If you've subscribed to an asymmetric DSL Internet connection (ADSL) in the United Kingdom, you may have gone for the service because of a provider's advertised "up to" 13.8Mbps speed.

Alas, according to the latest comparison survey from UK regulator Ofcom, you're probably getting much slower performance—something in the neighborhood of 6.2Mbps. In other words, around 45 percent of the advertised speed.

"Very few ADSL broadband customers achieved average actual download speeds close to advertised 'up to' speeds," the UK regulator says. A mere 14 percent of customers on "up to" 20Mbps or 24Mbps services enjoyed download rates over 12Mbps. Almost sixty percent received average download speeds of 6Mbps or even less.

On the other hand, cable broadband subscribers in the UK are getting speeds that are far closer to the advertised rates.

For example, Ofcom found that Virgin Media's "up to" 10Mbps cable service delivered average speeds that were 96 percent of their advertised "up to" throughput rates (not to mention "significantly" faster than ADSL). The cable provider's "up to" 20Mbps deal averaged at 18Mbps (90 percent of advertised), and its equivalent 50Mbps averaged at 45.6Mbps (92 percent).

British Telecom's Fiber-to-the-Cabinet (FTTC) service is also more accurately delivered to UK premises than is DSL. Ofcom's survey determined that download speeds averaged at around 31.8Mbps—80 percent of the advertised speed.

But BT's upload speeds rule—its 40Mbps offering delivering an average 7.8Mbps, "significantly higher than any other service we measured."

Upload speeds are typically much lower than downloads, of course. Virgin's Media's "up to" 50Mbps upload rate clocked in at 2.8Mbps on average, "with all other ISP packages delivering average upload speeds of less than half this."

Inescapable characteristics

These numbers were crunched for Ofcom by the SamKnows research company. SamKnows conducted a somewhat less-detailed survey of UK broadband speeds around seven months ago that reached similar conclusions. The outfit culled the data for this new report from November 1 through December 15, 2010—conducting 765 million automated tests across a cohort of 1,710 UK residential broadband subscribers.

The company is doing a similar study for the Federal Communications Commission. The FCC announced the project a day after its own research indicated that four out of five Americans have no idea what their actual or even advertised broadband speed is. We asked the Commission when its SamKnows survey will be completed, but we haven't gotten a response yet.

Meanwhile Ofcom says it's happy about the faster cable and fiber speeds, but notes that most broadband in the UK still gets delivered through ADSL—in other words, via copper lines originally designed for phone services "which have been stretched to the very edge of their capability in order to provide broadband."

An inescapable characteristic of ADSL broadband is that performance is constrained by the length and quality of the copper line. Our research finds that for many consumers the speeds available to them via ADSL are not sufficient for a high-quality experience of high-bandwidth services such as internet TV, or for connecting multiple devices to the Internet.

As a consequence of this, Ofcom recommends the following additions to the UK's Voluntary Code of Practice on Broadband Speeds: