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A ring network is a network of two or more network devices connected together physically or logically, one after another, so that they form a chain of devices, with the last device in the chain being hooked back to the first device. Ring networks typically are engineered as single-ring or dual-ring topologies. Multi-ring technologies involving two or more parallel rings also have been in development.
Networks generally are characterized in two ways: physically and logically. The term “physical topology” describes the way devices are physically connected together, therefore a physical ring network topology has devices physically connected together to form a ring. The logical topology view is concerned with how information flows. In the logical perspective, a ring network topology might have devices physically connected together as a bus network, star network, mesh network or tree network, but information flows from device to device as if they were connected in a physical ring. For instance, a network might physically be arranged as a star network, but information could flow from device to device as if it were a ring network.
One of the major disadvantages of a single ring network is that a break anywhere in the ring can cause total failure of information flow. To help prevent disruptions of this nature, a second parallel counter-rotating ring or pathway can be added that sends information in the opposite direction. This type of redundant network is called a dual ring network. If one of the rings in a dual ring network suffers damage, information can still reach all of the devices by using the undamaged alternate pathway.
A second disadvantage of ring networks is that information travels more slowly because data has to pass through each device as it makes its way through the network. Despite this limitation, ring topologies are still employed in fiber optic networks such as fiber distributed data interface (FDDI) networks, synchronous optical networking (SONNET) networks and synchronous digital hierarchy (SDH) networks. When these high-speed networks incorporate a physical dual ring topology, they further benefit from the redundancy provided by this type of topology.
Ring networks first became popular in the 1980s when logical ring network topologies were used in token ring technologies. Limitations inherent in a ring network, along with compatibility issues between token ring and other protocols, resulted in token ring technologies being largely replaced by newer data transport methods such as Ethernet. Although Ethernet has increasingly continued to supplant protocols used with fiber optic ring networks, ring network use and development for high speed data transport have continued.