X.25 network

X.25 is a Telecommunications Standardization Sector (ITU-T) recommendation that defines the physical level (physical layer), link level (data link layer), and packet level (network layer) of the OSI reference model. A X.25 network is an interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) that operates in the packet mode, which is connected to public data networks by dedicated circuits. X.25 networks use the connection-mode network service.

An iSeries™ X.25 line can be connected through a packet-switching data network (PSDN) and an adjacent remote system by using either a nonswitched or switched physical line. A switched line connection is one that is established on demand between the iSeries system and the X.25 network. On nonswitched line connections, the iSeries system supports both switched virtual circuits (SVCs) and permanent virtual circuits (PVCs). Only SVCs are supported on switched physical lines.

One X.25 line supports one or more virtual circuits. Each virtual circuit can support one of the following:

• One or more Systems Network Architecture (SNA) sessions that can include Advanced Program-to-Program communications (APPC), SNA upline facility , remote work stations, or finance communications
• One connection to an asynchronous communications host system (the primary or controlling computer in a communications network)
• One connection to an asynchronous device through the X.25 network packet assembler/disassembler (PAD) function
• One connection to an asynchronous communications host system through iSeries PAD emulation
• One user-defined communications facility
• One Transmission Control Protocol/Internet Protocol (TCP/IP) link to an adjacent IP node or gateway. (A gateway is a device that is used to connect two systems that use two different communications protocols)