Photonic Network Communications, 8:2, 191±207, 2004
# 2004 Kluwer Academic Publishers. Manufactured in The Netherlands.
Capacity Ef®ciency and Restorability of Path Protection and Rerouting
in WDM Networks Subject to Dual Failures
Dominic A. Schupke,* Robert G. Prinz
Munich University of Technology, Institute of Communication Networks, D-80290 Munich, Germany
Received July 28, 2003; Revised October 23, 2003; Accepted October 24, 2003
Abstract. Resilient optical networks are predominately designed to protect against single failures of ®ber links. But in larger networks,
operators also see dual failures. As the capacity was planned for single failures, disconnections can occur by dual failures even if enough
topological connectivity is provided.
In our approach the design of the network minimizes the average loss caused by dual failures, while single failures are still fully survived.
High dual failure restorability is the primary aim, capacity is optimized in a second step.
For WDM networks with full wavelength conversion, we formulate mixed integer linear programming models for dedicated path protection,
shared (backup) path protection, and path rerouting with and without stub-release. For larger problem instances in path rerouting, we propose
Computational results indicate that the connectivity is of much more importance for high restorability values than the overall protection
capacity. Shared protection has similar restorability levels as dedicated protection while the capacity is comparable to rerouting. Rerouting
surpasses the protection mechanisms in restorability and comes close to 100% dual failure survivability. Compared to single failure planning,
both shared path protection and rerouting need signi®cantly more capacity in dual failure planning.
Keywords: multiple failures, path protection, rerouting, path restoration, WDM networks
Optical networks are subject to many kinds of
disruptions, especially environmental disruptions
and those caused by hardware failures and operational
errors. The most important failure type is regarded as
the ®ber cut, for example, caused by a backhoe. As
such a cut typically breaks all ®bers of a ®ber link, the
end-nodes which were previously neighbors become
disconnected. In order to re-establish the connectivity
of disrupted paths, appropriate protection and restora-
tion mechanisms are deployed in the network. Often
these networks are designed to protect against single
failures of such ®ber links at a time.
But for larger and more connected networks dual
failuresÐthough much less probable than single
failuresÐshould be taken into account [1,2]. This
can be made plausible by the following calculation.
Consider a network with ®ber links of equal length l,
Then a ®ber link has the availability of:
where MTBF is the mean time between failures and
MTTR is the mean time to repair of one link-
kilometer. For a network with m ®ber links we
calculate the probability for two simultaneous link
1 À a
m À 2
Assume a MTBF value of 300 years per kilometer and a
MTTR value of 8 hours. The mean time of dual failures
in a network with 25 ®ber links of 500 km length is then