Circuit - Communication path

Enhanced Definition

In the mainframe context, a communication path refers to the logical or physical medium and associated protocols that enable data exchange between components within a z/OS system, between z/OS LPARs, or between a z/OS system and external network resources. It represents the established route for information flow, managed by communication software and hardware. A **communication path**, often referred to as a circuit in a broader networking context, represents a logical or physical connection established for data transmission between components within a mainframe system or between a mainframe and external devices or networks. In the z/OS environment, these paths are critical for I/O operations, network connectivity, and inter-processor communication, forming the backbone of data flow.

Key Characteristics

- Logical vs. Physical: Can represent a physical link (e.g., an OSA port, a FICON channel) or a logical connection (e.g., a VTAM session, a TCP/IP socket connection) established over physical media.
- Protocol-Driven: Governed by specific communication protocols such as SNA (Systems Network Architecture), TCP/IP, ESCON, or FICON, dictating how data is formatted, transmitted, and received.
- Managed by z/OS Components: Configuration and management are handled by z/OS communication subsystems like VTAM (for SNA), the TCP/IP stack, or I/O configuration definitions (for channels).
- High Availability: Often designed with redundancy and failover capabilities to ensure continuous data flow, using technologies like multipathing or redundant network adapters.
- Performance Attributes: Characterized by bandwidth, latency, and throughput, which are critical for mainframe workload performance and responsiveness.
- Security Implications: Communication paths are subject to security controls, including encryption, authentication, and access control, to protect data in transit.

Use Cases

- Terminal Access: Providing logical paths for 3270 terminal emulation sessions (e.g., via TN3270 or SNA) from end-user workstations to z/OS applications like CICS, TSO, or IMS.
- Application-to-Application Communication: Facilitating inter-application communication, such as CICS-to-CICS inter-region communication (IRC), IMS-to-IMS, or cross-LPAR data exchange using APPC/APPN or TCP/IP.
- Database Connectivity: Enabling remote clients and distributed applications to connect to mainframe databases like DB2 for z/OS or IMS DB via DDF (Distributed Data Facility) or DRDA.
- Network Integration: Connecting the mainframe to enterprise-wide IP networks (LAN/WAN) for file transfers (FTP), web services (z/OS HTTP Server), and general network services using OSA adapters and the TCP/IP stack.
- Storage and Peripheral Connectivity: High-speed communication paths (channels like ESCON or FICON) are used to connect the mainframe processor to external storage subsystems (DASD, tape) and other peripheral devices.

Related Concepts

Communication paths are fundamental to virtually all mainframe operations. VTAM and the TCP/IP stack are the primary software components that define, manage, and utilize these paths for SNA and IP -based communications, respectively. OSA (Open Systems Adapter) provides the physical hardware interface for TCP/IP communication paths, while ESCON and FICON define the physical and logical paths for high-speed I/O channels to storage and peripherals. LPARs (Logical Partitions) often communicate with each other over dedicated or shared communication paths, enabling workload distribution and resource sharing.

Best Practices:

Implement Redundancy: Configure multiple, independent communication paths (e.g., redundant OSA adapters, multiple VTAM major nodes, FICON multipathing) to eliminate single points of failure and ensure high availability.
Monitor Performance: Regularly monitor communication path performance metrics (e.g., bandwidth utilization, latency, error rates) using tools like RMF, SMF, or network management software to identify bottlenecks and ensure optimal throughput.
Secure Communications: Encrypt sensitive data in transit using technologies like AT-TLS (Application Transparent Transport Layer Security) for TCP/IP or IPSec, and implement robust access controls at the network and application layers.
Capacity Planning: Conduct thorough capacity planning for communication paths, considering peak workloads and future growth, to ensure sufficient bandwidth and processing power for all critical applications.
Document Configurations: Maintain detailed and current documentation of all communication path configurations, including physical layouts, logical definitions (e.g., VTAM definitions, TCP/IP profiles), and associated security policies.