2. Study of the Restoration Path Blocking Problem in Optical Networks Fang Yu EECS Berkeley Joint work with AT&T Research Labs

3. Mesh Restoration in Optical Network Mesh restoration through “intelligent” O-E-O cross- connects (XC) through two paths –Service Path –Restoration Path

4. Shared Risk Group (SRG) Shared fiber span is all the optical fibers collocated in the same –cable –conduit –two consecutive points of access (such as a manhole, central office, or amplifier site) SRG records the fiber span information To provide reliability, service path and restoration path should be SRG disjoint

5. Restoration Path Blocking Problem Restoration path may be blocked if –one or more links along the restoration path doesn’t have sufficient free bandwidth –two restoration paths try to reserve the same channel on the same link (glare problem) If blocked, crankback mechanisms are used to send them back to the originating XCs for further alternate routing on other paths Blocking significantly adds to the restoration time  Minimize blocking

6. Signaling mechanism for restoration path setup –Select an outgoing line –Select enough channels inside the line –On the other cross-connect, reserve corresponding channels on the incoming link. Glare: two path set up messages traveling in opposite directions trying to reserve the same channels Glare Problem Connection 1 Reserve channel 1&2 Connection 2 Reserve channel 1&2 Glare

7. Channel Selection Algorithms Channel selection objective: –Less Fragmentation –Less Contention Hi-Lo technique for selection of channel inside line –Pre-select Master and Slave –The Master XC selects from the highest indexed channels, while the Slave XC selects from the lowest –Glare only happens if two connections are using up all the channels in the line.

8. Line Selection algorithm Widely used maximal packing algorithm for line selection –selects the smallest line that can hold this request –minimize the bandwidth fragmentation –High connection Interleave Line Selection Algorithm to reduce glare –Select the link in the interleaved fashion One end select from 1,3,5,…, 6,4, 2 The other end select from 2,4,6, …,5,3,1 –A little more Fragmentation –Low probability of contention

9. Distributed vs Centralized Restoration Approach Distributed restoration –Fast speed –Avoidance of the high cost to customize centralized systems to control specific network elements –No coordination, high connection for network resource Centralized approach –Efficient use of resource through global optimization –High signaling cost and not scalable –Center server can possibly fail

10. Hybrid Centralized/Distributed Restoration Approach Hybrid Approach –Centralized restoration “path server” to optimize the restoration path selection –Distributed control to compute service paths and set up service/restoration path Advantages –Achieves both the speed of distributed restoration and the use of optimized restoration paths –Path server allows carriers to customize restoration to their own specifications –Can support non-realtime restoration path computation and optimization

11. Simulation Results