tween the re- authority, the remainder being reserved for GSM. Subsequently an ceived SAT and that expected, the audio path is muted to prevent the additional allocation of channels below the existing TACS channels interfering signal from being overhead. If the condition persists, the was made, namely the Extended TACS (ETACS) channels, and the call is aborted. standard was modified accordingly. AMPS underwent a long development period, and an extended TACS equipment availability and cost have both benefitted from trial (technical and commercial) which not only fixed the system the standard's similarity to AMPS, and TACS systems have been parameters but also contributed to the basic planning rules which adopted by several European countries (UK, Eire, Spain, Italy, hold true for all cellular systems. The system design was compre- Austria and Malta), in the middle east (Kuwait, UAE and Bahrain) hensively described in 1979 (Bell, 1979), but it was not until 1983 and the far east (Hong Kong, Singapore, Malaysia and China). In Detailed description of GSM 47/9 Europe, TACS is on an equal footing with NMT in terms of installed The time delay between a base station transmitting a data burst customer base. A variant of TACS (called J-TACS) has also been and receiving the response from the mobile is measured at the adopted in Japan. base station and used to calculate the distance between them. This distance is also taken into account in handover determina- 47.4.3 tion. 4. The data is used as a timing reference by the mobile to lock its NMT stands for Nordic Mobile Telephone (system), and was de- internal clocks. veloped jointly by the PTTs of Sweden, Norway, Denmark and Finland during the late 1970's/early 1980's. The system was de- C450 contains a number of advanced features made possible by signed to operate in the 450MHz band, and was later adapted to also the application of current developments in technology. Although use the 900MHz band. Although NMT was developed after AMPS, speech transmission is analogue, it can b