IMS TM network overview

IMS is a general-purpose database and transaction manager system that provides the necessary support for an advanced telecommunications network. Virtual Telecommunications Access Method (VTAM®) controls the physical transmission of data in the network, directing data to IMS from various logical units and from IMS to the appropriate logical units.

An IMS telecommunications network must include the following components:
  • IMS
  • VTAM (Although IMS uses the network facilities of VTAM, it can also control non-VTAM devices such as Basic Sequential Access Method (BSAM). VTAM is the preferred access method for IMS.)
  • Communications hardware (such as control units)
  • Terminals
The network can optionally include any of the following components:
  • IMS Transaction Manager (IMS TM) services, such as:
    • Extended Terminal Option (ETO)
    • Fast Path
    • Message Format Service (MFS)
    • Intersystem Communication (ISC)
    • Multiple Systems Coupling (MSC)
    • Advanced Program-to-Program Communication (APPC)
  • Common Queue Server (CQS) and a coupling facility with any of the following structures:
    • Shared-queues structures
    • Shared-data structures
    • Resource structure
  • VTAM generic resource groups
  • Open Transaction Manager Access (OTMA)
  • Common Service Layer (CSL) including:
    • Operations Manager (OM)
    • Resource Manager (RM)
    • Structured Call Interface (SCI)
In addition, an IMS telecommunications network can operate within one of the following frameworks:
  • IBM® Systems Network Architecture (SNA), which brings together multiple products in a unified design. SNA formally defines the functional responsibilities of the network components.
  • A client-server environment, using Advanced Program to Program Communications (APPC) or OTMA.
Definitions: 
  • A logical unit is an addressable resource that can be an application program, a terminal, or a subsystem such as Customer Information Control System (CICS®). A logical unit can also be a component of a general-purpose terminal system that consists of a programmable controller and its attached operator terminals, printers, and auxiliary control units.
  • The word terminal is used throughout this manual to describe devices and also applies to controllers and remote subsystems. The operator terminals can be keyboard printers, display stations with keyboards, communication terminals, or a mixture of these devices.

A network consisting of IMS and programmable logical units enables users to distribute functions throughout network components. This distribution of function reduces processing requirements that are placed on the central processor (also referred to as the host), and it can reduce the impact on the rest of the network when one component encounters a problem.

The following figure illustrates the components of a complete communications network system. The arrows in the figure indicate communications that occur between components. The figure shows the following components:
  • IMS and its application programs
  • Virtual Telecommunications Access Method (VTAM)
  • Tivoli® NetView® for z/OS®
  • z/OS operating system (including APPC/MVS™ if APPC/IMS is used)
  • IBM 37x5 Communications Controller and Network Control Program (NCP)
  • Terminal
Figure 1. Components of a network
A terminal, a Communications Controller (CC), and an operating system (OS) align horizontally. The terminal connects to VTAM through a network control program (NCP) that is on the CC. On the OS, VTAM communicates with IMS.
The following list summarizes how each of the components in the preceding figure participates in the network:
IMS
  • Checks transaction security
  • Schedules the proper application program
  • Directs output to the proper terminal
  • Provides checkpoint and recovery capabilities
Application program
  • Reads data from terminal
  • Writes data to processor
  • Reads data from processor
  • Writes data to terminal
VTAM
  • Connects and disconnects terminal from the network
  • Sends data from terminal to IMS
  • Permits both monitoring and modifying of the network
  • Sends data from IMS to the terminal
  • Manages physical network (with Tivoli NetView for z/OS)
Communications Controller
  • Adds line control characters
  • Transmits data
  • Receives data
  • Removes line control characters
  • Checks for transmission errors
  • Controls buffering
Network Control Programs (NCP)
  • Sends and receives data from communication lines and adapters
  • Checks for and records errors

Planning an IMS network requires an understanding of each component and of its relationship to the others.

The following topics provide additional information:

Programmable logical unit (LU)

Definitions:
  • A programmable logical unit (LU) is an input/output device that is in session with IMS. Application programs in remote logical units can be designed to control more than one terminal.
  • When a logical unit informs VTAM that it wants to communicate with IMS, VTAM notifies IMS using the VTAM Logon exit routine. IMS then accepts the request, and VTAM logically connects the logical unit to IMS. This logical connection is called a session.
Some of the functions performed by the remote application program include:
  • Reading from and writing to associated terminals
  • Editing and verifying the data that is received from a terminal
  • Reading from and writing to disk storage within the remote LU
  • Reading from and writing to the host in which IMS is running
  • Editing and verifying data that is received from the host in which IMS is running
  • Communicating with other network logical units
  • Formatting display and printer devices
  • Operating offline when the host, VTAM, IMS, or NCP is unavailable
Depending on the system type, each LU can consist of one or more terminals. An application program that controls an LU consisting of more than one terminal or component must be able to direct output to a specific device. Therefore the application program must be capable of some form of data interrogation in order to make the proper device selection. IMS assists the application program in this process by:
  • Allowing LU components to be defined and addressed individually
  • Providing, in the header of each output message, the identification of the component to which the message is directed

Communications Controller and Network Control Program (NCP)

VTAM uses the facilities of NCP, which runs in the 37x5 Communications Controllers. VTAM uses NCP to:
  • Control lines and devices that are attached to the controllers
  • Transmit data between the logical unit and the host CPC
  • Perform error recovery
  • Collect statistics about the network

Virtual Telecommunications Access Method (VTAM)

VTAM controls the allocation of network resources, and enables these resources to be shared among VTAM users. To VTAM, IMS is a single VTAM user; VTAM is unaware of the IMS application programs.

The IMS application programs use the IMS CALL interface to request IMS services; IMS uses VTAM macros in order to activate VTAM facilities.

If APPC/IMS is active, VTAM regards it as a separate user.

Using VTAM USERVAR with IMS

IMS uses VTAM user variables (USERVARs) in XRF complexes to help maintain and manage sessions when an active IMS subsystem fails. You can, in limited cases, also use VTAM USERVARs to point to a VTAM APPLID of an IMS that is not part of an XRF complex; however, IMS only supports this use of VTAM USERVARs if the IMS being pointed to is the session's primary logical unit (PLU).

For example, you might use a VTAM USERVAR in a non-XRF context if you change the APPLID of an IMS system, but want to temporarily allow LUs to continue connecting to the IMS system using the old APPLID.

Note: In an ISC or MSC environment, a VTAM USERVAR might behave unpredictably when pointing to an IMS subsystem that is not part of an XRF complex. This unpredictable behavior is due to the fact that IMS subsystems can be either the PLU or the secondary logical unit (SLU) in sessions between two IMS subsystems in ISC and MSC environments.

Related Reading:

  • For more information on VTAM and how it is used, see z/OS Communications Server: SNA Programming.
  • For more information on VTAM USERVARs, see the z/OS Communications Server: SNA Network Implementation Guide

IMS product features

IMS comprises two main product features:
  • The Database Manager (IMS DB), which can control your databases
  • The Transaction Manager (IMS TM), which can control your data communications and application programs
IMS provides:
  • Standard functions required by application programs
  • An execution environment for concurrently running application programs that serve many online users
  • Control of full-function and Fast Path databases

IMS and the application programs that it controls run under z/OS.