KEY POINTS:
So, you've got your super-duper ADSL2+ broadband internet connection with its potential speed of 24 megabits a second - and you're wondering why it's not delivering web pages to your desktop at quite that rate? Welcome to the world of contention rates, backhaul and bandwidth bottlenecks.
"Oversold" is the word that springs to mind in relation to internet service providers' latest broadband offerings. A gullible media - and I plead guilty here, too - must also cop some of the blame for building up unrealistic expectations of the new (in New Zealand, at least) generation of DSL services.
But who can blame us all for getting a little excited in a land so starved of bandwidth for so long?
The trouble is, the latest services are coming to us on the same creaky old copper phone lines that were installed decades ago for the purposes of voice communications. You have to admire the ingenuity of telecoms engineers who have managed to come up with ways of getting the same lines to carry internet traffic at faster and faster speeds.
But there is a limit, and that is what we are up against now. More accurately, there are several limits, and we're feeling the effects of them all.
The first is the length of your phoneline: DSL services, which most New Zealand broadband subscribers have, slow down the further you are from the telephone exchange.
For faster flavours of DSL, such as ADSL2+, this is even more marked: at a distance of 2km from the exchange, the top speed you can expect is less than half the theoretical 24Mbit/s.
When ADSL2+'s still-faster sibling, VDSL2, arrives (Vodafone and Orcon last week said they'd begun VDSL2 trials), the slowdown with distance will still be noticeable: its 52Mbit/s top speed is much reduced beyond about 500 metres from the exchange.
In other words, DSL is a clever way of getting a tired old phone system to do the work of an internet-age data network, but only if it's not asked to carry lots of video and do other high-bandwidth tasks.
The next limit is the number of subscribers among whom your ISP is dividing the available bandwidth - a measure known as the contention ratio.
If you're sharing the bandwidth with 19 other subscribers (a 1:20 contention ratio), you'll be better off than if you and 49 others (a 1:50 ratio) are fighting it out to download the latest YouTube sensation.
The irony is that as an ISP's popularity rises, its network speed falls, unless its investment in bandwidth is staying ahead of the rate at which it is adding subscriber numbers. In my experience, service tends to get worse before it gets better. And ISPs tend not to be too forthcoming with their contention ratios.
Two other factors are conspiring to make your new broadband connection less speedy than you were led to expect. One is the amount of backhaul bandwidth your ISP has from the exchange to the wider internet. The other - and a particular issue in New Zealand - is how much bandwidth your provider is prepared to fork out for on the Southern Cross submarine cable (half-owned by Telecom) linking us with the rest of the world.
The choking effect of these bottlenecks is particularly noticeable at rush hour on the information superhighway. And that, says Michael Cranna, of broadband performance measurement company Epitiro Technologies, reliably begins around 3.30pm each day - the time when tens of thousands of school kids get home.
"Basically," says Cranna, "at 3.30pm the whole thing slows down, and it stays that way till about 8pm. You can see the pattern in the data - clear as day."
Epitiro keeps an eye on what's happening at 11 sites around the country, from where it tests broadband speeds for the five biggest ISPs and a selection of second-tier providers.
It does so in Britain, too, where it provided the Advertising Standards Authority with data that led to Virgin Media last month being ticked off for making misleading download speed claims.
The company also provides a free tool, Isposure, that lets individual broadband users tell exactly what performance they're getting from their ISP and easily compare that with the speeds enjoyed by other ISPs' subscribers in their vicinity. Once Windows-based Isposure (download it at isposure.co.nz - there's no Apple version) is loaded on your PC, it will test your line speed, web browsing speed, responsiveness for gaming and web address look-up speed.
Cranna says this is more information than ISPs themselves have about how your connection is performing. They probably like it that way. If they really knew, it might cramp their style in the claims they make for their services. Then again, maybe it wouldn't hold them back.
Anthony Doesburg is an Auckland technology journalist
