Token Ring

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Overview

Token Ring is a protocol that is the second most widely-used protocol on Local Area Network (LAN) after Ethernet. In a Token Ring network, all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time.

The Token Ring network was originally developed by IBM in the 1970s. It is still IBM’s primary LAN technology, and was eventually standardized with protocol IEEE 802.5. The related IEEE 802.5 specification is almost identical to and completely compatible with IBM’s Token Ring network. In fact, the IEEE 802.5 specification was modeled after IBM Token Ring, and it continues to shadow IBM’s Token Ring development. The term Token Ring generally is used to refer to both IBM’s Token Ring network and IEEE 802.5 networks.

Background

Token Ring (IEEE 802.5) is deterministic star-wired ring architecture, and the sequence in which users gain access is predetermined. The controlling station, called the active monitor, generates a special signaling sequence called a Token that controls the right to transmit. This Token is continually passed around the network from one node to the next. When a host has something to send, it captures the Token, and changes it to a frame, setting its status to busy. It then adds the frame header, information and trailer fields. The header includes the address of the hosts that will copy the frame. All nodes read the frame as it is passed around the ring to determine if they are the recipient of a message. If they are, they extract the data, retransmitting the frame to the next host on the ring. When the frame returns to the originating station, it removes the frame and reissues a free token which can then be used by another host. The token-access control scheme thus allows all hosts to share the network bandwidth in an orderly and efficient manner. The advantages of Token Ring, in addition to deterministic, are excellent throughput and efficiency at high load. The major minus point is the presence of a centralized monitor function, which includes a critical component. Another disadvantage is that the ring is broken whenever one host is down or the cable breaks.

Application

Stations on a Token Ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. This token passing mechanism is shared by ARCNET, token bus, 100VG-AnyLAN(802.12) and FDDI, and has theoretical advantages over the stochastic CSMA/CD of Ethernet.

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