Why are there high and low bands? eg DCS/EGSM?

[SeriousTyro]

I'm wondering what the main reason is for having dual bands high and low ?

Can the phone use both high and low band at the same time, sort of like LTE carrier aggregation?
If so then there would be more BW so more data can be transferred?

 

[Mityan]

If you mean 900/1800 MHz, then the phone can use only that frequency which the base station allows him at the exact time. The band of GSM signal is 200 kHz, and increasing of throughput rate is provided by using several time slots if it is possible (if they are free from other users).

 

[SeriousTyro]

I see but why is it necessary to have a high band and a low band in a region? Like in the Americas for 3G Bands 2(High) and 5(Low) are used.