Virtual Telecommunications Access Method ( VTAM ) is the IBM subsystem that implements Systems Network Architecture (SNA) for mainframe environments. 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.
31-437: VTAM was introduced in 1974 after a series of delays 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
62-410: 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 APPN was considered overly complex and as of 2012 has been largely superseded by TCP/IP. 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
93-493: A USART . There was an interrupt for every transmitted or received character. Six different asynchronous character formats, two forms of Bisync and HDLC / SDLC were supported. A single scanner could attach up to 96 (64 for the first scanner) half-duplex lines. This is the basis of the theoretical maximum capacity of 352 lines. In practice the limit was lower as a scanner with more than 48 half duplex lines could not support any 9.6 kbit/s lines. The Type III Communication Scanner
124-589: A 3705-I. A 4704 (a 3704-compatible device) was planned, but was never introduced. The Amdahl 4745 was completely and in every respect a logical superset, or a physical subset of the IBM 3745. Whereas the IBM 3745 offered one or two physical processors, and one or two physical buses, both apparently for redundancy, the Amdahl offered one very highly reliable physical processor and one very highly reliable physical bus (but two logical buses), with no apparent need for redundancy. Whereas
155-529: 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
186-502: A maximum memory capacity of 1024 KB, all of which is contained within the base frame. Three types of peripherals were available: communications scanners, channel adapters and 8" floppy disk (only available in models without a channel adapter). The first Communication Scanner and the first Channel Adapter occupied the same frame as the CCU. Extra peripherals required extra frames. All Amdahl 4705s were provided with an 8" floppy drive, which
217-444: A mean speed of 120 cycles per second (cps). Alternatively sixteen half-duplex lines running at 1200 cps (9.6 kbit/s) would be the theoretical limit. IPSANET experience was that six full-duplex 9.6 kbit/s lines carrying a heavy load was the limit. IBM software may have had superior performance. For a Type III Communications Scanner cycle steal processing of a single character was fairly inexpensive—a single storage cycle
248-684: A new and supposedly converged access method for disk storage. 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 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.". A limited version of ACF/VTAM known as ACF/VTAME (Advanced Communications Function for
279-456: A single frame two feet wide and three feet deep. Up to three expansion frames can be attached for a theoretical capacity of 352 half-duplex lines and two independent channel adapters. The 3704 is an entry-level version of the 3705 with limited features. IBM intended it to be used in three ways: The storage word length is 16 bits . The registers have the same width as the address bus. Their length varies between 16, 18 and 20 bits depending on
310-427: A single line was limited to 56 kbit/s; with internal clocking the limit was 2.4 kbit/s. Aggregate line connection ability was limited by processing speed. For a Type II Communications Scanner, processing of a received or transmitted character might take fifty storage cycles (forty instructions). In a 3705-II this gives an aggregate capacity of 20 kB/s . This would allow about 160 half-duplex lines running at
341-420: A single memory cycle. There are eight storage reference instructions which require two or three storage cycles to complete. The only shift capability is to shift right one or to add a register to itself. Special hardware assists in the calculation of a cyclic redundancy check for detection of transmission errors. Both CRC-16 CCITT and CRC-16 IBM are supported. Assuming the running value is maintained in storage,
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#1732845623592372-407: A total of four channel adapters). Amdahl's 4705 Type 2 channel adapter also simulated a Type 3 channel adapter. The Amdahl 4705 was compatible with and ran about 2.5 times as fast as a comparable 3705-II. The IBM 3704 was an entry-level version of the 3705. It was restricted to 26 half duplex lines (assuming Type 2 Scanner). DMA devices were not supported on the 3704. Performance was about the same as
403-513: 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. Solid Logic Technology components, similar to those in S/370, were used. The 3705 normally occupies
434-416: The IBM 3745 (IBM 3725) was that complete. Later, a very high-speed scanner was offered with the Amdahl 4745 (and also with the Amdahl 4725, for which IBM had no such corresponding 3725 offering) which offered increased redundancy over the IBM 3745 very high-speed scanner (1.544 Mbit/s , T1, or 2.048 Mbit/s, E1) by having the line switchable to a backup 4725/4745 under operator console control. The 3705
465-423: The IBM 3745 offered two buses, the Amdahl 4745 offered a single bus which throughput capacity exceeded the aggregate throughput capacity of the two buses of the IBM 3745. The design of the Amdahl 4745 was such that the control program (NCP) could not tell whether it was operating in an IBM 3745 or in an Amdahl 4745 (or, for that matter, in an IBM 3725 or in an Amdahl 4725). Amdahl's logical and physical simulation of
496-635: The Virtual Telecommunications Access Method Entry) was made available for DOS/VSE systems running on the IBM 4300 . 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. VTAM supports several network protocols, including SDLC , Token Ring , start-stop, Bisync , local (channel attached) 3270 devices, and later TCP/IP. In
