Since, I used the GSM 1800, the signal of GSM 1800 is less than GSM 900 when I was in the room. I wonder about it, Hense, I try to ask the engineers and find the informations, then I got the answer from WiKiAnswer. Because, GSM 900 can hold more no. of subscribers as compared to GSM 1800 & 1900. Also, since c= f x (lambda) Which implies smaller frequency means longer wavelength and longer wavelength would travel a longer distance than a short wavelength in same environment requiring less transmit...
