As the telecommunication industry introduces new sophisticated
technologies, the nature of services and the volume of demands have
changed. Indeed, a broad range of new services for users appear,
combining voice, data, graphics, video, etc. This implies new planning
issues. Fiber transmission systems that can carry large amounts of data
on a few strands of wire were introduced. These systems have such a
large bandwidth that the failure of even a single transmission link: in
the network can create a severe service loss to customers. Therefore, a
very high level of service reliability is becoming imperative for both
system users and service providers. Since equipment failures and
accidents cannot be avoided entirely, networks have to be designed so as
to "survive" failures. This is done by judiciously installing spare
capacity over the network so that all traffic interrupted by a failure
may be diverted around that failure by way of this spare or reserve
capacity. This of course translates into huge investments for network
operators. Designing such survivable networks while minimizing spare
capacity costs is, not surprisingly, a major concern of operating
companies which gives rise to very difficult combinatorial problems. In
order to make telecommunication networks survivable, one can essentially
use two different strategies: protection or restoration. The protection
approach preas- signs spare capacity to protect each element of the
network independently, while the restoration approach spreads the
redundant capacity over the whole network and uses it as required in
order to restore the disrupted traffic.
