ree classes are defined in the specification, 1 mW, 2.5 mW and 100 mW. Most are either the first two classes that provide ranges of up to 100 mm and 10 m respectively. In order to enable effective communications to take place in an environment where a number of devices may receive the signal, each device has its own identifier. This is provided by having a 48-bit hard wired address identity giving a total of 2.815 X 10 1 unique identifiers and this should be quite sufficient! 284 Newnes Guide to Radio and Communications Technology Using Bluetooth there are two types of link that are possible. There is the Asynchronous Connectionless Communications Link (ACL) and this is used for file and data transfers. A second form of link is known as a Synchronous Connection-orientated Communications Link (SCL) and two of these are available. These are used for applications such as digital audio. The asynchronous link supports a maximum data rate of 732.2 kbps in an asymmetric mode, whereas in a symmetrical mode running the same data rate in both directions this rate is reduced to 433.9 kbps. The synchronous links support two bi-directional connections at a rate of 64 kbps. The data rates are adequate for audio and most file transfers. However, the available data rate is insufficient for applications such as high rate DVDs that require 9.8 Mbps or for many other video applications including games spectacles. Data is organized into packets to be sent across the link. The Bluetooth specification lists 17 different formats that can be used dependent upon the requirements. They have options for elements such as forward error correction data and the like. However, the standard packet consists of a 72-bit access code field, a 54-bit header field, and then the data to be transmitted which may be between 0 and 2745 bits. This data includes the 16-bit CRC if it is needed. As it is likely that interference will cause errors, error handling is incorporated within the system. For asynchronous links packet sequence numbers are transmitted.