The IBM Network Control Program, or NCP, was software that ran on a 37xx communications controller and managed communication with remote devices. NCP provided servces comparable to the Data Link Layer and Network Layer functions in the OSI model of a Wide area network.
The original IBM Network Control Program ran on the 3705 and supported access to older devices by application programs using Virtual Telecommunications Access Method (VTAM).
A subsequent version also supported access to Systems Network Architecture (SNA) networks, initially only by application programs using VTAM and later by application programs using Telecommunications Access Method (TCAM). This was the basis for the subsequent versions that ran on an IBM 3704, 3705, IBM 3725. IBM 3720, or 3745. It caused the machine to become an SNA Physical Unit Type 4 (PU4). A PU4 usually had SDLC links to remote cluster controllers (PU1/PU2) and optional SDLC links to other PU4s. Polling and addressing of the cluster controllers was performed by the NCP without mainframe intervention.
In 2005 IBM introduced Communications Controller for Linux (CCL), a software product that allows an unmodified NCP to run on the mainframe, eliminating the need for a separate communications controller in some cases.
A local NCP connected to a System/370 channel via single address.
A remote NCP had no direct connection to a mainframe. Instead a connection was made through a local NCP via SNA packet switching.
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