The Token ring , more known internationally under the term of Token Ring , is a protocol of Lan which functions on the layers Physique and Liaison of OSI model. It uses a special screen of three bytes, called token, which circulates in only one direction around a ring. The Token-ring screens traverse the ring in a direction which is always the same one.

The Paradigme is that of the roundabout , which is shown generally able to run out a flow larger than a crossroads , all things being equal. Moreover the fact of avoiding the time wasted in collisions in the CSMA/CD (where it could in the event of congestion account for 1/e=63,2% of the Band-width!) was to make the Token Ring 16 Mbit/s five times faster than Ethernet (10 Mbit/s at the time); important consideration for the night backup of the work stations. N the other hand, one created for itself topological constraints : Ethernet is conceivable on any support, including in theory by infra-red on a white ceiling; the Token Ring cannot function that on a buckles. Note: The first version of Token Ring made it possible to reach 4 Mbit/s.

A typical loop of Token Ring could make 6 km. It is useful to remember that to 16 Mbit/s each transmitted bit occupies 18 meters length on the wire.

The token materializes the right to transmit . Each station passes it (the repeats ) on the ring, at the station which was to him preset following station. A station wishing to transmit keeps it the time necessary to transmit a screen, then sends the token following this screen to indicate that the way is free. If a station breaks down, an interaction is made in order to block the access to the token for the station which is broken down. LAN Token-ring use usually the differential coding of Manchester of the bits on the medium.

A ring of Token Ring was limited to 250 stations (and not 256!), not for questions of binary classification, but because of considerations related to the frequency of transmission.

IBM popularized the use of Token-ring networks about the middle of the Années 1980, with the architecture IBM Token-ring based on units of multistation access active (MSAU or MAU) and the Cabling system Structured IBM. The IEEE standardized the Token-ring network later under the reference IEEE 802.5.

Thus, the original architecture of the Token Ring imposed a physical ring and logical. The appearance of the MAU made it possible to be freed from a physical topology out of ring, since wiring was then carried out out of star (all cables gathered on the same point). The MAU was then given the responsability virtually to reconstitute a ring network.

The work group IEEE 802.5 published various standards authorizing of the flows of 4 Mbit/s, 16 Mbit/s then 100 Mbit/s. This last however was only transitory because of massive arrival of Ethernet.

The Token Ring gave excellent results if one considers the first implementations on shared medium of Ethernet, and were regarded as high performance viable alternative to those. Contrary to the performances of Ethernet, which decrease with the probability of collisions of screens and thus with the number of stations, those of Token Ring are constant and thus foreseeable, since the collisions are impossible.

The development of the Ethernet commutated made again Ethernet more competitive, the structure which he asked being lighter. Indeed, Ethernet offered higher flows at a less cost, which caused the fall of the Token Ring. The higher sales of Ethernet allowed economies of scale drawing the prices the fall, in the long term making him replace Token-ring. Architecture out of ring remained however used in the fast transmissions FDDI and permanent CDDI with 100Mb/s. Today, IBM itself is equipped in-house in Ethernet.