9.1.3.2 Programming

Any coding done under VSE, such as VTAM exits, will almost certainly need rewriting (and will certainly need re-linking) for the OS/390 environment. Also, some VTAM exit routines may be implemented differently under VSE and OS/390. User-written VTAM programs and exits must be reviewed carefully for compatibility. Please refer to the chapter Operating System Facilities in the VTAM Programming manual.

9.1.4 Network Configuration

It is not unusual for a migrating customer to require multiple host access to the SNA network. Multiple host configurations can serve to reduce or even eliminate service disruptions during the migration process. Multiple host configurations can provide for both interactive and batch traffic between all connected images. This means that any terminal, regardless of to which host it is physically connected, can access any VTAM application in the network, regardless of in which host the application resides. In addition, SNANJE connections can be set up between JES and PNET. For a migrating customer, these capabilities can be particularly useful, providing simultaneous access to both old and new systems as well as file transfer capability between them.

VTAM provides for two different multiple host networking architectures, traditional SNA subarea (cross domain) and/or APPN (Advanced Peer-to-Peer Networking). VSE/VTAM V3R4 is limited to SNA subarea because it does not provide full APPN capability. The MultiDomain package of VSE/VTAM V4R2 is required to support multiple host connectivity in an SNA subarea network. VSE VTAM V4R2 provides APPN capability at all three functional levels, but with some limitations in the Client/Server and MultiDomain packages. OS/390 VTAM is full featured and provides both SNA subarea and APPN capabilities. Implementation requires careful planning and solid networking skills. Some points to consider in network design are:

Number of hosts Number of applications Number of terminal resources Number of NCPs

Traffic patterns Future growth Backup/redundancy requirements Bandwidth requirements Operation/management Security

Cost

Concepts and technical explanations, as well as implementation techniques for SNA networking in both subarea and APPN designs, can be found in the following manuals:

IBM Network Products Implementation Guide, GG24-3649VTAM Network Implementation Guide, GC31-8370

Chapter 9. Telecommunications Subsystems 191

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IBM OS/390 manual Network Configuration, Programming

OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.