Dynamic RWA

1. Dynamic RWA • Connection requests arrive sequentially. • Setup a lightpath when a connection request arrives and teardown the lightpath when a connection departs • Goal is to minimize connection blocking • Solve the routing subproblem and the wavelength assignment subproblem separately

2. Routing • Fixed routing • Fixed-alternate routing • Adaptive routing

3. Fixed Routing • Always choose the same fixed route (calculated offline) for a given source-destination pair • E.g. shortest-path routing • Advantage: simple • Disadvantages: • High connection blocking • Unable to handle faults

4. Fixed-Alternate Routing • Each node maintains an ordered list of a fixed set of routes to each destination node • E.g., k shortest-path routes • Primary route: the first route in the list • Alternate route: a route that does not share any links with the first route • Useful for fault tolerance

5. Fixed-Alternate Routing • When a connection request arrives, the source node tries each of the routes in the list in sequence until a route with a valid wavelength assignment is found • Advantages • Simple • Fault tolerance • Significantly reduce the connection blocking probability compared to fixed routing

6. Adaptive Routing • Route is chosen based on the current network state • Two approaches • Adaptive shortest-cost-path routing • Least-congested-path routing

7. Adaptive Shortest-Cost-Path Routing • Use layered graph • Link costs • 1 for unused link •  for used link • c for wavelength conversion link • When a connection request arrives, compute the shortest-cost path between source and destination • Advantage: low blocking • Disadvantage: nodes need update network state whenever a connection is setup/teardown

8. Least-Congested-Path (LCP) Routing • For each s-d pair, a set of routes is predetermined • When a connection request arrives, the least-congested path is chosen • Congestion on a link = # wavelengths available on the link • Fewer available wavelength  more congested • Congestion on a path = congestion on the most congested link in the path • Use shortest-path routing to break ties • An alternative: give priority to shortest paths, use LCP to break ties

9. Wavelength Assignment Heuristics • Assume fixed number of wavelengths • Minimize overall blocking probability for all connection requests • Single-fiber networks: R, FF, LU, MU • Multi-fiber networks: MP, LL, M, RCL • Protect multihop connections to achieve greater degree of fairness • Rsv, Thr

10. Wavelength Assignment Heuristics: Single-Fiber Case • Random Wavelength Assignment (R): • Find all wavelengths available on the required route • Randomly choose one available wavelength • First-Fit (FF) • Wavelengths are numbered • Choose the first available wavelength • Computation cost lower than R • Perform well in terms of blocking probability and fairness • Both R and FF require no global knowledge

11. Wavelength Assignment Heuristics: Single-Fiber Case • Least-Used (LU)/SPREAD • Choose the least used wavelength • Attempt to balance the load among all wavelengths • Favor short paths, not fair for long paths • Perform worse than random • Most-Used (MU)/PACK • Choose the most used wavelength • Pack connections into fewer wavelengths • Slightly outperform FF • Both LU and MU require global knowledge

12. Wavelength Assignment Heuristics: Multi-Fiber Case • Min-Product (MP) • Goal: minimize # fibers by packing wavelengths into fibers • First compute for each wavelength j that is available on p • Choose the lowest numbered wavelength in the set of wavelengths that minimize the above value • Become FF in single-fiber networks • Perform worse than the multi-fiber version of FF (both fibers and wavelengths are ordered)

13. Wavelength Assignment Heuristics: Multi-Fiber Case • Least-Loaded (LL) • Select the wavelength that has the largest residue capacity on the most loaded link along route p • Choose the minimum indexed wavelength j in Sp that achieves • Become FF in single-fiber networks • Outperform MU and FF

14. Wavelength Assignment Heuristics: Multi-Fiber Case • MAX-SUM (M) • Assume the set of possible connection requests is known in advance and the route for each connection is pre-selected • Attempt to maximize the remaining path capacities after lightpath establishment

15. Wavelength Assignment Heuristics: Multi-Fiber Case • MAX-SUM (M) • : a network state that specifies the routes and wavelength assignments of existing lightpaths • Link capacity r(, l , j) on link l and wavelength j in state : # fibers on which wavelength j is unused on link l • Path capacity r(, p , j) on path p and wavelength j: # fibers on which wavelength j is available on the most congested link along path p • Path capacity of path p in state , R(, p): sum of path capacities on all wavelengths

16. Wavelength Assignment Heuristics: Multi-Fiber Case • MAX-SUM (M) • ’(j): the next state of the network if j is assigned to the connection • P: set of all potential paths for connection requests in the current state • Choose the wavelength j that maximizes • Equivalently, choose wavelength j that minimizes the total capacity loss on this wavelength, which is

19. Wavelength Assignment Heuristics: Multi-Fiber Case • Relative Capacity Loss (RCL) • Improve on M by taking into consideration # available alternate wavelengths for each potential future connection • RCL chooses wavelength j that minimizes the sum of the relative capacity loss on all the paths

21. Heuristics for Protecting Multihop Paths • Longer lightpaths have a higher probability of getting blocked than shorter paths  want protect longer paths • Proposed schemes: Rsv and Thr • Only specify whether the connection request can be assigned a wavelength under the current wavelength-usage conditions  must be combined with other wavelength assignment schemes • Achieve a greater degree of fairness

22. Heuristics for Protecting Multihop Paths • Wavelength Reservation (Rsv) • A given wavelength on a specified link is reserved for a multihop traffic stream • Reduce blocking for multihop traffic while increasing the blocking for single-hop traffic • Protecting Threshold (Thr) • A single-hop connection is assigned a wavelength only if the number of idle wavelengths on the link is at or above a given threshold.