Editing Virtual Telecommunications Access Method

'''Virtual Telecommunications Access Method''' ('''VTAM''') is the [[IBM]] subsystem that implements  [[Systems Network Architecture]] (SNA) for [[Mainframe computer|mainframe]] environments.<ref>{{cite book|last=IBM Corporation|title=ABCs of z/OS System Programming Volume 4|year=2011|url=http://www.redbooks.ibm.com/redbooks/pdfs/sg246984.pdf}}</ref> 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.

==History==
VTAM was introduced in 1974 after a series of delays<ref>{{cite news|last=Frank|first=Ronald A|title=IBM Delays Second Virtual TP Release; SD:C Impact Expected|url=https://books.google.com/books?id=pWBoOXVjuZ0C&dq=vtam+announced+intitle%3Acomputerworld&pg=PP1|access-date=June 30, 2020|newspaper=Computerworld|date=October 17, 1973}}</ref> as a major component of SNA along with the  [[IBM 3705 Communications Controller|370x]] [[IBM Network Control Program|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 memory|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<ref>{{cite web|last=IBM Corporation|title=5735-RC2 IBM Advanced Communication Function for VTAM (ACF/VTAM) V1 R1 for OS/VS|date=8 August 2001 |url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=dd&subtype=sm&appname=ShopzSeries&htmlfid=897/ENUS5735-RC2#ToC_1|archive-url=https://web.archive.org/web/20160304002257/http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=dd&subtype=sm&appname=ShopzSeries&htmlfid=897/ENUS5735-RC2#ToC_1|archive-date=2016-03-04|access-date=June 30, 2020}}</ref> 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.".<ref>{{cite manual|title=Introduction to Advanced Communications Function|id=GC30-3033-1|publisher=IBM|date=May 1979|url=http://www.bitsavers.org/pdf/ibm/sna/acf/GC30-3033-1_Introduction_to_Advanced_Communications_Function_May79.pdf|archive-url=https://web.archive.org/web/20171126055958/http://www.bitsavers.org/pdf/ibm/sna/acf/GC30-3033-1_Introduction_to_Advanced_Communications_Function_May79.pdf|archive-date=2017-11-26}}</ref>

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/360_and_successors|DOS/VSE]] systems running on the [[IBM 4300]].<ref>{{cite manual|publisher=IBM|title=Network Program Products: General Information|id=GC27-0657-2|date=September 1985|url=http://www.bitsavers.org/pdf/ibm/sna/GC27-0657-2_Network_Program_Products_General_Information_Sep85.pdf}}</ref>

===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 [[Synchronous Data Link Control|SDLC]], [[Token Ring]], start-stop, [[Binary Synchronous Communications|Bisync]], local (channel attached) 3270 devices,<ref>{{cite manual|publisher=IBM|title=Introduction to VTAM|id=GC27-6987-5|date=April 1976|url=http://bitsavers.org/pdf/ibm/sna/vtam/GC27-6987-5_Introduction_to_VTAM_Apr76.pdf}}</ref> 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]]/[[IBM 3746|3746]] Communications Processor, with its own operating system, the Network Control Program [[IBM 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' resources<ref>{{cite web|title=z/OS Communication Server: SNA Network Implementation Guide - Static definition of cross-domain resources|url=https://www.ibm.com/docs/en/zos/3.1.0?topic=domains-static-definition-cross-domain-resources|publisher=IBM}}</ref> 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),<ref>{{cite web|title=z/OS Communication Server: SNA Resource Definition Samples - Connecting multiple networks using SNA network interconnection|url=https://www.ibm.com/docs/en/zos/3.1.0?topic=node-connecting-multiple-networks-using-sna-network-interconnection|publisher=IBM}}</ref> allowing networks of different organizational entities (firms) to be interconnected with a high degree of independence. Finally, Advanced Peer to Peer Networking functions ([[IBM Advanced Peer-to-Peer Networking|APPN]]) were added to VTAM.<ref>{{cite web|title=Networking on z/OS - Introduction to APPN|url=https://www.ibm.com/docs/en/zos-basic-skills?topic=appn-introduction|publisher=IBM}}</ref> 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.

==References==
{{Reflist}}

{{Mainframe I/O access methods}}

[[Category:Systems Network Architecture]]
[[Category:IBM mainframe operating systems]]