DFT - Distributed Function Terminal

Enhanced Definition

A Distributed Function Terminal (DFT) is a type of IBM 3270-compatible terminal connection that allows a single physical terminal device to establish and manage multiple concurrent logical sessions with different host applications or systems on an IBM mainframe. It achieves this by leveraging the capabilities of an intelligent terminal controller, such as an IBM 3174 or 3274. A Distributed Function Terminal (DFT) is a type of terminal connection in IBM mainframe environments that allows a single physical terminal device to establish and manage multiple independent logical sessions with different host applications or multiple sessions to the same application. It primarily functions by distributing terminal control functions from the host to a local controller, reducing host overhead and network traffic.

Key Characteristics

- Multi-session Capability: A primary feature allowing a single physical terminal to display and interact with multiple independent logical sessions, often through a session manager on the controller.
- Controller Dependent: Requires an IBM 3174, 3274, or compatible terminal controller to manage the physical connection to the host and distribute sessions to the terminal.
- Coaxial Connection: Typically connects to the terminal controller via a dedicated coaxial cable, providing a high-speed, reliable physical link.
- 3270 Emulation: Functions as an enhanced IBM 3270 display terminal, supporting the 3270 data stream protocol for screen-oriented interaction.
- Fixed Function: While offering multi-session capabilities, DFT terminals are primarily for display and data entry, with limited local processing power compared to intelligent workstations.
- Application Access: Can simultaneously access diverse mainframe applications like CICS, IMS, TSO, or even different LPARs from a single physical device.

Use Cases

- Application Development & Testing: Developers can concurrently access multiple CICS regions (e.g., development, test, production), a TSO session for JCL submission, and a DB2 session from one terminal.
- System Administration: System programmers can monitor different system consoles, VTAM sessions, or multiple LPARs simultaneously to manage and troubleshoot the z/OS environment.
- Customer Service Centers: Agents can switch between an order entry application, a customer inquiry system, and a knowledge base application without needing multiple physical terminals.
- Operations Monitoring: Operators can display various operational monitors, job queues, and console messages on different logical screens from a single workstation.

Related Concepts

DFT terminals are an evolution of the basic 3270 terminal concept, enhancing its utility by adding multi-session capabilities. They rely heavily on IBM 3174/3274 controllers which act as concentrators and session managers, translating the physical terminal's input/output to the host. Communication between the controller and the z/OS host is typically managed by VTAM over an SNA network, with DFT providing the end-user interface to applications running in environments like CICS or IMS.

Best Practices:

Leverage Session Managers: Utilize the session management features provided by the terminal controller (e.g., SWAP, LOGON, LOGOFF) for efficient navigation and control of multiple sessions.
Optimize Controller Configuration: Properly configure the 3174/3274 controller's parameters to balance the number of supported DFT sessions with network bandwidth and host resource availability.
Implement Robust Security: Ensure that each logical session accessed via a DFT terminal is protected by appropriate security mechanisms (e.g., RACF or equivalent) to control access to specific applications and data.
Plan for Network Capacity: Account for the aggregate traffic generated by multiple logical sessions when designing the network links between the controller and the z/OS host.
Consider Modern Alternatives: While DFT was critical, modern environments often use TN3270 emulators on PCs, which provide similar multi-session capabilities over TCP/IP, often with enhanced features and integration.