Fair Spectrum Degradation with QoS Assurance
for Lightpath Establishment in Elastic Optical Networks
Badr Mochizuki and Yutaka Takahashi（The Kyoto College of Graduate Studies for Informatics）
Abstract

In Elastic Optical Networks (EONs), lightpaths are established in order to transmit optical signals
through channels called spectrum grids. EON, with its variable spectrum grids, can use the spectrum
efficiently and flexibly because the spectrum can be allocated at a subwavelength level in order tomatch the required bandwidth. As the network gets congested, an increasing number of lightpath
requests get blocked because of the spectrum contiguity constraint. In order to address this issue,degraded spectrum provisioning has been proposed in the literature. In this paper, we propose a QoS
assurefd fair spectrum degradation algorithm for lightpath establishment in EON in order toimprove fairness. In our proposed method, degraded spectrum provisioning is performed only forlightpaths with large number of hops and large number of required FSs in order resolve the double
unfairness that occurs in EON. We evaluate the effectiveness of the proposed method by simulation
for the NSFNET topology and discuss its results.

■ 1. Introduction
Optical switching networks allow ultra-highspeed communication and are currently used innowadays networks. In these networks,
lightpaths are established in order to transmit
data as an optical signal through channels called
spectrum grids. Recently, the Internet traffic hasgrown exponentially and has become burstierthan ever due to bandwidth hungry network
applications as well as to the increasing numberof users getting access to the Internet. This hightraffic load has raised concerns in the researchcommunity over the capacity of the current
optical switching networks and whether its limit
would be reached in the near future.
This limit may be reached soon because current
optical networks allocate bandwidth in an
inefficient manner and because the size of thespectrum is fixed to 50 GHz by ITU. In addition
the spectrum unit is at the wavelength level.
Therefore, if the required bandwidth is not a
multiple of 50 GHz, then part of the unused
spectrum is wasted because it is not available toother users.
This introduced the need of flexible new48

NAIS Journal

generation optical networks that can support
high data bit rates. As a cost effective solution,
Elastic Optical Network (EON) has been proposed[1].
In EON, the spectrum grid size can be changed
in order to match the required bandwidth byusing bandwidth variable transponders that cantransmit, receive and switch optical signals at the
sub-wavelength unit called Frequency Slots (FS).
Hence, EON allocates the spectrum much more
efficiently and flexibly without adding more
bandwidth. However, in EON all the FSs
belonging to the same lightpath have to beallocated adjacently, hence satisfying thespectrum contiguity constraint. Consequently, as
the network gets congested, many lightpath
requests get blocked because the spectrum
contiguity constraint cannot be satisfied [2-3].
In order to address this issue, degraded
spectrum provisioning has been proposed in theliterature as an effective solution [4-5]. Most of
the research work in the literature proposed
degraded spectrum provisioning for alreadyestablished lightpaths and these methods reduce
the number of allocated FSs during the lifetimeof the established lightpath [6-10]. These methods

succeeded in decreasing the blocking probability,
however extra costs are incurred due to constantmonitoring and delay caused by FSs deallocation.
On the other hand, there is few research work in
the literature that considered degraded spectrum
provisioning method during the lightpath
establishment process such as in [11-12].
Performing degraded spectrum provisioning at
the lightpath establishment process reduces costs
such as constant monitoring and deallocation

Figure 1. Elastic optical networks.

delays. Therefore, in this paper spectrum
degradation at the lightpath establishment phaseis considered.
In [12], the authors proposed an algorithm that
allocates a smaller number of FSs than requiredby the lightpath request and increases the

In the following, EON is introduced in section 2
and some related work on degraded spectrum
provisioning is summarized in section 3. Section
4 presents and explains the proposed method.
Numerical results are shown in Section 5, andholding time in order to keep the total amount ofconclusions are presented in Section 6.

the traffic unchanged. In the numerical results, it■ 2. Elastic Optical Networks (EONs)

was shown that the blocking probability has beensignificantly decreased. However, this method did
not improve fairness between requests which is

