The IMS/ESA® database and transaction management facility is used throughout the world. For many enterprises, IMS™ is the data processing backbone, supporting large personnel and financial databases, manufacturing control files, and inventory management facilities. IMS backup and recovery features protect valuable data assets, and the IMS Transaction Manager provides high-speed access for thousands of concurrent users.

Traditionally, many IMS users have used 3270-type protocol to communicate with the IMS Transaction Manager. In that environment, all the processing, including display screen formatting, is done by the IMS mainframe. During the decade of the 1980s, users began to move some of the processing outboard into personal computers. However, these PCs were typically connected to IMS through SNA 3270 protocol.

During that period, although most IMS users were focused on 3270 PC emulation, many non-IMS users were busy building a network based on a different protocol, called TCP/IP. As this trend developed, the need for an access path between TCP/IP-communicating devices and the still-indispensable processing power of IMS became clear. IMS TCP/IP provides that access path. Its role can be more easily understood when one distinguishes between traditional 3270 applications (in which the IMS processor does all the work), and the more complex client/server applications (in which the application logic is divided between the IMS processor and another programmable device such as a TCP/IP host).

MVS™ TCP/IP supports both application types:

• When a TCP/IP host needs access to a traditional 3270 Message Format Service (MFS) application, it does not have touse the IMS TCP/IP feature; it can connect to IMS directly through Telnet which provides 3270 emulation services for TCP/IP-connected clients. Telnet is a part of the base TCP/IP Services product. (See z/OS Communications Server: IP User's Guide and Commands for more information).
• When a TCP/IP host has to support a client/server application, it should use the IMS TCP/IP feature of TCP/IP Services. This feature supports two-way client/server communication between an IMS message processing program (MPP) and a TCP/IP host.

As used in this information, the term client means a program that requests services of another program, which is known as the server. The client is often a UNIX-based program; however, DOS, Windows, Linux, CMS, and MVS-based programs can also act as clients. Similarly, the term server means a program that is often an IMS message processing program (MPP); however, the server can be a TCP/IP host, responding to an IMS MPP client.