ESCON Director

Enhanced Definition

An ESCON Director is a specialized fiber optic switching device used in IBM mainframe environments to provide high-speed, any-to-any connectivity between `ESCON` channels on `z/OS` mainframes and `ESCON`-attached peripheral devices such as `DASD` (Direct Access Storage Devices), tape libraries, and other mainframes. It acts as a central hub, enabling flexible and dynamic I/O path management and extending the reach of mainframe I/O. An ESCON Director is a specialized fiber optic switching device used in IBM mainframe environments to provide dynamic, non-blocking connectivity between mainframe ESCON channels and peripheral control units. It acts as a central hub, enabling multiple mainframes or Logical Partitions (LPARs) to share access to various I/O devices like DASD (Direct Access Storage Device) and tape libraries.

Key Characteristics

- Fiber Optic Technology: Utilizes ESCON (Enterprise Systems Connection) fiber optic cables for data transmission, offering significantly greater distances (up to 60 km with appropriate components) and higher speeds (17 MB/s per port) compared to older copper-based Bus and Tag channels.
- Any-to-Any Switching: Allows any ESCON channel port on a mainframe to connect to any ESCON device port, providing flexible configuration and dynamic path switching without physical recabling, managed through a director control unit.
- High Port Density: Typically offers a large number of ports (e.g., 64, 128, 256) to consolidate I/O connectivity for multiple mainframes and numerous peripheral control units within a data center.
- Distance Extension: Overcomes the inherent distance limitations of Bus and Tag channels, allowing peripherals to be located much further from the mainframe, facilitating disaster recovery and remote site configurations.
- Redundancy and Reliability: Often deployed in redundant pairs with multiple paths to ensure high availability and fault tolerance for critical I/O operations, including redundant power supplies and control cards.
- Zoning Capabilities: Supports logical partitioning (zoning) to isolate traffic and enhance security by restricting which channels can access specific control units or devices, preventing unintended or unauthorized access.

Use Cases

- Storage Connectivity: Connecting z/OS mainframes to ESCON-attached DASD subsystems (e.g., IBM 3390 series, DS8000 series via ESCON interfaces) and automated tape libraries for high-volume data storage and retrieval.
- Inter-System Communication: Facilitating high-speed communication between multiple z/OS mainframes for technologies like PPRC (Peer-to-Peer Remote Copy) for synchronous or asynchronous disaster recovery, or XCF (Cross-System Coupling Facility) for sysplex operations.
- Peripheral Sharing: Enabling multiple mainframes to share access to common peripheral devices, such as tape drives or specialized controllers, optimizing resource utilization and reducing hardware costs.
- I/O Path Consolidation: Centralizing and managing numerous I/O paths through a single director or a pair of directors, simplifying cabling, reducing physical infrastructure, and streamlining I/O configuration compared to direct channel-to-device connections.

Related Concepts

The ESCON Director is a fundamental component of the ESCON architecture, which succeeded Bus and Tag channels and was itself largely superseded by FICON (Fiber Connectivity). It directly interfaces with ESCON channels on the mainframe and ESCON ports on control units. Its configuration is defined within the z/OS I/O configuration, typically managed via HCD (Hardware Configuration Definition) or IOCP (I/O Configuration Program), which define the logical paths that the director facilitates. It plays a crucial role in enabling z/OS features requiring high-speed, long-distance I/O, such as GDPS (Geographically Dispersed Parallel Sysplex) and PPRC for business continuity.

Best Practices:

Redundant Paths: Implement redundant ESCON Directors and multiple paths from the mainframe to each control unit to ensure continuous operation and