In this section, we explain how data is

another major issue in EON. Lightpath requests

transmitted in EONs and how spectrum grids arewith a large number of hops and a large number

utilized. As shown in Fig. 1, EONs are circuitof required FSs tend to be blocked more often

switched optical networks, therefore, a lightpath,than other requests. Consequently, requests withwhich is an all optical connection, is establishedthe same priority class may have different QoS,

between a source and a destination before data

which is not a desirable situation.

transmission. First a lightpath establishment

In this paper, a QoS assured fair spectrum

request arrives at an EON source node withdegradation algorithm for lightpath

information such as required destination node,

establishment in EON is proposed in order tobandwidth (number of FSs) and the holding time

improve fairness. In the proposed method,(connection duration). Then, the source nodedegraded spectrum provisioning is performed

computes the best path until the destination,

only for lightpaths with large number of hops andchecks if there are enough available contiguous

large number of required FSs in order resolve theFSs at each link along the best path until

double unfairness that occurs in EON. In

destination, and reserves the FSs at each link ofaddition, the proposed algorithm is QoS assured

that path. Next, data is transmitted all opticallywhich means that all the requested traffic is

without Optical/Electrical/Optical conversion,transmitted without data loss because theallowing ultra-fast transmission. Finally, all theholding time of the lightpath is increased.

reserved FSs are deallocated and the lightpath isNumerical results show that not only fairness

released.

was improved but also that the blocking

The main advantage of EON over conventional

probability was decreased compared to

WDM optical networks is its flexible spectrum

conventional EON lightpath establishment

grids. Figure 2 shows the difference between themethod that do not perform degraded spectrum

spectrum grids of EON and conventional opticalprovisioning.

networks. The size of every spectrum grid is fixed

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49

category is called QoS assured degradation andwhile a smaller number of FSs is allocated, theholding time is increased in order to keep thetotal amount of traffic unchanged [4-5].
Degraded spectrum degradation methods have(a) Fixed grid optical networks.

been proposed in the literature [6-12]. In [6], theauthors proposed a method that performs
spectrum degradation in order to reduce the costs
incurred from network failure caused by natural
disasters. In this method, the main goal is tomaintain network survivability. In [7], the(b) Elastic optical networks.
Figure 2. Comparison of spectrum grids.

authors proposed a method that performs
spectrum degradation as a means to provide
service differentiation. Here, the algorithm

and is set to 50 Ghz for conventional optical

determines the number of reduced FSs depending

networks (See Fig. 2 (a)). Here, the spectrum ison the lightpath priority level. In [8], the authors

not fully utilized if the required bandwidth of the

proposed a double optical and electricallightpath request is not a multiple of 50 GHz.

degradation method in order to reduce theblocking probability. In electrical degradation, theAs illustrated in Fig. 2 (a), the whole spectrum

number of allocated FSs is decreased and thegrid is not fully utilized and part of the spectrum

holding time is increased, whereas in opticalgrid is unused, which leads to inefficientdegradation the modulation format is changed inspectrum use. On the other hand, Fig. 2(b) showsorder to decrease the number of FSs.

that EON spectrum grids have variable sizes

In [9-10], the authors proposed spectrum

such as 15 GHz and 40 GHz in order to match thedegradation methods for time varying traffic ofrequired bandwidth by the lightpath request.

already established lightpaths. Here, theThis is possible because in EON a FS is at the

spectrum allocated to an end-to-end connectionsubwavelength level and the spectrum is

between source and destination nodes varies

efficiently used, hence EON can accommodate

dynamically with time for the duration of amore lightpaths compared to conventional WDM

lighpath. These methods use spectrum expansion/

optical networks. Therefore EON can support

degradation by increasing/decreasing the amounthigh data bit rates flexibly.

of allocated FSs of an already established■ 3. Related Worklightpath as the traffic decreases/increases. While

Next, we introduce some related work on
degraded spectrum provisioning in EON. In this50

these methods succeeded in decreasing theblocking probability, extra costs are incurred due
to constant monitoring and delay caused by FSs
deallocation.

p a p e r, w e c o n s i d e r t h a t o u r l i g h t p a t h

On the other hand, there are few research work

establishment requests are delay tolerant andin the literature that proposed degraded spectrum

can accept a lower Quality of Service (QoS) which

provisioning method during the lightpath

can be classified into two categories. The firstestablishment process such as in [11-12].

