Editing Virtual Telecommunications Access Method (section)

==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.