ains the user data, the envelope P.O.Box VIA AIR MAIL Sydney South carries the information which ensures the data reaches Australia the recipient. The envelope must therefore carry the address of the recipient as well as the sender. It must further inform the post office how the letter is sent (express, standard, etc.), the date it is posted and should it be returned after a certain waiting period and to whom, and SO on. The envelope may even indicate which post office it may not travel through. It may also carry information on the language used, and whether any processing of the data (including encryption) has been carried out. A data packet is constructed similarly. Each data packet needs to carry information that ensures its correct delivery. This information is sometimes known as control data. The struc- ture of a data packet, including control and user data is shown in Figure 8.3. A packet 'header' field acts as the 'envelope' and contains all control information to ensure desired delivery of the data payload. This control information may include one or more of the following fields: Source address Destination address Total packet length Number in the series Time to live Protocol (language) used The user data (information) is the payload of the packet and is usually of a limited size and depends on the protocol being used. A 'tail' field may be included to inform the receiver of certain extra control information (such as to indicate that a data payload has ended, include a check sum for error detection, and/or to act as a buffer between two successive packet transmissions). Header Information data Figure 8.3 Structure of a packet-switched data transfer Case Study 8.2: Internet Protocol and OSI Packet-switched data communications in the late 1970s held the promise of a merger between two fast growing industries: telecommunications and computers. A number of companies, including network operators and computer manufacturers and service pro- viders, had a major interest in the emergence of a global standard, and 