I. J. Communications, Network and System Sciences, 2008, 2, 105-206
Published Online May 2008 in SciRes (http://www.SRPublishing.org/journal/ijcns/).
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
Routing and Wavelength Assignment in GMPLS-based 10
Gb/s Ethernet Long Haul Optical Networks with and
without Linear Dispersion Constraints
Le Nguyen BINH1,2
1 Department of Electrical and Computer Systems Engineering, Monash University, Australia
2 Lehrstuhl für Nachrichen- und Übertragungstechnik,
Technische Fakultaet der Christian Albretchs Universitaet zu Kiel, Germany
E-mail: le.nguyen.binh@eng.monash.edu.au
Given a set of lightpath connection requests in an all-10 Gb/s optical dense wavelength division multiplexed
(DWDM) Ethernet network, lightpaths are designed. In addition the wavelength channels are assigned
subject to minimization of the channel blocking and provisional requests satisfying the limits due to
accumulative linear dispersion effects over the hops.
This paper proposes a routing and wavelength assignment scheme for DWDM long-haul optical networks
that includes routing, assignment and reservation of different wavelength channels operating under the
Generalized Multiprotocol Label Switching (GMPLS) environment. The GMPLS framework can offer an
approach to implement IP over DWDM with variable weighting assignments of routes based on the
limitations due to residual dispersion accumulated on the lightwave path.
The modeling is implemented under the framework of an object-oriented modeling platform OMNeT++.
Network performance tests are evaluated based mainly on a long-haul terrestrial fiber mesh network
composed of as well as three topologies structured as chain, ring, and mesh configurations. Blocking
probability of lightpath connection requests are examined with the average link utilization in the network
employing variable number of wavelength channels in association with the limits of route distance due to
linear chromatic and polarization mode dispersion effects.
Keywords: DWDM Optical Networks, Optical Transmission Systems, GMPLS, Routing and Wavelength
Assignment (RWA), Wavelength Routers.
1. Introduction
In recent years, since the invention of optical amplifiers
in late 1980s advances in dense wavelength division
multiplexing (DWDM) technology has enabled the
implementation of multi-Tera/bits/sec. capacity in
intelligent optical networks (ION) [1]. In particular 10
Gb/s Ethernet optical networks over DWDM have been
standardized and emerges as the most likely ultra-fast
information networks in the near future. Undoubtedly
systems and networks need to be interconnected in the
optical wavelength domain or layer via optical routers
and photonic switches and managed under appropriate
protocols. The DWDM technology offers the capability
of building very large wide area network (WAN)
consisting of thousands of nodes with per-node
throughputs of the order of tens of Gb/s.
However, at 10 Gb/s and possibly higher with the
possibility of upgrading certain number of channels to
40 Gb/s or even 100 Gb/s, the detrimental effects of
linear dispersion effects such as the chromatic dispersion
(CD) and polarization mode dispersion (PMD) and are
critical for error free transmission and interconnect
between the IP routers. Furthermore, the number of
wavelength channels are high with the average total
power may be over the nonlinear threshold for the
transmission lightpaths that would lead to the reduction
of the eye opening of the receiver signals. We must note
that these effects are not very critical for 2.5 Gb/s
transmission bit rate which is now commonly used in
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
present optical networks.
In this paper we include only the residual linear
dispersion and the PMD effects into the investigation of
traffic engineering in IP over WDM optical networks,
especially the routing and wavelength assignment (RWA)
of wavelength channels over fiber paths of the networks.
We understand that these transmission issues have not
ever been taken into account in published research
works. Due to the linear dispersion tolerance and
nonlinear effects currently several advanced modulation
formats have been studied [1] in order to combat these
impairments. Usually these distortion effects are
dependent on the length of the transmission that is
commonly faced in all optical networks, especially in
backbone WDM networks.
Routing and wavelength assignment (RWA), i.e. a
cross-connecting of several wavelength allocated
channels between DWDM optical networks, is very
important [3]. One unique feature of DWDM networks
is the tight coupling between routing and wavelength
selection. Routing and wavelength assignment (RWA)
problem is a major and complex problem associated with
optical networks. Thus, given that (i) a set of dynamic
and randomly chosen lightpaths that need to be
established; (ii) constraints of the total number of
wavelengths propagating in an optical fiber; and (iii)
wavelength continuity constraint (WCC), which means a
lightpath must use the same wavelength on all the links
along its path from source to destination edge node, The
controller for routing and assignment of wavelength
channels must determine the routes over which these
lightpaths should be set up and determine the
wavelengths to be assigned to these lightpaths so that an
optimum number of lightpaths may be established for
the routing. Each wavelength channel is associated with
a dispersion factor. Due to the dispersion slope of the
transmission fiber, the dispersion compensation using
dispersion compensating fiber is normally fully
compensated at the center wavelength of the wavelength
band. Thus there exists residual chromatic dispersion
(CD) at other channels. At 10 Gb/s these residual
dispersion effects are very critical. Furthermore the
dispersion due to different traveling velocity of polarized
modes of the single mode fiber, the polarization mode
dispersion (PMD) is also important and must be taken
into account.
Lightpaths that cannot be set up due to constraints on
routes or wavelengths or the linear CD and PMD at
which the eye opening penalty (EOP) at the receive end
would suffer 3 dB, are said to be blocked, so the
corresponding network optimization problem is to
maximize the probability of set-up for a current
connection request while minimizing the blocking
probability of future connection requests. Furthermore in
long haul terrestrial and or intercontinental optical
networks, a generic management of the whole network
status is preferred.
The concept of GMPLS offers an approach to
implement IP over DWDM [4–10]. Indeed, GMPLS
becomes evident that it is the best control plane solution
for next-generation optical networking. In GMPLS, the
MPLS label is generalized so that a label can also be
encoded as a time slot, a wavelength, or a spatial
identifier. The embrace by GMPLS is extremely large;
the most critical part of it is to solve the general problem
of dynamic lightpath establishment.
