an office without the need to move all the wiring. It also provides almost instant access for people bringing laptop computers into an office to connect without having to make physical wire connections each time. Ad hoc networks may also be used. Here computers and peripherals may be brought together and communicate with each other without being part of a larger system. This type of configuration does not need the use of an access point. For this type of system, algorithms have been 286 Newnes Guide to Radio and Communications Technology Wired backbone distribution system Access Access point (AP) point (AP) Approximate limit of access point communications Figure 11.1 IEEE 802.11 system as used to provide connectivity for a large network system written to enable one of the computers to elect a base station or master, with the others remaining as slaves. This type of network may be established when several people come together and need to share data between their computers or if they need to access a printer without the need to use wire connections. Unlike Bluetooth, 802.11b uses direct sequence spread spectrum. This allows it to operate in an environment with other users, provided that the levels of interference do not rise too high. A variety of different types of modulation are used. Systems running at 1 Mbps use BPSK (binary phase shift keying) modulation, and those running at 2 Mbps QPSK (quaternary phase shift keying) modulation. For higher data rates, systems running at 5.5 Mbps and 11 Mbps use CCK (complementary code keying) and QPSK modulation. CCK involves 64 unique code sequences, each of which supports 6 bits per codeword. The CCK codeword is then modulated onto the RF carrier using QPSK, and this allows another 2 bits to be encoded for each 6-bit symbol. In this way each 6-bit symbol contains 8 bits, i.e. 1 byte. Around the world different countries have slightly different legislation and frequency allocations. As a result there are differences between the channels and hence centre frequencies that are