Analogue networks are becoming obsolete and are being replaced by digital cellular networks. There are no analogue networks operating in the UK and the frequency bands that they used to occupy are now reassigned to GSM operators.
However some countries still have an analogue service based on
AMPS or TACS. These provide national coverage with roaming facility and hand-off from cell to cell as a mobile travels across the network, calling for more complex signalling than local trunking.
The control system must know at all times the location of all operational mobiles. It does this by continuously monitoring signal strength and instructing the mobile to change channel as it crosses cell boundaries.
To permit the maximum re-use of frequencies mobiles are instructed by the system controller to reduce transmitted power to the minimum necessary for acceptable communications in either eight or five steps, depending on the mobile classification, to 0.01 W. Mobiles are classified by their maximum effective radiated power output:
Class 1 10 W erp, vehicle mounted Class 2 4 W erp, transportable Class 3 1.6 W erp, transportable Class 4 0.6 W erp, hand-portable
The channel separation is 25 kHz with a peak speech deviation of ±9.5 kHz. Signalling is by Manchester encoded PSK at 8 kbit/s with 6.4 kHz deviation.
To discriminate between stations transmitting on the same RF channel an FM supervisory audio tone (SAT) is transmitted concurrently with speech at ±1.7 kHz deviation. There are three such tones: 5970, 6000 and 6030 Hz.
An FM signalling tone (ST) of 8 kHz transmitted for 1.8 seconds (deviation=±6.4 kHz) indicates the clear-down of a conventional telephone call.
The advanced mobile phone service (AMPS) is the American cellular system designed for 30 kHz channel spacing with a 10 kbit/s signalling rate. TACS is based on the AMPS system but modified for reduced channel spacing. Narrower channel spacing systems (NAMPS and NTACS) are also found in some countries.