What does "H" mean in the signal indicator? 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?
Let's state all the possible letters in the signal indicator, in descending speed (indicated speeds are roughly theoretical maximum):

5G = 20 Gbps
LTE+ = 300 Mbps
LTE = Long Term Evolution (commonly and falsely known as 4G), 100 Mbps
H+ = HSDPA Plus, 10 Mbps
H = either HSDPA (3.5G) or HSPA+ (3.95G), depending on your network provider, 2 Mbps
3G = UMTS (3G), 384 kbps
E = EDGE (2.5G), 40 kbps
G = GPRS (2G), 14 kbps
(no letter) = voice only, no data connection, 2.4 kbps

and also:

R = roaming (this means you're not on your home network)

(source for some of this.)