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Token Ring Network

Physical Ring Topology

Token Ring technology was invented by IBM in 1984 and defined in standard IEEE 802.5 by the Institute of Electrical and Electronics Engineers. The token ring network has a logical ring topology, and may be setup with a physical ring topology, but is usually implemented in a physical star topology.

Physical Star Toplogy

The central device of a token ring, called a Media Access Unit (MAU) or Multistation Access Unit (MSAU), can be thought of as a "Ring in a Box". It allows multiple network stations in a logical ring to connect as a physical star. The loop that used to make up the ring is integrated into a chip.

Multistation Access Unit

In a physical Token ring topology, when a cable is open or a station is not operating, the entire network goes down. However with a MAU, the broken circuit is shorted out, closing the loop so the network can continue to operate and the nonoperating stations may be unplugged without crashing the entire network.

Token ring protocol operates at the data link layer of the OSI model. In a token ring network, the first computer to come online creates a three-byte data frame called a token. The token is sent on the cable to the next node in the ring. The token continues around the ring until it arrives at a node that wants to transmit data. The node that wants to transmit data takes control of the token.

A node can only transmit data on the network cable when it takes control of the token. Since only one token exists, only one node can transmit at a time. This prevents the collisions that might occur with the Ethernet CSMA/CD access method.

After a node takes control of the token, it transmits a data packet. A Token Ring packet contains four main parts: The data, the MAC address of the packetís source, the MAC address of the packetís destination and a Frame Check Sequence (FCS) error checking code.

The data packet continues around the ring until it reaches the node with the destination address. The receiving node accepts the data and marks the packet that the data was received. The data packet then continues around the ring until it reaches the source node again. The source node removes the packet from the cable and releases the token so that another node may transmit.

Initially token ring ran at 4 Mbit/s, In 1989 IBM introduced the 16 Mbit/s token ring. Other companies introduced proprietary 10 Mbit/s and 12 Mbit/s versions of token ring. Speeds of 4 Mbit/s, 16 Mbit/s, 100 Mbit/s and 1 Gbit/s have been standardized by IEEE 802.5.

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