t widely used for asynchronous display terminals a destination point on the network. PSEs are usually linked in a but can support printers, terminals servers and other devices. There mesh topology, providing at least two circuits to each node, their need not be a PAD at both ends of the call - one end may be a PAD bandwidth determined by traffic demand. (Figure 54.5). tackling a terminal's asynchronous signals, the other might be a While the CCITT standards offer guidance for access to a PSN, mainframe host with a direct connection through an X.25 port. what goes on inside it is left entirely to the network suppliers and PADs invariably support three CCITT standards: X.3, X.28 and PSE manufacturers such as Northern Telecom. The internal protocol X.29, allowing each PAD port to match the requirements of incom- usually resembles X.25 but adds functionality such as flow control, ing terminals and remote host computers. X.3 defines PAD par- network management and accounting. ameters, such as whether or not it echoes keystrokes back to the Internetworking between discrete PSNs is increasingly important, screen; X.28 and X.29 control how those parameters are installed, such as between private and public networks. It takes place via X.28 from terminal to PAD via a keyboard command and X.29 from virtual circuits through gateways. The CCITT X.75 recommenda- host to PAD via a so-called qualifier bit in a data packet. tion supports X.25 to provide such a gateway in special nodes Each X.25 port in the network and all the devices associated with known as signalling terminal equipment (STE).(Figure 54.5). They it such as modems and cables, constitute a DCE. provide connectivity services but operate at the first three OSI Mainframe Modem 25 Interface Modem End user terminal X.25 Interface Packet Switched Network X.25 Interface Mini-computer DTE = Data Terminal Equipment DSE = Data Switching Exchange DCE . Data Circuit-terminating Equipment Figure 54.6 Inside and outside the X.25 cloud Packet switching and the OSI model 54/7 DTE