Highly Available Networks

Figure 2: Star Topology: Using Twisted-Pair
With Twisted-Pair, the SPOF is changed from the cable to
the hub. If a single segment from a system to the hub or if
the LAN card on that system fails, only that system is unable
to use the network.
Optical fiber based networks use a concentrator, or a switch,
rather than a hub. The two most common protocols running
over optical fiber are Fiber Data Distributed Interchange
(FDDI) and Fibre Channel (FC) with FDDI currently the
largest percentage. This concentrator remains the SPOF for
the LAN.
Hewlett-Packard (HP) makes two types of FDDI controller
cards: Dual-Attach Stations (DAS) and Single-Attach
Stations (SAS). SAS FDDI is a point-to-point connection
between the system and the concentrator. Therefore, SAS configurations have the same SPOFs as the
twisted-pair. DAS can be either star topology or ring topology. DAS ring topology is discussed in the
next section.
DAS FDDI eliminates the segment as a SPOF. When there are no failures, only the first segment is
being used. When a failure of a segment occurs, FDDI is able to transparently switch to the second
segment and continue LAN operations.
Figure 3: Star Topology: Dual-Attached FDDI with a concentrator
In Figure 3 we can see there are still two SPOFs with DAS FDDI in a ring topology.
The concentrator.
The DAS FDDI controller card on each system.
Ring Topologies
A ring topology is one where each system connects directly to the next system and the next system
connects to the next and so on forming a ring.