category is called QoS affected degradation that

Performing degraded spectrum provisioning at

consists in allocating a smaller number of FSs

the lightpath establishment process reduces costs

than required and keeping the same holding

such as constant monitoring and delays due totime, hence resulting in data loss. The second

deallocation of FSs. In [12], the authors proposed

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double unfairness that occurs in EON. In
addition, the proposed algorithm is QoS assured
which means that all the requested traffic istransmitted without data loss because theholding time of the lightpath is increased.
4.1 System ModelIn this subsection, the system model for the
proposed lightpath establishment in EON ispresented. EON is modelled as a graph GEON (NEON,
LEON) where NEON is the set of EON nodes and LEON

is the set of EON links. The maximum number of
EON nodes is N and the maximum number of

Figure 3. Example of degraded
spectrum provisioning.

an algorithm that allocates a smaller number of

EON links is L . The i th( i =1,…, N ) EON node isdenoted as n iEON ∈ N EON. The i th ( i =1,…, L ) EON

link is denoted as l iEON ∈ L EON , and the total
number of FSs for link l iEON is denoted as M.
In this network, a lightpath is establishedFSs than required by the lightpath request andbased on the information of a lightpath request.

increases the holding time in order to keep theThis latter contains information about thedestination node denoted as n dest, the required

total amount of the traffic unchanged. In the
numerical results, it was shown that the blocking
probability has been significantly decreased.

number of frequency slots fR ( fR ≤ M), and theholding time hR for the lightpath.

However, this method in [12] did not improve

A lightpath is established if there are enoughfairness between requests which is anotheridle and contiguous FSs at each link of thecomputed path p with p hops hops between the

majorissue in EON. Lightpath requests with alarge number of hops and a large number ofrequired FSs tend to be blocked more often thansource and the requested destination node ndest.
of available FSs at link i is
The number f availablei

priority class may have different QoS. After ancollected for each link i in p until the requested
destination node ndest. Here, we define the number

extensive literature research, it was concluded

f availableof available FSs for path p as follows:pother requests. Hence, requests with the samethat this paper is the first to propose a QoS
assured degraded provisioning during lightpath
establishment, and this is an extension of myprevious work in [12].

■ 4. QoS Assured Fair Spectrum Degradation
This section presents the proposed QoS assured
fair spectrum degradation algorithm for lightpath

= min
∈

(1)

≥ fR, the lightpath will be established
If f availablepwith fR FSs and with a holding time hR. Otherwise,
the lightpath is either established with adegraded spectrum provisioning or blocked. The
next subsection explains the conditions that mustbe satisfied in order for to perform degraded
spectrum provisioning for a lightpath.

establishment in EON in order to improve
fairness. In the proposed method, degraded
spectrum provisioning is performed only forlightpaths with large number of hops and large
number of required FSs in order resolve the

4.2 Fairness-awareness: Conditions for spectrum
degradation
In EON, lightpath requests with a large
number fR of FSs and a high number phops of hops

NAIS Journal51

at path p tend to be blocked more often thanother lightpaths with smaller fR and phops. This isbecause it is more difficult to find available andcontiguous FSs when f R and p hops are large.
Consequently, this results in different blocking
probabilities for lightpath requests within thesame priority class.
In the proposed method, fairness awareness is
implemented by providing almost the same
blocking probability for all requests regardless ofthe values of fR and phops. In the proposed method,
fairness awareness is implemented by allowing
spectrum degradation only for lightpaths withlarge number of fR and phops and by blocking other< f R , then therequests. Therefore, if f availablep

Figure 4. Flow chart of the proposed method.

following two conditions are checked.
Condition 1:
The number of hops phops of path p should belarger than or equal to a certain threshold Mhop.
Condition 2:
The number f R of requested FSs should belarger than or equal to MFS.
If conditions 1 and/or 2 are satisfied, then
spectrum degradation is performed as explainedin the next subsection.

=ℎ ⎛
∙

ℎ

⎝

／

⎞
⎠

(3)

Finally, the lightpath is established with f degradpdegrad.
FSs at each link of p with holding time h p
4.4 Overview
In this subsection we give an overview of our
proposed QoS assured fair spectrum degradation
method. Our proposed method establishes alighpath according to the following steps (Fig. 4).

