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whats the diffrence between interleave and fast

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You're stats show that your modem error correction is switched off. Requesting interleaving is the easiest way to get error correction switched on, although you will lose a little line speed and interleaving itself will increase your latency. Alternatively it may be enough just to increase your margin, although this again will lower your speed. In practice if you just tell them you're getting a lot of errors then they'll probably do whatever they think best.

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if i change my router to adsl 2 mode will it have any effect

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It wouldn't do any harm to try. Most routers have a default setting which will attempt ADSL2 first and fall back to ADSL1 if ADSL2 isn't supported on the line, and this is the best setting to use. Most 8Mb services are provided by ADSL1 anyway, but it's just possible that it's a capped ADSL2+ service in which case using ADSL2+ should improve your line a little. Even then I think you'll end up needing interleaving and/or margin increase.

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Requesting interleaving is the easiest way to get error correction switched on, although you will lose a little line speed

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Just to clarify, I thought that you only lost speed if you were able to sync at 7616 or above due to 512 Kb overheads which interleaving uses?

As I am not able to sync that high, I thought that the only penalty I have by having interleaving enabled is the increased latency?

Strictly interleaving and error correction are separate, but they are usually turned on together. Interleaving causes a latency increase, but has no impact on sync speed. However error correction imposes an overhead which reduces the net line speed. The actual overhead varies depending on how much error correction is switched on by the ISP, trading speed for reduced errors. The only exception is when the line can achieve 8Mbps sync with capacity to spare, in which case the correction overhead can be taken from the spare capacity without reducing the net line speed below 8Mbps.

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Strictly interleaving and error correction are separate, but they are usually turned on together. Interleaving causes a latency increase, but has no impact on sync speed. However error correction imposes an overhead which reduces the net line speed. The actual overhead varies depending on how much error correction is switched on by the ISP, trading speed for reduced errors. The only exception is when the line can achieve 8Mbps sync with capacity to spare, in which case the correction overhead can be taken from the spare capacity without reducing the net line speed below 8Mbps.

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Sorry I don't think you're right there see http://kitz.co.uk/adsl/interleaving.htm

BT (and to be fair most UK ISPs) frequently use the term 'interleaving' to refer to both interleaving and error correction, as the BT standard profiles that have error correction all have interleaving as well so it keeps things simple to use a single term. However they are technically quite distinct. It's perfectly possible to have error correction without interleaving (although not the reverse as interleaving has no benefit without error correction).

As for whether you can have error correction whilst syncing at 8Mbps - see these line stats as a recently posted example. This shows 20 bytes of error correction check bytes in every 254 (actually corresponding to an overhead of 640kbps of the line speed), but with the net line speed being 8096kbps.

No he's not, the confusion arises because what BT calls interleaving is not simply interleaving but interleaving with FEC. Interleaving on its own simply rearranges the data, as in your link, so that the transmitted data is no longer contiguous. Thus any errors are not on contiguous blocks when rearranged or deinterleaved at the receive end meaning that it is less likely that there are too many errors for the standard error correction routine to fix.

However, on ADSL MAX when BT applies interleaving it also applies the maximum amount of FEC (Forward Error Correction), which adds an extra 640Kb of error correction information to the data stream. Interleaving adds latency due to the time it takes to interleave and deinterleave the data stream while the extra FEC data added reduces the data rate because some of the data rate is taken up by the extra 640Kb of FEC data transmitted.

Either method can be used in isolation or combined but BT uses them combined with the maximum amount of FEC applied. If interleaving was used without FEC then the only effect would be an increase in latency with the data rate staying the same. If FEC was used on its own then the effect would only be reduced data rate with the latency staying the same. Of course, when FEC is used the actual amount of data moved is the same except that 640Kb of it isn't usable data but error correction data. When the two are used together as in BT's case then you get both an increase in latency and a decrease in data rate.

EDIT: Typo

Edited by deleted (Sun 21-Jan-07 13:49:06)

Sorry, must type faster, I was typing while you posted

No problem - your explanation was more detailed anyway!

Thank you for explaining that so clearly, but I think the confusion about interleaving arises also from the terms used to describe it, including your own. AFAIK a fast ADSL connection has some error correction built in anyway. I assume this is also known as FEC. As you describe BT interleaving includes additional FEC error correction. This could be better described.

Surely the additional error correction introduces latency as well as the interleaving process itself?

I am trying to get my own LLU interleaving turned off. If I eventually succeed, it will be interesting to see whether TT uses a different mix from that deployed by BT.

It occurs to me that a better system to use for correcting transmission errors would be to firstly increase error correction and then secondly switch on interleaving if necessary. ie to make a one stage process into a two stage process.

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a fast ADSL connection has some error correction built in anyway

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Only if it's enabled by the ISP. It's very common for a line not to have error correction enabled, as it wastes line speed if it's not required. On such a line when an error does occur it is uncorrected by the modem and results in a CRC error. On modems that display the ADSL parameters you can see this very clearly - the 'R' paremeter (ie: the number of check bytes) is set to zero. Here's an example.

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I assume this is also known as FEC.

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Correct - Forward Error Correction.

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As you describe BT interleaving includes additional FEC error correction.

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Sorry, but no. BT have error correction either on or off.

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Surely the additional error correction introduces latency as well as the interleaving process itself?

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No. ADSL error correction is performed by hardware and introduces no significant delay. Latency due to interleaving is inherent in the process as you have to wait for all the interleaved frames to arrive before you can recover one.

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It occurs to me that a better system to use for correcting transmission errors would be to firstly increase error correction and then secondly switch on interleaving if necessary. ie to make a one stage process into a two stage process.

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It would certainly be good to have the option, as it would allow errors to be reduced without the latency, which would be adequate for many cases.