Therefore, in this paper various schemes for RWA
algorithms are studied, compared and selectively
implemented on a long haul optical terrestrial mesh
network based on a common control plane GMPLS
framework, especially when residual chromatic
dispersion and polarization mode dispersion effects are
taken into account. The nonlinear phase noises and
impairments in network routing and wavelength
assignment will be reported in a future article. Other
network topologies also are investigated in this work
such as a 4-node/8-node chain, a 5-node ring, and a
terrestrial mesh network. We studied two kinds of
wavelength assignment methods in all-optical networks:
First-fit (Fixed-order) and random wavelength
assignment, and two kinds of link metrics: simple total
and available wavelength (TAW) and enhanced TAW.
We use OMNeT++ [12] to simulate GMPLS-based all-
optical networks and compare the blocking probabilities
of these RWA methods on different topologies and
traffic models. The simulation results show that first-fit
wavelength assignment has a lower blocking probability
than random wavelength assignment, and enhanced
TAW performs better than simple TAW when link
utilization becomes high. We also found that under some
circumstances the difference in blocking probability may
be widened and worsened due to additional limitations
due the total accumulated dispersion along a routed
The paper is organized as follows: Section 2 outlines
the physical constraints on the routings of wavelength
channels over long haul optical networks whose
parameters are then declared. Also an ultra-fast
automatic wavelength router is proposed. The specific
characteristics of GMPLS are given. Section 3 and 4
propose the RWA algorithm and structures of simulation
platform. The issues involved RWA problems in such
linear dispersion (CD and PMD) effects are given. Our
simulation model and simulation results are given in
Section 5. Traffic performances of the proposed RWA
under different network scenario under with and without
dispersion effects are obtained and described. Finally,
conclusion and further works are found in Section 6.
2. Network Physical Constraints
Routing and wavelength selection are coupled tightly in
156 L. N. BINH
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
WDM all-optical networks. When establishing a
connection in optical networks, both routing (path
selection) and wavelength assignment (allocating a
wavelength along the selected path) must be considered.
This is referred to as routing and wavelength assignment
(RWA) problem. This problem is more complex than
routing problem in electronic networks. Optical
networks operating at lower bit rates less than 2.5 Gb/s
can be routed under constraints of [3]: (i) Wavelength
continuity constraint (WCC): a lightpath must use the
same wavelength on all the links along its path from
source to destination edge node; (ii) Distinct wavelength
constraint: all lightpaths using the same link (fiber) must
be allocated distinct wavelengths; (iii) Limited number
of wavelength per fiber link is restricted within the C-
band and by the nonlinear threshold, mainly due to the
self phase modulation (SPM) effects. Furthermore for
higher bit rates 10 Gb/s and above additional constraints
are (iv) Total fiber routing path is limited due to the total
residual dispersion allowable on the total routing
distance that suffer an eye opening penalty of 3 dB; and
(v) Total accumulated noise and hence EOP due to
amplified stimulated amplification within optical
amplifiers placed in each fiber spans that is important for
backbone long haul networks. These noise contributions
are ignored in this work.
It is noted here that the dispersion compensation can
normally be made for a specific wavelength of the C-
band and there are always residual dispersion
mismatched at other wavelength channels due to
dispersion slopes. Thus the effects of residual dispersion
are very critical in the design of the DWDM network
routing and wavelength assignment as these dispersion
effects are length dependent.
2.1. Wavelength Continuity Constraint (WCC)
The WCC is a unique constraint of WDM all-optical
networks. In WDM networks, if a connection is accepted,
it should be assigned a path through the network and a
wavelength. This wavelength must be the same
wavelength on all links along the path. Consequently, it
is likely that any algorithm engaging WCC would suffer
higher blocking probability. If another lightpath has to
be set up between node 1 and node 3, it will be blocked,
because there is no available wavelength along its
lightpath (i.e. no common available wavelength on both
links). Due to high costs of all-optical wavelength-
conversion photonic components, it is assumed that none
of the optical cross connectors (OXCs) has wavelength
conversion capability.
The algorithm must establish dynamically an end-to-
end path between any ingress nodes and any egress
nodes in the all-optical network. Our main goal is to
minimize the blocking of lightpaths in the network.
Different metrics for dynamic routing algorithms are
examined. For those parameters which are not
wavelength-based then constraints are not taken into
account, unlike in other published works physical
constraints are integrated them in the routing decision
process [1].
2.2. Distinct Wavelength Constraint
Different lightpaths on the same physical link (fiber)
must use different wavelengths. Each fiber in WDM
networks may contain several WDM wavelengths
subject to power below specific power threshold due to
nonlinear effects. However these nonlinear effects are
ignored in this report. It is assumed that the operating
wavelengths are placed in a 50, 100 or 200 GHz grid
(0.4nm wavelength spacing) with a nominal center
frequency of 193.1THz (1552.52 nm) in the middle of
the 1.55µm fiber and EDFA pass-band [4]. More details
are specified in ITU G.692.
2.3. Number of Wavelengths per Fiber Link
Consider one link between two adjacent nodes. Each
link cable may be composed of several fibers in which
several multiplexed wavelength channels are transmitted.
Figure 1 shows a typical automatic wavelength optical
cross connect (OXC) including wavelength converters,
wavelength switching matrix whose control signals
come from the central management systems complying
the state of the designed algorithm and global
information obtained from the nodes of the networks.
The wavelength converter is a semiconductor optical
amplifier biased to operate for wavelength conversion.
This conversion is associated with the wavelength
multiplexer so that once the conversion is completed to a
wavelength output port the channel is routed
immediately to the output. Thus this structure allows an
ultra-fast routing. The limit of the operation of this OXC
depends on the conversion speed of the SOA which is,
under present technology, in order of picoseconds for
GHz operation.
Let λi, j be the wavelength number i in the fiber j.
Assuming the operating wavelength range is the C-band.
The wavelengths range in the C-band is 1,530nm to
1,565 nm. Each wavelength is assigned a specific
emission wavelength that is specified by the ITU-T.
