Token ring

Token ring local area network (LAN) technology is a for local area networks. It uses a special three-byte frame called a "token" that travels around a logical "ring" of workstations or servers. This token passing is a channel access method providing fair access for all stations, and eliminating the collisions of contention-based access methods.

Introduced by IBM in 1984, it was then standardized with protocol IEEE 802.5 and was fairly successful, particularly in corporate environments, but gradually eclipsed by the later versions of Ethernet.

(The main focus of this article is the IBM/IEEE 802.5 version, but there were several other earlier implementations of token rings).

A wide range of different local area network technologies were developed in the early 1970s, of which one, the Cambridge Ring had demonstrated the potential of a token passing ring topology, and many teams worldwide began working on their own implementations. At the IBM Zurich Research Laboratory Werner Bux and Hans Müller in particular worked on the design and development of IBM's token ring technology, while early work at MIT led to the Proteon 10 Mbit/s ProNet-10 token ring network in 1981 – the same year that workstation vendor Apollo Computer introduced their proprietary 12 Mbit/s Apollo token ring (ATR) network running over 75-ohm RG-6U coaxial cabling. Proteon later evolved a 16 Mbit/s version that ran on unshielded twisted pair cable.

IBM launched their own proprietary token ring product on October 15, 1985. It ran at 4 Mbit/s, and attachment was possible from IBM PCs, midrange computers and mainframes. It used a convenient star-wired physical topology, and ran over shielded twisted-pair cabling, and shortly thereafter became the basis for the (ANSI)/IEEE standard 802.5.

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