A QoS-aware wavelength assignment scheme for optical networks

Optik 124 (2013) 4498–4501

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Optik journal homepage: www.elsevier.de/ijleo

A QoS-aware wavelength assignment scheme for optical networks Bijoy Chand Chatterjee a,∗ , Nityananda Sarma a , Partha Pratim Sahu b a b

Department of Computer Science and Engineering, Tezpur University, Assam, India Department of Electronics and Communication Engineering, Tezpur University, Assam, India

a r t i c l e

i n f o

Article history: Received 7 September 2012 Accepted 12 January 2013

Keywords: Alternate path routing Dispersion QoS RWA

a b s t r a c t Dispersion in fiber optic is wavelength dependent and it degrades the quality of service (QoS) in an optical network. Although use of dispersion compensating fiber reduces the effects of dispersion but it is very costly. In this paper, we propose a QoS-aware wavelength assignment (QWA) scheme to improve the quality of service in an optical network by reducing the overall dispersion in the network. In this scheme, the connection requests with longer lightpath are assigned the wavelengths having lesser dispersion and the wavelengths having higher dispersion are assigned to the lightpaths with shorter distance. The lightpaths are computed using alternate path routing to achieve the lower blocking probability. The performance analysis of QWA scheme is done in terms of total dispersion using step-index fiber (SIF). Results of our experiments show that QWA scheme outperforms conventional wavelength assignment scheme based on First-Fit method (WAFF). © 2013 Elsevier GmbH. All rights reserved.

1. Introduction In recent years, to fulfill the ever increasing demand of bandwidth, research interests have grown towards high speed optical network. Optical network consists of nodes linked with optical fiber where dispersion creates the signal distortions during transmission. In a wide area optical network, dispersion of a signal increases with increase of fiber length which results in degradation of quality of service (QoS) in an optical network [1]. Although, dispersion compensating devices like dispersion compensating fiber (DCF), optical phase conjugation, pulse prechirping and duobinary transmission are normally used to reduce dispersion, they are expensive. As an alternative solution, routing and wavelength assignment (RWA) [2–7] scheme can incorporate mechanisms to reduce the total dispersion in the network. In RWA, many conventional wavelength assignment based approaches such as First-Fit (FF), Random Wavelength Assignment, Least-Used (LU), Most-Used (MU), MinProduct (MP), Least-Loaded (LL), MAX-SUM, and Relative Capacity Loss (RCL) have been reported in [8–14], but none of these approaches takes care of the reduction of total dispersion in the network. In this direction, Zulkifli et al. [15] has reported a dispersion optimized impairment constraint (DOIC) based RWA in which DCF is used to optimize the dispersion. Use of DCF is not very effective because DCF is very expensive and its propagation loss is very high compared to step-index fiber. To the best of our knowledge, no

∗ Corresponding author. E-mail addresses: [email protected] (B.C. Chatterjee), [email protected] (N. Sarma), [email protected] (P.P. Sahu). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.01.035

study has been done to use RWA without using dispersion compensating devices for reducing the total dispersion in optical network which can improve the the quality of service. In this paper, we have attempted to reduce the total dispersion in the network by using a QoS-aware wavelength assignment (QWA) scheme and compared its performance with conventional wavelength assignment scheme based on FF method [10] (WAFF). The rest of the paper is organized as follows. The proposed QoSaware wavelength assignment scheme is presented in Section 2. Section 3 evaluates the performance of the proposed scheme and finally, Section 4 concludes the paper. 2. Proposed QoS-aware wavelength assignment (QWA) scheme First, we define some basic terms that are used throughout this paper. The total dispersion1 [1] in optical fiber denoted by D, is the combination of material and waveguide dispersions. Material dispersion denoted by Dm , occurs because the refractive index varies as a function of the optical wavelength, whereas waveguide dispersion denoted by Dwd , occurs due to wavelength dependence of the group velocity on the mode. The total dispersion in optical fiber can be computed as |D| = |Dm | + |Dwd |

(1)

1 Defined as the pulse spread on a function of wavelength and is measured in picoseconds per kilometer per nanometer [ps/(nm km)].