Consequently, it is assumed that there is a maximum of
44 wavelengths per fiber with 100 GHz spacing between
channels. The wavelengths are ordered as follows: λ1, j =
1,530.33nm and λ44, j = 1,564.68nm, with j being the
number of fibers used. In a nutshell, the physical-based
wavelength constraints are:
1, 44,
..........1 44
1,530.33 ;1,564.68
wavelength i injfiberi
nmnm C band
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
wavelength channels propagating in the same fiber path.
Figure 1. Schematic of an optical cross connect (OXC)
incorporating wavelength conversion and routing by an
SOA and demultiplexer and switching matrix for routing a
specific wavelength to a designated output port for further
Still, it can be shown that a multi-fibre RWA problem
is algorithmically equivalent to a single fibre based
optical network. Thus, in the following, an ordered list
of wavelengths with only one fibre is considered.
Further it is very unlikely that the 44 wavelengths
transmitted or available in the C band will be all
practically employed. Usually only a few wavelength
channels are lit. Thus it is reasonable to assume that the
population of the WDM slots is limited to a multiple of 8
wavelengths, up to 32 or 40 with 100 GHz spacing
between wavelength channels. As the number of fibers
and the optical power launched in each wavelength,
there are possibilities that the total average power
propagating through a fiber path would reach above the
nonlinear threshold (normally 3 dBm) and hence
nonlinear phase distortion. In this work we set the limit
at 10 dBm taking into account the randomness of the
chance of simultaneous transmission of all channels.
2.4. Residual Linear Dispersion Constraints
For high capacity long-haul transmission employing
external modulation, the dispersion limit can be
estimated in the following equation [38,39].
= (2)
where D is the dispersion factor and BR is the bit rate, LD
is the dispersion length.
2.5. Polarization Mode Dispersion Limit
The length limit by the PMD effect can be defined as
follows. Constraint on the dispersion length due to PMD
must be included and an estimate of the transmission
length limit due to PMD effect is given as [39]
with LPMD is the maximum length due to the PMD effect
that generates a 1dB penalty on the eye opening.
2.6. GMPLS Framework
Generalized Multiprotocol Label Switching (GMPLS)
extends the label switching architecture proposed in
MPLS to other types of non-packet based networks,
such as SONET/SDH based networks and WDM
networks. It has recently become evident that GMPLS is
the unified control plane solution for ultra-high capacity,
ultra-fast optical networking. In GMPLS, the MPLS
label is generalized so that a label can also be encoded as
a time slot, a wavelength, or a spatial identifier. The
basis of the GMPLS framework is defined in [5,9],
including two main parts of routing and signaling.
GMPLS routing proposes extensions to interior
gateway protocols (IGP) of routing being the open
shortest path first extended for traffic engineering
(OSPF-TE) [7] and intersystem to intersystem extended
for traffic engineering (ISIS-TE) [8]. The protocols
provide the distribution of link state information with
traffic engineering information and attributes such as
network topology, resource availability, and
administrative constraints.
GMPLS signaling protocols include reservation
protocol extended for traffic engineering (RSVP-TE) [10]
and the constrained label distribution protocol (CR-LDP)
of the MPLS-TE framework [11]. The GMPLS signaling
protocols extend certain base functions of the RSVP-TE
and CR-LDP signaling, which originally configure and
control the distributed label switched paths (LSP). The
extensions impact basically on LSP properties on how
labels are requested and communicated, unidirectional
nature of LSPs, how errors are propagated, and
information provided for synchronizing the ingress and
egress nodes.
3. Routing and Wavelength Assignment
RWA algorithm must be able to identify the maximum
of explicit routes in IONs in order to satisfy dynamic
routing requests. The RWA algorithm separates routing,
wavelength assignment (WA) and reservation
158 L. N. BINH
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
mechanisms. The search of an explicit route in the
network that would meet the physical constraints of the
ION is the principal aspect. Such a separation between
routing and reservation protocols eases further works.
RWA algorithm is designed so that it gives an output
an explicit route in the ION that determines each node to
be traversed by the lightpath. Besides, a WA algorithm
allows the determination of a wavelength channel
satisfying the WCC that is reserved on the lightpath.
Explicit routes and their associate wavelengths are then
treated as the inputs of a reservation protocol in order to
establish the optical circuit in the network. Present work
develops a RWA scheme model of a GMPLS-based ION
based on an adaptive link-state routing using global
network knowledge including the rise time and
dispersion budget of proposed routing link/distance.
This choice is due to the followings. Table 1 shows a
summary of the WA algorithms on a RWA problem.
Table 1. WA (wavelength assignment) algorithms in the
RWA problem
heuristics Characteristics Advantage Drawback
The wavelengths are
indexed, and a
lightpath will attempt
to select the
wavelength with the
lowest index before
attempting to select a
wavelength with a
higher index
If global
blocking if
Select one of the
wavelengths at
If local
Very simple.
Does not
The wavelength
which is the most
used in the rest of the
network is selected.
Spreads the
load evenly
across all
The wavelength
which is the most
used in the rest of the
network is selected.
reuse in the
3.1. Routing
Several types of RWA algorithms have been studied.
Three main types of routing schemes are fixed, fixed-
alternate and dynamic routing. Fixed and fixed-alternate
routing principles are very simple. However, they have
poor capabilities when dealing with self-healing
capabilities and can yield a very high blocking
probability under dynamic network operations. Thus,
dynamic routing is considered.
The main disadvantage of dynamic routing is
possibility for high overhead due to route advertisements.
Nevertheless, the choice of an algorithm is always a
trade-off between network performance and traffic
overhead. Considering earlier research studies presented
in [3,12,13], it has been thought that a dynamic routing
is one of the best ways to solve the RWA problem.
