VTAM
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Virtual Telecommunications Access Method (VTAM) is the IBM subsystem that implements
Systems Network Architecture Systems Network Architecture (SNA) is IBM's proprietary networking architecture, created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols but, in itself, is not a pi ...
(SNA) for mainframe environments. VTAM provides an
application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how t ...
(API) for communication applications, and controls communication equipment such as adapters and controllers. In modern terminology, VTAM provides a communication stack and device drivers.


History

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 Synchronous Data Link Control (SDLC) is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture (SNA). SDLC supports multipoint links as well as error correction. It also runs under the assumption th ...
(SDLC). In IBM terminology, VTAM is an
access method An access method is a function of a mainframe operating system that enables access to data on disk, tape or other external devices. Access methods were present in several mainframe operating systems since the late 1950s, under a variety of nam ...
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 Telecommunications Access Method (TCAM) is an access method, in IBM's OS/360 and successors computer operating systems on IBM System/360 and later, that provides access to terminals units within a teleprocessing network. Features TCAM provides si ...
(TCAM), which were maintained for compatibility reasons. As such, VTAM is comparable with the
Virtual Storage Access Method Virtual Storage Access Method (VSAM) is an IBM DASD file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now ...
(VSAM), which was at that time a new and supposedly converged access method for disk storage.


ACF/VTAM

Originally, VTAM was provided free of charge like most
systems software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android and Microsoft Windows, computational science software, game engines, search engin ...
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.". 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 The IBM 4300 series are mid-range systems compatible with System/370 that were sold from 1979 through 1992. They featured modest electrical and cooling requirements, and thus did not require a data center environment. They had a disruptive effect ...
.


SNA Services

VTAM has been renamed to be the SNA Services feature of ''Communications Server for
OS/390 OS/390 is an IBM operating system for the System/390 IBM mainframe computers. Overview OS/390 was introduced in late 1995 in an effort to simplify the packaging and ordering for the key, entitled elements needed to complete a fully functional ...
''. This software package also provides
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
functions. VTAM is used in multiple enterprises as of September 2019.


Features

VTAM supports several network protocols, including SDLC,
Token Ring Token Ring network IBM hermaphroditic connector with locking clip. Screen contacts are prominently visible, gold-plated signal contacts less so. Token Ring is a computer networking technology used to build local area networks. It was introduc ...
, start-stop,
Bisync Binary Synchronous Communication (BSC or Bisync) is an IBM character-oriented, half-duplex link protocol, announced in 1967 after the introduction of System/360. It replaced the synchronous transmit-receive (STR) protocol used with second gener ...
, local (channel attached) 3270 devices, and later TCP/IP. In a VTAM network, communication took place through an integrated communication adapter in the mainframe itself, or by a separate programmable front-end processor, the
IBM 3745 The IBM 3745 is the latest and last of a 37xx family of communications controllers for the IBM mainframe environment. As of mid-2009 there were an estimated 7,000+ of the larger 3745 models still in active production status, down from 20,000 or m ...
/ 3746 Communications Processor, with its own operating system, the Network Control Program
NCP NCP may refer to: Science and medicine * Novel Coronavirus Pneumonia (a temporary name for COVID-19), an outbreak that was officially identified in late 2019. * HIV-1 nucleocapsid protein 7 (NCp7), a target of zinc finger inhibitors * Nucleosome ...
. These machines are no longer actively marketed by IBM, but are still supported. 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. 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 networks with more than one mainframe. A subsequent development was SNA Network Interconnect (SNI), allowing networks of different organizational entities (firms) to be interconnected with a high degree of independence. Finally, Advanced Peer to Peer Networking functions (
APPN IBM Advanced Peer-to-Peer Networking (APPN) is an extension to the Systems Network Architecture (SNA) "that allows large and small computers to communicate as peers across local and wide-area networks." Goals and features The goals of APPN were: * ...
) 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 DECnet is a suite of network protocols created by Digital Equipment Corporation. Originally released in 1975 in order to connect two PDP-11 minicomputers, it evolved into one of the first peer-to-peer network architectures, thus transforming DEC ...
-killer, but by the time APPN was actually shipped the popularity of DECnet had already declined. APPN was considered overly complex and as of 2012 has been largely superseded by TCP/IP.


References

{{Mainframe I/O access methods Systems Network Architecture IBM mainframe operating systems