B.C. Chatterjee et al. / Optik 124 (2013) 4498–4501

where  d2 (n1 ) |Dm | = · c d2 |Dwd | = −

2(n1 − n2 )u2 cv2

 u2 = a

a =

2

(2)



42 2 · n1 − ˇ2 2

 (3)

(4)

(5)

42 (n21 − ˇ2 )

  3  bi 2 nj = 1 + i=1

d



 d(n2 ) · n2 d

2 v2

2

d(nj )

1−

2 − a2i

3

 = −

1+

i=1

where j = 1 and 2

2

(a2i bi /(2 − a2i ) )

where j = 1 and 2

3

3 ⎢ d (n2 ) ⎢ = − ⎣ d2 i=1

(7)

(b 2 /(2 − a2i )) i=1 i

⎡ 2

(6)



(a2i bi (a2i

+ 3 ))/(( − 2

2

{1 +

3 a2i ) ) ·

3 i=1

1+

3 i=1

4499

such constraint on dispersion, the overall dispersion in the network can be reduced to a great extent, which will in turn lead to better QoS of the network in terms of overall signal quality without increasing network setup cost. The detailed descriptions of the wavelength assignment is given in Algorithm 1 where K number of alternate paths are computed on the basis of link state information for the entire session of a connection request. Time complexity analysis of Algorithm 1 To analyze the time complexity, we use the symbols and notations already defined in Table 1. Total time required consisting of the following components. • The time to arrange all the wavelengths in the network according to increasing order of their dispersion is O(WlogW · E). • The time to compute K number of the shortest paths for Z connection requests and sort them in descending order of their primary path lengths is O(((E + NlogN + K))·Z) +ZlogZ). • The time to perform wavelength assignment for Z connection requests using K alternate path is O(L · W · K · Z). Therefore, the total time required for Algorithm 1 is O(WlogW · E) +O(((E + NlogN + K))·Z) +ZlogZ) +O(L · W · K · Z).

⎤ bi

2 /(2

−

a2i )

(bi 2 /(2 − a2i ))}



⎡ ⎤

3 2 3 2 2 3 (a2i bi /(2 − a2i ) ) ·  (a bi /((2 − a2i ) ))/( 1+ bi 2 /(2 − a2i )) ⎥ i=1 i i=1 ⎥− ⎣ ⎦  3  ⎦ 1+ (bi 2 /(2 − a2 )) i=1

i=1

i

(8) Algorithm 1. In the above equations, n1 , n2 , c, , a , ˇ, bi and ai represent the refractive index of core, refractive index of cladding, speed of light in vacuum, wavelength, radius of core, propagation constant, constants related to material oscillator strengths and oscillator wavelengths respectively. The used notations in the paper are summarized in Table 1. The proposed QoS-aware wavelength assignment scheme is intended to improve the overall quality of services (QoSs) in an optical network by reducing the overall dispersion in the network. To achieve our goal, in this paper, the connection requests with longer lightpath are assigned the wavelengths having lesser dispersion and the wavelengths having a higher dispersion are assigned to the connection requests with shorter lightpath distance. Use of a conventional RWA approach may lead to a situation where the connection requests with longer lightpath are assigned the wavelengths having a higher dispersion and the wavelengths having lesser dispersion are assigned to the connection requests with shorter lightpath distance. As a result, the overall dispersion in the network may increase which will degrade the signal quality. Therefore, if the connection requests are assigned the wavelengths with Table 1 Used notations. Notation

