l office to the transmitter site. As there is no incremental cost for the transmissions that are made, individual calls are not charged, but instead there is a rental for overall use of the system. For those users with their own licences they naturally have to pay for the licence and the cost of the equipment. While many systems operate with the remote or mobile stations being able to hear all the calls being made, this may not always be satisfactory and a system of selective calling may be required. There are two ways of achieving this. One is to use dual tone multiple frequency (DTMF) signalling whereas the other uses the continuous tone coded squelch system (CTCSS). DTMF is a system that is widely used for telephone signalling and is almost universally used for touch tone dialling for landline telephones today. Set pairs of tones are used to carry the information. The eight frequencies used are 697, 770, 852 and 941 Hz (termed the 'low tones') and 1209, 1336, 1477 and 1633 Hz (termed the 'high tones'). One high and one low tone are used and the combinations are used to represent different numbers. The relevant code consisting of one or more digits is sent and the station is programmed to respond to the number, typically one or two digits respond by opening the squelch on the receiver to let the audio through. The disadvantage of this system is that if the receiver does not pick up the code at the instant the DTMF signalling takes place then it will not respond to any of the message. This can be a significant disadvantage because mobile stations often lose the signal for short periods as they are on the move. The other widely used system is CTCSS. This is also called sub-audible tones or PL tones (a Motorola trademark). It is a system where a 240 Newnes Guide to Radio and Communications Technology subaudible tone (below about 250 Hz) is transmitted in addition to the normal voice channel. Only when the correct tone for the required station is transmitted will the squelch for that receiver be opened and the tran