ICTON 2013 We.A1.4

978-1-4799-0683-3/13/$31.00 ©2013 IEEE 1

Dispersion Constraints in Optical Burst Switched Metropolitan Networks with WDM/OCDM Technology

Luiz Henrique Bonani1, Member, IEEE, Alex Bernaz dos Santos1, Lidia Galdino2 1CECS, Universidade Federal do ABC, Santo André-SP, Brazil

2FEEC, Universidade Estadual de Campinas, Campinas-SP, Brazil Phone: (+55)11 4996-8289, e-mail: luiz.bonani@ufabc.edu.br

ABSTRACT In this work we study the influence of Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) in the performance of Optical Burst Switched (OBS) Metropolitan networks using the hybrid WDM/OCDM (Wavelength Division Multiplexing/Optical Code Division Multiplexing) technology with Optical Orthogonal Codes (OOC). The analysis is carried on the context of a metropolitan network, since its resources are limited and the WDM/OCDM technology is becoming an interesting way to improve the utilization of wavelengths. However, the pulse broadening is also increased when using WDM/OCDM technology, since the chip rates are higher than the bit user rates due to the process of code construction, and the dispersion mechanisms must be taken into account even when modeling metropolitan optical networks, in which the route distances are much smaller than that in a core optical network. Keywords: Chromatic Dispersion, PMD, OCDM, OBS Networks.

1. INTRODUCTION WAVELENGTH division multiplexing and optical code division multiplexing (WDM/OCDM) technology has been pointed as a promising candidate to implement the next generation of optical networks, since an extraordi-nary improvement in the design of optical encoder/decoder devices has been observed along the last years. Be-sides, this technology allows improving the capacity and the scalability of optical networks, enabling the scaling number of usable channels in two dimensions: code and wavelength. However, the number of users in the net-work is limited by the code cardinality and it is very important to figure out that by increasing the number of codes, and consequently the cardinality, the dispersion constraints will act to degrade the performance of the system due to the pulse broadening.

The main advantages of using WDM/OCDM scheme is to increase the number of effective transport chan-nels allowed to set up a connection and to allow the sharing of a single wavelength bandwidth with low rate users. Furthermore WDM/OCDM networks can be wavelength routed, supporting also dynamic OBS (Optical Burst Switched) Networks, that leads to a more efficient use of the network resources.

The most conventional schemes for OBS assume unacknowledged one-way reservation of network resources, suffering from high loss rates [1] when experiencing high traffic loads, since the bursts will be sent without the guarantee of resource availability along the route. The signalling message will try to reserve the resources in a hop by hop way. In order to reduce the losses, full wavelength conversion can also be assumed, and wavelengths are not used only for routing, but also to provide point-to-point connections. On the other hand, this is an expen-sive solution because the economic costs involved with the wavelength conversion are higher than the gain ob-tained in performance, since we have not mature technologies to perform wavelength conversion in the optical domain. One obvious option is to increase the number of wavelengths and centralize the routing operation in such way that every application from a given node to a destination one is delivered using a single channel, but as we have a limited number of wavelengths, the Blocking Probability (BP) will increase with the traffic loads. In addition, the insertion of new wavelengths implies the change of tunable lasers and (de)multiplexers in network nodes, which is expensive and not possible in all the cases. To overcome these limitations, a metropolitan OBS network operating with a limited number of wavelengths and some optical orthogonal codes in the links is con-sidered. The network resources comprising wavelengths and optical codes are used to route the bursts hop by hop in a best-effort strategy, since no technique for contention resolution is assumed and a given resource must be chosen for the first link. Moreover, since the number of available optical codes (cardinality) and their weighs impact in the effects coming from the dispersion mechanisms, the performance evaluation must be made in order to define the operation ranges.

This work is organized as follows. Section 2 describes the mathematical models to compute the pulse broad-ening in the presence of chromatic dispersion and polarization mode dispersion mechanisms in an OBS network operating with WDM/OCDM technology. Section 3 presents the simulation tool and simulation scenarios as-sumed to achieve the proposed performance evaluation. In Sections 4 we present the results achieved according to the studied scenarios and, finally, in Section 5 we present the conclusions and the planning future works.

