Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Performance Tradeoffs of Shared Limited Range Wavelength Conversion Schemes in Optical WDM Networks Fahad A. Al-Zahrani, Abdulgader A. Habiballa, Ayman G. Fayoumi, Anura P. Jayasumana Electrical and Computer Engineering Department Colorado State University, Ft. Collins, CO 80523 (Fahad, Habiball, Ayman)@engr.colostate.edu, [email protected] Abstract— Performance of all-optical switches that employee different types of limited-range wavelength converters (LRWC) are investigated. Previous work has shown that there is a remarkable improvement in blocking probability while using LRWC over full range conversion, but has not considered the coincident effect of conversion resources sharing. We consider the case where an incoming wavelength can be converted to a range of outgoing wavelengths, where d is the range of conversion. The simulation results demonstrate that the performance improvement obtained by full range wavelength conversion can almost be achieved by using a fractional ranged ranged LRWC. I. I NTRODUCTION The optical switches used to aggregate data through the optical backbone must exhibit increased efficiency while maintaining low cost. Wavelength Converter (WC) [4], which is used to manipulate optical channels in Dense Wavelength Division Multiplexing (DWDM), optical switches, routers, and add/drop multiplexers, represent a very important resource that is essential for exploiting the capacity of the optical network. WC is used to eliminate the severity of ”wavelength continuity constraint” [10], and improving wavelength agility for dynamic reallocation of optical channels, which in turn enhances the scalability of the network to match the demand for bandwidth. The proper use of converters decreases the number of necessary wavelengths in an optical network. Moreover, tuneability of a WC further enhances utilization and flexibility. Since the number of available wavelengths is a limited resource, and it also affects the complexity of the switches, different WC ranging architectural configurations must be studied and analyzed to achieve an adequate level of wavelength utilization while maintaining the efficiency. Conventional optoelectronic conversion, which uses Optical/Electrical/Optical (OEO) line cards that incorpo0-7803-9019-9/05/$20.00 ©2005 IEEE. rates a tunable laser for its output, incurs a high cost due to the complex implementation of such converters. Optical networks with limited range wavelength conversion, LRWC, will be less costly to implement than networks that have no constraints on conversion [8], [12]. The conversion within each node could be accomplished alloptically instead of using OEO technique. All-optical WC [9] is of increasing interest for future all-optical telecommunication networks. In these all-optical devices [13], the conversion efficiency is a function of the input and output wavelengths, leading to limited conversion capabilities. Even otherwise, the number of such shared devices in the node can be reduced while maintaining comparable efficiency requirement. However, it is difficult to provide a full capability of WC without OEO conversions under the current technology. Converters sharing, [5], [1], [2], has led the way for new generation of wavelength routers, called wavelength convertible switches or WCSs. The different types of WCS [14] architectures have introduced different WC sharing schemes in order to minimize the cost and maintain efficiency. It has been shown that these WCSs with shared conversion resources can achieve very close performance to full WC when every channel has its own fully ranged dedicated converter [1], [2]. Previous work has shown that LRWC can considerably improve the performance of all-optical networks, [8], [12]. However, the concept of conversion resource sharing in conjunction with different wavelength range configurations has not been explored. In this paper, we perform a simulation based analysis of different LRWC configurations that implement conversion resources sharing. This paper focuses on minimizing the cost associated with converters and maintaining an appropriate level of performance by quantifying the penalty for mismanaging WC resources. This paper is structured as follows. Section 2 presents LRWC. Sections 3 presents different LRWC ranging architectures. Simulation results are discussed in Section 4 followed by conclusion in Section 5. II. WAVELENGTH C ONVERSION The optical layer transports a large numbers of channels, each operating at rates such as 2.5 