Definition

token ring

A token ring network is a local area network (LAN) in which all computers are connected in a ringor star topology and pass one or more tokens from host to host. Only a host that holds a token can send data, and tokens are released when receipt of the data is confirmed. Token ring networks prevent data packets from colliding on a network segment because data can only be sent by a token holder, and the number of tokens available is controlled.

Token ring topology

In a token ring network, computers, or nodes, are wired in a star formation to a central multistation access unit (MSAU), also known as a hub, MAU or concentrator. For example, six nodes might be connected to an MSAU in one office and that MSAU would be connected to an MSAU in another office that served eight other modes. In turn that MSAU could be connected to another MSAU that is connected to the first MSAU. Such a physical configuration is called a star topology, although the actual configuration is a ring topology because every message passes through every computer one at a time, until it forms a ring.

An advantage of an MSAU is that if one computer fails in the ring, the MSAU can bypass it and the ring will remain intact.

Typically, each node connection cannot exceed 382 feet, depending on the cable type. However, this distance can be increased up to a mile and a half using repeaters token ring repeaters.

Type 1 and Type 3

Token ring networks are generally considered either Type 1 or Type 3 configurations. Type 1 networks can support up to 260 stations per ring, while Type 3 can support up to 72 stations per ring. 

History

The most broadly deployed token ring protocols were IBM's, released in the mid-1980s, and the standardized version of it known as IEEE 802.5, which appeared in the late 1980s. The IEEE standard version provides for data transfer rates of 4, 16 or 100 Mbps. They were at one time extremely popular, but their popularity has since been overtaken by Ethernet.

Token rings were popular because they worked well with large amounts of traffic. But they were not well suited to large networks, particularly if those networks were spread widely or had physically remote nodes. Some of those limitations were overcome with the addition of MSAUs, which are similar to hubs on Ethernet.

Token ring vs. Ethernet

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 -- and thanks to the development of Ethernet switching and full-duplex links, which eliminated collisions as a practical concern in most situations.

Carrier Sense Multiple Access/Collision Detect (CSMA/CD), specified in the IEEE 802.3 standard, is the protocol for carrier transmission access in Ethernet networks. On Ethernet, any device can try to send a frame at any time. Each device senses whether the line is idle and therefore available to be used. If it is, the device begins to transmit its first frame. If another device has tried to send at the same time, a collision is said to occur and the frames are discarded. Each device then waits a random amount of time and retries until successful in getting its transmission sent.

Token ring, specified in the IEEE 802.5 standard, is the original token-passing standard for twisted-pair, shielded copper cables. Supports copper and fiber cabling from 4 Mbps to 100 Mbps. It's sometimes called "IBM Token-Ring."

How Does a Token Ring Work?

Very briefly, here is how token ring works:

1) Systems in the LAN are arranged in a logical ring; each system receives data frames from its logical predecessor on the ring and sends them to its logical successor. The network may be an actual ring, with cabling connecting each node directly to its neighbors, but more often is a star, with the ring existing only logically in the wiring closet within the "multiaccess unit" to which all the hosts connect.

2) Empty information frames are continuously circulated on the ring, along with frames containing actual data; any node receiving an empty frame and having nothing to send simply forwards the empty frame.

3) Any station with data to transmit waits until it receives a free token. It then changes the free token to a busy token and transmits a block of data as a frame.

4) When a computer has a message to send, it waits for an empty frame. When it has one, it does the following: 

a) Inserts a token indicating that it is sending data in the frame -- this may be as simple as changing a zero to a one in the token section of the frame, although other schemes are possible, and is said to "hold the token" for that frame.

b) Inserts the data it wants to transmit into the payload section of the frame.

c) Sets a destination identifier on the frame.

5) When a computer receives a frame containing data (indicated by the token) it knows it cannot transmit data of its own and so it does the following:

a) If it is not the sender or the destination, it simply retransmits the frame, sending it to the next host in the ring.

b) If it is the destination for the message, it copies the message from the frame and clears the token to indicate receipt.

c) If it is the sender (and assuming the destination node has indicated receipt of the frame by clearing the token) it sees that the message has been received, removes the message payload from the frame (restoring it to "empty" status) and sends the empty frame around the ring.

Token passing

The token ring topology uses an access method called token passing. In order for any station on the ring to transmit, it must first possess a token. A token is a frame -- that is, data that is transmitted between network points as a unit complete with addressing and necessary protocol control information -- consisting of three fields that circulate on the ring until a station captures and removes it in order to transmit.

The first field in the token is called the starting delimiter and contains a pattern of bits indicating the start of the frame.

The second field is the access control field. The bits within this field control the priority of bits to be communicated. Stations are given priority for every token, from 0 to 7. When the frame passes what's known as the active monitor, described below, the bit is set to 1 and is used to detect a continuously circulating frame on the ring.

Every station in a token ring network is either an active monitoror standby monitor. There can be only one active monitor on a ring at a time. The active monitor performs a number of ring administration functions: It operates as the master clock for the ring in order to provide synchronization of the signal; it inserts a 24-bit delay into the ring to ensure sufficient buffering; it make sure that one token circulates whenever there is no frame being transmitted, and to detect a broken ring; and removes circulating frames from the ring 

The third field in the token frame is the ending delimiter.

The Fiber Distributed Data Interface (FDDI) also uses a token ring protocol.

Advantages of token rings

  • Token rings reduce the chances of data collision;
  • Token passing performs better than bus topology under heavy traffic
  • A server is not needed to control connectivity among the nodes

Disadvantages of token rings

  • Data packets must pass through all the nodes, slowing routing.
  • Token ring networks have suffered from network management problems and poor network fault tolerance.
This was last updated in July 2016

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