Virtual Telecommunications Access Method (VTAM) is IBM's software package that provides communications via telecommunication devices for mainframe environments. It is the implementation of Systems Network Architecture (SNA) for mainframes. VTAM provides an API for communications applications, and controls communications equipment such as communications adapters and communications controllers. In modern terminology, VTAM provides a communications stack and device drivers.
In a VTAM environment, the actual network communication took place either by a communications adapter in the mainframe it self, or (for the larger mainframes), by a separate front-end processor, the [IBM 3745]]/3746 Communications Processor, with its own operating system, the Network Control Program NCP. These machines, usually costing several hundred thousand dollars, are now obsolete and are no longer actively marketed by IBM, but support continues. IBM provides hardware maintenance and microcode updates for the estimated 20,000 installed 3745/3746 controllers. A robust third party industry of smaller 3745/3746 specialty companies provide such controllers, upgrades, features and related support services. VTAM and SNA are still in use by many enterprises.
The VTAM subsystem is controlled via a commands issued through the system console (or in exceptional cases through special control cards in the JES).
Initially, VTAM only allowed communication between mainframes and peripheral equipment, such as terminals, distributed processors and minicomputers. Later, "cross-domain" services were introduced (not to be confused with TCP/IP domains!) allowing SNA network with more than one mainframe. A subsequent development was SNA Network Interconnect, allowing networks of different organisational entities (firms) to be interconnected with a high degree of independence. Finally, Advanced Peer to Peer Networking functions were added to VTAM. In APPN the mainframe is no longer the heart of the network, but all nodes in the network are considered as peers. One of the reasons why APPN was introduced was to act as a DECnet-killer, but by the time APPN was actually shipped the popularity of DECnet had already declined. APPN was a very complicated way to introduce simplicity, and as such it did not survive the rise of TCP/IP and internet in the early 1990s.
The name "VTAM" can be explained as follows: VTAM is an access method, the name used in mainframe operating systems for software allowing application programs to read and write data to and from external devices, and it is called "virtual" because it was introduced at the time that 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 for various older telecommunications access methods, notably BTAM and TCAM (that were maintained for compatibility reasons). As such, VTAM is comparable with VSAM, at the time a new and supposedly converged access method for disk storage.
When it was introduced, VTAM was provided free of charge, just as all systems software at the time. Later, VTAM became the first piece of systems software that was provided for a licence fee. Then it was renamed to ACF/VTAM (where ACF stands for Advanced Communication Function). Contrary to the popular belief, IBM did not stop providing free systems software because of antitrust reasons, but because non-IBM plug-compatible mainframe processors emerged at the time. Of course, there is no point for a commercial company in providing free software to be used for competitive equipment. In later years, ACF/VTAM licences became very expensive, in the $ 10000 bracket per month, but later the price was reduced again because mainframe cost of ownership became incompetitive due to software pricing.