What is Token Ring in Networking?

July 22, 2018

The robustness of token ring has won it a place in the heart of many large enterprise network The communication capabilities provided by local-area networks (LANs) are affected, to a large extent, by the performance of the medium access control protocol. In geographically concentrated areas, communication subnetworks usually realize a bus or ring topology. The IEEE 802 standardization body has established e.g. three standards for LAN medium access control protocols: CSMA/CD, token bus and token ring. The term “token ring” is used to describe a computer network configuration where each computer is linked to the computer next to it to form a closed ring.

Basic Overview of Token Ring

Token ring or IEEE 802.5 is a network where all computers are connected in a circular fashion. The term token is used to describe a segment of information that is sent through that circle; when a computer on the network can decode that token, it receives data.

A token ring network is a local area network (LAN) topology where nodes/stations are arranged in a ring topology. Data passes sequentially between nodes on the network until it returns to the source station. To prevent congestion and collision, a token ring topology uses a token to ensure that only one node/station on the line is used at a time, thereby easily denoting media users of its activity. A token ring LAN is physically wired as a star topology but configured as a ring topology. The token ring LAN system was standardized by the Institute of Electrical and Electronics Engineers as IEEE 802.5.

Token ring was once widely used on LANs, but has been nearly entirely displaced by Ethernet thanks to pricing — token ring products tended to be more expensive than Ethernet at similar speeds

An understanding of the fundamental architecture of token ring can help in the understanding of why token ring continues to handle the constant increase and growth of local-area networks, and handle this traffic well. In many environments, shared token ring hubs continue to deliver mission-critical networking traffic without the need for expensive and painful upgrades.

The token-passing protocol for ring access control is based on a predefined 24-bit pattern, called a token, which continuously circulates around the ring.

Figure: Token Ring View

The Token is a special packet of data that floats around the network. The token can travel in either direction around the ring, but only in one direction at a time carrying data.

Token ring communication steps:

• As the Token passes a device, it can check to see if the Token is free.
• If it is free, the device can attach its data.
• The Token will then pass from device to devices. Each device will look at the data to determine if it was meant for them.
• When the destination device gets the Token, it strips off the data and attaches a reply to the Token and sends it out.
• The original sender strips off the reply and sends out the free Token.

How Token Ring Works?

Unlike all other standard forms of LAN interconnects, Token Ring maintains one or more common data frames that continuously circulate through the network.

These frames are shared by all connected devices on the network as follows:

• A frame (packet) arrives at the next device in the ring sequence.
• That device checks whether the frame contains a message addressed to it. If so, the device removes the message from the frame. If not, the frame is empty (called a token frame).
• The device holding the frame decides whether to send a message. If so, it inserts message data into the token frame and issues it back onto the LAN. If not, the device releases the token frame for the next device in sequence to pick up.

In other words, in an effort to minimize network congestion, only one device is used at a time. The above steps are repeated continuously for all devices in the token ring. Tokens are three bytes that consist of a start and end delimiter that describe the beginning and end of the frame (i.e. they mark the frame’s boundaries). Also within the token is the access control byte. The maximum length of the data portion is 4500 bytes.

References

[1] Follows, Jonathan. “Token Ring Solutions.” IBM International Technical Support Organization, available at www. redbook.ibm.com (2000).

[2] “1.2 Network Topologies”, available online at: https://ccnabasics.wordpress.com/2012/08/01/network-topologies/

[3] “Token Ring Network”, available online at: https://www.techopedia.com/definition/26095/token-ring-network

[4] “What Does Token Ring Mean”, available online at: https://www.lifewire.com/what-is-token-ring-817952

[5] “Token ring”, available online at: https://www.computerhope.com/jargon/t/tokering.htm

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