Editing Virtual Telecommunications Access Method (section)

==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.