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Line speed v Sync speed

glloyd
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Line speed v Sync speed

Why is the line speed so much lower than the sync speed. It can't be all overheads otherwise some people on lower connect speeds would not get any throughput at all. It seems to me that there is a certain amount of throttling.
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Re: Line speed v Sync speed

As you ought to know "Line speed" (if you are talking about that at the member centre) is the equivalent of the BT IP profile which on 21CN is 88.2% of the sync speed. On 20CN the profile is defined in bands by the table that's been published for years - I thought you knew all this. If not you should pop over to kitz and have a good read.
glloyd
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Re: Line speed v Sync speed

Is there any need to be so damn rude. Of course I know the IP profile is about 88% of sync speed and the reason given is overheads. BUT, and the reason for my post, that is a big chunk of the sync speed. Surely there could be a way of cutting those overheads down, I don't think they were always that high.
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Re: Line speed v Sync speed

Interesting question
A quick search threw up this http://www.dslreports.com/speed/
Quote
In an ideal world, you should be able to see in your browser download window, during a sustained transfer, a rate equal to your purchased speed, divided by 8 (to get bytes), less 13% (TCP/IP and ATM header overhead).
another search found this nugget
Quote
ATM cells are 53 bytes long: 5 header and 48 payload
I wonder if the ATM overhead is 5/48 or 10.4%
This is a much better explanation http://aa.net.uk/kb-broadband-how-atm.html
Superuser
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Re: Line speed v Sync speed

Yes, the bulk of the 11.8% overhead is from ATM. As mentioned ATM cells are 53bytes long with 5byte header and 48byte payload. That makes the ATM overhead 5/53 * 100% = 9.4%. One of the reasons ATM packets are so small is because they are designed to carry different types of information - audio as well as TCP/IP data. Note this is telephone type audio, not VoIP which uses TCP/IP for transport.
TCP/IP overhead is 40bytes so 40/1500 * 100% = 2.7% in a 1500byte packet. PPP also incurs a small overhead. There is also wastage because I don't think an ATM cell is permitted to carry data from more than one TCP/IP packet.
In fixed speed ADSL services these overheads were still present but "built in". So, for example with a 512kbps (0.5Mbps) service the sync speed would actually be 576kbps; a 1Mbps service would sync at 1152kbps; and a 2Mbps service would sync at 2272kbps.
David
glloyd
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Re: Line speed v Sync speed

I wonder if the full fibre exchanges i.e. no copper make any difference to overheads?
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Re: Line speed v Sync speed

It won't as the transmission protocols will remain the same
Superuser
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Re: Line speed v Sync speed

I think fibre might make a difference since I believe end-to-end PPPoE is used (ie no ATM). I've seen mention in the Fibre Trials board that the overhead on FTTC is only 3% which looks like just TCP/IP and PPP.
FTTC knows conventional analogue voice is being used so ATM audio needn't be allowed for. I assume FTTP operates similarly at the moment, with "no copper" to come (on BT services).
David
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Re: Line speed v Sync speed

Quote from: glloyd
Is there any need to be so damn rude. Of course I know the IP profile is about 88% of sync speed and the reason given is overheads. BUT, and the reason for my post.........

As you obviously knew that, then if your question had been phrased differently and asked in the right place - it's not feedback on Plusnet's service, then you wouldn't have got such a sarcastic response. Plusnet are, as is any ISP, only operating a system to Internationally agreed standards, and you won't change that! (as frustrating as it maybe, nor do I disagree with your sentiment).
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Re: Line speed v Sync speed

jim:post Perhaps if posters have nothing positive to contribute and only want to make a  sarcastic comment then they should refrain from responding mod:end
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Re: Line speed v Sync speed

If that remark is aimed at me Jim, then I suggest you re-read my initial response where I did point the OP to a web site that would have led to plenty of information on the issue. And this board is still the wrong board for this topic IMHO  Crazy
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Re: Line speed v Sync speed

if the cap fits  Lips are sealed
If it was thought to be the wrong board a Report to Moderator would have been appropriate and I am not going to move it now
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Re: Line speed v Sync speed

I did a report to moderator  Angry
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Re: Line speed v Sync speed

5 days after your original sarcastic response
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Re: Line speed v Sync speed

Jim - you, Dick and Adie are normally quite on the ball when it comes to moving posts. If I were to stop and report every thread I thought was on the wrong board everytime I spotted one, it would waste a fair bit of my time, never mind yours as your moderation boards would get cluttered with stuff you'd have to read when you may have already dealt with the matter by the time your read it.
However your comments are noted -  I will refrain in future from posting at all in any threads I consider are on the wrong board and I shall click on report to moderator.