etwork 54/5 Source Destination Call request Incoming call establishment phase Call accepted Call connected transfer Data transfer phase Data transfer Clear request Clear indication clearing phase Clear confirmation Clear confirmed Figure 54.3 Phases of a PSN call Each user-originated call over the network demands its own ware and offering an X.25 port, for example. It could also be a virtual circuit, whether a SVC, emulating the PSTN, or permanent Packet Assembler/Disassembler (PAD), converting data between link, such as a leased line. For a permanent-circuit, the DTEs are X.25 and other communications protocols, allowing devices unable permanently connected, obviating the need for a call set-up phase. to support X.25, such as ASCII terminals, to access an X.25 based The term PAD is often applied too widely, covering any device The packet switched network allowing non-X.25 devices to communicate over an X.25 based network, such as devices supporting SNA to X.25 conversion. A DTE is a generic term for any equipment designed to manipulate A PAD assembles packets as data arrives from attached users in packets that is attached to the network (Figure 54.4). It could be an asynchronous, character by character, form. It assigns relevant ad- end-user terminal or a standalone minicomputer running X.25 soft- dresses and error correction data and forwards the packets on to the DTE = Data Terminal Equipment DSE = Data Switching Exchange DCE = Data Circuit-terminating Equipment Figure 54.4 PSN mesh topology 54/6 Packet switching Public Network Public Network Private STE = Signally Terminal Equipment Network Figure 54.5 Interworking PSNs via X.25 and X.75 switch. Conversely, it sorts data arriving from the switch in packets, No CCITT specification exists for DCE-to-DCE communication- converting the packets into a character stream again for onward s, SO data must pass through an intermediate device known as a Data passage to the recipient computer. or Packet Switching Exchange (PSE), either at an intermediate or at PADs are mos