systems and have already become well established in fixed System Technical Journal, January. networks for the provision of 'free fone' or 'toll-free' type services, BS (1990) Total Access Communication System (TACS) BS6940 for example. However, the ability of an IN architecture to provide Parts 1 & 2. customised services is particularly valuable to a mobile user, who Hansen, S. (1988) Voice Activity Detection (VAD) and the Oper- can have improved control over the handling of incoming calls. IN ation of Discontinuous Transmission (DTX) in the GSM Sys- techniques also provide the ability to create a wide variety of tem. In Proceedings of the Digital Cellular Radio Conference, advanced services. October 12-14, Hagen, Germany. Second generation cellular systems such as GSM are already Hata, M. (1980) Empirical Formula for Propagation Loss in Land designed around an architecture which can support IN type applica- Mobile Radio Services, IEEE Transactions, VT- 29, (3), Au- tions. In particular, the HLR function is closely related to the IN gust. service control point. We can expect further developments in the Hodges, M.R.L. (1990) The GSM Radio Interface, Br Telecom near future which will bring a range of IN features to both the Technology Journal, 8, (1). mobile user and the service provider. Mallinder, B.J.T. (1988) An Overview of the GSM System. In Proceedings of the Digital Cellular Radio Conference, October 47.7.3 Personal communications 12-14, Hagen, Germany. Okumura, Y. et al. (1968) Field Strength and its Variability in u.h.f. The term PCN, Personal Communications Network, is used widely and v.h.f. Land Mobile Radio Service, Review of the Electrical in the UK, whilst PCS, Personal Communications Services is used Communication Laboratory, (6). in the USA. Both aim at the same objective of serving the mass Oschner, H (1988) Overview of the Radio Subsystem. In Proceed- consumer market with mobile communications. The key challenge ings of the Digital Cellular Radio Conference, October 12-14, is to provide a very hig