Note: SDLC supports traditional AS/400 communication protocols, such as APPC, but does not support
TCP/IP.
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 AS/400 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 AS/400 system and the X.25 network. On nonswitched line
connections, the AS/400 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:
v 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
v One connection to an asynchronous communications host system (the primary or controlling computer
in a communications network)
v One connection to an asynchronous device through the X.25 network packet assembler/disassembler
(PAD) function
v One connection to an asynchronous communications host system through AS/400 PAD emulation
v One user-defined communications facility
v 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)
X.21 network
In data communications, a specification of the International Telegraph and Telephone Consultative
Committee (CCITT) that defines the connection of data terminal equipment to an X.21 (public data)
network.
The AS/400 system supports short—hold mode (SHM) operation for use with X.21 circuit—switched
networks. X.21 short—hold mode is characterized by a series of connections and disconnections with a
remote controller or system on an X.21 circuit—switched line. When there is no data traffic, the connection
is broken, but the SNA sessions remain active. When either side has data to send, the connection is
established again.
Chapter 10. Common networking standards 95
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