Gb/sec or 10 Gb/sec, between two nodes using optical DWDM technology [4], [8], which requires WC mechanisms to resolve conflicts. Ideally, the conversion process should be independent of bit rate or signal format, entirely optical, require little power, not degrade the signal, and be tunable. All-optical WC promises better response, simplicity, and transparency to the signal format whereas optoelectronic converters are inherently dependent on input transmission format and bit rate, so a change in data rate requires changing the converter, [4]. A. All-optical Wavelength Converters All-optical networks will eliminate the electronic bottleneck and deliver scalability, and flexibility. Related cost constraints mean that WCs must be compact, and low-cost relative to existing optoelectronic WC solutions. Low cost and small size make the semiconductor optical amplifier an attractive option. The nonlinear phenomena in semiconductor optical amplifiers have been widely utilized for WC and optical switching as follows: • A semiconductor optical amplifier, or SOA [11], can be used to improve a signal that has weakened. It outputs a new wavelength that has same modulation as input wavelength. The wavelength can be engineered to vary within a very large range between 1300 and 1600 nm. • Four Wave Mixing, [6], is made possible by nonlinear effects between two closely spaced wavelengths, that produce conjugate outputs from both initial wavelengths. Cross-phase modulation converters use the input signal to modulate the phase of a second wavelength. This effect can be used to convert an input signal at a specific wavelength to an exact copy of the signal at a different wavelength. • Good results have been obtained with parametric conversion in periodically poled lithium-niobate waveguides [3]. It is a very fast conversion method relying on interaction between a signal and highpower pump beam inside the nonlinear crystal. Single or multi-channel WC is achieved in the optical chip. The WC chip is protocol independent and currently supports 10 Gbit/s, 40 Gbit/s, and the future extension to 160 Gbit/s high-speed network applications. The operation of this type covers the entire C-band. B. Limited Range Wavelength Conversion, LRWC LRWC means that each input wavelength may be converted to any wavelength of a specific range of wavelengths. All-optical LRWC decreases blocking probability, increases NxN switch scalability, lowers power consumption, and operates at speeds important for telecommunication. Even though LRWC does not eliminate the severity of wavelength continuity problem completely, it enhances the performance to levels close to full range WC and reduces the cost in term of implementation and control. The cost of conversion not only depends on the number of converters in the network, but also considers the architectural complexity of each individual converter. A significant cost penalty can be incurred if WC is not maintained in an efficient manner. However, we show how one could significantly mitigate this penalty by allowing limited range WC, using a small subset of wavelengths as the range, complemented with the ability to share these resources. III. WC R ANGING A RCHITECTURES This study examines how WC is related to efficient utilization of an optical channel. A recent advancement in all-optical WC technology makes it possible to convert the input wavelength to different output wavelength if those are within different limited ranges. Implementing all-optical full WC is quite difficult due to technological limitations. The transmitters and receivers for all-optical wavelength converters are tunable [7]. The tuning range is, perhaps, determined at the time of fabrication and can not be changed thereafter. Presumably, range tunable transmitters and receivers are cheaper and simpler to construct than fully ranged ones. Our aim is to determine the range configuration that is most practical for tunable all-optical devices in the presence of WC resources sharing. In [8], [12], the authors have analyzed all-optical networks without consideration for WC sharing or the architectural differences in defining the conversion range. Notably, our study allows the observation of the interactive behavior of LRWC with conversion sharing and the tradeoff scenarios that could result in a cheaper conversion cost. We defined a measure of WC called the conversion range. If a network node has a conversion range d then a wavelength may shift to any one of a pre-defined set of d A. Simulation of WC Ranging Schemes Networks with different conversion ranging architectural definitions and different conversion sharing degrees were simulated considering c, number of shared converters in the conversion bank, W, total number of wavelengths available on a fiber, and d. If the requested wavelength is not available in any outgoing fiber, the request will be forwarded to one of the shared converters within the pre-specified range d for the given architecture. The following assumptions are used in our simulation models. 