QoT-aware RWA algorithms for Fast Failure Recovery in All-Optical Networks Amir Askarian1 , Yuxiang Zhai1 , Suresh Subramaniam1 , Yvan Pointurier2 , Ma¨ıt´e Brandt-Pearce3 1 Department
of Electrical and Computer Engineering, The George Washington University, Washington, DC {amiran,yzhai,suresh}@gwu.edu 2 Department of Electrical and Computer Engineering McGill University, Montreal, Quebec, Canada
[email protected] 3 Charles L. Brown Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA
[email protected]
Abstract: We study the performance of restoration algorithms in realistic physically impaired all-optical networks from time complexity viewpoint. We present two new algorithms that exhibit low blocking probability while having moderate vulnerability ratio and computational time. c 2008 Optical Society of America OCIS codes: 060.4250 Networks; 060.4510 Optical communications
1 Introduction As the need for high transmission rates in the backbone of data networks increases, all-optical networks which are well capable to meet this demand draw attention. Such high-speed networks require elaborate protection and restoration schemes with low time complexity overhead, to avoid substantial data loss in case of failures. On the other hand, due to the lack of OEO regenerators, connections in all-optical networks encounter noise accumulations and crosstalk unique to all-optical networks. This issue becomes more critical in the design of protection and restoration algorithms, since the connections are no longer vulnerable only to the failure of a link, but can be vulnerable to the physical layer impairments as well. This feature illustrates the need for considering the physical layer characteristics into account while designing higher layer protocols, in other words, a cross-layer design. Cross-layer Routing and Wavelength Assignment (RWA) has been studied in recent research such as [1] and [2]. Looking into the failure recovery algorithms in this context was first done by us in [3]. Failure recovery can be categorized into protection and restoration schemes and it can be done link-wise or path-wise. We have looked at the performance of different types of path protection schemes in optical networks with physical layer impairments in [3], where we adapted a new cross-layer RWA algorithm called HQ from [4]. In this paper we look at the time complexity overhead of several QoT-aware and non-QoT-aware algorithms for failure recovery. We propose two new compound algorithms that exhibit low blocking probability without sacrificing recovery speed.
2 Network Model and RWA Algorithms In our physical layer model we account for four dominating impairments: intersymbol interference (ISI), amplifier noise (ASE noise), interchannel nonlinear effects (nonlinear crosstalk), and optical leaks at the nodes (node crosstalk). We measure the QoT of a lightpath by its BER, which should remain below a threshold set by the network manager to ensure almost error-free data transmission. To estimate BERs, we use the relation between BER and the so-called corresponding Q factor. Assuming Gaussian distributions for the ‘0’ and ‘1’ samples after photodetection, the Q factor for a signal on a lightpath is given by: μ1 − μ0 Q= (1) σ0 + σ1 where μ0 and μ1 are the means of the ‘0’ and ‘1’ samples, respectively, and σ 0 and σ1 are their standard deviations. In the QoT-aware scheme, which we refer to as Highest Q (HQ) [3,4], in order to establish a connection, we look at the shortest path on all available wavelengths between source and destination, compute the Q-factor for all these candidate paths and pick the one with the highest Q-factor. As we will see from the numerical results in the next section, this algorithm has good performance in terms of blocking probability and vulnerability ratio (a measure of restorability; exactly defined later), but it is computationally expensive and is rather slow for high-speed all-optical networks. We look at two types of non-QoT-aware RWA algorithms. In the first type, a connection is routed according to the shortest path algorithm and the first available wavelength on that path (if any) will be assigned to that connection. We refer to this method as First Fit or FF in short. In the second approach, for each working wavelength we try to find the shortest path (may not be available, due to wavelength unavailability somewhere along that path) and assign the shortest one among
all to the connection. We refer to this approach as Best Fit or BF. Non-QoT-aware algorithms have low computational time overhead, but in a realistic all-optical network with physical layer impairments, they have poor performance in terms of blocking probability and vulnerability ratio. In this paper, we look at the performance of the HQ path restoration algorithm used in combination with the non-QoTaware schemes. In the first algorithm, every arriving connection is routed based on the HQ method. In case of a failure, the restoration path for every affected connection is found according to the FF algorithm. We refer to this algorithm as HQ-FF (in this terminology, the first part refers to the algorithm used to set up the primary path, and the second part to the restoration algorithm). As opposed to the HQ-HQ method in which the restoration paths are also found according to the HQ algorithm, HQ-FF is expected to be noticeably faster. Our simulation result in the next section supports this expectation. In the second approach, we establish the primary path according to HQ algorithm, and for the restoration paths we use the BF approach. We refer to this algorithm as HQ-BF. For link protection schemes, we need an offline algorithm to reserve a protection path and wavelength around each link. In this paper have adopted the loopback recovery algorithm proposed in [5]. 1 In link restoration scheme, in case of link failure, for each connection using that link a restoration path is found around that link on the same wavelength used by that connection. This is due to the assumption that there is no wavelength conversion in the network, hence link protection and restoration do not fall into the categories of FF or BF (there is only one candidate wavelength). In all restoration algorithms, it is obvious that the working wavelength of the affected connection may not be available in the restoration path. We take this event into account in computing the vulnerability ratio. Path protection schemes were studied in detail by us in [3]. In our evaluation, the QoT-aware algorithm HQ Dark Backup performed best. In that method, for every connection in the network there is a backup path reserved but it is kept dark until a link used by the primary path fails. In this algorithm both the primary and the backup paths are found by the HQ algorithm. We present our results for HQ dark backup here for comparison.
