ication level cascade down through the logical layers and result in a series of data frames to be transmitted across the physical network medium. In a wireless network that physical layer is provided by radio frequency (RF) or infrared (Ir) communications and in Part II the basics of these methods of wireless communication will be covered. Starting with the RF spectrum, the regulation of spectrum use is briefly described and spread spectrum techniques are then introduced. This is a key technology that enables high data link reliability by making RF communications less susceptible to interference. Multiple access methods that enable many users to simultaneously use the same communication channel are then discussed. Signal coding and modulation is the step that encodes the data stream onto the RF carrier or pulse train, and a range of coding and modulation techniques applied in wireless networking will be covered, from the simplest to some of the most complex. The various elements that impact on RF signal propagation will be described, enabling a calculation of the link budget - the balance of power available to overcome system and propagation losses to bring the transmitted signal to the receiver at a sufficient power level for reliable, low error rate reception. The link budget calculation is an essential part of the toolkit of the wireless network designer in defining the basic power Part Two requirements for a given network installation. This will usually be supplemented by practical techniques such as site surveys, which will be covered in Parts III and V. Ultra wideband (UWB) radio has emerged as a key technology for short range wireless network applications, and some of the varieties of UWB radio that are applied in wireless USB, wireless Fire Wire and ZigBee will be described. Chapter 5 gives a similar overview of aspects of infrared communications, as used in IrDA connections, covering the infrared spectrum, Ir propagation and reception, and Ir link calculation. CHAPTER Radio Communication Basics The RF Sp