ure of mobile communications equipment using standardised technology in larger volumes; and 'open' telecommunications industries with increased competition in the supply of network equipment and handsets. These same forces have continued to drive further convergence in the context of 3G and 4G technologies. Convergence of technologies has been one of the main reasons driving the reduction in the cost of mobile devices and mobile communications tariffs over the past three decades. Backwards Compatibility Mobile networks and mobile devices are usually backwards compatible with earlier standards and/or technologies because: a network operator is obliged to continue to serve mobile devices using older technologies; network roll-out is a process that may take several months or years and a network operator needs to support multiple technologies during this process; and mobile devices need to operate with many older technologies because mobile networks are not homogeneous in many parts of the world. A mobile device may need to roam to a net- work that uses an older technology, for example because there is no network coverage by the newer technology, including temporarily when there is insufficient capacity on the newer technology network. In practice, a base station and a mobile device communicate with each other and inform each other of the standards they support. This allows both to communicate using the standards they share. Terrestrial wireless telecommunications systems resolve two challenges: one operating in channel conditions where signals are blocked by physical objects, and where multiple signal arrivals can lead to fading. Furthermore, multiple users transmit together and therefore their signals mutually interfere. The other is allocation of scarce bandwidth resources among large number of simulta- neous users. Technology development over some four decades has not only made terrestrial wireless telecommunications possible, but highly competitive with fixed broadband. Review Questions 1. What was the fundament