In the first instance, both adaptive routing schemes,
that is link-state routing and distance-vector routing are
considered. Contrary to link-state based routing
algorithms that flood packets onto the network distance-
vector algorithms send their route advertisements only to
their neighbors. In link-state algorithms, the link state
update is flooded onto the whole network or area of
3.1.1. Distance-vector Routing Algorithm
A distance-vector routing approach needs to use a
distributed algorithm such as the popular Bellman-Ford
algorithm. It is possible that a distance-vector algorithm
can minimize the traffic overhead in the network while
still giving relatively low blocking probabilities under
high loads. A distance-vector algorithm is also
interesting when considering constraint routing. Indeed,
it is a property of the Bellman-Ford algorithm that, at its
h-th iteration, it identifies the optimal (in our context:
maximal number of wavelengths) path between the
source and each destination, among paths of at most h
hops. This is recommended for QoS routing
implementations [14]. Furthermore the distance vector is
also tagged with the dispersion and distortion estimated
for the routing distance.
Bellman-Ford algorithm progresses by increasing hop
count, it essentially provides for free the hop count of a
path as a second optimization criterion. This property is
very interesting when applied to all-optical networks.
However, even if the shortest path is sacrificed for a
longer – in hops – route, the lightpath should not be too
long as this could lead to a poor optical power and
dispersion budgets.
Routing Information Protocol (RIP) is the most
popular implementation of a distance-vector based
protocol because it is simple and it is well suited to small
networks. However, RIP has several flaws that make it
unsuitable for ION. Particularly, RIP is unsuitable for
large configurations and the convergence of the
algorithm can also be lengthy; it suffers also from the
count-to-infinity problem, of which the best remedy is to
implement link-state algorithms.
3.1.2. Link-state Based Routing Algorithm
In link-state based routing, information is only sent
when changes occur. A node builds up first a description
of the topology of the network. Then it may use any
routing algorithm to determine the route.
In the context of the ION, in order to compute an
explicit route, it is also much easier to use a link-state
based routing algorithm. Indeed, the lightpath to be
established is more optimal if each node has a global
knowledge of the network. It is also a property of the
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
Dijkstra algorithm that a complete route from a source to
any other node in the network can be easily found by
recursive iteration on the graph.
Link-state based algorithms can use any routing
algorithm. Different RWA algorithms can be
implemented based on different optimization schemes
(partial or total wavelength knowledge).
The open shortest path first (OSPF) protocol, known
mostly for its second version [23], is the most widely
known link-state based routing protocol and is employed
for the simulation platform of this work. It has been
increasingly popular over RIP, because it is most
suitable for large networks. OSPF is an open source
algorithm that can be found in different languages,
including C++. This is a particular advantage for
simulation program based on OMNeT++, a C++ based
3.1.3. Route Advertisements
The route advertisement messages are built according to
the OSPF extensions for GMPLS. This work proposes to
extend the sub type length value (TLV) field relative to
the interface switching capacity descriptor (ISCD) field
of the GMPLS OSPF extensions.
In order to dynamically monitor the state of the
network, each GMPLS router keeps track of all the
wavelength capabilities of the whole network in a links
database. This database is constantly updated each time
a “routing” message is received. The “routing” message
contains the description of the wavelength capabilities of
a certain link that have changed very recently.
Those “routing” messages are actually flooded on the
whole network by the node which one of its links’
capabilities changed containing the address of the
extremities of the link of which the wavelength
capability has changed and the state of the changed
The links database is composed of records, where
each record describes one link of the network. Each
record contains the node addresses of the link’s
extremities, a wavelength capability field and a metric
field. The wavelength capability field describes
explicitly the wavelength resources of the considered
link. The metric field is the cost to use this link when
performing the shortest path calculation. In this work,
two different metrics have been implemented. They are
both function of the total number of wavelengths and the
number of available wavelength(s) on the link as
described in the next part.
3.1.4. Link Metrics
The link metrics represent the costs to use certain links
in the network. Intuitively, the link cost is a linear
function of the total number of wavelength and the
number of available wavelengths and the residual
dispersion if taken into account.
The routing scheme is tested under different link
metrics: simple total and available wavelength (TAW) or
enhanced TAW. Let a
be the number of available
(unused) wavelengths on the link (i,j) and the total
number of possible wavelengths on that link. The simple
TAW metric represents the load assigned to a link and is
defined by:
Ejiw T
ji ∈∀−= ,,1
The enhanced TAW metric is to test the metric
proposed by [24] that equivalently minimizes the
probability of blocking on an explicit route in which the
weights are assigned following a log scale as:
ji ∈∀
−−−= ,,11log
If residual dispersion is accounted then a constant or
a linear or quadratic function of dispersion factor can be
added into (4) or (5).
3.1.5. Path Calculation
The links database that has been freshly updated by the
GMPLS router serves as the basis of the path discovery
calculation based on the Dijkstra algorithm. This path
computation is performed by each node in the network
when it receives a “routing” message, which is an
update of a certain link capability in the network.
The routing algorithm actually allows each node to
build its own photonic database that contains N records,
where N is the number of nodes of the network. Each
record is based on the following structure: (i) a node
destination address — this identifies the record; it is the
node to reach; (ii) a total cost to this destination node —
this is the total cost when taking the shortest path route
to the destination node; (iii) an address of the last-but-
one node on the shortest path route to the destination
node; (iv) an end-to-end available wavelength capability,
which determinates explicitly the possible wavelengths
to be assigned, if any, on the shortest path route; and (v)
an explicit route holding an ordered list of all the
addresses of the nodes on the shortest path route.
Given the links database, the first three fields are
actually the direct output of the Dijkstra shortest path
algorithm [14]. The last two fields are the result of a
sub-routine of the Dijkstra algorithm. Indeed, it is a
property of the Dijkstra algorithm that it is possible to
find the list of the nodes of each shortest path by a
simple recursive call.
We have also set constraints on the dispersion and
attenuation of the fiber paths, in particular the
polarization mode dispersion factor (PMD) as PMD is
160 L. N. BINH
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
unlike the chromatic dispersion dependent on the fiber
length. For chromatic dispersion we assume that the
(standard single mode fiber) SSMF is compensated with
dispersion compensating fiber and only residual
dispersion is used. At 10 Gb/s these dispersion effects
are very critical. Additional nonlinear phase and hence
distortion effects are not considered in this work.