3 Simulation Results and Conclusions We use Vulnerability Ratio [3] as a metric to describe the performance of our algorithms in the context of random single link failures. Vulnerability Ratio is the probability that a randomly picked ongoing connection (at the time of failure) cannot be restored because of unacceptable QoT, if a random link fails at a random point of time during the operation of the network. This applies both to connections that were using the failed link as well as other connections. In order to compute the Vulnerability Ratio, we note that the vulnerability of a connection stays the same between network state changes (i.e., connection admissions and departures). Therefore, we can calculate the vulnerability for each network state, and average it over all network states by using the durations of the network states. We run simulations to compare various protection and restoration algorithms on the NSF network with 14 nodes and 21 bidirectional links. Compared with the original topology, we scaled down the distances by a factor of 10 so that every node is reachable from any other node without electrical regeneration. Traffic arrives at each node according to a Poisson process and call durations are exponentially distributed; calls are uniformly distributed over all possible pairs of nodes. We are interested in the difference of blocking probability and vulnerability ratio between the various RWA algorithms and also in the time consumed to process the connection requests. The typical value for switch port crosstalk ratio and adjacent wavelength crosstalk ratio are -30dB and -25dB. The physical parameters of the simulated network are the same as what we used in [4]. Fig. 1 shows the blocking probability for various RWA schemes. We can see that the HQ-HQ path restoration scheme and the combined algorithms (HQ-FF and HQ-BF) considerably outperform other methods. The HQ-HQ path restoration also has significantly lower vulnerability ratio, as we can see in Fig. 2. The vulnerability ratio of the combined algorithms are of the same order as that of non-QoT-aware path restoration algorithms and HQ dark backup path protection. The drawback of the HQ-HQ path restoration is its high computational time. Fig. 3 shows the processing time for HQ-HQ and combined algorithms versus the arrival rate (offered load) 2 . We can see that the combined algorithms can significantly increase the speed of restoration. Fig. 4 shows the time complexity for the non-QoT-aware algorithms. It can be seen that the combined algorithms can decrease the computational time to the same order as the non-QoT-aware ones. Our results show that using a combination of QoT-aware primary path establishment algorithms and non-QoT-aware restoration algorithms has the advantage of achieving low blocking probability and moderate vulnerability ratios, while avoiding high computational time complexity. This work was supported by the National Science Fundation grants CNS-0519911 and CNS-0520060. 1 To the best of our knowledge, this algorithm is the only one available in the literature that does not require wavelength conversion and is applicable to an arbitrary network topology. 2 Timing is obtained on a general purpose computer.
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4 References 1. Session OWR in Proceedings of OFC (IEEE/OSA 2007). 2. Sessions: ONS03, ONS04 in Proceedings of ICC (IEEE 2007). 3. Y. Zhai, Y. Pointurier, S. Subramaniam, and M. Brandt-Pearce, “Performance of dedicated path protection in transmissionimpaired DWDM networks,” in Proceedings of ICC (IEEE 2007). 4. Y. Pointurier, M. Brandt-Pearce, T. Deng, and S. Subramaniam, “Fair QoS-aware adaptive Routing and Wavelength Assignment in all-optical networks,” in Proceedings of ICC (IEEE 2006). 5. M. Medard, R. A. Barry, S. G. Finn, W. He, and S. Lumetta, “Generalized loop-back recovery in optical mesh networks,” IEEE/ACM Trans. Networking, vol. 10, no. 1, pp. 153–164, Feb 2002.
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