Physical ring topology

From Hill2dot0
Jump to: navigation, search
Physical ring topology

The definition of a physical ring topology is simple: Devices are attached via a series of point-to-point links that form a closed loop. In most physical ring topologies, the links were typically simplex, resulting in transmissions that always moved in one direction around the ring. Each device took the signal it received on its input link and repeated the signal to its output link. The devices attached to the ring were the LAN adapters, which provided this repeating function. Because of this feature, LAN adapters attached to a ring were said to be actively attached.

Note that a physical ring also constitutes a broadcast environment, even though it is a collection of point-to-point links. Every LAN adapter sees what every other LAN adapter transmits because each transmission is propagated around the ring, from adapter to adapter, until it returns to the originating LAN adapter. This original transmitter does not repeat the signal again, effectively “taking it off the ring.”

It should be evident that the ring depicted on the visual has serious problems. Note that each of the links and each of the points of attachment is a point of failure. If any one of these fails, the ring is open and transmissions can no longer flow completely around the ring. For this reason, physical rings were seldom deployed in production networks. Where they are used, most LANs implemented using physical rings use two rings, and a mechanism for using one of the rings as an alternate path. This is true of Fiber Distributed Data Interface (FDDI) and can also be applied to some token ring LANs.

The end of both FDDI and token ring as pervasive LAN technologies has meant that the physical ring, and even the logical ring, has largely disappeared from the market.