ICTON 2013 We.A1.4

2

2. DISPERSION MODELING FOR WDM/OCDM-OOC We propose to evaluate a hybrid WDM/OCDM network with Optical Orthogonal Codes (OOC). In this system, every optical orthogonal code is reusable and can be sent simultaneously at different wavelengths. Considering a WDM grid with M wavelengths, each one can support a total of C codes. The number of resources seen by the users is given by:

MCR ×= (1)

It should be noticed that the maximum number of codes that can be generated is limited by the OOC cardi-nality, which is a function of the code parameters. The OOC codes can be characterized by (N, W, θa, θc), where N is code length, W is the code weight, θa and θc are the maximum values of out-of-phase autocorrelation and cross-correlation, respectively. The theoretical upper bound on the cardinality of an strict OOC (N ,W), when θa = θc = 1 is given by [2]:

−−

≤)1(

1),(WW

NWNC (2)

High values of N increase the number of codes C that may be utilized (cardinality) and thus allow the OCD-MA domain to support a higher number of simultaneous users. For the present analysis we will neglect the rise time of MAN NRZ-OOK signals in all-optical networks and we will assume N = Bc/B, where B is the bit rate of the user signal and Bc is the code chip rate. Considering that: (i) the user bit rate B is the same in all network links, (ii) all transmitted signals present the same spectral width Δλ and (iii) the code cardinality is the same in all the links, we can model the pulse broadening T due to chromatic dispersion of a burst travelling along a given route with K links, which lengths are L(k), k = 1, ..., K, as:

∑=

∆=K

kic kLDBT

1

)()( λ (3)

In (3) we have made an adaptation from the formulation given in [3], where Di is the chromatic dispersion coefficient in the wavelength λi, which can be calculated by (4) [4]. Moreover, DR is the chromatic dispersion coefficient in the reference wavelength, λR is the reference wavelength and S is the chromatic dispersion slope.

( )RiRi SDD λλ −×+= (4)

The pulse broadening due to Polarization Mode Dispersion (PMD) can be also modelled using (5), where DPMD is the PMD dispersion coefficient.

)(1

2 kLDBTK

kPMDcPMD ∑

=

= (5)

In order to compute the Average Pulse Broadening (APB) we can compute the mean of T and the mean of TPMD for all the generated busts. It should be noticed that since we have burst losses in the network due to the missing contention resolution schemes and absence of resources conversion, the average pulse broadening will be computed for all the bursts, including that blocked (Total Accounting) and only for the successfully received bursts (Received Accounting).

3. SIMULATION SCENARIOS AND RESULTS We consider that the network topology is the well-known South of Finland Topology given by Fig. 1a, which Average Route Distance (ARD) is 66,17 km. We have considered that each network node is able to receive and generate Poissonian traffic with mean burst size of 5000 bytes, which are routed by the minimum distance path, discovering the next node in a hop by hop way. It was used the ONSim simulation tool [5] performing OBS switching paradigm with the JIT signalling. ONSim uses the model described in [6] and the configuration pa-rameters were offset time of 1 ms, setup time of 150 μs and switch time of 500 ns. The resources are assigned in the first node using the Random Fit algorithm considering the current available resources [7].

For the physical layer, we suppose transmission in the C-band, using λ1 = 1550.1161 nm and λ2 = 1550.9180 nm. The reference wavelength λR was 1544.5300 nm, where the chromatic dispersion coefficient is DR = -0.75 ps/nm·km and the chromatic dispersion slope is S = 0.06 ps/nm2·km. The transmitter linewidth Δλ was chosen to be 2.5 GHz, which is the same as 0,02 nm. The assumed bit rate for each user, B, is 2.5 Gb/s and the PMD coefficient is 0.12 ps/√km. We have also investigated three scenarios for the availability of resources: M = 2 and C = 2 (Scenario I); M = 2 and C = 3 (Scenario II); M = 2 and C = 4 (Scenario III).

The BPs for the Scenarios I, II and III are shown in Fig. 1b. From this figure we can see a gain around 25% in BP for the Scenario II, related to Scenario I and a gain around 10% for the Scenario III related to Scenario II,

ICTON 2013 We.A1.4

3

basing our decision on using the hybrid WDM/OCDM technology (that is cheaper and easily scalable) instead of increasing the number of available wavelengths in the system. In other hand, the dispersion impairments should be evaluated, since the WDM/OCDM technology increases the bit rate seen by the network.

