TOKEN RING:
According to Black (1983), "a token is a time slot or frame that is passed to the next station on the ring network. The frame contains the address of a station on the ring. The token is available to any station that has traffic to place in it. Upon using the token, the station sets a flag and places a destination address in the header to indicate the token is full. The token moves around the physical ring, is checked at each intermediate station for relevant address, and is eventually passed to the destination node. The receiving station relays a receipt back to the originator by changing a bit flag in the token. The originating station must then make the token empty or free, and pass it out onto the ring".
(Picture from http:// www.webopedia.com)
In a Token Ring network, an electronic token (named a control token) is circulated around a closed electronic loop (token passing). The Token is then read and repeated by each node on the network to the next node. The circulation continues even when no data is is being transmitted. If any node on a Token Ring network is wishing to transmit data, it must await a Token. When the node gets it, it fills a frame with data, add the source and destination address, it then sends it to the next node. This (data frame) circulates around until it reaches the destination node. The destination node then reads the data into its local memory area (or buffer), and makes an acknowledgement mark on the data frame. This then circulates back to the originating node (source). The originating node receives and verify whether it contains an acknowledgement. This (acknowledgement) allows the source node to know that the data frame was received correctly. On the other hand, if the data frame does not contain an acknowledgement, than the destination node is not responding. The source node will then transmit a new Token if it has finished transmitting data. The new Token can then be used by other nodes.
TOKEN RING MAINTENANCE
According to William Buchanan (2000), a Token Ring system requires considerable maintenance and must perform the following functions:
(a) Ring initialization: The ring must be re-initialized at the start-up of a network, or after the ring has been broken. The node which starts a new Token, or which goes next, and so on is sorted out by a co-operative decentralized algorithm.
(b) Adding to the Ring: The network must be short down and re-initialized if a new node is to be physically connected to the Ring.
(c) Deletion from the Ring: Also, the network may have to be short down if a node disconnects itself from the Ring This disconnection is done by joining its predecessor and its successor.
(d) Fault Management: Typical errors that occurs in a Token Ring network is when two nodes at the same time think it is their turn to transmit, or when the Ring is broken, as none of the nodes think that it is their turn.
TOKEN RING MULTISTATION ACCESS UNITS (MAUs)
Multistation access unit (MAU) allows the switching in and out of a network by nodes, using a changeover switch or by automatic electronic switching (that is, auto-loopback). This has significantly reduced the problems of adding and deleting nodes to or from a Ring network, as it has the advantage of not shutting down the network when nodes are added, deleted or develop faults (Buchanan, 2000).
TOKEN BUS
In Bus Topology, a Token can be used through a bus path (Token-Bus Topology). Here, the stations passes the Tokens by placing the address of the next logical recipient in the header of the packet. The signal is physically monitored while it passes along the Bus. The receiving station gets the Token, based on the sending station's placement of a station address in the destination header. The Token (that is, right to transmit) is passed from stations in descending numerical order, based on station address. A station may transmit data frames if it hears a Token frame addressed to itself. On completion, it passes the Token to the next station in the logical ring. While holding the Token, a node may temporarily delegate its right to transmit to another station by sending a request-with-response data frame or issue a solicit-successor frame (the address in the frame is between it and the next successor station).
On completion of transmitting any data frames it may have, each station passes the Token to its successor by sending a Token control frame. The station then listens for the evidence that its successor has heard the Token frame, and is active. The sender assumes that its successor has the Token and is transmitting, if it hears a valid frame, following the Token. If not, the sender attempts to assess the stations on the network, and may implement measures to pass around the problem station by establishing a new successor. Attempts are made to re-establishing the Ring if there are more serious faults (Black, 1983).
References:
Black,U.D. (1983).Data Communications and Distributed Networks. New Jersey: Simon & Schuster Company. p. 300.
Buchanan,W. (2000). Distributed Systems and Networks. Berkshire: McGraw-Hill Publishing Company. pp. 219-222.
For details check the following:
http://www.rad.com/networks/1997/nettut/token_ring.html http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/tokenrng.htm#xtocid1 http://inews.webopedia.com/TERM/T/token_ring_network.html http://fcit.coedu.usf.edu/network/glossary.htm#token http://fcit.coedu.usf.edu/network/chap2/chap2.htm#TokenRing
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