4.3 QoS assured degradation
In this paper, the case of delay tolerant trafficthat can accept a lower QoS is considered. In
technique is QoS assured, which means that evencontaining the destination node ndest, therequired number fR of FSs, and the holding

though a smaller number of FSs are allocated,time hR.

addition the proposed spectrum degradation

the holding time is increased in order to keep theoriginal traffic amount unchanged, hence no data
loss occurs. In the proposed method, spectrum

Step 2: Compute the shortest path p with thesmallest number of hops between n iEON
and ndest.

degradation is performed at lighpathofestablishment by allocating a number f degradp. If f available≥ fR, go to Step
Step 3: Compute f availablepp

FSs which is smaller than the required f R as

Step 4: Check Conditions 1 and 2 of Section 4.2.

follows:
f degrad＝ kd・f available, kd ∈ (0,1]pp

6. Otherwise go to Step 4.
If either one or both conditions are(2)

kd is a coefficient that represents the degradationlevel.

52

Step 1: A new lightpath establishment request
arrives at a node n iEON with information

satisfied, go to Step 5. Otherwise go to
Step 7.
and h degrad. Then go to Step
Step 5: Compute f degradpp
6.

On the other hand, the original holding time hR
asof the lightpath request is increased to hdegradp

Step 6: Establish the lightpath.

follows:

Step 7: Block the lightpath.

NAIS JournalFigure 5. The NSFNET topology.
Table 1. Simulation parameters.
Figure 6. Blocking probability vs. network load forsimple degradation of [12].

The performance of all three methods arecompared using the blocking probability and thefairness index as performance metrics.
The blocking probability BP is derived as follows:■ 5. Numerical Results
In this section, the performance of the proposed
method is evaluated by Monte Carlo simulation
for the NSFNET topology as shown in Fig. 5 andTable 1. In this topology, the number N of nodes is
14 and the number L of links is 21. The number M
of FSs at a link is 64. Lightpath establishment

=

(4)

+

Where R blocked corresponds to the number ofblocked lightpath requests and Rccepted refers to thenumber of accepted lightpath requests.
The fairness index FI , as defined in [13], isderived as follows:=（ ∑ℎ =1ℎ

× ∑ℎ =1
（

）
ℎ

2

）
2

(5)

according to a Poisson process with rate λW h e r e B hp
loss is the blocking probability forlightpath requests whose path p has hp hops, and[Requests/ms], and the utilization time of a

H is the maximum number of hops any path canlightpath follows an exponential distribution withhave for a specific EON topology.

requests are assumed to arrive at an EON

rate 1.0.
The performance of the proposed method is

5.1 E ffect of the network load on the blocking

compared with two other lightpaths

probability and the fairness index for eachestablishment methods under the spectrum

number of hops

contiguity constraint. These methods are“Simple

In this subsection, the effect of the network

Degradation”proposed in [12] and“Conventionalload on the blocking probability and the fairness

EON”which is a lightpath establishment method

index is analyzed. Figure 6 shows the blocking

that does not perform spectrum degradations,rate against the network load for “Simple

therefore if there are not enough idle anddegradation”at hops 1, 2 and 3. Recall that incontiguous FSs, the lightpath request is dropped.

the NSFNET topology, the shortest path between

Simulations were performed for the values ofparameters kd, Mhop and MFS as shown in Table 1.

any two nodes has a maximum of three hops,
therefore Fig. 6 shows the blocking probability forhops 1,2 and 3 against the network load. From

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53

Figure 7. Blocking probability vs. network load forthe proposed fair degradation.

Figure 8. Fairness index vs. network load.

this figure, we find that the blocking rate for all

Moreover, fairness of the proposed method has

hops becomes large as the network load increases.

been evaluated using the fairness index against

Moreover, Fig. 6 shows that the blocking rate of 1

the network load as illustrated in Fig. 8 for the

hop requests is the smallest and that of 3 hop is

proposed method, “Simple Degradation” andthe largest. This difference of blocking probability“Conventional EON”. Please note that the closer

occurs because lightpath requiring 1 hop only

the fairness index is to 1, the better the fairness.

have to allocate FSs at one link whereas lightpath

From this figure, it is shown that the fairness

with 3 hops will have to allocate FSs at threeindex of the proposed method is larger than thelinks. Therefore, lightpaths with 1 hop will haveother two methods for all values of the networka higher chance of finding available FSs andload. Furthermore, the fairness index of thehence the blocking probability of 1 hop lightpaths

proposed method is the closest to one. Therefore,is smaller than 3 hop lightpaths. Consequently,

we confirm that the proposed method is indeed

this implies that fairness has not been improved

effective in improving fairness using the fairness

with “Simple Degradation”.

index.

