reserve power in the reflection it will be Chapter Five Angular offset 1/3° (exaggerated) Collimating Gathering source Beam offset 6cm detector at 10 metre distance Figure 5-5: Focussed In Link Alignment over a 10 m Range important to use a high reflectance, low absorption material to reflect the beam. A white painted ceiling is a good reflector of sunlight, with a reflection coefficient for visible light of around 0.9, but is a poor reflector at Ir wavelengths, absorbing about 90% of the incident Ir radiation. To achieve a comparable 0.9 reflection coefficient for Ir, an aluminium or aluminium foil covered panel would be a suitable reflector. Summary of Part II The radio frequency or infrared communication technologies described in Part II are at the heart of the physical layer of wireless networks. An understanding of the basics of these technologies will provide a firm foundation from which to discuss the implementation of wireless networks, whether local, personal or metropolitan area (LAN, PAN or MAN). In particular, the link budget calculation will be important in establishing power requirements and coverage in LAN and MAN applications. Spread spectrum and digital modulation techniques are key to understanding how a wide range of data rates can be accommodated within the limited available bandwidth, for example in the 2.4 and 5.8 GHz ISM bands. Ultra wideband radio - using a bandwidth approaching 7 GHz with a transmitted power density below the FCC allowable noise emission level - stretches the conceptual boundaries of what radio frequency communications can achieve, and the increasing number of practical applications including wireless USB and ZigBee, are testament to the practicality of this remarkable technology. Infrared Communication Basics Infrared communication links are perhaps the most "transparent" in terms of a very low requirement for user configuration, and to some extent this means that the user can be unconcerned about the underlying technology. However, even infrared links can be stretched t