Virtual Telecommunications Access Method
| Company type | Software subsystem |
|---|---|
| Industry | Mainframe telecommunications |
| Predecessor | Basic Telecommunications Access Method (BTAM), Telecommunications Access Method (TCAM) |
| Founded | 1974 |
| Products | Virtual Telecommunications Access Method (VTAM), ACF/VTAM, ACF/VTAME |
| Owner | IBM |
Virtual Telecommunications Access Method (VTAM) represents the IBM subsystem responsible for implementing Systems Network Architecture (SNA) within mainframe settings. It offers an application programming interface (API) tailored for communication applications and manages communication hardware such as adapters and controllers. Fundamentally, VTAM functions as both a communication protocol stack and a set of device drivers.
History
VTAM was introduced in 1974 following some postponements and became a fundamental element of SNA, alongside the 370x Network Control Program (NCP) and Synchronous Data Link Control (SDLC).
Within IBM's framework, VTAM is classified as an access method software that permits application programs to perform data input and output operations with external devices. The label 'virtual' was applied during IBM's shift toward virtual storage operating systems for the System/360 family. VTAM was designed to supersede earlier telecommunications access methods such as Basic Telecommunications Access Method (BTAM) and Telecommunications Access Method (TCAM), which continued to be supported for backward compatibility. It is analogous to the Virtual Storage Access Method (VSAM), introduced as a unified access method for disk storage.
ACF/VTAM
The initial release of VTAM 2 was distributed without charge; subsequent versions were not free. In 1976, IBM released ACF/VTAM (Advanced Communication Function/Virtual Telecommunications Access Method) as a commercial product. This version introduced the Multisystem Networking Facility, allowing communication across multiple S/370 mainframes.
A specialized variant named ACF/VTAME (Advanced Communications Function for the Virtual Telecommunications Access Method Entry) was developed specifically for DOS/VSE environments running on the IBM 4300 series.
SNA Services
Within Communications Server for OS/390, VTAM has been rebranded as SNA Services, which also incorporates TCP/IP capabilities. As of September 2019, VTAM continues to be operational within numerous organizations.
Features
VTAM supports a range of network protocols, including SDLC, Token Ring, start-stop transmission, Bisync, local (channel-attached) 3270 terminals, and later additions such as TCP/IP.
Communication within a VTAM-configured network is facilitated either through integrated mainframe communication adapters or via dedicated programmable front-end processors like the IBM 3745/IBM 3746 Communications Processor, which operate under the Network Control Program (NCP). Although IBM has discontinued marketing these controllers, they remain supported with hardware maintenance and microcode updates for roughly 20,000 units still deployed. Additionally, specialized third-party vendors offer enhancements and ongoing support services for these devices. Both VTAM and SNA maintain active usage among many enterprises.
Initially, VTAM enabled data exchange between mainframes and peripheral devices including terminals and minicomputers. The introduction of 'cross-domain' capabilities expanded SNA networks to interconnect multiple mainframes. Following this, SNA Network Interconnect (SNI) allowed networks from different organizations to link while preserving substantial autonomy. Eventually, Advanced Peer-to-Peer Networking (APPN) was integrated into VTAM, transitioning from a centralized mainframe model to a peer-oriented network architecture. APPN was intended to replace DECnet, but by its launch DECnet's prominence had diminished. Due to its complexity, APPN has largely been overtaken by TCP/IP since approximately 2012.