(a)

(b)

Figure 1. (a) The south of Finland topology (b) The network Blocking Probability

Therefore, Fig. 2 and Fig. 3 present the evolution of APB due to CD and PMD for the presented WDM/OCDM architecture with W = 3. We can see that their values increase with the number of optical codes C, and smoothly decreases with the network load increasing. This decreasing due to network load occurs because the bursts that are blocked and do not reach their destinations will also contribute for the APB (Total Account-ing). Moreover, this behaviour can also be seen for the Received Accounting analysis, since for higher loads the BP will be higher for the bursts in longer routes, increasing the probability of successfully reach the destination for the bursts in the smaller routes. As the pulse broadening due to the dispersion mechanisms depends on the travelled distance, the APB for the received bursts will also decrease with the increasing of network load, but will present a higher value when compared with the results achieved with the Total Accounting.

Figure 2. Average pulse broadening due to chromatic dispersion for Total and Received Accountings

Figure 3. Average pulse broadening due to polarization mode dispersion for Total and Received Accountings

In a general analysis, all the studied scenarios will be in the range of the accepted pulse broadening (around 10%), which guarantees a good signal quality. However, we have to notice that the maximum number of codes

ICTON 2013 We.A1.4

4

used in our scenarios was C = 4, that will lead to N = 25, considering W = 3. Moreover, it is known that the mul-ti-access interference (MAI) for WDM/OCDM systems will be quite severe for small values of N. As a result, in our analysis the dispersion mechanisms must be strongly considered when the WDM/OCDM technology is a candidate to be used in metropolitan optical networks.

In order to show the dangerous influence of APB, we have constructed a basic topology using the original skeleton with the all the links presenting 1% of their original values. We have made steps increasing in 30% the link lengths, evaluating the ARD and the APB for both studied dispersion mechanisms using the Received Ac-counting and the studied scenarios. We have also considered W = 5, which will lead to N = 81 using C = 4.

(a) (b) Figure 4. Average Pulse Broadening for Received Accountings due to (a) chromatic dispersion and (b) PMD

The first results in Fig. 4 show the impact of ARD on APB. Basically, the main relation that we can take is that the increasing of C or W will impact in the size of ARD to keep the APB in a satisfactory level to guarantee a good signal quality. For instance, for the Scenario III with W = 5, the APB will reach 10% when the ARD is less than 20 km, limiting the network covering area.

4. CONCLUSIONS Our study have included a model to evaluate the Average Pulse Broadening (APB) when using WDM/OCDM technology and we have shown that the dispersion mechanisms must be taken into account even when modeling metropolitan optical networks, with small Average Route Distances (ARD). Our results show the tradeoff be-tween the APB and the WDM/OCDM parameters as W and C that will impact in the maximum ARD that can be reached. The next steps of this study will be the evaluation of the dispersion in the several routes individually and evaluate the actual value of BER due to MAI in these scenarios, since in a dynamic network like OBS the total available number of codes is rarely used simultaneously in all the network links.

ACKNOWLEDGEMENTS L. H. Bonani would like to thank the financial support of FAPESP, under grant 2010/07382-8.

REFERENCES [1] M. Yoo, C. Qiao, and S. Dixit: QoS performance of optical burst switching in IP-over-WDM networks,

IEEE Journal on Selected Areas in Communications, vol. 18, pp. 2062–2071, Oct. 2000. [2] H. Yin and D. J. Richardson: Optical Code Division Multiple Access Communication, Springer, 2007. [3] R. Ramaswami, K. N. Sivarajan and G. H. Sasaki: Optical Networks – A practical Perspective, 3rd Ed.,

Morgan Kaufman, 2010. [4] N. Zulkifli, C. Okonkwo, and K. Guild: Dispersion optimised impairment constraint based routing and

wavelength assignment algorithms for all-optical networks, in Proc. of ICTON 2006, Nottingham, UK, Jun. 2006.

[5] L. H. Bonani. (2013, March 30). ONSim: The Optical Networks Simulator [Online]. Available: http://professor.ufabc.edu.br/~luiz.bonani/projects/ONSim.html.

[6] J. Teng, G. Rouskas: A detailed analysis and performance comparison of wavelength reservation schemes for optical burst switched networks, Photonic Network Communications, vol. 9, no. 3, pp. 311-335, Jan. 2004.

[7] D. Saha: A comparative study of distributed protocols for wavelength reservation in WDM optical net-works, Optical Networks Magazine, vol. 3, no. 1, pp. 45-52, Jan. 2002.