3g - What does "H" mean in the signal indicator?

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I understand that the colour of the signal indicator tells me whether or not I've got a connection to Google servers or not, but what is the "H" that sometimes pops up?

Usually my indicator says "3G":

But, in addition to having an "H" now and again, sometimes it has no letters at all:

I assume this is telling something about what kind of data network I'm on, or not on, but what exactly is happening?

Answer

It's HSDPA which if I'm not mistaken is referring to 3.5G.