Quad band refers to mobile phones which support 4 frequency bands, to allow global roaming.
GSM initially started in Europe with 2 frequency bands: one in 900MHz range and 1800 MHz. When GSM technology crossed into America, two extra frequencies were allocated (since the existing European frequency bands were already allocated to devices by the FCC).
GSM pioneered SMS (Short Message Service) or "text messaging" which has become incoporated into other mobile standards as well.
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