Virtual Telecommunications Access Method (VTAM) is the IBM subsystem that implements Systems Network Architecture (SNA) for mainframe environments.
In IBM terminology, VTAM is an access method software allowing application programs to read and write data to and from external devices.
[4] A limited version of ACF/VTAM known as ACF/VTAME (Advanced Communications Function for the Virtual Telecommunications Access Method Entry) was made available for DOS/VSE systems running on the IBM 4300.
VTAM supports several network protocols, including SDLC, Token Ring, start-stop, Bisync, local (channel attached) 3270 devices,[6] and later TCP/IP.
A robust third party industry of smaller 3745/3746 specialty companies provide such controllers, upgrades, features, and related support services.