Unit 2

Lesson 5 - Ring Topology

   Topology Characteristics

A pure ring is a collection of separate point-to-point links, arranged to make a ring. Each node attached to the ring has one input and one output connection, so that each node is connected to two links. A node attached to a ring may be a single computer or a device used to attach many computers to a common ring.

Signals received on the input connection are passed, immediately and with little buffering, to the output connection by "repeater" circuitry in each node. Thus, data flows only in one direction on some rings. Each node has the ability to put new bits onto the ring to send messages and, if a message is addressed to that node, to copy bits off the ring as messages go by. The most common type of cabling used in pure ring topologies is fiber optic cable.

Ring Topology

Ring Topology

As shown on the Ring Topology Diagram, each node (or access device) is attached to a ring in ring topology. If a node fails (e.g., power is removed), it does not repeat signals from its input. If any node fails to repeat signals, the ring is broken, and data transfer stops until the failing node is restored or removed from the ring. Some ring topologies incorporate a dual-ring structure. Using two rings provides for recovery in case of node or link failures.

Rings are often used as the backbone of networks. A ring backbone provides for connectivity in systems between floors of a multistory building, or between buildings in a metropolitan area network (MAN).

The Star Ring Design

The IBM Token Ring (and the essentially identical IEEE 802.5) is built by combining ring and star topologies to obtain a reliable and serviceable configuration. As depicted on the Star Ring Topology Diagram, one wire runs from each of four nodes to a central ring wiring concentrator, also called a MAU or wiring hub. The concentrator performs two important functions:

It detects when a node is not responding and "locks it out" so that the ring can continue to operate when a node fails. This happens automatically when the concentrator senses a node is not responding.

It provides a "bridge" to other rings, sending information addressed to nodes on other rings across bridge circuits to those rings, and accepting messages from other rings for its nodes. Rings joined in this manner effectively become a single ring. By bridging wiring concentrators, ring size is effectively unlimited.

Star Ring Topology

Star Ring Topology

Star ring topology requires more wire to implement than a pure ring or bus, but the problems of a pure ring are solved, and a ring topology has advantages over a bus topology. Note that the star ring is a physical star. However, information travels from node to node in a logical ring. Each node attached to the central hub receives information in turn, until all nodes attached to the MAU have received the information. Because these networks are physical stars but information travels in a logical ring, they are termed star rings. Twisted pair cable, both shielded and unshielded, is the most common cable used to create star ring networks.

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