Traditionally adsl used what is known as "FAST" method for transmitting data.
MaxDSL can make use of a technology called "Interleaving" to help maintain the tolerance against noise on longer length lines.

If during transit more than a certain amount of data has been lost then the data cannot be correctly decoded.
Short bursts of noise on the line can cause these data packets to become corrupt and the modem has to re-request data which in turn can slow down the overall rate at which data is transmitted.

Interleaving is a method of taking data packets, chopping them up into smaller bits and then rearranging them so that once contiguous data is now spaced further apart into a non continuous stream. Data packets are re-assembled by your modem.
The diagram below is an example of how interleaved traffic is transmitted.

When adsl data packets are transmitted they contain additional control bits that enable your modem/router to check and correct a certain amount of erroneous bits. This type of error checking is known as FEC or Forward Error Correction.

If your line is particularly susceptible to bursts of noise then interleaving should improve your adsl experience simply because if you lose a whole batch of data then this could cause your modem to loose sync with the exchange.

We can see in the diagram below what effect bursts of noise can have on packets that are being transmitted.
Using the more traditional FAST method, the modem would not be able to interpret the packet and these would be lost, or even worse your connection would drop.

Using Interleaving, the modem is able to re-assemble the data or if necessary just re-request the part of the data that it is unable to recover. By improving the efficiency of the error detection and correction correction codes, BT state that interleaving has been "shown to significantly improve error performance and stability of marginal lines".

By now you may be asking if interleaving is so good, why isn't it used by default?
The answer to that is that interleaving has a downside - chopping up, rearranging and decoding of the data adds a small amount of additional time it takes for data transmission.

BT state that it can increase latency by an additional 20-40ms. Whilst this will not be noticeable to the vast majority of users, ardent gamers are the ones most likely to complain about additional latency and therefore prefer a slower sync speed than higher latency.

It should also be pointed out that BTw state that applying interleaving reduces the maximum line rate achievable from 8128kbps to 7616kbps.
Interleaving and error correction are always switched on at the same time and the 7616 profile has a 512kbps error correction overhead with an increased latency of around 16 ms.
Note: although BT state 7616 is maximum sync speed with Interleaving, several instances of higher sync speeds have been reported by users.

Interleaving is set to "auto" by default and interleaving is then controlled by the DLM process.
It is possible to arrange to have interleaving set to permanently "on" or "off" via your ISP. Some ISPs will make a charge for doing this.


Interleave Depth: defines the number of bits (or bytes) in each block of data.
(My diagram shows an example of Interleaving depth of 4).
ADSL supports a various levels of interleaving, the depth of which can range from 1 (no interleaving) to 64.

Interleave Delay: defines the mapping (relative spacing) between subsequent input bytes at the interleave input and their placement in the bit stream at the interleave output.
(In my diagram this would be the time between say 2 yellow boxes).

Maximum Interleave Delay: configurable attribute on some dslams/routers as the maximum time for the Interleave Delay. - The higher the Interleave Delay the greater the Interleaving Depth.

Latency- It is important to note that Interleaving Depth & Interleaving Delay do not appear to be the same thing as the additional amount of latency you will see when interleaving is switched on.
Nor is latency affected by speed -eg it does not decrease when you go from 1Mb to 2Mb.

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