Duplex
Enhanced Definition
Key Characteristics
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- Full-Duplex: Allows simultaneous, independent data transmission in both directions over a single communication link. This is the standard for modern TCP/IP networks and provides optimal throughput and efficiency.
- Half-Duplex: Permits data transmission in both directions, but only one direction at a time. Devices must alternate between sending and receiving, often requiring a "turnaround" mechanism, which can introduce latency.
- Hardware/Software Dependency: Duplex mode is determined by both the physical communication hardware (e.g., OSA-Express network adapters, modems) and the software protocols (e.g., VTAM, TCP/IP stack) configured on the mainframe and connected devices.
- Performance Impact: Full-duplex significantly improves network performance by eliminating wait times for channel turnaround, which is critical for high-volume transaction processing and interactive terminal sessions on z/OS.
- Simplex (for contrast): While not duplex, simplex communication is one-way only (e.g., a broadcast). Most mainframe communication requires two-way interaction, making simplex rare for primary data exchange.
Use Cases
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- 3270 Terminal Emulation: Modern 3270 emulators connecting to z/OS via TCP/IP operate in full-duplex mode, allowing users to type commands while the mainframe simultaneously sends screen updates, ensuring a responsive interactive experience.
- Application-to-Application Communication: Programs communicating via APPC (Advanced Program-to-Program Communication) or TCP/IP sockets (e.g., CICS-to-CICS, CICS-to-distributed servers) leverage full-duplex for efficient, high-speed data exchange.
- Network Printer Communication: Modern network printers connected to z/OS via TCP/IP typically use full-duplex to receive print data while concurrently sending status, error, or job completion messages back to the mainframe.
- Data Transfer Protocols: File Transfer Protocol (FTP) and other data transfer mechanisms between z/OS and other systems benefit from full-duplex to send data and receive acknowledgments or control commands concurrently, improving transfer rates.
Related Concepts
Duplex settings are fundamental to network protocols like SNA (Systems Network Architecture) and TCP/IP on z/OS. In SNA, older communication methods, particularly those involving modems or certain types of physical units, might have been half-duplex, managed by VTAM (Virtual Telecommunications Access Method) and LU (Logical Unit) types. With TCP/IP on z/OS, duplex mode is configured at the network interface card (NIC) level (e.g., OSA-Express adapters) and is crucial for the performance of applications running under CICS, IMS, DB2, and other subsystems that rely on network connectivity. A mismatch in duplex settings between the mainframe and its connected network devices can lead to severe performance degradation.
Best Practices:
- Ensure Full-Duplex Configuration: For all modern mainframe network interfaces (e.g., OSA-Express adapters) and connected network switches/routers, always configure and verify full-duplex operation for optimal performance and throughput.
- Avoid Duplex Mismatches: A common cause of network performance issues on the mainframe is a duplex mismatch between the z/OS network adapter and the connected switch port. This can lead to excessive collisions (if one side is half