3.1.6. Lightpath Establishment
There are two categories of lightpath establishment, one
is the static lightpath establishment (SLE), and the other
is dynamic lightpath establishment (DLE).
3.1.7. Static Lightpath Establishment
Static lightpath establishment (SLE) is used in the design
and capacity planning phase of architecting an optical
network. It can be logically decomposed into four sub-
problems. Assuming no wavelength conversion, the sub-
problems are listed as follows [33]: (i) Topology sub-
problem: determine the logical topology to be imposed
on the physical topology, that is, determine the
lightpaths in terms of their source and destination edge
nodes; (ii) Lightpath routing sub-problem: determine the
physical links which each lightpath consists of, that is,
route the lightpaths over the physical topology; (iii)
Wavelength assignment sub-problem: Determine the
wavelength each lightpath uses, that is, assign a
wavelength to each lightpath in the logical topology so
that wavelength restrictions are obeyed for each physical
link; and (iv) Traffic routing sub-problem: Route packet
traffic between source and destination edge nodes over
the logical topology obtained.
The SLE is also referred as static RWA problem, can
be formulated as an integer linear program (ILP) where
the objective is to maximize the number of connections
that are successfully routed. A number of studies have
investigated the static RWA problem.[12,15,17,29]. The
ILP formulations are NP-complete and therefore may
only be solved for very small systems. For large systems,
heuristic methods must be used. A number of heuristics
were proposed for the problem, they can be divided in
four classes [17]: (i) heuristic solutions of the mixed
integer linear programming problem; (ii) maximizations
of the single-hop traffic flows; (iii) heuristic
maximizations of the single-hop and multi-hop traffic
flows; (iv) algorithms based on the adoption of a pre-
established regular logical topology and on the
optimization of the nodes placement according to the
traffic pattern.
3.1.8. Routing Sub-problems
Fixed Routing — The most straightforward approach to
routing a connection is to always choose the same fixed
route for a given source-destination pair. This is called
fixed routing. Fixed routing approach is easy to
implement. However, it is very limited in terms of
routing options. If resources (wavelengths) along the
path are tied up, it can potentially lead to high blocking
probabilities in the dynamic case, or may result in a
large number of wavelengths being used in the static
case. Moreover, fixed routing may be unable to handle
fault situations in which one or more links in the
network fail.
Fixed-Alternate Routing — An approach to routing
that considers multiple routes is fixed-alternate routing.
It increases the likelihood of establishing a connection
by taking into account network state information. In
fixed-alternate routing, each node in the network is
required to maintain a routing table that contains an
ordered list of a number of fixed routes to each
destination node. Fixed-alternate routing provides
simplicity of control for setting up and tearing down
lightpaths and it may also be used to provide some
degree of fault tolerance upon link failures. It can
significantly reduce the connection blocking probability
compared to fixed routing.
Adaptive Routing — In adaptive routing, route form
a source node to a destination node is chosen
dynamically, depending on the network state. The
network state is determined by the status of all
connections that are currently in progress. In order to
choose an optimal route, a cost is assigned to each link
in the network based on current network state
information, such as wavelength availability on links,
the total residual dispersion amount. A least-cost
algorithm is then executed to find the minimum-cost
route. Whenever a connection is established or taken
down, the network state information is then updated.
3.2. Wavelength Assignment
In general, if there are multiple feasible wavelengths
between a source node and a destination node, then a
wavelength assignment algorithm is required to select a
wavelength for a given lightpath. The wavelength
selection may be performed either after a route has been
determined, or in parallel with finding a route.
Since the same wavelength must be used on all links
in a lightpath (noted that wavelength conversion is out
of scope of this paper), it is important that wavelengths
are chosen in a way which attempts to reduce blocking
for subsequent connections. For the case that there are
multiple feasible wavelengths between a source node
and a destination node, heuristic methods must be used
to assign wavelengths to the lightpath. A review of
wavelength-assignment approaches can be found in [13]
as follows: (i) Random: this scheme first searches the
space of wavelengths to determine the set of all
wavelengths that are available on the required route,
randomly choose one wavelength (usually with uniform
probability). (ii) First-Fit or fixed-order, in which all
wavelengths are numbered. After searching for the
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
available wavelengths, a wavelength with lowest index
is chosen. The idea behind this scheme is to pack all of
the in-use wavelengths toward the lower end of the
wavelength space so that more wavelengths with higher
indexes can be used for longer-hop connections. (iii)
Least-Used selects the available wavelength that is the
least used in the network, so the load is balanced on all
the wavelengths. (iv) Most-Used is the opposite scheme
of Least-Used, it always choose the available
wavelength that is most used in the network. It packs
connections into fewer wavelengths. (v) Min-Product is
used in multi-fiber networks. In single-fiber networks, it
is the same as First-Fit. It packs wavelengths into fibers,
minimizes the number of fibers in the networks. (vi)
Least-Loaded is also used in multi-fiber networks. It
chooses the wavelength that has the largest residual
capacity on the most-loaded link along route p. (vii)
Max-Sum is proposed for multi-fiber networks but can
also be used in single-fiber case. Generally speaking, the
scheme considers all possible paths (lightpaths with their
pre-selected routers) in the network and attempts to
maximize the remaining path capacities after lightpath
establishment. (viii) Relative Capacity Loss (RCL) is
based on Max-Sum. RCL calculates a Relative Capacity
Loss for each path on each available wavelength and
then chooses the wavelength that minimizes the sum of
the relative capacity loss on all the paths [36]. (ix)
Wavelength Reservation reserves some wavelengths for
multi-hop connections. This scheme reduces the
blocking for multi-hop traffic but sometimes may over-
punish single-hop traffic; and (x) Protecting Threshold,
in which a single-hop connection is assigned a
wavelength only if the number of idle wavelengths on
the link is at or above a give threshold.
In this work the first-fit and random schemes are also
selected for modeling the lightpath setup and RWA
schemes in GMPLS-based IP-over-DWDM optical
network. First-fit chooses the first wavelength available
in an ordered list of wavelength in contrast to the
random scheme which chooses a wavelength randomly
between the different available wavelengths. Those two
schemes are used directly at the output of the path
calculation algorithm in order to determine the
wavelength to be reserved on the path to each
destination in the network from a source node.