Meaning

N, E

Total number of nodes and total number of links or edges in the network respectively Total number of wavelengths per fiber link and total number of links between a s–d pair Connection request from source s to destination d within holding time tH Wavelength and total number of wavelengths per fiber link respectively Total number of connection requests in the network Total number of paths (including primary path) for a connection request C(s, d, tH ) Dispersion in [ps/(nm km)] of an optical fiber, the total dispersion in [ps/nm] of the network and the total propagation loss in [dB] of the network respectively

W, L C(s, d, tH ) , Y Z K D, ND and PL

QWA

Input: Network configuration and set of connection requests. Output: Wavelengths assignment and total dispersion of the network. Assumption: (a) Connection requests C(s, d, tH ) arrive to the system based on Poisson process. (b) Each fiber link carries equal number of wavelengths and the network is without wavelength conversion capabilities. Arrange the wavelengths of each fiber link in the Step 1: increasing order of their dispersion, estimated using Eqs. (1)–(8).   W1 =

Step 2:

Step 3:

1 , 2 , · · ·, Y

|D(1 ) ≤ D(2 ) ≤ · · · ≤ D(Y )

where W1 is the ordered set of wavelengths and D(i ) indicates the dispersion of the wavelength i . Compute K number of shortest paths (including primary path) for each of the connection request using Dijkstra’s algorithm and sort them in descending order of their primary path lengths. R = {r1 , r2 , · · ·, rZ } |dis(r1 ) ≥ dis(r2 ) ≥ · · · ≥ dis(rZ ) where R represents the ordered set of connection requests and dis(ri ) indicates the length of the shortest lightpath of connection request ri . For each of the connection request in R, perform the following in the given sequence: (a) First, try to assign a wavelength with less dispersion to the primary path. (b) If no wavelength assignment is possible in Step 3(a), consider the alternate paths in the ascending order of their lightpath distance for assigning a wavelength (with similar constraint on dispersion like in Step 3(a)) till one alternate path is assigned a wavelength. (c) If no wavelength assignment is possible either in Step 3(a) or Step 3(b) within tH , the connection request is treated as blocked one. Otherwise, compute the dispersion (with the assigned wavelength) for the connection request and add this dispersion to the total dispersion of the network. (d) Drop the connection request from the network.

3. Performance analysis In this section, we have evaluated the performance of the proposed scheme through a simulation of NSFNET T1 backbone [8] as shown in Fig. 1. The following assumptions are made for simulation purpose.

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Fig. 1. NSFNET with its distance matrix.

• Measurements can be performed on materials contemplated for core such as 13.5 GeO2 :86.5 SiO2 and constants related to material oscillator strengths and oscillator wavelengths are given in [16]. • Measurements can be performed on materials contemplated for cladding such as Quenched SiO2 and constants related to material oscillator strengths and oscillator wavelengths are given in [16]. • Wavelength range is considered from 1.520 ␮m to 1.590 ␮m because of lower propagation loss in optical fiber link. • The spacing between two wavelengths is 0.8 nm for 100 GHz frequency spacing (ITU G.694.1) [17]. • Connection requests are generated randomly based on a Poisson process and the arrival time between the two successive requests follows an exponential distribution. We chose the Poisson model because the burstiness of traffic on the backbone is usually suppressed by the huge amount of aggregation of services and the actual traffic distribution remains unknown.

Fig. 3. Dependence of total dispersion and number of wavelengths on number of paths for the QWA scheme.

We have simulated the proposed scheme with connection requests varying from 100 to 500, distributed randomly among all the possible s–d pairs and compared its performance with similar conventional wavelength assignment scheme based on FF method [10] (WAFF). For comparison purpose, we have taken only FF method because among other wavelength assignment approaches, FF is better in terms of blocking probability and its computational complexity is also low. Fig. 2 shows dispersion versus wavelength for step-index fiber (SIF) and dispersion compensating fiber (DCF). In the figure, the solid and dashed lines represent the dispersion for SIF and DCF respectively. The dispersion for SIF is estimated using Eqs. (1)–(8)

