Is it just me, or is 3G either really good or really bad, but rarely "OK"

I've started noticing that my experience of mobile broadband (iPhone 3GS on Voda UK) is much more polarised than that of fixed broadband.

With ADSL, performance seems to vary over a fairly wide spectrum of possibilities, because of the vagaries of contention at my local exchange's DSLAM, the connection of the server at the other end, and especially WiFi in different parts of my house. Overall, it's generally OK - I've had one major outage in 5 years (after a massive fire/flood at a BT Exchange that took out half of central-west London's phones).

But on mobile, it seems to be all or nothing. Either very good... or frustratingly bad.

I've been playing around with the Speedtest app on my iPhone (3GS, on Voda UK). And the results are really polarised. The downlink speeds (in Kbit/s) for my last 20 or so tests:

1940
1238
1 (yes, really - and I got 236kbit/s uplink!)
2638
13
2249
36
1554
33
690
1125
75
1088
717
3465

In other words, I either get 1-2Mbit/s+, or less than 100kbit/s (and not on EDGE, either, according to the indicator). I've only got two "middling" results around 700kbit/s, and none at all in the 200-500kbit/s range. Several times I've had results that make GPRS look poor. On several occasions recently I've had to switch the radio on/off via the "flight mode" switch to see if it helped to re-register. A couple of times I've done a full on/off of the phone.

Now, I recognise that I probably tend to do speed tests when I'm most frustrated, so there's a bias there. There could also be some bugs in the phone's firmware. But it's really conspicuous that I almost never get a "sub-par, but sort-of OK" performance. It's essentially all or nothing, even when I supposedly actually have coverage.

That's a very different user experience to fixed broadband.

I'm sure there are commenters who have much more test/measurement expertise than my amateur armchair efforts. But to my uneducated eye, it looks like there are problems beyond raw air-interface capacity. I'd expect a much more graceful decline in performance if it was just due to 2, 5, 10 or 100 other people in the same cell at the same time checking Facebook at the same time as me.

Is it a glitch somewhere in the RNC or GGSN? Who knows? Am I being policy-managed in response to my usage (middling, maybe 300-500MB/month) or my coruscating blog post on Voda's roaming practices a few weeks ago? [No conspiracy stories here to be fair - the variability was there long before].

Either way, it can't be long before we get some massive crowd-sourced programme to track network quality and deconstruct the root causes - the open question in my mind is whether this happens before or after the operators themselves get on top of it.

The theme of using DPI, probes and data-mining to analyse network and service-assurance problems is one of the top 10 technologies I introduce in my recent research paper on Mobile Broadband Traffic Management.