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)

)( pl

ljD

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• 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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• 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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• 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

),,(min),,()(

jlrjprpl

W

j

jprpR1

),,(),(

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• 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

)),('( pjRPp

)),),('(),,(( jpjrjprPp

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• 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.

dynamic rwa

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