Upon completion of this 'hand-shaking', packetised data communications between the two devices commence. This process is illustrated in Figure 12.5. Request Clear (RTS) (CTS) Figure 12.5 CSMA-CA process in wireless LAN systems Personal Wireless Communications Systems Access point Figure 12.6 Wi-Fi transmission through access point wireless LAN centralised control In systems with a centralised access point, the access point acts in a similar way to a base station regulating which user devices may transmit at any particular interval through a polling process. When a device is in the proximity of an access point, it responds to the polling signal when it has traffic to send. Based on this signalling channel resources are shared among all active Wi-Fi equipment. This process is illustrated in Figure 12.6. This centralised control mechanism is now the most common 802.11 network configuration: while the peer-to-peer networking mechanism is fully specified, it is rarely utilised. Case Study 12.1: Wi-Fi Applications and Roll-outs Although Wi-Fi systems are generally designed to operate for short range communica- tions, they can and have been proposed for long range and compete with mobile tele- communications systems such as GSM or CDMA. The main driver in the early 2000s was the significant cost advantage that Wi-Fi systems enjoyed in comparison with GSM or CDMA base stations. The difference in cost was at times several orders of magnitude. Moreover, the transmission rate capability of IEEE 802.1 11a or 802.11g was up to Content and services layer 54 Mbps which was several times higher than what a GSM or CDMA standard base station could support. A number of operators consid- Connectivity retail layer ered rolling out Wi-Fi networks to provide a broadband infrastructure. Indeed a number of Infrastructure layer city-wide networks have been and continue to be rolled out. These systems rightly belong to the infrastructure layer of the telecommunications ecosystem. Subscriber/end-user Broadband Telecommunications Technologie