Virtual Telecommunications Access Method

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Virtual Telecommunications Access Method (VTAM) is the IBM subsystem that implements Systems Network Architecture (SNA) for mainframe environments. [1] 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.

Contents

History

VTAM was introduced in 1974 after a series of delays [2] 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 the Virtual Storage Access Method (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 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 [3] 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.". [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. [5]

SNA Services

VTAM has been renamed to be the SNA Services feature of Communications Server for OS/390 . This software package also provides TCP/IP functions. VTAM is used in multiple enterprises as of September 2019.

Features

VTAM supports several network protocols, including SDLC, Token Ring, start-stop, Bisync, local (channel attached) 3270 devices, [6] 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 3745/3746 Communications Processor, with its own operating system, the Network Control Program NCP. 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) 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 considered overly complex and as of 2012 has been largely superseded by TCP/IP.

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MVS

Multiple Virtual Storage, more commonly called MVS, was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. IBM developed MVS, along with OS/VS1 and SVS, as a successor to OS/360. It is unrelated to IBM's other mainframe operating system lines, e.g., VSE, VM, TPF.

z/VSE is an operating system for IBM mainframe computers, the latest one in the DOS/360 lineage, which originated in 1965. Announced Feb. 1, 2005 by IBM as successor to VSE/ESA 2.7, then-new z/VSEwas named to reflect the new "System z" branding for IBM's mainframe product line.

OS/390 is an IBM operating system for the System/390 IBM mainframe computers.

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 and is, in itself, not a piece of software. The implementation of SNA takes the form of various communications packages, most notably Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.

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 that an SNA header is present after the SDLC header. SDLC was mainly used by IBM mainframe and midrange systems; however, implementations exist on many platforms from many vendors. The use of SDLC is becoming more and more rare, mostly replaced by IP-based protocols or being tunneled through IP. In the United States, SDLC can be found in traffic control cabinets.

Interlink Computer Sciences, of Fremont, California, was a developer of hardware and software that allowed IBM mainframe computers running the MVS operating system to be connected to non-IBM networks.

Remote job entry is the procedure for sending requests for non-interactive data processing tasks (jobs) to mainframe computers from remote workstations, and by extension the process of receiving the output from such jobs at a remote workstation.

3270 emulator

A 3270 Emulator is a terminal emulator that duplicates the functions of an IBM 3270 mainframe computer terminal on a PC or similar microcomputer.

In computing, Advanced Program to Program Communication or APPC is a protocol which computer programs can use to communicate over a network. APPC is at the application layer in the OSI model, it enables communications between programs on different computers, from portables and workstations to midrange and host computers. APPC is defined as VTAM LU 6.2

IBM 3705 Communications Controller

The IBM 3705 Communications Controller is a simple computer which attaches to an IBM System/360 or System/370. Its purpose is to connect communication lines to the mainframe channel. It was a first communications controller of the popular IBM 37xx series. It was announced in March 1972. Designed for semiconductor memory which was not ready at the time of announcement, the 3705-I had to use 1.2 microsecond core storage; the later 3705-II uses 1.0 microsecond SRAM. Monolithic System Technology components, similar to those in S/370, were used.

Binary Synchronous Communication 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 generation computers. The intent was that common link management rules could be used with three different character encodings for messages. Six-bit Transcode looked backwards to older systems; USASCII with 128 characters and EBCDIC with 256 characters looked forward. Transcode disappeared very quickly but the EBCDIC and USASCII dialects of Bisync continued in use.

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."

Teleprocessing Network Simulator (TPNS) is an IBM licensed program, first released in 1976 as a test automation tool to simulate one or many network terminal(s) to a mainframe computer system, for functional testing, regression testing, system testing, capacity management, benchmarking and stress testing. In 2002, IBM re-packaged TPNS and released Workload Simulator for z/OS and S/390 (WSim) as a successor product.

IBM 37xx

IBM 37xx is a family of IBM Systems Network Architecture (SNA) programmable communications controllers used mainly in mainframe environments.

The IBM Network Control Program, or NCP, was software that ran on a 37xx communications controller and managed communication with remote devices. NCP provided services comparable to the Data Link Layer and Network Layer functions in the OSI model of a Wide area network.

Basic Telecommunications Access Method (BTAM) is a low-level programming interface specified by IBM for use on the IBM System/360 for start-stop and binary synchronous telecommunications terminals. Later, IBM specified higher-level interfaces QTAM and TCAM.

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 more in 2007. The 3745 and associated 3746 models were once heavily used within financial, insurance and retail industries as well as within government agencies globally. However, today most organizations have migrated away from the use of 3745s. IBM's Enterprise Extender and the Communication Controller for Linux on System z (CCL) have largely displaced the older 3745s. IBM announced in September 2002 that it would no longer manufacture new 3745s, but IBM continues to support the hardware by providing worldwide maintenance service, by providing microcode releases and by supporting the associated software including NCP and the virtual telecommunications access method (VTAM). IBM has announced end-of-service dates for Japan, Europe and the Middle East, but has not yet announced end-of-service for the Americas and parts of Asia.

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 names; the name access method was introduced in 1963 in the IBM OS/360 operating system. Access methods provide an application programming interface (API) for programmers to transfer data to or from device, and could be compared to device drivers in non-mainframe operating systems, but typically provide a greater level of functionality.

OS/360 and successors Operating system for IBM mainframes

OS/360, officially known as IBM System/360 Operating System, is a discontinued batch processing operating system developed by IBM for their then-new System/360 mainframe computer, announced in 1964; it was influenced by the earlier IBSYS/IBJOB and Input/Output Control System (IOCS) packages for the IBM 7090/7094 and even more so by the PR155 Operating System for the IBM 1410/7010 processors. It was one of the earliest operating systems to require the computer hardware to include at least one direct access storage device.

IBM 3767

IBM 3767 Communication Terminal is a serial printer terminal that employed dot matrix print-head technology and, for the first time, the Synchronous Data Link Control (SDLC) communications protocol set under IBM's Systems Network Architecture (SNA). It was introduced in 1974 and was used widely during the late 1970s to 1990s, for attachment to IBM System/360 and System/370 mainframe computers and IBM System/7 as an alternative to a 2741 typewriter terminal.

References

  1. IBM Corporation (2011). ABCs of z/OS System Programming Volume 4 (PDF).
  2. Frank, Ronald A (October 17, 1973). "IBM Delays Second Virtual TP Release; SD:C Impact Expected". Computerworld. Retrieved June 30, 2020.
  3. IBM Corporation. "5735-RC2 IBM Advanced Communication Function for VTAM (ACF/VTAM) V1 R1 for OS/VS" . Retrieved June 30, 2020.
  4. Introduction to Advanced Communications Function (PDF). IBM. May 1979. GC30-3033-1.
  5. Network Program Products: General Information (PDF). IBM. September 1985. GC27-0657-2.
  6. Introduction to VTAM (PDF). IBM. April 1976. GC27-6987-5.