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Overview

The Token-Ring Data Link

The discussion of the 802.5, Token-Ring Data Link progresses from a broad overview of token passing architectures, through the frames that are in use in 802.5, to the ways these frames are used by the communicators.

It's important to remember that all behavior that is "802.5" is implemented by the chipset on the adapter card itself, and the attached CPU (the one that is running Unix or Windows, and the user's application program) is not involved in the processing of 802.5.

To fully understand the interaction between the 802.5 standards and the protocol stack (IP, IPX, NetBIOS, etc.) that's running on top of the Token-Ring, you must understand the associated Logical Link Control (802.2) standards that are discussed in a separate section of the compendium.

Topics In This Discussion

• Introduction
  There are several technologies that use token passing. This introduction puts 802.5 Token-Ring into perspective and talks about how it is implemented by various vendors.
• Frame Formats
  A fundamental part of understanding how Token-Ring works and what types of problems it may have is the basic understanding of the parts (fields) of an 802.5 frame. This reference section explains the 802.5 and 802.5 SNAP frame formats.
• Ring Monitors
  The Active Monitor and Standby Monitor functions are the basis for normal ring operation. They have a sophisticated interactive behavior discussed in this section.
• Contention
  When the Active Monitor is powered down, fails, or detects certain problems then the negotiation process is started to elect a new Active Monitor. This section describes the causes of Contention and the expected behaviors during the process.
• Ring Poll
  This section describes the normal interaction between stations on the ring.
• Ring Purge
  The Ring Purge process is the predecessor to generating a new Token. This section discusses the process and its relationship to troubleshooting.
• Token Priority
  This section discusses the prioritization of the token message and how it has been implemented in today's environment.
• Soft Errors
  Each adapter will report a variety of error conditions. By understanding what is being reported, the engineer can understand what types of errors are occurring and what may be causing them.
• Beaconing
  When the ring is unable to negotiate the election of an Active Monitor station it enters the state called Beaconing. This section discusses the events that cause Beaconing, how to isolate the Fault Domain, and what can be done to fix a Beaconing or intermittently Beaconing ring.
• Troubleshooting Methodology
  There is a field-proven methodology that WildPackets uses when analyzing or troubleshooting a Token-Ring network. This section describes the methodology in detail.
• State Machines
  The 802.5 standards discuss ring operation in terms of simple process interrelationships. These simple processes form 'state machines'. This section prepares the reader to understand the IEEE 802.5 standards document by providing an introduction to the concept of 'state machines' - the actual spec is full of this type of reference.
• Timers
  There are a number of timed and time-out events that occur during Token-Ring operation. This section lists and discusses the 802.5 timers and their implications.
• Protocol Analysis
  If you are using a protocol analyzer then this section talks about what kinds of frame-level events you are going to see when you capture from a Token-Ring network.