 protocols and devices, enabling users to choose equipment from multiple vendors, unrestricted by proprie- tary vendor protocols. The protocols at either end of a resulting Packet switching equipment DPN-100 network do not have to be the same to communicate. Examples include SNA/SDLC, 3270 display system protocol for IBM 3270 bisynchronous terminals, and asynchronous polled inter- An example of a successful packet switching system is DPN-100 face (API) for relevant point-of-sale terminals. from Northern Telecom. As well as private corporate networks over 30 PTT's have installed DPN networks as the basis of their national The switch and network architecture focuses on users' need for X.25 services. DPN switches are modular, so that they can be transparent access to applications, shielding them from control and purchased to match the capacity requirements of the network, and security mechanisms necessary to maintain the integrity of the data subsequently grow with future needs. In addition to X.25, a variety passing across the network. of access protocols, (all major international and most de facto The DPN-100 range gives the network controller the ability to standards such as SNA) are supported, enabling the support of a define network service targets based on user needs: fast application multi-vendor data processing environment. Trunks can be directly access, no interruptions during application processing and fast sys- attached to the DPN-100 at speeds of up to 1.544Mbit/s (UST1) or tem response. There is also an emphasis on secure information flow 2.048Mbit/s (UK MegaStream and equivalent). Enhancements to through the establishment of Closed User Groups and Network User the DPN range include the support of Frame Relay as an access Identifiers, plus passwords as desired. It also highlights the import- protocol. ance of comprehensive network management, providing a real time A major advantage of the DPN-100 range is its conformance to network control system with dynamic updates of switch and overall international 