About GSM spec.

In GSM spec 0505 ,

a. Spectrum due to the switch , why the power measured is 400/600/1200/1800Khz from channel , not other frequence offset?

b. Specturm due to the modulation , why the power measured is 100/200/250/400/>600Khz from channel,not other frequence offset?

please post the GSM here, i canot judge what y said

If u learned about the GSM 0505 , you can do it . as the apec. is so long , it's not convenient to paste it here.