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 sourcedestination 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 leastcongested 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  Dlj for each wavelength j that is
available on p l ( 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
min ( M l  Dlj )
Sp that achieves max
jS p l ( p )
– Become FF in single-fiber networks
– Outperform MU and FF
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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 r ( , p, j )  min r ( , l , j )
l ( p )
– Path capacity of path p in state , R(, p): sum of path
W
capacities on all wavelengths
R( , p)   r ( , p, j )
j 1
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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
R( ' ( j ), p)
– Choose the wavelength j that maximizes 
pP
– Equivalently, choose wavelength j that
minimizes the total capacity loss on this
wavelength, which is  (r ( , p, j)  r ( ' ( j), p, j))
pP
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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
 (r( , p, j)  r( ' ( j), p, j)) / r( , p, j)
pP
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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 wavelengthusage 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.
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