GSM performs SDMA using cells with BTS and assigns an MS to a BTS. Furthermore, FDD is utilized to separate downlink and uplink. Media access combines TDMA and FDMA. In GSM 900, 124 channels, each 200 kHz wide, are used for FDMA, whereas GSM 1800 utilizes, 374 channels. Typically, 32 channels are reserved for organizational data; the remaining 90 are used for customers. Channels 1 and 124 are not used for transmission in GSM 900.