Duel-Ring Networks-

                Duel-Ring Networks commonly run the Fiber Distributed Data Interface (FDDI).  FDDI is limited to a maximum ring length of 62 miles and operates at 100Mbps.

                There are difference between token ring and FDDI when it comes to token passing.  A computer on an FDDI network can transmit as many; frames as it can produce in a preset time interval before letting the token go.  In addition, several frames can circulate the ring at once.  This gives an overall edge in speed to FDDI over token ring.

                One ring called primary ring; the other ring is the secondary ring.   Under normal operation, all data flows on the primary ring, and the secondary ring remains idle.  The secondary ring is used only if a break occurs in the primary ring.  The ring automatically reconfigures itself to use the secondary ring when necessary and continues to transmit. 

                Workstation generally is connected only to the primary ring.  These single-attachment hosts connect to the ring using a dual-attached concentrator (DAC).  These clients only have a connection to the primary ring.  The DAC and dual-homed stations have connections to both the primary and secondary rings.  When the primary ring is broken, only stations with dual connections are involved in calculating an alternative route.

The primary medium for an FDDI network is fiber-optic cable.  This means

·         An FDDI network is more secure because it does not emit electromagnetic-field signals that can be tapped.

·         An FDDI network can transmit over longer distances without the use of repeaters to strengthen the signal.

·         An FDDI network is immune to electromagnetic noise.