Token ring :: essays research papers

Token ring is a local area network protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels unidirectionally around a star-wired logical ring. Token ring frames travel completely around the loop. The name 'Token Ring' is misleading since the physical topology is a loop. Each station passes or repeats the special token frame around the ring to its nearest downstream neighbor. This token-passing process is used to arbitrate access to the shared ring media. Stations that have data frames to transmit must first acquire the token before they can transmit them. Token ring LANs normally use differential Manchester encoding of bits on the LAN media. Token ring was invented by Olof SÃ ¶derblom in the late 1960s. It was later licensed to IBM, who popularized the use of token ring LANs in the mid 1980s when it released its IBM token ring architecture based on active multi-station access units (MSAUs or MAUs) and the IBM Structured Cabling System. The Institute of Electrical and Electronics Engineers or IEEE (http://www.ieee.org) later standardized a token ring LAN system as IEEE 802.5 (http://www.ieee802.org/5/). Token ring LAN speeds of 4Mbps, 16Mbps, 100Mbps and 1Gbps have been standardized by the IEEE 802.5 working group. Token ring networks had significantly superior performance and reliability compared to early shared-media implementations of Ethernet (IEEE 802.3), and were widely adopted as a higher-performance alternative to shared-media Ethernet. However, with the development of switched Ethernet, token ring architectures lagged badly behind Ethernet in both performance and reliability. The higher sales of Ethernet allowed economies of scale which drove down prices further, and added a compelling price advantage to its other advantages over token ring. Token ring networks have since declined in usage and the standards activity has since come to a standstill as switched Ethernet has dominated the LAN/layer 2 networking market. The Token Frame When no station is transmitting a data frame, a special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to transmit data. When a station needs to transmit a data frame, it converts the token frame into a data frame for transmission. The special token frame consists of three bytes as follows: Starting Delimiter - consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations.