Vodafone backflipped on its plans to introduce per-MB charging on its prepaid plans yesterday. Given many plans offer gigabytes of data, why does this actually matter?
The decision for Vodafone to flip back to per-KB charging on its plans is quite a welcome one, but not for the reasons you may be thinking. There's a widespread belief that per-unit charging -- whatever the unit of measurement is -- means that your account gets charged that unit for every online activity. Send ten emails from your phone on a per-MB plan, and it'd cost you 10MB and so on. This isn't generally so, but there's still a problem with charging at a per-MB level.
Evan Smith outlines the exact nature of the problem over on his blog. What you have to be careful of is your data session terminating; at that time, your telco will assess how much actual data you've used, and then round it up to the per-unit price. So if you've only used 0.5MB, it will be charged as a full 1MB. Most phones should try to keep a data connection active for push services such as email and social networking, even if they're not using much data.
Where this gets tricky is that it's very easy to drop on and off networks without realising it; Smith's example is of travelling on a train through Sydney's train network, picking up a connection at each station even if you're not actually getting anything done, and racking up the megabyte charges as you go. Charging at a per-Kb rate doesn't stop this issue, but it makes it significantly less of a problem for customers, as you're more likely to get the actual data you've paid for.
Data Sessions On Your Mobile [Evan Smith]