not very difficult to obtain a scanner that does not have these restrictions; it is also possible to re-program a scanner SO that the 'deleted' bands can be accessed. Later versions of the NMT specifications defined optional analogue encryption, which was based on two-band audio frequency inversion. If both the base station and the mobile station supported encryption, they could agree upon using it when initiating a phone call. Also, if two users had mobile stations supporting encryption, they could turn it on during conversation, even if the base stations did not support it. In this case audio would be encrypted all the way between the two mobile stations. While the encryption method was not at all as strong as encryption in newer digital phones, it did prevent casual listening with scanners. The cell sizes in an NMT network range from 2 km to 30 km. With smaller ranges the network can service more simultaneous callers; e.g. in a city the range can be kept short for better service. NMT used full duplex transmission, allowing for simultaneous receiving and transmission of voice. Car phone versions of NMT used transmission power of up to 6 watts and handsets up to 1 watt. Signalling between the base station and the mobile station was implemented using the same RF channel that was used for audio, and using the 1200 bps (bits per second) FFSK modem. This caused the periodic short noise bursts that were uniquely characteristic of NMT sound. 1.2.2 AMPS (Advanced Mobile Phone System) The first cellular licences in the US were awarded in 1981, and the cellular services started in 1983 in Chicago and The Baltimore-Washington area using the AMPS. The AMPS was based on the FDMA (frequency division multiple access) technology, which allowed multiple users in a cell or cell sector. Initially, cell size was not fixed and an eight mile radius was used in urban areas and a twenty-five mile radius in rural areas. However, as the number of users began to increase, new cells were added. With the addition of every new cell, the fre