Token Ring Network

The Token Ring LAN technology was developed by IBM. In a token ring network all devices are connected in a ring. There are no collisions or contests to grab hold of the shared medium. Instead frames are circulated in an orderly manner in one direction.

Physically, a token ring network is wired in star topology with IBM Type-1 shielded twisted pair with unique connectors called IBM data or Boy George connectors.

Token ring network uses a token frame to control media access. A token frame is generated when the ring is initialized. This token is circulated in one direction through the ring. Devices that have nothing to transmit just pass the token to the next device. When the token arrives at a device that is waiting to transmit, it captures the frame and replaces it with a message frame and passes it to the next device. When a frame carrying a message arrives at a device it checks the destination address of the message. If it identifies itself as the destination then it copies the message and sets certain flags in the frame to notify that the frame has been received and copied. Then it passes it to the next device. This frame continues to circulate until it reaches the sender, who sees that the message has been copied and removes the message from the frame. The empty frame or token is then passed on to the next device and circulated in the ring until someone grabs hold of it.

The frame structure also provides a provision for prioritizing the transmissions in the ring. Devices can set the priority bits in the token and circulate them in the ring.

Token ring has been standardized as IEEE 802.5. Initially it had a speed of 4Mbps. Later a 16Mbps variant was released and IEEE 802.5 standard was extended to support this. Higher speeds of 100Mbps were also standardized and marketed but these did not become very popular. Fast Ethernet (100 Mbps) with switching became the dominant layer 2 technology and token rings soon became a thing of the past.

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