GSM900 refers to the GSM system operating in the 900 MHz frequency range, encompassing the primary GSM band (GSM-P), its extension (E-GSM), and the segment reserved for railways (R-GSM). The standard defines the GSM900 band spanning 876 – 915 MHz for uplink and 921 – 960 MHz for downlink, with mobiles transmitting in the lower band and base stations in the upper band. In practical terms, GSM900 typically refers to the portion used by GSM operators for public services, excluding the R-GSM band, which covers the range of 880 – 915 MHz (uplink) and 925 – 960 MHz (downlink).