3.3. Signaling and Resource Reservation
In order to set up a lightpath, a signaling protocol is
required to exchange control information among nodes
and to reserve resources along the path. In most cases,
the signaling protocol is closely integrated with the
RWA algorithms.
Signaling and resource reservation protocols may be
categorized based on whether the resources are reserved
on each link in parallel, reserved on a hop-by-hop basis
along the forward path, or reverse path (backward).
Algorithms will also differ depending on whether global
information including the dispersion factors of each hop
is available or not.
4. Model Implementation
4.1. OMNeT++ Simulation Model
OMNeT++ is a free discrete event network simulation
tool, which was primarily designed for the simulation of
communication networks, multi-processors and other
distributed systems [5]. The OMNeT++ based model is
built to test two link metrics mentioned previously: the
simple TAW and the enhanced TAW; and two
wavelength assignment heuristics: first fit and random
assignments. Different topologies are included for
simulation such as single link, chain, ring, and mesh
The model is composed of simple modules referred to
as GMPLS router and compound modules referred to as
EndSystem. The GMPLS router module contains all the
RWA and reservation algorithms, while the EndSystem
module is responsible of generating and analyzing the
response to connection request messages.
The EndSystem module composes of Generator and
Sink simple modules.
The Generator module generates a certain number of
connection requests to randomly chosen destination
nodes in the ION.
The Sink module receives the responses to each
connection request generated by its counterpart
Generator module. The principal task of the Sink module
is to analyze the responses and to generate a statistical
Note that the Endsystem compound module actually
models an end system in an ION. Effectively, the
EndSystem module represents one or several end
systems such as switches or routers, e.g. such end
systems may be an ATM switch or IP routers using
MPLS-based protocols. It is also essential to standardize
the physical interface between the clients (end systems)
and the transport network (ION) which can be
designated as the UNI interface. An implementation
agreement for the UNI has been proposed by the OIF
Each GMPLS router module is associated to only one
Endsystem module. Those modules have the same
address. The network model is based on the USA
Abilene network that composes of 12 GMPLS router
modules scattered at different GigaPoPs; each GMPLS
router module is associated to its own Endsystem
module. An object-oriented framework has been
developed with an UML class diagram (implemented
with Rational Rose) of the RWA model which describes
the relation and contents of different of classes of the
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Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
simulation framework which are Generator, Sink,
GmplsRouter, Node, LinkInfo, NodeInfo, ExplicitRoute
and LambdaCap. The details of these classes can b
explained on request of readers.
4.2. Reservation Protocol
For simplicity, a reservation protocol based on parallel
reservation has been implemented in order to test the
different routing and wavelength assignment schemes
implemented. The reservation scheme and process of
messages exchanges are shown in Figure 2 and
described as follows:
The Generator sends a “request” message for a new
connection to another randomly chosen node. When
GMPLS Router receives a “request” message, it
calculates an explicit route to the destination requested
and assigns a wavelength for this connection and then
estimation of the total residual and PMD dispersion,
thence broadening time.
If both route and wavelength are available, the
GMPLS router reserves in parallel the route and the
wavelength. It sends in parallel a “reserve” message to
each node of the explicit route, excepted itself. When a
node receives the “reserve” message, it checks if the
requested wavelength is available on the link to its
predecessor in the explicit route. Then it sends a
“response” message back to inform the source whether
the reservation is successful.
Figure 2. Parallel reservation model
Figure 3. Illustration of the use of TAKEDOWN messages
If the requested wavelength is not available in one or
several node on the explicit route, the source will send a
TAKEDOWN message to all the nodes, which have
already reserved the wavelength on the explicit route to
reset it as available (see Figure 3).
4.3. System Parameters
The network traffic is generated in terms of connection
request, which is a request to establish a lightpath from a
source node to a randomly chosen destination node. The
connection request arriving at each router is assumed to
follow an exponential distribution with mean
poisson per
unit of time. The system variable parameter is T
, the
total number of wavelengths on each link and the
connection requests arrivals. T
changes between 8
and 24 by an increment of 8. Performance parameters
considered are blocking probability P and link utilization
The blocking probability P is the probability that a
connection request is blocked due to no available
wavelength for the selected path. The average link
utilization U is defined as the percentage of time that all
wavelengths of each link in the network are fully utilized,
ijij ij
ij ij
ijij T
Under the residual dispersion, the constraint for
routing is
with the subscript k indicating fiber path and Nl is the
total number of the fiber paths that a routing path l is
selected. The length limit is set by the dispersion length
LD and the PMD effect, LPMD, the maximum length due
to the PMD effect that enforces a 1dB penalty on the eye
opening. It is noted that in calculating the total length of
the routing route both the linear chromatic and
polarization mode dispersion impairments are summed
up as they would be superimposed on each other. PMD
is a random process and it is modeled with the total
average group velocity delay and a Maxwellian
distribution. Since the lightpath travels the whole
distance from one node to the other, it is reasonable that
the average and standard deviation is sufficient for our
estimation of the distance and distortion effects.
In addition to the CD and PMD, nonlinear phase
distortion is also critical that would be investigated in a
future article. The nonlinear effects [38] include the self
phase modulation (SPM), the cross phase modulation
(XPM), the four waves mixing (FWM), stimulated
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
Raman scattering (SRS) and stimulated Brillouin
scattering (SBS). Of these effects SPM and XPM are
generated with the change of the phase of the carrier due
to the total intensity of the light paths exerted over the
core of the fiber and length dependence. The XPM
would create a low frequency noise component in the
frequency spectra of other lightpaths from lightwave
signals of a lightpath. When the utilization is increased
the nonlinear effects SPM and XPM are enhanced and
thus distortion would affect the quality of the transmitted
signals in the lightpaths. Similarly for SRS but the
distortion is generated to the lightpaths whose spectra
are about 100 nm away from the signal bands. This
would be significant when both C- and L- bands are
employed. These nonlinear effects are not included in
this work but will be reported in a future article.
