LAT Architecture Network Managers Guide Jul85

This "Local Area Transport (LAT) Architecture Network Manager's Guide" (Order No. AA-DJ18B-TK, July 1985) is a reference manual for network and system managers dealing with Digital Equipment Corporation's LAT architecture, primarily used in Ethernet environments for terminal connections.

The guide is structured into four main chapters:

1. LAT Architectural Overview: Introduces the LAT architecture, which is designed for efficient bandwidth use in local area networks (LANs). It operates on a requester/provider model with terminal servers and service nodes as building blocks. The architecture is layered, including a Virtual Circuit layer (for reliable data paths, using START, RUN, STOP messages) and a Slot layer (for user sessions/connections, multiplexing data over virtual circuits, using various slot types like START, DATA, STOP). It also covers Service Class 1 for interactive terminals, which provides directory services (node/service names, ratings, group codes for network partitioning).

2. LAT Configuration Guidelines: Details how to control and configure LAT products on both service nodes and terminal servers. This includes starting/stopping LAT operations and setting various control parameters such as:

   • Circuit Timer: Defines the interval between Virtual Circuit layer messages, impacting terminal response time and service node load.
   • Keepalive Timer: Monitors virtual circuit status.
   • Retransmit Limit: Specifies retransmission attempts before a circuit is declared "down."
   • Group Codes: Logically partitions the network.
   • Datagram and Slot Size: Affects memory usage and protocol overhead.
   • Node Names and IDs, Service Names and IDs: For identification and service discovery.
   • Service Ratings: Used by terminal servers to choose among multiple service providers (e.g., DYNAMIC for load balancing).
   • Maximum Circuits/Connects/Sessions: Sets resource limits. The chapter also explains how to use SHOW commands to display operating characteristics and access performance counters.

3. LAT Performance Issues: Discusses performance measurement for terminal servers (response time, throughput) and Ethernet utilization. It explains how a higher circuit timer can improve service node performance by reducing interrupt load, but may increase response time. Configuration guidelines for maximum performance include minimizing terminal servers and accessed service nodes, using group codes, and adjusting the circuit timer based on usage patterns (e.g., interactive vs. file transfer).

4. LAT Troubleshooting: Provides techniques for diagnosing LAT network problems, assuming individual server products have already been tested. Key tools and procedures include:

   • LAT Counter Information: Checking Illegal Messages Received and Illegal Slots Received for faulty nodes.
   • Data Link Counters: Monitoring Send Failures, Receive Failures for Ethernet cable/interface issues, and Total Frames Sent counters (Deferred, Single/Multiple Collision) to assess Ethernet traffic load.
   • Network Testing Procedures: Using the Ethernet loopback facility with the LOOP CIRCUIT command (via NCP for DECnet or terminal server commands) to test connectivity to target nodes, often with "assistant nodes" to isolate problems.
   • DECnet Remote Console Facility (RCF): For remote diagnostics and management of terminal servers.
   • DECnet Event Logging: To monitor down-line load status of terminal servers.

In essence, the document serves as a comprehensive guide for managing, optimizing, and troubleshooting LAT networks, detailing the architecture's layers, configuration parameters, performance considerations, and diagnostic tools.