On the other hand, Fig. 7 illustrates the same
situation as in Fig. 6 but for the proposed Fair
spectrum degradation method. From this figure,

5.2 I m p a c t o f t h e n u m b e r o f s p e c t r u mdegradations

we can observe that all three blocking

Next, the impact of the network load on theprobabilities also become large as the network

average number of spectrum degradations isload increases. In addition, the blocking

investigated in this subsection. Figure 9 shows

probabilities for hops 1, 2 and 3 are almost equal.

the average number of spectrum degradations

This implies that lightpath requests have about

against the network load for the proposed method

the same chances of succeeding in establishing aand for “Simple Degradation”. This figure shows

lightpath regardless of the number of hops.

that the number of spectrum degradations

Hence, we can conclude that fairness has been

increases as the network load increases for alli mprov ed b y the pro po se d m e t ho d fo r al lmethods. This is because as the network loads

lightpaths by using the blocking probability

increases, the network gets more congested due

metric.

to the fact that there are less idle and contiguous
FSs. Hence, lightpath requests tend to getblocked more frequently when the network loadincreases.

54

NAIS JournalFigure 9. Average number of degradations vs.
network load.

In addition, the average number of spectrum

Figure 10. Overall blocking probability vs.
network load.

probability compared to“Conventional EON”

degradations for the proposed method is smallerthat do not perform spectrum degradations.

because “Simple Degradation” performs spectrum■ 6. Conclusion

than that of “Simple Degradation”. This isdegradation for all lightpath establishment
requests, whereas the proposed method performs

In this paper, we proposed a QoS assured fair

spectrum degradations only for lightpaths which

spectrum degradation algorithm for lightpath

require a large number of FSs and hops.

establishment in EON in order to improve
fairness. In the proposed method, degraded

5.3 Evaluation of the overall blocking probability

spectrum provisioning is performed only for

Finally, we investigate how the overall blocking

lightpaths with large number of hops and large

probability is affected by the proposed method.

number of required FSs in order resolve the

Figure 10 shows the overall blocking probabilities

double unfairness that occurs in EON. We

of the proposed method,“Simple Degradation”

evaluated the performance of the proposed

and“Conventional EON”against the network

method by simulation for the NSFNET topology.

load. From this figure, we can see that the overall

In the numerical results, we show that not only

blocking probabilities for all three methods

fairness was improved but also that the blocking

increase as the network load becomes large,

probability was decreased compared towhich confirms the same tendency as for local
blocking probabilities shown in Fig. 6 and Fig. 7.
Moreover, it is shown that the proposed method
has a slightly higher overall blocking probability

than “Simple Degradation”. This is because theproposed method performs spectrum degradation
only for some lightpath requests, and lightpaths
that require small number of FSs and hops areblocked in order to improve fairness for all
r e q u e s t s . H o w e v e r, t h e o v e r a l l b l o c k i n gprobability of the proposed method is still smaller

than that of “Conventional EON”. Therefore our
proposed method can also decrease the blocking“Conventional EON”.

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◆著者紹介

望月

バドル

Badr Mochizuki

京都情報大学院大学助教
福井大学大学院工学研究科科修了 工学博士
奈良先端科学技術大学院大学情報科学研究科
元 CNRS 研究所（フランス） 研究員
Al Akhawayn 大学（モロッコ） 工学士

髙橋

豊

工学修士

Yutaka Takahashi

京都情報大学院大学教授
京都大学名誉教授
元奈良先端科学技術大学院大学情報科学研究科教授
元フランス・パリ大学客員教授
元フランス国立情報制御研究所（INRIA) 客員研究員
元通信・放送機構（TAO）「多段接続された CATV 網による通信・放送統合 技術に関する研究開発」統括責任者（プロジェクトリーダー）