5. Simulation Results and Discussions
One of our goals is to estimate the performances of
different RWA algorithms by simulations under without
and with residual dispersion effects. The blocking
probability P versus the average link utilization U is
used as the performance criterion. Under the condition
that the wavelength conversion is not used, the
wavelength continuity constraint would lead to a high
level of blocking when the load increases when no
dispersion constraints are imposed. We then investigate
the performance under the dispersion constraints. Firstly,
wavelength assignment schemes are compared. The
blocking in the network as a function of the average link
utilization is described, analyzed and discussed. Then,
various RWA algorithms are tested based on some
specific network topologies.
5.1. Performance under No Constraints of
Dispersion Effects
The first-fit and random wavelength assignment schemes
are simulated and presented in this sub-section under
non constraints of linear dispersion and nonlinear effects.
The blocking probability P is plotted as a function of the
link utilization U for different values of T
, the total
number of wavelengths in a fiber. T
is varied between
8 and 24 by increment of 8 for different schemes,
including simple TAW metric, enhanced TAW metric,
first-fit and random wavelength assignment are tested.
Figure 4 shows the difference of performance between
the two wavelength assignments. They represent
blocking probability P versus the normalized link
utilization for: Figure4(a) and (b) T
=8/simple TAW,
=8/enhanced TAW, (c) and (d) T
=16/enhanced TAW, and (e) and (f)
=24/simple TAW and T
=24/enhanced TAW, as
It is found that the first-fit wavelength assignment
performs superior, with much less blocking problem,
than the random scheme when the average link
utilization is between 40% and 60%, and when the link
utilization becomes relatively high, the difference
between these two wavelength assignments is very small.
The results are independent with either the TAW type or
the number of total wavelengths.
(a) (b)
(c) (d)
(e) (f)
Figure 4. Blocking probability versus link utilization with
total number of wavelength for First-fit and random
wavelength assignments — no linear dispersion and
nonlinear effects.
Shown in Figure 5 is the difference of performance
between two metrics used simple and enhanced TAW.
When the link utilization is relatively low, the difference
is not much, but with link utilization becomes high,
enhanced TAW performs better. In general, it has been
observed that the enhanced TAW metric performs better
than the simple TAW metric at higher utilization. One
reason is that the enhanced metric tries to minimize the
weight of the explicit route while still trying to minimize
the probability of blocking for further requests.
Conversely, the simple TAW metric does not take into
account any probabilities. It just tries to minimize the
total link utilization on a possible explicit route from the
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Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
ingress node to the egress node.
(a) (b)
(c) (d)
(e) (f)
Figure 5. Blocking probability versus link utilization with
total number of wavelength for simple and enhanced TAW.
5.2. Performance under Constraints of
Dispersion Effects for Specific Networks
The purpose of this section is to test the affection of
network topologies on different RWA algorithms under
the constraints of linear dispersion and nonlinear effects.
Specific networks of 5 node ring and a specific long haul
backbone mesh network (see Figure 6) are chosen so
that the lengths of the fiber physical paths can be
Now let
A = {A1÷A4} = {4-node chain, 8-node chain, 5-node
ring, Specific Net} (8)
be the set of four topologies (Networks with assigned
codes of A2, A3 and A4) to be investigated including a
4-nodes (A1) and 8-nodes chains (A2), a 5-nodes ring
(A3), and the Specific Net (Figure 6 as A4), respectively.
A total wavelengths number of 24 which is chosen with
50 GHz channel spacing for the study of the traffic
performance o the networks under no dispersion effects
and with linear CD and PMD effects. Note that the
dispersion due to nonlinear self phase effect is not
considered as a constraint [40]. The network routing is
subjected to the linear dispersion constraints given in
eq.(6) and (3) and (4). For the same reason as mentioned
above, we compare two wavelength assignment
heuristics the first-fit and random wavelength
The simulation results are shown in Fiugre 8 and we
can make the following observations. Firstly, under the
case of the simplest topology – single link, suppose there
are n total wavelengths on the link, then when the
number of requests is smaller than n, the block will
never happen. This is because there must be unused
wavelength(s) on the link, and no matter which
wavelength assignment heuristic is adopted, the source
chooses the wavelength from those unused. Secondly,
when the link utilization is low, there is no difference
between First-fit and random wavelength assignments,
since the connection requests can always be satisfied.
When link utilization becomes higher, the block happens.
In the same topology, the First-fit WA has a lower
blocking probability than random wavelength
assignment. But when link utilization is very high, the
difference between these two wavelength assignments
becomes narrower.
It also can be seen that the difference between first-fit
and random wavelength assignment heuristics of the 4-
nodes chain is quite small, but that of 8-nodes chain
becomes larger. Naturally the reason is destinations of
connections are chosen randomly, so with 8-nodes chain
there should be more multi-hop connection requests than
that with 4-nodes chain. The first-fit heuristic tends to
pack all of the in-use wavelengths toward the lower end
of the wavelength space so that more wavelengths with
higher indexes can be used for longer-hop connections.
Thus on 8-nodes chain the advantage of First-fit
heuristic becomes more obvious.
Figure 6. Specific Net with Cities and Distance (in number
of spans). A typical long haul back bone optically amplified
Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
dispersion managed network — the number indicates the
number of spans in multiples of 100 km spans with
dispersion compensating module and optical amplifiers.
Capital letters indicate the sites for IP routers and OXCs.
Residual dispersion is taken as 2% of the link distance.
The ring and 4-node chain topologies give much
lower blocking probability as compared with 8-node
chain and mesh topologies (The Specific Net of Figure
We now demonstrate the two link metrics: simple
TAW and enhanced TAW. Link metrics are used in path
selection. In the chain topologies, there is only one path
between the source and the destination, so there is no
difference between simple TAW and enhanced TAW.
