Traditionally adsl used what is known as "FAST" method for transmitting
MaxDSL & LLU 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 by your router.
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
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
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 data and the information would have to be re-requested, or even worse, your
connection could 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, then why isn't it used by
The answer to that is that it can also have a downside - chopping
up, rearranging and decoding of the data adds a small amount of additional
time it takes for data transmission. Forward Error Correction will also add to the delay, since the check bytes will take additional transmission time.
BT state that interleaving 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 synch speed than higher latency.
It should also be pointed out that whilst BTw state that applying interleaving shouldn't reduce your line speed, it does
reduce the maximum line rate achievable from 8128kbps to 7616kbps due to the additional overhead required for check bytes.
Note: although BT state 7616 is maximum sync speed
with interleaving, many instances of higher sync speeds have been reported
by users. This is dependent upon your router being able to support S=1/2 mode which effectively combines two RS code words into a larger logical code word of 510 bytes (ANSI T1.413).
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.
Whilst Interleaving on its own may not reduce your maximum sync speed, Forward Error Correction can and does affect your maximum sync rate.
Interleaving is set to "auto"
by default and interleaving is then controlled by the DLM process, turning it on if needed.
It is possible to arrange to have interleaving set to permanently
"on" or "off" via your ISP. Some ISPs may make a charge
for doing this.
~ Interleave Depth
- Interleave Depth defines the number of bits (or bytes) in each block
of data, for example my diagram above shoes an example Interleaving depth of 4.
- ADSL supports a various levels of interleaving, the depth of which
can range from 1 (no interleaving) to 64.
- Steps Ive seen are 4, 8, 16, 32, 64 for the downstream and 2, 4, 8 for the upstream.
- BTw makes little mention of Interleave Depth, and this would appear to be something that is controlled entirely by the DLM as there certainly are various depths of interleaving applied by the system.
- Some LLU providers have configurable Interleaving Depths and will change your profile upon request.
- Some routers will show you the depth of Interleaving applied to your line
eg: D (interleaver depth) 32 4
~ Interleave Delay
- 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.
- For example, in my diagram this would be the time between say
2 yellow boxes.
- Some routers will show the interleave delay in ms.
~ 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.
- Some LLU providers (such as Sky) will configure this figure upon request
- It is important to note that Interleave Depth & Interleave
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 connection speed -eg it does not decrease when
you go from 1Mb to 2Mb.