s: X.400, the message handling system (MHS) of ITU-T: electronic mail; IS 8571, file transfer access and management (FTAM) of ISO: file transfer protocol of OSI; FTP and other application layer protocols of the Internet: see Section 6.6. FTP is not defined strictly according to OSI. The importance of the OSI protocols just discussed is decreasing as the Internet expands. OSI protocols are official standards; to meet all needs they Introduction to Telecommunications Network Engineering are very complex, and their usage is restricted to public telecommunications networks. However, their design principles are valid for all protocols and that is why we have spent some time with OSI. OSI is also valuable model for analysis and comparison of different protocols. 6.3.4 TCP/IP Protocol Stack Instead of OSI protocols, a major share of data communications use TCP/IP, which is used in the global Internet. The relationship of TCP/IP to OSI lay- ers is shown in Figure 6.11 [3]. The Internet as a network, its services, and its operation is introduced in Section 6.6. Section 6.6 also illustrates operation of the most important protocols belonging to the TCP/IP protocol stack. Those readers who are not familiar with any data communication pro- tocol may have found our discussion quite abstract. To make the operation of protocol layers more concrete, we illustrate in the next section how actual data packets are handled when they are transferred down and up through the protocol stack. Further clarification is given in Section 6.6, in which the TCP/IP stack is described from the bottom up. 6.3.5 Data Flow Through a Protocol Stack Let us assume that the user of the source machine performs an action that creates the message, M(A), which is produced by a process running in the application layer (OSI layer 7) in the source machine (see Figure 6.12). This message could be an e-mail that we transmit to the other computer through the network. The message is passed from the application layer directly to the transport layer in the TCP/IP pr