the true nature of DSL

Im out in the wilds of Orkney (specifically, Sanday Island - KW17 2BW) and trying to do video streaming to "broadcast" an H.264 camera stream (via a content delivery network). understandably, the weak link in the signal path is the BT Consumer broadband uplink from the house to the wider internet. Ive clocked it using speedtestuk.net (and similar) at 300 Kbps. The download side varies between 2 - 3.5 Mbps.

Naturally I am interested in improving the uplink speed, to enable higher resolution etc.

How does A-DSL really work?

I thought that ADSL technology was developed specifically to enhance the difficult "local loop" wiring, that final stage of phone circuits that connect the subscriber to the service premises, often with very old copper wiring.
I further thought that one of the wizard wheezes of ADSL is to scavenge bandwidth from the uplink side in order to broaden the downlink side; which is "probably what most people want".

Am I right so far?

if I am right, then is there some sort of "optimising" algorithm that runs in the ADSL modems in order to decide what uplink speed to plump for?
I mean: if the modems are going all-out to get max downlink speed, the logical conclusion is that you will end up with zero uplink. However, that wont work due to the duplex nature of IP traffic. And they dont. you never find a link with less than about 300Kilobits of uplink.
is there then a lower limit of uplink speed that you can set a modem to negotiate for? if so is there an upper limit?

and, most importantly can you program an ADSL modem to increase the uplink speed at the expense of the downlink?

I fully realise that the first answer is "no, you cant", thats certainly what BT sales say. But, if you knew how to drive an ADSL modem properly, could you tweak it for more upload?

The short answer is, as BT says, "No, you can't." The 'A' in ADSL stands for 'asymmetric'. The downlink signal is given the lion's share of the available bandwidth and that's the way it is. You cannot change it.

If your exchange is enabled for it (i.e. 21CN from BT or a suitable LLU operator) and your line characteristics allow, you could use ADSL2+ Annex M which allows a higher speed upstream connection to be negotiated, albeit at the detriment of downstream speeds. Lookg at SamKnows that may not be available on your exchange.

As you're getting 300kbps on a speed tester, that implies that you're on a "home" service with a 448kbps ("raw"rate) upstream. Moving to a"Business or "Pro" service would (line characteristics permitting) increase your upstream to 832kbps("raw" rate) This shojld be available on your exchange but not all ISPs allow this.


If you can post line stats, someone here would be able to confirm what you'd likely be able to get out of your line in both scenarios. Given that the upstream traffic usies lower frequencies and is less suceptible to attenuation by line lenght, you'll probably be able to realise an improvment uinless you're on a very long line.

On domestic-grade ADSL, your short answer is "No".

You can get a higher upload from a Business-grade service, but that's likely to just rob you of some of your download speed instead.

I's suggest you have a chat with one of your knowledgeable neighbours - specifically, this chap, http://sandaysoft.com/contact


Detail, but paraphrasing - The "upload" part of ADSL uses the lowest frequencies, mostly so that you always get an upload of some sort. The download uses the higher frequencies, which tail off first, so further away you are, the slower your download speed. Trading some of the lowest-frequency parts of the "download" section gives you more upload...

thanks for informative & rapid answers
Ionic - I had surmised as such. As you correctly assume, our local exchange is not 21cn enabled yet (although the next one up the line - a 30km oversea hop to Kirkwall - is)

I am indeed on a home BT service.

I had run into this 448kbps number elsewhere. What do you mean by "raw"? is *this* the maximum uplink allowed by the modems?
is the 300kbps uplink that I see the result of line loss from the 448kbps which I would get with an ideal channel to the exchange?
I strongly suspect that this "448" number then is a variable which is programmed into the DSLAMs (?) and could - theoretically - be modified...?

I say DSLAM - I dont really know what Im talking about, I gather from googling that the DSLAM is the device at the excchange that the home ADSL modem communicates with.

It is certainly possible for me to change to a business broadband service using my local exchange, and the BT sales staff did indicate that upstream would increase. I am pretty sure that they will be able to change me to business without sending a man to the excchange therefore they must be able to do this over-the-wire which reinforces my supposition that the rate limits can be tweaked in exchange firmware.

what do you mean by line stats?

on a related matter:
last weekend I was experimenting with uploading this video stream to a content delivery service. I was also messing around with portforwarding (ie, opening unusual ports on my home hub's firewall). after doing this the internet service went unusable (DNS errors) for a few hours but gradually picked up. I phoned BT residential (Indian call center) and they confirmed there was a fault on the line. However I wondered whether BT sensed that I was messing and "shut me down". is this possible?

448 Kbps is the link speed over your copper wiring, and this is talking in ATM protocol. The internet uses IP, so maximum IP will be around 380 Kbps.

On a Max service which you are on, with the right provider you can double your upstream speed for a fee. The regrade to a Max Premium service is usually a remotely controlled thing.

On the Home Hub locking up, if you are playing around with lots of open ports it is possible you may have just crashed the hub, and it recovered after a short while. As for BT shutting you down, nice idea but not something they do.

if you don't know exactly what you're doing, opening random ports is not a good idea.

In reply to a post by MM0TJR:

I had run into this 448kbps number elsewhere. What do you mean by "raw"? is *this* the maximum uplink allowed by the modems?

The 448K is the max upstream connection (sync) speed that BT has defined for its A(asymmetric)DSL Max product throughout the UK. Similarly it has defined 832K for its ADSL Max Premium product. There is nothing you can do to change these, nor do they offer any other products at your exchange (but see below). They are both subsets of the (ANSI/ITU) Standard ADSL (G.DMT) which offers an upstream sync speed of up to 1.3 Meg.

Sync speeds, both up & down, are the raw speeds the bits are carried between your exchange and your modem. These include both the actual data that arrives at your PC, usually counted in Bytes, and all the protocol overheads needed to carry them, like an envelope. Usually the actual data throughput speed is about 83% of sync speed. Hence your upstream speedtest of 300K+.

The modem plays no part in limiting these speeds other than adhering to the Standards and negotiating for a stable connection.

I believe at one time BT offered a Symmetric DSL of 0.5, 1 or 2 Megs, but not sure they still do.

Please post your router stats (see here: How to get your router line stats for your router) so we can see if you are getting what you should.

I doubt BT would shut you down whatever you were doing as long as it was lawful.

Edited by XRaySpeX (Sat 27-Oct-12 01:25:41)