.co.uk/adsl/IPprofile.htm
Quote : in practice I don't think many would say they actually achieve a throughput rate
Must admit it is rare . Never had a speed test as fast as Profile . Yet it should be as quote from Kitz
The IP profile is a "net" figure after an allowance has been made for any processing overheads such as TCP/IP and ATM overheads.
Read more: http://www.kitz.co.uk/adsl/IPprofile.htm#ixzz0X9128xOT
Regards Jeff
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The speed testers are useful, but I'd not take the results too literally. As we all know, certain speed testers seem to be pure works of fiction, but even the better ones are only a snapshot of a few moments in the day. I was actually impressed today when downloading a show from the BBC iplayer...
As the big download chugged along, once it got into its stride, I opened task manager and displayed the networking graph, which showed an almost perfectly flat horizontal bar, at 0.3% LAN utilization. 0.3% of my gigabit LAN comes out at 3 megabits, which is exactly my profile. :)
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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 (http://www.kitz.co.uk/adsl/IPprofile.htm#IP_and_bRAS_profiles).
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.
Warning - while you were typing 6 new replies have been posted. You may wish to review your post.
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Thanks for all the answers guys, you'd be surprised how many people in the industry simply don't understand IPStream. I know a lot of support staff from various companies don't learn about BT Wholesale's technology specifics nerely as much as they should do. One thing I plan to do is roll out training with that aspect in mind.
Our BRAS is actually built into our individual DSLAMs on LLU, so our IP profiles are whatever the sync speed is. This is what lead to my confusion as to why BT's profiles are lower than the sync speed... it seemed like a bit of a rip off on face value :P
From RAS to the exchange there will be lots of different overheads going on aside from ATM & TCP/IP
Yeah, there's L2TP in the mix there as well to tunnel the session from the LTS to the tunnel endpoints. I remember once we had a case where a customer had set their MTU to 1500 and it wasn't allowing fragmentation. Took ages trying to explain that 1500 won't work because of the L2TP overhead :P
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Hi Tirion,
Reading between the lines it seems as if you might be associated with a Communications Provider.
If so it would be nice to know which one and whether you intend to join the cognoscenti here?
Kind regards,
Walter
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>> how many people in the industry simply don't understand IPStream.
They dont - well certainly not first line CS from the larger ISPs. Theyre not expected to and have the "reboot scripts".
The smaller/medium ones seem to fair much better when it comes to knowing how adsl works.
Actually if you are who I think you are, then I can quite forgive you for not knowing too much about BTs DLM ... since you primarily deal with LLU .. and were one of the first LLU ISPs in the UK?
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Reading between the lines it seems as if you might be associated with a Communications Provider.
If so it would be nice to know which one and whether you intend to join the cognoscenti here?
Its up to the member if they wish to post in official capacity or not.. We do have a few ISP reps that arent officially tagged who do post as they prefer to keep their employer anonymous.
For the sake of members security, any tagged ISP reps (http://forum.kitz.co.uk/index.php/topic,1737.0.html) are those whom I can prove work for the ISP they say they do.
As mentioned above though, Im pretty sure I know who Tirion is.. and shouldnt take me too long to confirm it.
Tirion please PM me if you wish tagging and I can arrange this for you.
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I handle technical escalations and complaints for Easynet and UK Online, but I'm posting here purely out of personal interest. I do post on the TBB UK Online forum in my official capacity, but haven't had a great deal of time to do so recently (it's purely voluntary).
The info on this site is second to none, and easily puts wikipedia to shame. I'm helplessly drawn in by my inner-geek ;)
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btw... came across this when looking at something else.. so thought I'd quote the official BT explanation.
The ATM payload rate is defined as the rate available for the transport of IP packets in the payload of
ATM cells; note however that each IP packet will incur ATM adaptation layer, PPPoA/oEoA and
encapsulation layer overheads. The maximum IP throughput rate that can actually be achieved will
hence vary considerably depending upon the IP packet size. No allowance is required for the
overhead introduced by the ATM layer, as this has already been taken into account in these figures.