By: Nick Pidgeon. The most common local area network alternative to Ethernet is a network technology developed by IBM, called token ring. Where Ethernet relies on the random gaps between transmissions to regulate access to the medium, token ring implements a strict, orderly access method. A token-ring network arranges nodes in a logical ring, as shown below. The nodes forward frames in one direction around the ring, removing a frame when it has circled the ring once. Token-ring nodes do not look for a carrier signal or listen for collisions; the presence of the token frame provides assurance that the station can transmit a data frame without fear of another station interrupting.
Because a station transmits only a single data frame before passing the token along, each station on the ring will get a turn to communicate in a deterministic and fair manner. Token-ring networks typically transmit data at either 4 or 16 Mbps. Fiber-distributed data interface FDDI is another token-passing technology that operates over a pair of fiber optic rings, with each ring passing a token in opposite directions. Below is a brief outline of Token Ring's self maintenance system.
Token Ring Self Maintenance When a Token Ring network starts up, the machines all take part in a negotiation to decide who will control the ring, or become the 'Active Monitor' to give it its proper title.
This is won by the machine with the highest MAC address who is participating in the contention procedure, and all other machines become 'Standby Monitors'. The job of the Active Monitor is to make sure that none of the machines are causing problems on the network, and to re-establish the ring after a break or an error has occurred.
The Active Monitor performs Ring Polling every seven seconds and ring purges when there appears to be a problem. The ring polling allows all machines on the network to find out who is participating in the ring and to learn the address of their Nearest Active Upstream Neighbour NAUN.
Ring purges reset the ring after an interruption or loss of data is reported. Each machine knows the address of its Nearest Active Upstream Neighbour. This is an important function in a Token Ring as it updates the information required to re-establish itself when machines enter or leave the ring.
When a machine enters the ring it performs a lobe test to verify that its own connection is working properly, if it passes, it sends a voltage to the hub which operates a relay to insert it into the ring. This unit concentrates wiring in a star topology but internally forms a logical ring topology over which network traffic can travel.
Lobes connect the individual stations to the MAU. The maximum cable length for a lobe is MAUs typically support 8 or 16 connections for attaching lobes. You can extend a Token Ring network by connecting MAUs to ring-out and ring-in ports to form larger rings that can support larger numbers of stations. Stackable MAUs simplify this process. You can connect up to 33 MAUs to form a network. Many MAUs support being connected by fiber-optic cabling to create networks that span a building or campus.
Token Ring networks typically operate at speeds of 4 or 16 Mbps, although speeds of up to Mbps are possible with equipment from some vendors.
Token Ring networks come in two types, both of which can operate at 4 or 16 Mbps:. Type 1 is often considered more reliable than Type 3, but the larger installed base of UTP cabling makes Type 3 a viable option for new Token Ring installations. Type 1 configurations support up to stations per ring, while Type 3 can support up to 72 stations per ring.
Token Ring stations pass a single data packet called a token from one computer to the next rather than let each node transmit independently, as in a contention-based network such as Ethernet. Only one token can be on the network at a time, so collisions do not occur in Token Ring networks as they do in Ethernet networks.
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