527-419: The amount of storage installed. A particular interrupt level has eight registers. Register zero is the program counter which gave the address of the next instruction to be executed; the other seven are accumulators. The four odd-numbered accumulators can be addressed as eight single-byte accumulators. Instructions are fairly simple. Most are register-to-register or register-immediate instructions which execute in
558-474: The command from the channel and either reject the command or process it according to specific rules. Interrupt processing required about fifty storage cycles to process a four byte transfer. The 3705 could only have a single Type 1 Adapter but two Type 4 Adapters were permitted. An Amdahl 4705 could have four Type 4 channel adapters (Type 1 channel adapters were not offered). Type 2 and Type 3 Channel Adapters were designed for NCP use only. Only one device address
589-568: 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 were introduced (not to be confused with TCP/IP domains) allowing SNA networks with more than one mainframe. A subsequent development
620-493: The execution time to accumulate one more byte is five storage cycles (three instructions). Rapid context switching was a design objective. The register file is divided into four sections. The three commonly used interrupt levels and the background level have distinct sets of registers. Therefore, entry into most interrupt levels does not require saving the registers of the interrupted program. The infrequently used level which processed program and hardware errors shares registers with
651-451: The family ran one of three IBM-supplied programs. The 3710 communications controller was introduced in 1984. The 3725 and the 3720 systems were announced in 1983. The 3725 replaced the hardware line scanners used on previous 370x machines with multiple microcoded processors. With the expansion unit, the 3725 could support up to 256 lines at data rates up to 256 kbit/s, and connect to up to eight mainframe channels. Marketing of
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#1732845623592682-409: The next highest level and thus has to save and restore registers. The five program levels are: The 3705 has a flashing light style control panel which is superior to most other IBM panels of that style. The two unusual (for IBM) features are: The 3705 has a maximum memory capacity of 512 KB, but this requires four physical frames (the base frame and three expansion frames). An Amdahl 4705 has
713-537: 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
744-408: 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
775-545: Was a high performance device for attachment of Bisync and HDLC / SDLC lines. It operated on entire frames. DMA was used to fetch and store the bytes of a frame. In theory the line attachment capacity was the same as for Type II Communication Scanner (352 line limit). The need to restrict scanner size to 48 lines to support 9.6 kbit/s was still present. Restricting scanner size to 16 lines allowed line speeds of up to 30 kbit/s. A scanner size of 8 lines allowed speed of about 60 kbit/s. With modem supplied clocking,
806-470: Was an entry-level device which presented an interrupt on every received bit. Transmission also required an interrupt for every bit. In theory this would have allowed for rather imaginative uses such as Morse Code and connection to devices with unusual framing methods. A maximum of 64 half-duplex lines could be attached. The aggregate bandwidth was restricted due to the heavy processing requirements. The Type II Communication Scanner performed functions similar to
837-470: Was artificially limited to 48 and 96 for marketing reasons. Two types of channel adapter were offered: Type 1 and Type 4 Channel Adapters were designed for 270x emulation. They recognized up to 256 channel addresses and transferred data in small bursts (four bytes for the Type 1; 32 bytes maximum for the Type 4). The hardware could accept almost all of the 240 possible channel command codes. Software had to analyse
868-516: Was eventually replaced by the 3720 and 3725. The 3745 was similar but not entirely compatible. The 3705 was withdrawn from marketing in December 1985. Hardware maintenance service was withdrawn in January 1999. IBM 37xx IBM 37xx (or 37x5 ) is a family of IBM Systems Network Architecture (SNA) programmable communications controllers used mainly in mainframe environments . All members of
899-648: Was recognized. Data transfer was via cycle steal which made the overhead low if large buffers were used. The hardware accepted seven channel commands of which four were completely processed by the adapter. This left a read and two write commands for the software to process. These adapters could be connected to any of the three System/370 channel types but a block multiplexor channel was preferred. The Type 3 Adapter could be simultaneously operational (but not in mid-command) on two different channels which might be connected to different hosts. An Amdahl 4705 could have Type 2 channel adapters as well as Type 4 channel adapters (up to
930-553: Was required. This would give a theoretical limit of a million cps (eighty 9.6 kbit/s half-duplex lines). In practice the limit was probably lower as some processing would be required for end of frame. Also if the frame contents were moved about in storage this would require 3.5 storage cycles per byte. IBM communications scanners were limited to 48 lines on the first communications scanner and 96 lines on communications scanners two through four. Amdahl communications scanners were theoretically capable of 128 lines per scanner, but this
961-485: Was used for diagnostics as well as for remote program loading. This floppy drive and all Channel Adapters were contained in the Base Frame, although additional S/370 channel I/O interfaces could require an Expansion Frame. That is, more than four I/O interfaces required an Expansion Frame, which accommodated four additional I/O interfaces. Three different communication scanners were offered: The Type I Communication Scanner