s shared between sev- eral organizations; Introduction to Telecommunications Network Engineering National or even international coverage; Standardized multivendor equipment; Support of the VPN for each user organization of the network, each of which can modify their resources, such as the usage of channels (mobile-to-mobile, mobile-to-base station) and priorities; Each user organization has its own "dispatcher station" from which an operator can communicate with all terminals; A number of channels that can be permanently or temporarily allo- cated for a certain organization (quarantined share of recourses); Open channel (mobile-to-mobile and point-to-multipoint) commu- nication supported; Prioritization of organizations and user groups; The standardization of TETRA took place after GSM was up and run- ning, and the requirements were slightly different from those for public cel- lular networks. The standardization work was carried out by ETSI, which had also specified the GSM. For these reasons TETRA technology is closely related to that of the GSM but differs in details. Some key technical specifications of the TETRA system are as follows: TDMA/FDMA channel access method; A 25-kHz carrier spacing (FDMA); Four user channels per carrier (TDMA); Frequency-division duplex (FDD) principle with 10-MHz duplex distance; A 28-Kbps maximum user bit rate (all four time slots of one carrier used by a single user), packet- or circuit-switched; Speech coding at the 4.8-Kbps data rate. 5.4.3 Radio Paging Paging systems are simplex systems and they transmit short texts or simply generate an audible beep. Pagers are small and inexpensive wireless commu- nication devices that are used by subscribers to receive messages without dis- turbing their current activities. There are two basic types of radio paging networks, on-site pagers and wide-area pagers. On-site pagers cover a local Mobile Communications area like a building or a hospital. Wide-area pagers may cover a whole country. New paging technologies are available such as the 