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Introduction

Introduction And Technology Perspective

"Token Ring" is the term normally used to refer to the IEEE 802.5 standard ring network implementation. When we say "Token Ring" we usually think of IBM. In fact, a token passing ring is employed by other network technologies:

• MAP (Manufacturing Automation Protocol)
  This was the original network technology implementation that came out of the 1970's and early 1980's and is typically associated with General Motors Corporation.
• IEEE 802.4
  The 802.4 standards define a broadband network architecture that uses a token message to allow media access. 802.4 derived much of its design from General Motors MAP.
• ArcNET
  Yes, good old outdated ArcNET was, in essence, a baseband implementation of 802.4 and used a token for media access.

Token Passing Concepts

The concept of passing a token message to grant the right to access the physical network media is not unique to the 802.5 standards. Our convention, when referring to token passing rings, is to hyphenate the words "Token" and "Ring" to specifically refer to the IBM/802.5 implementation of token passing.

An 802.5 Token-Ring differs from the other token passing technologies (MAP, 802.4, ArcNET) in that the ring in 802.5 is a physical, circular, interconnected ring of stations. Each station has a separate transmitter and receiver port. The transmitter port on one station is connected to the receiver port on the next station. The term "Upstream Neighbor" is used to refer to the transmitting station and that station transmits to its "Downstream Neighbor". A Downstream station receives bits on the cable attached to its Nearest Upstream Neighbor station. This station repeats bits to the next station.

PHYSICAL AND LOGICAL RING ARCHITECTURES

The MAP, 802.4, and ArcNET implementations of token passing were bus architectures. Whether a physical bus (like 802.4) or a star-wired bus (like ArcNET) the fundamental definition of a "bus" applied - all stations could hear all transmissions from all other stations. This is different from a physical ring where a station only hears the transmissions coming from its immediate upstream neighbor.

The term applied to the bus implementation of token passing is, "Token Passing Bus". In a Token Passing Bus architecture the token message must have an address, since it is being sent to the 'next' station in a logical, numerically sequential, ordering of devices. The token message is sent from one station to the next and finally back to the first station. This creates the logical ring for media access.

Implementation Of 802.5 Token-Ring

The software that carries out the frame transmission and ring management consists of two components: Media Access Control (MAC) and Logical Link Control (LLC). The MAC software is normally implemented as ROM-based software code that controls a CPU that is physically on the Token-Ring adapter card. Often the LLC code is also implemented in hardware on the adapter as well. Because of this, the computer that is using the Token-Ring adapter is not participating in the processing of ring management, ring error recovery, or actual frame transmission. The software that is in ROM on the adapter is referred to as "Microcode".

The Token Ring Chipset that consists of the main IC (Integrated Circuit) and the necessary support IC's and ROM is manufactured competitively by three primary companies: IBM, Texas Instruments, and Madge. While each manufacturer is using the same IEEE 802.5 standards for designing their own microcode the actual code varies from IBM to Texas Instruments to Madge. Each vendor has their own patents and copyrights and they can not use the exact same code implementation (or hardware implementation on the card, for that matter). This competitive environment results in slight differences in the actual behavior of each chipset.

To fully understand the specifics of a particular Token-Ring adapter it is necessary to get a copy of the Chipset Architectural Reference Guide. IBM, Texas Instruments, and Madge have this document available through their technical support departments.