Need an explanation on data throughput versus bandwidth and frequency

[sinnadyr]

I am trying to understand what difference the frequency of a carrier channel versus the bandwidth of the channel is related to the actual speed of a network link. I see that for 2.4GHz most systems operate at much larger frequencies than on e.g 868MHz, but sometimes, e.g with ZigBee, they use transfer rates up to 250kb/s on ALL frequencies. What also confuses me is that there are so many different speed for different systems at 2.4GHz.

Is it so that data rate is only decided by the modulation scheme of the digital signal? How much does bandwidth (i.e. frequency max - frequency min) impact the data rate?

Thank you very much in advance