BT interleaving

[tynie1874]

Do i need to Phone up BT to get it turned off? because when i speak to an advisor there english is terrible i cannot understand them and i would love if there was a way on the website i could do this.

[Swimmer]

Do i need to Phone up BT to get it turned off? because when i speak to an advisor there english is terrible i cannot understand them and i would love if there was a way on the website i could do this.

 

I am not sure of what you mean but when you phone BT you get placed through to India. I would phone them again and demand that you speak to the UK call centre.

 

I had the same problem before and they put me through upon request.

[systemx]

Why would you want it switched off?Interleaving or multiplexing is a means of sending different signals over the one channel by subdividing timeslots(I think)

[systemx]

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. 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 sync 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.

 

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

 

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 configuarable 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

~ 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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[DikT.]

Appreciate the info systemX. I thought it was something along these lines :-)

[Stuart Lyon]

Thes o2 techies - don' you just love their plain English replies.

[tynie1874]

well i play games online and my ping/latency should go up according to other people so say i had a 50ping ingame with interleaving on....

 

then i get interleaving off i get a 15-25 ping which is better for me or so i have been told. well my dad phoned up today and they said it should be off by tommorow evening.