1) The offered traffic is in the form of connection requests for an entire optical channel. These requests arrive at each node according to an independent stationary Poisson process with rate λ. 2) Connection holding time is exponentially distributed with the mean 1/µ. 3) Connection request that cannot be serviced are blocked. 4) Wavelengths are assigned randomly from the set of free wavelengths on the associated path. Existing requests cannot be reassigned different wavelengths. If the input/output range of the converter is defined as Any- to-Any, the request will be forwarded to any one of the shared output converters, where d=W for the Any-to-Any case. In this case, if there is no free wavelength available on the outgoing range, or there is no free converter, then the request is blocked. Both Any-to-Any and Any-to-Range conversion architectures, which are easily implemented as all-optical devices, have the same conversion bank behavior, but they differ in the available output wavelength range that can serve the path setting request. The Any-to-Range scheme is expected to result in a blocking probability performance comparable to the earlier scheme, where this performance can be further enhanced by adjusting or managing the conversion resources sharing degree. In the Range-to-Any and Range-to-Range LRWC simulations, the behavior of the conversion bank differs from the previous two cases due to the fact that each request uses just one specific converter that has the incoming wavelength in its range. IV. S IMULATION R ESULTS In this section, we discuss different simulation results for different LRWC ranging schemes. In figure 1, the blocking is plotted as a function of offered load per wavelength for 4 differently ranged LRWC architectures. We considered 3 different Any-to-Any switch sizes, W=64, 32, 16, that have similar shared conversion degree of d=2, where d=W/c. The results for this architecture consistently show that switches with higher number of wavelengths, W, have better blocking performance given similar d. The graph also shows that at lower offered 0.45 0.4 0.35 Blocking probability wavelengths. The cost of all-optical devices, packaged in a way that both transmitter and receiver are in the same chip, is a function of the complexity in ranging both the transmitters and receivers. The ranging architectures proposed and simulated in this paper have LRWC in the following sense. • Any-to-Any: The conversion device based on this architecture can not fall under all-optical category due to technology limitations. This device can convert any incoming input wavelength to any outgoing one, corresponding to full range conversion. Even in optoelectronic converters, wider range tunable transmitters are more difficult to build and accordingly more expensive. • Any-to-Range: This conversion device can convert any incoming wavelength to a wavelength that falls in a pre-specified range for full non overlapping coverage of wavelengths, which is considered in this paper, d=W/c, where W is the total number of wavelengths in the fiber and c is the number of converters shared in the node. • Range-to-Any: This device behaves in an opposite manner compared to the previous ones. The incoming request is required to use one specific conversion device depending on its wavelength and the range it falls into. • Range-to-Range: The range in both sides are defined by d=W/c. 0.3 0.25 Any−to−Any, W=16, C=8 Any−to−Range, W=16, C=8 Range−to−Any, W=16, C=8 Range−to−Range, W=16, C=8 Any−to−Any, W=32, C=16 Any−to−Range, W=32, C=16 Range−to−Any, W=32, C=16 Range−to−Range, W=32, C=16 Any−to−Any, W=64, C=32 Any−to−Range, W=64, C=32 Range−to−Any, W=64, C=32 Range−to−Range, W=64, C=32 0.2 0.15 0.1 0.05 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Offered load per wavlength Fig. 1. Blocking Vs. Offered Load per Wavelength 1 0.5 Any−to−Range, C=32, d=2 Any−to−Any, C=2 Any−to−Any, C=4 Any−to−Any, C=8 Any−to−Any, C=16 Any−to−Any, C=30 Any−to−Any, C=32 Any−to−Range, C=30, d=2 0.45 0.4 0.35 Blocking Probability loads, all 3 cases have