h no transitions in the output. Chapter Two Data bits Non Return to zero (NRZ) Non Return to zero inverted (NRZI) period Figure 2-12: USB NRZI Data Encoding Scheme NRZI has the advantage of a somewhat improved noise immunity compared with the straight encoding of the input data stream as output voltages. Physical Layer Technologies - Wireless Networks The PHY layer technologies that provide the Layer 1 foundation for wireless networks will be described further in Parts III, IV and V, where LAN, PAN and MAN technologies and their implementations will be covered in detail. Each wireless PHY technology, from Bluetooth to ZigBee, will be described in terms of a number of key aspects, as summarised in Table 2-8. The range and significance of the issues vary depending on the type of technology (Ir, RF, Near-field) and its application (PAN, LAN or WAN). Operating System Considerations In order to support networking, an operating system needs as a minimum to implement networking protocols, such as TCP/IP, and the device drivers required for network hardware. Early PC operating systems, including Windows versions prior to Windows 95, were not designed to support networking. However, with the rise of the Internet and other networking technologies, virtually every operating system today qualifies as a network operating system (NOS). Individual network operating systems offer additional networking features such as firewalls, simplified set-up and diagnostic tools, remote access, Wireless Network Logical Architecture Table 2-8: Aspects of PHY and Data Link Layer Wireless Technologies Technology aspect Issues and considerations Spectrum What part of the electromagnetic spectrum is used, what is the overall bandwidth available, how is this segmented into channels? What mechanisms are available to control utilised bandwidth to ensure coexistence with other users of the same spectrum? Propagation What power levels are permitted by regulatory authorities in the spectrum in question? What mechanisms are available to control the tra