The blocking probability for a 5-nodes ring (A3) and
Specific Net (A4) topologies with a total of 24
wavelength channels. A3 network is set under the
condition of equal number of spans per link of 5
between nodes with 100 km completely dispersion
compensated per span that is a total of 500 km of
dispersion compensated fibers and optical amplifiers. A
residual dispersion is set at 2% of the dispersion of the
uncompensated dispersion of the lowest dispersion value
(e.g. 2% of 17 ps/nm/km of standard single mode optical
fiber). An average PMD first order value of 0.1
ps/(km)1/2 is also included. As can be seen from Figure 7,
the enhanced TAW performs better than simple TAW,
because the enhanced TAW takes into consideration the
further requests and tries to minimize the probability of
blocking for future, while simple TAW only choose the
link with a light load. The effects of CD and PMD have
shown clearly an increase in the blocking probability
when the utilization of the link paths is increased. This
is expected due to the limitation of the dispersion budget
given in (2) and (3). We note here that no statistical
property of the PMD is taken into account and that we
may expect some sudden fluctuation of the blocking
probability in practice.
Figure 7. Blocking probability versus link utilization with
total number of wavelength is 24 for 2 kinds of topology
under with and without constraints of linear chromatic
dispersion (CD bold circle dotted line) and CD + PMD
(random) effects (square dotted line). Legend: dotted line
indicates “random”.
Figure 8. Simple TAW vs. Enhanced TAW on node-ring
and sample network (Figure 6) (a) without (b) with
dispersion constraints using random and first-fit strategy.
Also, numerous simulations have been conducted to
test these RWA heuristics on different traffic schemes
and total available wavelengths. The traffic schemes
include uniform, exponential and Poisson distribution,
and the number of total available wavelengths is various
among 8, 16 and 24. There is no obvious impact on the
blocking probability due to different traffic schemes and
the number of total wavelengths. The first fit WA
performs better than random WA, and enhanced TAW
performs better than simple TAW. Figure 8(a) and (b)
shows a contrast of the blocking probability of the
simple TAW and enhanced TAW under without and
with dispersion constraints with different algorithms.
Concerning with TAKEDOWN message of
reservation scheme, in our previous simulations, the
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Copyright © 2008 SciRes. I. J. Communications, Network and System Sciences, 2008, 2, 105-206
TAKEDOWN message never appears. This is because
when a wavelength is reserved on some link(s), the
UPDATE messages will be sent to every node in the
network. When a source calculates the explicit routes, it
requires the knowledge of whether a wavelength is
unused or not, so it would not seek other nodes to
reserve those unavailable ones. There is one case that the
TAKEDOWN message would be used: after a
wavelength is reserved on some link, and before the
UPDATE message reaches a source, this source receives
a connection request and estimates some explicit route
using that wavelength on that link. Then the source
seeks for the downstream node of that link to reserve the
wavelength, which is already reserved for some other
connection. Clearly, the node responds negatively, and
TAKEDOWN messages are to be sent from the source
to other nodes on that explicit route.
6. Concluding Remarks
In this paper we have presented a simulation model on
GMPLS-based all-optical networks using OMNeT++ to
study the traffic performance of RWA of channels
within the C-band of optical fibers under the influences
of the dispersion effects due to chromatic and
polarization mode dispersion. The blocking probabilities
of two types of wavelength assignment heuristics and
two types of link metrics are investigated.
It has been found that when the link utilization is low,
there is no difference in blocking probability between
first fit and random wavelength assignments. Almost all
the connection requests can be satisfied. When link
utilization becomes high, first fit performs better than
random wavelength assignment. However, when link
utilization is close to 100%, the difference between the
two wavelength assignment heuristics becomes small
again. At higher link utilizations, blocking increases
exponentially when link utilization increases.
For the link metrics, we find, in a practical network
such as a ring or a mesh network, the enhanced TAW
link metric has a lower blocking probability than simple
TAW. Further the enhanced TAW metric performs
better at high link utilizations than the simple TAW
metric particularly when the number of wavelengths per
fiber is low. Conversely, at low link utilization, both
metrics have very similar results whatever the number of
wavelengths are used. Moreover, we also design a
contention case to illustrate the use of TAKEDOWN
message. Under this situation, the random wavelength
assignment performs better than first fit wavelength
assignment by spreading the chosen wavelengths.
In the simulation we only examine two simplest
wavelength assignment heuristics: first-fit and random
wavelength assignment schemes. They yield similar
performance when associated with any routing and
reservation schemes implemented in this work. We
believe more heuristics should be tested and compared.
Also, the wavelength convention (both full-convention
and part-convention) can be considered. For the
contention case, we wonder some improvements can be
done on the first fit wavelength assignment heuristic, to
make its blocking probability not worse than random
The effects of chromatic dispersion and PMD are also
included as constraints of the routing and wavelength
assignment. These effects are significant for 10 Gb/s
optical networks. The random scheme may not offer the
best due to the effects of the residual dispersion. So the
first-fit would offer better performance under these
dispersion constraints. Nonlinear dispersion effects in
RWA in optical networks such as self phase modulation
will be included in future works. These effects are
critical when the number of wavelength channels are
increased, thus the total average power and intensity
imposed on the fiber guiding region. This is very critical
for network management in practice as the symbol rates
are increased. The 40 Gb/s is emerging as the rate of
next generation networks and the linear dispersion and
nonlinear phase distortion are even much more critical
with much lower tolerance for distortion and must be
taken into account in the RAW management strategy.
This will be reported in a future article. The randomness
and statistical property of the PMD is included only with
its mean value. This clearly allows the expected traffic
performance and not to the details of the blocking that
may happen higher or lower from time to time.
We have also assumed that the lightwaves of the
lightpaths are amplitude modulated with either NRZ or
RZ formats. It is more likely that the RZ format is used
as it is the preferred format. Other modulation formats
such as different phase shift keying [39], continuous
phase shift keying or frequency shift keying have also
been intensively investigated. They would be taken as
the critical feature to combat impairments in long haul
optical DWDM networks and RWA management
7. Acknowledgement
The author acknowledges the OMNeT++ programming
assistance from C. Cieutat.
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