Token Ring is a communication protocol for local area networks (LANs) that connects devices using a star or ring topology. In this computer network configuration, each computer is linked to the computer next to it to form a closed ring.
Also, computers cannot transmit data until they receive permission from the network in the form of an electronic token. It functions at layer 2, i.e., Data Link layer of the OSI model.
Token ring networks prevent data packets from colliding on a network segment because a token holder can only send data and only a limited number of tokens are available.
IBM initially developed the token ring network architecture. Later on, it was standardised by the Institute of Electrical and Electronics Engineers as IEEE 802.5. A standard Token Ring will support only up to 16 Mbps.
To know how a token ring works, let’s first understand the meaning behind token and token passing mechanism used by Token ring networks.
What do you mean by Token?
A token is a unique series of bits that travel around a token ring network. As the token circulates, computers linked to the network can seize it.
It is just like a ticket, enabling its holder to transmit a message across the network.
Each network has only one token, so there is no chance that two computers will try to send data packets at the same time.
Which access method does Token Ring use? How does Token Passing work?
Token ring networks use the token passing access method. In this, a particular type of packet known as token circulates a cable ring from computer to computer in a logical direction.
When any computer on the ring wants to send data across the network, it has to wait for a free token. When a free token is detected, that computer will take control of it.
The computer can now transmit data in frames. These frames can also contain other information, such as address, attached in the form of headers and trailers.
While this computer uses the token, other computers cannot send data. That’s why, no contention and no collision happens.
Also, no time is exhausted waiting for computers to retransmit tokens due to network traffic on the cable.
How Token Ring works?
Now that you know about token and token passing access method, we will discuss how a Token ring works in detail.
In a Token ring, the connection between all computers (nodes) is in a circular fashion or ring format.
It uses a unique three-byte frame called a “token” that travels around a ring. It makes use of Token Passing controlled access mechanism.
Tokens consist of a start and end delimiter that describes the beginning and end of the frame.
The idea behind a token ring network is that a token is circulating in the ring and whichever node captures that token can transmit the data.
At a time, only one token is there in the network which travels from one node to the next in one direction only. Therefore, there can at most be one transmission at a time.
Since the token circulates in the ring, every node gets the token within some set time. That’s why there is an upper bound on the time of waiting to capture the token so that there is no starvation.
A particular sequence is assigned to the token packet to distinguish it from the regular data packets.
A frame (packet) arrives at the next node in the ring sequence. That node checks whether the frame contains a message addressed to it.
If so, the node separates the message from the frame. Otherwise, the frame is empty and is known as a token frame.
Empty information frames continuously circulate on the ring, along with frames containing actual data.
The node owning the frame decides whether to send a message. If so, it attaches message data into the token frame and sends it back onto the LAN. If not, the node frees the token frame for the next node in sequence to pick up.
The above procedure is repeated continuously for all nodes in the token ring.
Token Ring using Ring Concentrator/Hub
Although Token ring works efficiently, there exists a problem that if the cable breaks somewhere, the ring dies. A Ring concentrator/Hub elegantly solves this problem.
A Token Ring concentrator changes the topology from a physical ring to a star-wired ring. But the network remains a ring logically. The Token still circulates the network and is still controlled in the same manner.
However, employing a hub or a switch boosts reliability as the hub can automatically bypass any ports that are not linked or have a cabling problem.
What is Token Bus and Token Ring?
The IEEE 802.4 standard defines token bus networks. It was created by IBM to connect their terminals to IBM mainframes.
Token Bus is a type of local-area network (LAN) that has a bus topology and applies the token -passing method to control traffic on the bus.
A token bus network is somewhat similar to a token ring network. However, unlike the token ring, the endpoints of the bus do not meet to form a physical ring in a token bus.
Token Bus has two main issues. If the bus fails, all the devices beyond the failure are unable to communicate with the rest of the network. Second, adding more stations to the bus is a bit tricky.
Any new station that is improperly connected is unlikely to be able to communicate. Also, all devices beyond it are also affected. Due to this unreliability of token bus networks, the token ring came into existence.
Comparison between Token Ring and Ethernet
Token ring was once widely used on LANs but has been nearly entirely displaced by Ethernet due to high pricing and other limitations.
Starting in the 1990s, Token Ring significantly decreased in popularity and was gradually phased out of business networks as Ethernet technology began to dominate LAN designs.
Here are some notable differences between a Token Ring and the Ethernet
- Unlike an Ethernet network, devices within a Token Ring network can have multiple same MAC address without causing issues. Switched Ethernet cannot support duplicate MAC addresses without reprimand.
- Token Ring access is more deterministic, as compared to Ethernet’s contention-based CSMA/CD.
- Cabling for Token Ring networks is more expensive than Ethernet’s CAT 3/5e. Also pricier are Token Ring network cards and ports.
- Token Ring employs ‘access priority’ in which some nodes can have priority over the token. It is not allowed with unswitched Ethernet as all nodes have an equal contest for traffic.
- Like mentioned above, Token Ring networks utilise tokens to avoid a collision, while Ethernet networks (mainly if they use hubs) are more prone to them. That’s why Ethernet networks alleviate collision by carrier sense multiple access and by the use of an intelligent switch.
To summarise, a Token ring eliminates the common network problem of data collision by using tokens. Despite that, token ring is expensive as its hardware is more complex and more expensive to manufacture.
As a network technology, token ring is phasing out because it has a maximum speed of 16 Mbps which is slow by today’s Gigabit Ethernet standards. Although Token ring cost more per port to install, it provides numerous benefits over Ethernet and other CSMA/CD topologies. It is also still in the application where high reliability and redundancy are essential.
Both Ethernet and Token ring have their respective strengths and weaknesses, and each is suitable for a particular environment. Therefore, a network analyst must be completely aware of both the systems before selecting either of them.
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