Virtual telecommunications access method

Virtual Telecommunications Access Method (VTAM) is the IBM subsystem that implements Systems Network Architecture (SNA) for mainframe environments. VTAM provides an application programming interface (API) for communication applications, and controls communication equipment such as adapters and controllers. In modern terminology, VTAM provides a communication stack and device drivers.

VTAM was introduced in 1974 after a series of delays as a major component of SNA along with the 370x Network Control Program (NCP) and Synchronous Data Link Control (SDLC).

In IBM terminology, VTAM is an access method software allowing application programs to read and write data to and from external devices. It is called 'virtual' because it was introduced at the time when IBM was introducing virtual storage by upgrading the operating systems of the System/360 series to virtual storage versions. VTAM was supposed to be the successor to the older telecommunications access methods, such as Basic telecommunications access method (BTAM) and Telecommunications Access Method (TCAM), which were maintained for compatibility reasons. As such, VTAM is comparable with VSAM, which was at that time a new and supposedly converged access method for disk storage.

Originally, VTAM was provided free of charge like most systems software of that time. However, VTAM 2 was the last version to be freely available. ACF/VTAM (Advanced Communication Function/Virtual Telecommunications Access Method) was introduced in 1976 and was provided for a licence fee. The major new feature of ACF/VTAM was the Multisystem Networking Facility, which introduced "implementation of intersystem communication among multiple S/370s.".

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