I wonder how many customers are unaware that they are only receiving half the benefit of a 21cn connection. Obviously it must benefit the Company in some way.

Think about it.

If someone has slower speeds they're more likely to go elsewhere.

Not really. on Max ADSL the max sync PN allow is 7150 instead of the 8128. It would be interesting to know how much download speed PN estimates would be lost by uncapping the upload on 21cn.

The only reference I can find so far to upload speeds is that mentioned in ADSL2+ FAQ, see paragraph 2, and that's the theoretical maximum.

In reply to a post by wingco1:

on Max ADSL the max sync PN allow is 7150 instead of the 8128.

That's impossible.

This potentially means download speeds up to 20Mb and upload speeds anything up to 2.5Mb

My bold. Which although it's part of the spec, the faster upload speed is only available to PN customers on request. Maybe Bob or Jelv could offer an opinion as to how much dl speed would be lost on an uncapped upload.

There was a thread quite some time ago in which one of the reps stated that on 8mbps max adsl the sync rate from PN to the customer was capped at 7150.

Edited by wingco1 (Sun 30-Jan-11 15:19:30)

ADSL Max sync speeds are in multiples of 32kbps.

The rep was clearly talking about the IP Profile earned by an 8128kbps sync. You said PN cap the sync at 7150kbps, which would only attain a 6000kbps profile.

As I'm sure you know when you think about it .

You are correct. Thinking about it more, it was to do with a 8128 sync being capped at a speed lower than 7150.

In reply to a post by wingco1:

This potentially means download speeds up to 20Mb and upload speeds anything up to 2.5Mb

My bold. Which although it's part of the spec, the faster upload speed is only available to PN customers on request. Maybe Bob or Jelv could offer an opinion as to how much dl speed would be lost on an uncapped upload.

I'm not sure whether ot not Plusnet offer Annex M on ADSL2+. I seem to remember it isn't yet available from BT Wholesale, but I could be a bit out if date. I believe it is planned at some stage.

The ADSL2+ we all know of tops out at 1.3/1.4Mbps depending on what you read. That is Annex A, which I assume is the name of the relevant section of the ADSL2+ specification.

Annex M takes a chunk out of the upstream frequency range. If you look at your own DMT graphs, or any that have been uploaded and linked to by anyone else, you will see the attenuation and bit/frequency loading graphs. The top bit of the upstream is next to the bottom bit of the downstream.

Because that quite useful chunk of downstream frequencies is removed and used for upstream, the downstream sync speed necessarily falls, unless something else compensates. The fall can be as much as 2.5Mbps.

On Be products the compensation kicks in on lines which sync at about 16Mbps and higher - in other words quite low attenuation of the downstream frequencies. I know of no reason it shouldn't be the same on 21CN if/when it is available.

Because of the low attenuation it is possible to allocate more bits to each frequency/bin. (The question of why that doesn't happen anyway on Annex A has only occurred to me whilst typing this). This extra allocation supplies the lost sync speed.

The lower the attenuation the more extra can be allocated in there.

As I said, the cross-over point is around 16Mbps on Annex A. Below that you lose the full 2.5Mbps and are unlikely to gain anything worthwhile over 1.3Mbps upstream, as of course the attenuation on that prevents it even with the extra frequencies. Much below 16Mbps Annex A you don't get any upstream improvement at all by moving to Annex M.

The further above 16Mbps the Annex A sync is, the more the benefits of Annex M come through. Because more bits can be added to the remaining downstream bins the less you lose, and because the upstream attenuation is also lower the more you gain on that. It is possible to attain a full 24/2.5Mbps sync.

So if the Annex A sync is below (remember - about) 16Mbps, Annex M is lose-lose. Above it you start to gain upstream but lose 2.5Mbps, but the faster the line normally is the less you lose. Above some figure I haven't worked out, win-win!