Highly Available Networks

Single Points of Failure in Network Topologies
The key to eliminating communication downtime is to identify and eliminate the SPOFs within the
network. There are many different types of network topologies, each with their own SPOFs. This paper
will discuss three main topology types:
Bus
Star
Ring
Bus Topologies
A bus type network is one where each system taps directly into the same cable, has a matched
impedance, has only two ends, and all signals are terminated at the cable ends. The most common bus
topology uses ThickLAN and ThinLAN cabling. Both use coaxial type cable, commonly shortened to
"coax". With bus topologies, a single segment can be extended by using a repeater. A repeater takes a
signal from one segment and boosts the signal onto the next segment.
Figure 1: Bus Topology:
A SPOF with a bus topology is the cable. If the cable fails, no
system connected to it can communicate on the network. With
bus topologies, it can be difficult to isolate the exact location of
the problem. A fault could be caused by a bad connector or cable
break. Often with coax cables, finding the location of the fault is
done with a Time Domain Reflectometer, indicator lights on the
network interface cards, or tracing the cable segments.
The repeater in the diagram improves availability by segmenting
cable, impedance and termination faults. If the repeater goes
down, one LAN segment will be unable to communicate to other
LAN segment. In addition, a LAN controller card failure isolates
only one system from the LAN, unless the system is also a
gateway to another LAN.
Star Topologies
A star topology is a point-to-point cabling scheme usually connecting a system to a network hub or
concentrator with no connection in between. The two most common star topologies are twisted-pair
cables connecting to hubs, and optical fiber connecting to optical concentrators.