VTAM - Virtual Telecommunications Access Method

Enhanced Definition

VTAM (Virtual Telecommunications Access Method) is a core IBM z/OS software component that manages communication between applications (like CICS, IMS, TSO) and network devices, primarily within the System Network Architecture (SNA) framework. It acts as the central control point for SNA networks, enabling logical sessions between users, terminals, and host applications.

Key Characteristics

- SNA Foundation: VTAM is the host-based implementation of IBM's System Network Architecture (SNA), providing the necessary services for SNA communication.
- Session Management: It establishes, maintains, and terminates logical sessions between application programs and logical units (LUs), such as 3270 terminals or other applications.
- Resource Management: VTAM manages and controls all defined network resources, including physical units (PUs), logical units (LUs), and application programs (APPLs).
- Application Interface: It provides an Application Programming Interface (API) that allows host applications (e.g., CICS, IMS, TSO) to communicate with remote terminals and other applications.
- Network Control Program (NCP) Interaction: VTAM works in conjunction with NCP (Network Control Program) running in communication controllers (like 3745 or 37x5) to manage remote devices and data flow.
- Dynamic Reconfiguration: Supports dynamic additions, deletions, and modifications of network resources without requiring a full VTAM restart.

Use Cases

- 3270 Terminal Access: Enabling users to connect from 3270 terminals (or emulators) to CICS transactions, IMS databases, or TSO/ISPF sessions.
- Application-to-Application Communication: Facilitating communication between different mainframe applications or between mainframe applications and distributed systems using APPC (Advanced Program-to-Program Communication).
- Printer Connectivity: Managing the connection and data flow to SNA-attached printers, allowing applications to send print jobs to remote devices.
- Network Monitoring and Control: Providing a framework for network operators to monitor the status of SNA resources and perform control actions (e.g., activating/deactivating links).
- Inter-System Communication: Supporting communication between multiple z/OS systems within an SNA network, often for data sharing or distributed processing.

Related Concepts

VTAM is fundamental to SNA, serving as the primary host component that defines and controls the network. It interacts closely with NCP in communication controllers to extend network reach. Major mainframe subsystems like CICS, IMS, and TSO are VTAM applications, relying on VTAM to provide terminal and application-to-application connectivity. While TCP/IP has become prevalent, VTAM and SNA environments often coexist, sometimes with gateways or Enterprise Extender (EE) bridging SNA over IP networks.

Best Practices:

Resource Definition Management: Maintain accurate and well-documented VTAM resource definitions (APPL, LU, PU) to ensure network stability and ease of troubleshooting.
Performance Tuning: Regularly review and tune VTAM buffer pools (IOBUF, CRPLBUF, LPBUF) and other parameters to optimize network performance and reduce contention.
Security Implementation: Utilize RACF (or equivalent security manager) to protect VTAM resources, especially APPL definitions, controlling which users or groups can access specific applications.
Monitoring and Automation: Implement robust monitoring tools (e.g., NetView, OMEGAMON for VTAM) to proactively identify network issues and automate routine operational tasks.
High Availability Design: Design the SNA network with redundancy (e.g., multiple paths, backup controllers) and utilize VTAM features like CDRSC (Cross-Domain Resources) for resilience and disaster recovery.