"High availability (HA) means different things to
different people. This article defines availability as the
percentage of time that a computer system is capable of providing
the service that it is assigned to do. A good figure of
availability for computer systems that are used for business
critical tasks, such as running a telephone switch or enterprise
data communication network, is 99.999% of the time (five nines).
This translates to less than six minutes per year that the service
is not available.
CompactPCI traditionally has been the platform of choice for
these five nines systems, because hot swap of components in to and
out of a running system is usually also a requirement.
In a highly available network there should be multiple
independent paths to each system in the network to avoid single
points of failure (SPOF). Physical separation is also a good idea
because if both paths are in the same conduit, and the conduit gets
cut by accident, the network will go down."