This paper derives and evaluates mathematical models for supporting differentiated availability services in WDM networks protected by shared backup path protection (SBPP) schemes. These models aim to enable network operators to allocate working and backup resources for dynamic IP traffic connections based on their priority and availability requirements. The resource allocation process is based on optimal trade-off between the amount of allocated backup resources and degree of service availability (quality-of-recovery (QoR)) required for traffic connections arriving dynamically. Using a probabilistic approach based on Markov chain process, mathematical models have been conceived to quantify service availability, disruption rate and blocking probability of M:N and 1:N SBPP schemes in WDM networks. These models have been analytically and through simulation evaluated in terms of network service availability, disruption rate, network utilization and blocking probability.