Token Ring networks

The Token Ring version of the network was introduced to IBM in 1984 as part of a method proposed by the company for networking the entire range of computers and computer systems manufactured by IBM. In 1985, Token Ring became the ANSI/IEEE standard (ANSI is the ISO representative in the USA).

The Token Ring network is an implementation of the IEEE 802.5 standard

The Token Ring network is an implementation of the IEEE 802.5 standard. It is distinguished from other networks not only by the presence of a unique cable system, but also by the use of an access method with a token transfer. The typical network topology is a star/ring. The connection is performed through a hub in the form of a star, and the physical ring is implemented in the hub. Cable system - UTP and STP. The transfer rate is 4 and 16 Mbit/s.

When the first computer starts working on the network, it generates a marker. The marker goes around the ring from the computer to the computer (the direction of movement of the marker depends on the equipment) until one of them indicates that it is ready to transfer data and does not take control of the marker. A marker is a predefined bit sequence that allows a computer to send data via cable. When a marker is captured by any computer, others cannot transfer data. Capturing the marker, the computer sends a frame of data to the network. The frame passes through the ring until it reaches the node with the address corresponding to the address of the receiver in the frame. The receiving computer copies the frame to the receive buffer and marks the frame status information field. The frame continues to be transmitted around the ring until it reaches the computer that sent it, which makes sure that the transfer was successful. The computer removes the frame from the ring and returns the marker there. Only one marker can be transmitted in a network at a time, and only in one direction.

Token passing is a deterministic process. This means that the computer cannot independently start working on the network (as with the ŃSMA/CD access method). It can transfer data only after receiving a token. Each computer acts as a unidirectional repeater, regenerating the marker and sending it further along the ring.

The main component of Token Ring networks is a hub that implements a physical ring. In a network with the transfer of a marker, a failed computer or connection stops the movement of the marker, which leads to the termination of the entire network. Hubs are designed to detect a failed CA board and turn it off in time. This procedure allows you to bypass the failed computer, so the marker continues to circulate through the network.