VTAM
Encyclopedia
Virtual Telecommunications Access Method (VTAM) is IBM
's software package that provides communications via telecommunication devices for mainframe
environments. It is the implementation of Systems Network Architecture
(SNA) for mainframes. VTAM provides an API for communications applications, and controls communications equipment such as communications adapters and communications controllers. In modern terminology, VTAM provides a communications stack and device drivers.
VTAM supports several network protocols, including SDLC
and Token Ring
.
VTAM has been renamed to be the SNA Services feature of Communications Server for OS/390
. This software package also provides TCP/IP functions.
In a VTAM environment, the actual network communication took place either by a communications adapter in the mainframe itself, or (for the larger mainframes), by a separate front-end processor, the IBM 3745
/3746 Communications Processor, with its own operating system, the Network Control Program NCP
. These machines, usually costing several hundred thousand dollars, are now obsolete and are no longer actively marketed by IBM, but support continues. 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.
The VTAM subsystem is controlled via commands issued through the system console (or in exceptional cases through special control cards in the JES
).
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 network with more than one mainframe. A subsequent development was SNA Network Interconnect, allowing networks of different organisational entities (firms) to be interconnected with a high degree of independence. Finally, Advanced Peer to Peer Networking functions 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 a very complicated way to introduce simplicity, and as such it did not survive the rise of TCP/IP and internet in the early 1990s.
The name "VTAM" can be explained as follows: VTAM is an access method
, the name used in mainframe operating systems for software allowing application programs to read and write data to and from external devices, and it is called "virtual" because it was introduced at the time that 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 for various older telecommunications access methods, notably BTAM and TCAM (that were maintained for compatibility reasons). As such, VTAM is comparable with VSAM, at the time a new and supposedly converged access method for disk storage.
mainframe processors emerged at the time. Of course, there is no point for a commercial company in providing free software to be used for competitive equipment. In later years, ACF/VTAM licences became very expensive, in the $10000 bracket per month, but later the price was reduced again because mainframe cost of ownership became incompetitive due to software pricing.
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's software package that provides communications via telecommunication devices for mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
environments. It is the implementation of Systems Network Architecture
Systems Network Architecture
Systems Network Architecture is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program...
(SNA) for mainframes. VTAM provides an API for communications applications, and controls communications equipment such as communications adapters and communications controllers. In modern terminology, VTAM provides a communications stack and device drivers.
VTAM supports several network protocols, including SDLC
Synchronous Data Link Control
Synchronous Data Link Control is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture . 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...
and Token Ring
IBM token ring
thumb|Two examples of token ring networks: a) Using a single [[Media Access Unit|MAU]] b) Using several MAUs connected to each otherthumb|Token ring networkthumb|IBM hermaphroditic connector with locking clipthumb|An IBM 8228 MAU...
.
VTAM has been renamed to be the SNA Services feature of Communications Server for OS/390
OS/390
OS/390 is an IBM operating system for the System/390 IBM mainframe computers.OS/390 was introduced in late 1995 in an effort, led by the late Randy Stelman, to simplify the packaging and ordering for the key, entitled elements needed to complete a fully functional MVS operating system package...
. This software package also provides TCP/IP functions.
In a VTAM environment, the actual network communication took place either by a communications adapter in the mainframe itself, or (for the larger mainframes), by a separate front-end processor, the IBM 3745
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 more in 2007...
/3746 Communications Processor, with its own operating system, the Network Control Program NCP
IBM Network Control Program
The original IBM Network Control Program ran on the 3705 and supported access to older devices by application programs using Virtual Telecommunications Access Method ...
. These machines, usually costing several hundred thousand dollars, are now obsolete and are no longer actively marketed by IBM, but support continues. 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.
The VTAM subsystem is controlled via commands issued through the system console (or in exceptional cases through special control cards in the JES
Job Entry Subsystem 2/3
IBM's MVS and z/OS operating systems use a job entry subsystem to receive jobs into the operating system, schedule them for processing by MVS or z/OS, and to control their output processing.There are three job entry subsystems; Master, JES2 and JES3...
).
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 network with more than one mainframe. A subsequent development was SNA Network Interconnect, allowing networks of different organisational entities (firms) to be interconnected with a high degree of independence. Finally, Advanced Peer to Peer Networking functions 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
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 into a networking powerhouse in the 1980s...
-killer, but by the time APPN was actually shipped the popularity of DECnet had already declined. APPN was a very complicated way to introduce simplicity, and as such it did not survive the rise of TCP/IP and internet in the early 1990s.
The name "VTAM" can be explained as follows: VTAM is an access method
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. They were introduced in 1963 in IBM OS/360 operating system...
, the name used in mainframe operating systems for software allowing application programs to read and write data to and from external devices, and it is called "virtual" because it was introduced at the time that IBM was introducing virtual storage
Virtual storage
Virtual storage can refer to:* virtual memory; IBM uses "virtual storage" rather than "virtual memory"* The storage seen by clients when storage virtualization is used...
, by upgrading the operating systems of the System/360 series to virtual storage versions. VTAM was supposed to be the successor for various older telecommunications access methods, notably BTAM and TCAM (that were maintained for compatibility reasons). As such, VTAM is comparable with VSAM, at the time a new and supposedly converged access method for disk storage.
ACF/VTAM
When it was introduced, VTAM was provided free of charge, just as all systems software at the time. Later, VTAM became the first piece of systems software that was provided for a licence fee. Then it was renamed to ACF/VTAM (where ACF stands for Advanced Communication Function). Contrary to the popular belief, IBM did not stop providing free systems software because of antitrust reasons, but because non-IBM plug-compatiblePlug-compatible
A plug-compatible machine is one that has been designed to be backwards compatible with a prior machine. In particular, a new computer system that is plug-compatible has not only the same connectors and protocol interfaces to peripherals, but also runs the same CPU software as the old system...
mainframe processors emerged at the time. Of course, there is no point for a commercial company in providing free software to be used for competitive equipment. In later years, ACF/VTAM licences became very expensive, in the $10000 bracket per month, but later the price was reduced again because mainframe cost of ownership became incompetitive due to software pricing.