similar performances due to the abundance of wavelengths and conversion resources with respect to load serviced. In the same graph, we also considered 3 different Any-to-Range switch sizes, W=64, 32, 16, that have similar shared conversion degree, d, to the preceding case. The results showed that Any-toRange architecture’s blocking performance behaves in a comparable manner to the Any-to-Any case implying that the front ranging mechanism does not degrade performance while minimizing the architectural complexity of the switch. This behavior, also, creates an opportunity for network cost, that depends on number of converters, and conversion cost, that depends on converter architecture, tradeoffs. In a similar fashion, we considered switch sizes of W=64, 32, 16 and d=2 for Range-to-Any and Range-to-Range cases. These two architectures’ blocking performances are considerably worse than Any-to-Any and Any-to-Range due to the one converter limitation imposed, resulting from the end ranging mechanism, on all incoming requests. This limitation makes the switch behave the same regardless of the size, W, implying that the end ranging scheme can severely degrade performance. The Range-to-Range scheme is the least practical due to the fact that its behavior is severely affected by the ranging limitation imposed on both the input and the output sides. From pervious results, we can expect that Rangeto-Any and Range-to-Range architectures to be less sensitive to the change in number of converters, and accordingly to d, due to the already imposed ranging limitation on their input sides. This raises a question about how sensitive the Any-to-Any and Any-to-Range schemes to the change in number of converters c and the conversion range d. In figure 2, for an LRWC with W=64 and c=32, 30, 16, 8, 4, and 2, the blocking of Anyto-Any and Any-to-Range architectures decreases with converter count, even though, the width of the conversion range, d, gets smaller. The graph also shows that both Any-to-Any and Any-to-Range architectures behave comparably in each scenario indicating that Any-to-Range ranging scheme is a valid architectural abstraction that provides an equivalent performance behavior to Any-toAny scheme. This reaffirms the preceding results and show that Any-to-Range is the most practical and the most comparable to the Any-to-Any architecture. According to the relationship between the conversion range d and the number of shared converters, the range of each converter is expected to be less with increased sharing. Accordingly, the Range-to-Range architecture 0.3 0.25 0.2 0.15 0.1 0.05 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Offered load per wavelength Fig. 2. Blocking Vs. Offered Load per Wavelength suffers the most due to ranging restriction imposed on both sides when the number of shared converters increases resulting in performance degradation. It also can be deduced that these conversion ranging schemes are more responsive to the change in number of shared converters than the change in the conversion range, even though, both are interrelated according to d=W/c. Finally, the offered load versus the gain, which is defined as the ratio of utilization of the Any-to-any architecture versus that of the architecture of interest, shown in figure 3, for W=64, c=32 and d=2, confirms that the Anyto-Range architecture performs in an analogous fashion to the fully ranged Any-to-Any method. It also shows that Range-to-Any architecture lags in performance due to the input limitation imposed on the incoming traffic where as the Range-to-Range method is the poorest due the dual limitation imposed on both sides of the switch as explained earlier. These performance differences are clearer at higher offered loads due to the facts that limited range architectures are more susceptive to lag in utilization performance with higher traffic demand. The differences in these architectures define the practicality of each case with respect to a range of offered loads. This indicates that more limiting, conversion range wise, architectures can be employed in lightly loaded networks increasing the cost efficiency on both the WC hardware and control architecture of the switch. It should be noted that ranging of LRWC architectures can be implemented in an overlapping fashion causing a relative improvement in performance at the expense of control complexity. R EFERENCES 1 Any−to−Range with respect to Any−to−Any Range−to−Any with respect to Any−to−Any Range−to−Range with respect to Any−to−Any 0.95 Gain 0.9 0.85 0.8 0.75 