<P> Rings can be unidirectional , with all traffic travelling either clockwise or anticlockwise around the ring , or bidirectional ( as in SONET / SDH ) . Because a unidirectional ring topology provides only one pathway between any two nodes , unidirectional ring networks may be disrupted by the failure of a single link . A node failure or cable break might isolate every node attached to the ring . In response , some ring networks add a `` counter-rotating ring '' ( C - Ring ) to form a redundant topology : in the event of a break , data are wrapped back onto the complementary ring before reaching the end of the cable , maintaining a path to every node along the resulting C - Ring . Such `` dual ring '' networks include Spatial Reuse Protocol , Fiber Distributed Data Interface ( FDDI ) , and Resilient Packet Ring . 802.5 networks - also known as IBM token ring networks - avoid the weakness of a ring topology altogether : they actually use a star topology at the physical layer and a media access unit ( MAU ) to imitate a ring at the datalink layer . </P>
<P> Rings can be unidirectional , with all traffic travelling either clockwise or anticlockwise around the ring , or bidirectional ( as in SONET / SDH ) . Because a unidirectional ring topology provides only one pathway between any two nodes , unidirectional ring networks may be disrupted by the failure of a single link . A node failure or cable break might isolate every node attached to the ring . In response , some ring networks add a `` counter-rotating ring '' ( C - Ring ) to form a redundant topology : in the event of a break , data are wrapped back onto the complementary ring before reaching the end of the cable , maintaining a path to every node along the resulting C - Ring . Such `` dual ring '' networks include Spatial Reuse Protocol , Fiber Distributed Data Interface ( FDDI ) , and Resilient Packet Ring . 802.5 networks - also known as IBM token ring networks - avoid the weakness of a ring topology altogether : they actually use a star topology at the physical layer and a media access unit ( MAU ) to imitate a ring at the datalink layer . </P>
a fiber break in the link between the two nodes
a fiber break in the link between the two nodes
node failure
node failure