X.25 to TCP/IP Gateway

All of the previous options have one disadvantage: the application at the branch offices is still burdened with using a proprietary X.25 API. This may be a temporary solution to replace at least the X.25 PSDN with a TCP/IP WAN. Ultimately it will be far better to eliminate the X.25 applications altogether from the branch offices and replace them with a socket application.

X.25 to TCP/IP Gateway with PXS

(Click to enlarge) X.25 to TCP/IP Gateway LayGO Protocol Stack Diagram

The PXS handles and terminates the X.25 connection (and possibly Application Layer protocols) to the mainframe. Data received from the mainframe (payload) are forwarded via TCP/IP. As in XOT, a 2-byte header or a terminating character can be used to define the length of the data. Data received from the TCP/IP WAN are forwarded via X.25 to the mainframe.

A similar architecture can be used to replace PSDN X.25 virtual circuits connecting terminal emulators to a mainframe. A PXS running a telnet server listens to connection requests arriving via TCP/IP. One of the synchronous ports connects to the mainframe, using X.25. Each telnet session is assigned an X.25 virtual circuit. All the X.25 protocol is terminated at the mainframe site. Any terminal emulator that supports telnet can be used.