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Offered load per wavelength Fig. 3. Gain Vs. Offered Load per Wavelength V. C ONCLUSION This paper investigated issues associated with LRWC in the presence of conversion resources sharing. It analyzes the impact of using different architectures for wavelength conversion to mitigate the cost penalty. The throughput improvement due to the use of LRWC, in the presence of conversion sharing, depends more on the degree of sharing than the conversion range. The study shows that smaller conversion ranges can approach the same performance as full-range WC at slightly higher degree of sharing. The implementation of Any-to-Range architecture in all-optical wavelength converters would be more efficient and elegant solution, allowing fully integrated optical cross-connect/wavelength conversion boxes. Any-to-Range means less complex and less costly hardware than full range conversion, yet results in acceptable channel utilization. This introduces the issue of cost effectiveness when considering different switching architectures, conversion options, and hop configurations. The limited ranged architecture reduces the need for extremely complex ”any-to-any” control algorithms. Nevertheless, these ranging architectures do give up some flexibility and overall network efficiency. The complexity and the cost of converters depend on its conversion ranging mechanism. Switches with limited range conversion will be less costly to implement than the ones that employ full ranged converters. The cost of implementing optical layer can be expected to scale roughly in proportion to the number of conversion resources and how the resources are architected. [1] F. Al-Zahrani, A. Habiballa, and A. Jayasumana, Path Blocking Performance in Multi-Fiber Wavelength Routing Networks with and without Wavelength Conversion, Proc. 12th International Conference on Computer Communications and Networks, 2003, 580-583. [2] F. Al-Zahrani, A. Habiballa, and A. Jayasumana, Performance Merits of Multi-Fiber DWDM Networks Employing Different Shared Wavelength Conversion Resources Architectures, Proc. IEEE region 5 conference, 2004, 59-67. [3] D. Caccioli, A. Paoletti, et al, Field Demonstration of in-Line All-Optical Wavelength Conversion in a WDM Dispersion Managed 40-Gbit/s Link, Selected Topics in Quantum Electronics, IEEE Journal of, 10 (2) , 2004, 356-362. [4] J. M. H. Elmirghani and H. T. Mouftah, All-Optical Wavelength Conversion: Technologies and Applications in DWDM Networks, IEEE Commun. Mag., Mar ,38 (3), 2000, 86-92. [5] A. Habiballa, F. Al-Zahrani, A. Jayasumana, Wavelength Conversion Resources Allocation Algorithms for Share-per-Link Wavelength Convertible Switch, Proc. IEEE region 5 conference, 2004, 131-139. [6] K. Inoue, T. Hasegawa, K. Oda, and H. Toba, Multichannel Frequency Conversion Experiment Using Fiber Four-Wave Mixing, Electron. Lett., 29 (19), 1993, 1708-1710. [7] T. Lin and G. Sasaki, Nonblocking WDM Networks with Fixed-Tuned Transmitters and Tunable Receivers, Proc. 37th Annual Allerton Conference on Communication, Control, and Computing, 1999, 400-401. [8] R. Ramaswami and G. Sasaki, Multi-Wavelength Optical Networks with Limited Wavelength Conversion, IEEE/ACM Trans. Networking, 6 (6), 1998, 744-754. [9] L.H. Spiekman, U. Koren, M.D. Chien, B.I. Miller, J.M. Wiesenfeld, J.S. Perino, All-Optical Mach-Zehnder, Wavelength Converter with Monolithically Integrated DFB Probe Source, IEEE Photonics Technology Letters, 9 (10), 1997, 1349-1351. [10] T. Tripathi and K. N. Sivarajan, Computing Approximate Blocking Probabilities in Wavelength Routed All-optical Networks with Limited Range Wavelength Conversion, IEEE J. Select. Areas Commun., 18 (10), 2000, 2123-2129. [11] D. Wolfson, P. B. Hansen, T. Fjelde, A. Kloch, C. Janz, A. Coquelin, I. Guillemot, F. Gaborit, F. Poingt, and M. Renaud, 40Gbit/s All-Optical Wavelength Conversion in an SOA-Based Allactive Mach-Zehnder Interferometer, ECOC’99, 1999, II170-171. [12] G. Xiao and Y. W. Leung, Algorithms for Allocating Wavelength Converters in All-optical Networks, IEEE/ACM Trans. Networking, 7 (4), 1999, 545-557. [13] L. Xu, L.K. Oxenlowe, N. Chi, J. Mork, P. Jeppesen, K. Hoppe, J. Hanberg, Basic Characterization of Wavelength Conversion at 40 Gb/s Based on Electroabsorption Modulators, Proc. IEEE Lasers and Electro-optics Society 2002 Annual meeting, 2002, 111-112. [14] J. M. Yates and M. P. Rumsewicz, Wavelength Converters in Dynamically Reconfigurable WDM Networks, IEEE Communications Surveys, Second quarter, 1999, 2-15.