Dynamic RWA
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Transcript of Dynamic RWA
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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
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Routing
• Fixed routing• Fixed-alternate routing• Adaptive routing
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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
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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
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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
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Adaptive Routing
• Route is chosen based on the current network state
• Two approaches– Adaptive shortest-cost-path routing– Least-congested-path routing
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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
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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
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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
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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
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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
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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)
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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
)(minmax)( ljlplSj
DMp
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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
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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
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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
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)),),('(),,(( jpjrjprPp
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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
),,(/)),),('(),,(( jprjpjrjprPp
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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
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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.