Cluster Controller
A Cluster Controller, historically exemplified by devices like the IBM 3274 or 3174, is a hardware device that manages and controls a group of IBM 3270-type display stations (terminals) and printers. It acts as an intermediary, consolidating communication between these peripheral devices and the host mainframe, typically running z/OS. Its primary purpose is to offload communication management from the host and provide local buffering and control for attached terminals. A Cluster Controller, in the context of IBM mainframes, is a dedicated hardware device (such as the IBM 3274 or 3174) that manages and controls a group of non-programmable 3270 display terminals and printers. It acts as an intermediary, concentrating terminal traffic, handling protocol conversion, and buffering data between these devices and the host mainframe, thereby optimizing communication lines.
Key Characteristics
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- Hardware Device: A dedicated physical unit (e.g., IBM 3274, 3174) designed to interface with 3270-family terminals.
- Terminal Management: Capable of managing multiple 3270 display stations and printers, handling polling, addressing, and data flow for each attached device.
- Communication Concentration: Consolidates multiple terminal connections into a single, higher-speed link to the mainframe, reducing the number of communication lines required at the host.
- Local Buffering: Provides local data buffering capabilities, improving response times and reducing network traffic by handling screen formatting and data validation locally before transmission to the host.
- Protocol Support: Supports various communication protocols, primarily SNA (Systems Network Architecture) and BSC (Binary Synchronous Communications), for communicating with the host mainframe.
- Channel or Network Connection: Historically connected directly to the mainframe via a channel, later models supported network connections using SNA over SDLC or Token Ring, and eventually TCP/IP.
Use Cases
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- Office Terminal Connectivity: Connecting a group of 3270 "dumb" terminals in a data entry department or administrative office to a central z/OS mainframe application (e.g., CICS, TSO).
- Printer Sharing: Allowing multiple users on connected terminals to share a common set of 3270-compatible printers managed by the same controller.
- Remote Site Access: Providing mainframe access to users at remote branch offices by connecting the cluster controller via a leased line or other WAN technology to the central mainframe.
- Legacy Application Access: Essential for accessing character-based, screen-oriented mainframe applications that were designed for the 3270 data stream architecture.
Related Concepts
The Cluster Controller is intrinsically linked to 3270 Terminals, as it's the device that enables their connection and communication with the mainframe. It communicates with the host mainframe through VTAM (Virtual Telecommunications Access Method), using SNA protocols to establish and manage sessions for applications like CICS (Customer Information Control System) or TSO (Time Sharing Option). In modern environments, physical cluster controllers have largely been replaced by TN3270 servers and TN3270 emulators, which provide the same 3270 data stream functionality over TCP/IP networks, effectively virtualizing the role of the physical controller.
- Migration to TN3270: For new deployments or modernizing existing infrastructure, prioritize migrating from physical cluster controllers to TN3270 servers and TN3270 emulators to leverage standard IP networks and reduce hardware maintenance.
- Network Design: When physical controllers are still in use, ensure proper network design, including appropriate cabling, line speeds, and error handling, to maintain reliable communication with the host.
- Configuration Management: Carefully configure the controller's parameters, such as device addresses, polling intervals, and buffer sizes, to optimize performance and resource utilization for attached terminals.
- Security: Implement physical security measures for the cluster controller hardware and ensure network connectivity is secured, especially if it's exposed to external networks, to prevent unauthorized access to mainframe resources.
- Redundancy Planning: For critical applications, consider redundant cluster controllers or alternative access methods to ensure high availability in case of hardware failure.