Cellular Radio, mobile radio telephone system, which has rapidly supplemented landline telecommunications as a means of two-way personal communications.   Cellular radio works on the principle and uses the physics of two-way radio communications and is named after the unit “cell” into which an area is divided. Each cell has a radius of about 1.5 to 2.4 km (1 to 2.5 mi) and is equipped with a radio transmitter that employs its own range of radio frequencies. The same range is then repeated several times across a land mass. As a mobile radio telephone moves through this pattern of cells, its user’s calls, made as on an ordinary telephone, are switched from one cell to the next by a computerized system. By 2001, there were over 40 million users of mobile cellular phones in the United Kingdom, with four network operators providing services: Cellnet, One 2 One, Orange, and Vodaphone. Cellnet was bought by British Telecom in July 1999. The United Kingdom was one of the first countries to offer personal communications network (PCN) services, operating on a frequency of around 1.8 GHz. PCNs, and other mobile phones operating around 900 MHz, use a world standard digital technology GSM or Global System for mobile communications. This enables the mobile phone to be taken across country boundaries and still operate where a compatible network exists. The growth of the Internet has spurred on the development of fast data access technology for mobile phones. The GSM standard has been extended to enable Internet service to be delivered over a GSM network. This extension, known as the General Packet Radio Service (GPRS), allows mobile devices to send and receive packets of information just like a computer connected to a telephone line. The rate at which this exchange of information occurs is set to rise rapidly over the coming years; GPRS provides a 64kbit/sec channel which is shared among users in a cell but the Universal Mobile Telecommunication Service (UMTS) will increase the available data rate by a huge factor, to between 1 and 2Mbit/sec. At the same time that mobile data networks are introduced, the capability of mobile devices will grow. Early 2000 saw the introduction of "WAP enabled" mobile phones. These are capable of retrieving information from the Internet using the Wireless Access Protocol, which is designed to allow a mobile device to exchange packets (and hence, information) just like a static computer. Because the display on a mobile phone is limited, the information delivered has to be restricted and this is what WAP, and its associated protocols, does. BT Cellnet became the first mobile phone operator to offer GPRS phones in the UK in 2001, while later the same year the world’s first UMTS (“3G” or “third generation”) mobile phone service was launched in Japan by NTT DoCoMo. 3G, which enables callers to see each other, or view video footage, via a tiny screen incorporated into the handset, was introduced in the United Kingdom in March 2003. The first network to offer the service was Hutchison 3.