Wavelength division multiplexing (WDM) technology is considered to be the ultimate answer to the rapidly growing
capacity demand of next generation networks. Many routing and wavelength assignment (RWA) algorithms, proposed
for lightpath provisioning, base their routing decisions on the availability of network resources, and assume that optical
fibers and components are ideal. In reality, physical impairments degrade the quality of the optical signal propagating
through fiber segments and optical components. To cope with this problem, Impairment Constraint Based Routing
(ICBR) algorithms, that consider physical impairments during connection provisioning, are currently proposed to prevent
selecting lightpaths with poor signal quality. However, these algorithms support only a single (highest) quality of
transmission threshold, the same one for all connection requests. This does not fit well with the variety of services, with
potentially disparate QoS requirements, that the next generation networks are expected to support. Consequently, the
efficiency of network resource utilization is reduced.
This paper demonstrates that a significant improvement, in terms of blocking probability, can be achieved when using an
ICBR algorithm with differentiated physical layer constraints. Performance is compared with conventional impairment
aware routing approaches when unprotected, shared path protected (SPP), and dedicated path protected (DPP)
connection requests are considered. The achieved improvement is a result of more efficient resource utilization as
unnecessary connection blocking can be avoided by selecting network resources offering optical signal quality that is
"good enough" to satisfy a specific connection request.