whereas the dispersion of DCF is taken from previous authors [18]. It is seen that the dispersion using SIF increases with increase of wavelength but it is almost constant for DCF. The figure (small graph) also shows propagation loss per kilometer versus wavelength for SIF and DCF. It can be seen from the figure that the dispersion per kilometer for DCF is lower than that of SIF but propagation loss per kilometer in DCF is 3 times higher than that of SIF. Use of DCF in optical network results in increase of overall setup cost due to its higher propagation loss. SIF with addition of QWA scheme may be a better choice. Therefore, we use SIF in our proposed scheme. Fig. 3 shows the dependence of dispersion and number of wavelengths on number of paths for QWA (with 100 number of connection requests). It reveals that the number of wavelengths decrease with increase of number of paths. This is because of more paths are used to establish connection requests successfully. It can be seen from the figure that as number of paths increase, the total dispersion also increases due to increase of path length. The number of wavelengths and total dispersion of the network cross each other at K = 2, which gives the optimal value. Therefore, in the rest of the simulation study, we took K = 2 for each connection request. Fig. 4 shows total dispersion versus number of connection requests for the QWA and WAFF schemes under non-blocking condition where all the connection requests in the network are established successfully. In the figure, the solid and dotted lines represent total dispersion for QWA and WAFF schemes respectively. It is seen that the growth rate of total dispersion for QWA is less than that of WAFF. This is because in QWA, the connection requests

Fig. 2. Dispersion versus wavelength for SIF and DCF (in small graph, propagation loss versus wavelength for SIF and DCF).

Fig. 4. Total dispersion versus number of connection requests for QWA and WAFF schemes under non-blocking condition (in small graph, total propagation loss (PL ) versus number of connection re-quests (Z) for QWA and WAFF).

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the network which can improve the QoS in terms of signal quality without increasing network setup cost.

4. Conclusion

Fig. 5. Blocking probability versus number of wavelengths for QWA and WAFF schemes.

In this paper, we have proposed a QoS-aware wavelength assignment (QWA) scheme to improve the quality of service (QoS) in an optical network by reducing the overall dispersion in the network. The effectiveness of the proposed scheme is examined through its performance evaluation in NSFNET. Results of our experimental studies show that proposed QWA scheme with SIF, significantly reduces the dispersion in the network which can improve the quality of service (QoS) in terms of signal quality without increasing network setup cost. Therefore, SIF with QWA scheme will be a cost-effective solution with to use in practical networks.

Acknowledgment This work is supported by IETE Research Fellowship Scheme (No. IETE/J-282-10/BOR/2011) of The Institution of Electronics and Telecommunication Engineers.

References

Fig. 6. Total dispersion versus number of connection requests, obtained by using QWA andWAFF schemes under different blocking conditions.

with longer lightpath are assigned the wavelengths having lesser dispersion. The figure (small graph) also shows total propagation loss versus number of connection requests for QWA and WAFF. It can be seen from the figure that the growth rate of total propagation loss for both schemes is almost same with increase the number of connection requests. This is because propagation loss per kilometer of SIF is almost constant with increases of wavelength. Fig. 5 shows that the blocking probability2 versus number of wavelengths for both QWA and WAFF schemes (with 100 number of connection requests). It is seen from the figure that as number of wavelengths increase, blocking probability decreases for both schemes but the decreases rate of blocking probability is almost same for the both schemes. Fig. 6 shows total dispersion versus number of connection requests, obtained by using QWA and WAFF schemes for NSFNET respectively. In the figures, we have included the simulation result of total dispersion for WAFF using DCF (under non-blocking condition). It is seen that in the network, the total dispersion for QWA using SIF with 15% blocked connection is almost close to that for WAFF using DCF without blocking. From the above simulation study, it can be concluded that the use of QWA scheme with SIF significantly reduces the dispersion of

2 Defined as a ratio of total number of blocked connections and total number of connection requests in the network.

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