the network, even arriving out of sequence at the far end and relying that to a modern device they appear unnecessary. However, in an on the protocol to reassemble them in the correct sequence. Connec- international context, such a device may still be in use and con- tion oriented networks such as X.25 packet switching derive their nected to the network in some remote region of the world. The X.28 design methodology from telephony techniques, while connection- set of commands and messages are essential for controlling and less techniques, which are now widely used on LANs, were derived monitoring the PAD functions. from Ethernet radio broadcast systems, such as the Aloha system developed by the University of Hawaii. Today the single most important multiplexing technique is packet 40.6 X.25 and OSI switching as defined in the CCITT X.25 world wide accepted standard. From 1976 when it was first ratified it has become a X.25 goes a long way towards being OSI compatible and is a fundamental part of the Open Systems Interconnection model as one recognised standard. Figure 40.11 shows how X.25 and its associ- of the primary standards in layer three, the network layer. ated layers fits into the OSI model. The physical layers of both X.25 In its basic form, as an independent networking standard, it is and the OSI reference model are similar though X.21 and X.21bis designed to allow asynchronous devices to make and clear network only cater for point to point full duplex links. These physical layer connections by a defined calling procedure. It uses the CCITT standards, which are common to other forms of multiplexing, are standards for defining how the asynchronous device should inter- described in a later section. face (X.3); how to interpret network messages (X.28); the numbe- The data link layers are very close with minor differences. The ring system for setting up international calls (X.121); and most significant difference is that X.25 link layer only deals with negotiation parameters for end to end session compliance