a1768_1.pdf JThA64.pdf
Cross Layer Routing in Transparent Optical Networks Christina (Tanya) Politi, Chris Matrakidis, Alexandros Stavdas (1) Department of Telecommunications Science and Technology, University of Peloponnese, Karaiskaki St, Tripolis, 22100, Greece {tpoliti, cmatraki, astavdas}@uop.gr
Vasilis Anagnostopoulos (2) School of Electrical and Computer Engineering, NTUA, 9 Heroon Polytechniou Street, 15773, Zografou, Athens, Greece
[email protected]
Matthias Gunkel (3) T-Systems Enterprise Services GmbH, Deutsche-Telekom-Allee 7, D-64295 Darmstadt, Germany
[email protected]
Abstract: New algorithmically simplified WRA that incorporate physical layer criteria in the path establishment procedure are suggested and applied in the Deutsche-Telecom network. Their blocking performance superiority and physical performance guarantee are discussed. © 2006 Optical Society of America OCIS codes: (060.4250) Networks; (060.4510) Optical communications
1. Introduction In transparent optical networks, transmission and switching is performed at the optical layer and hence various impairments are imposed on the transmitted signals. The corresponding degradation becomes crucial as the number of channels and/or the transparent length is increased. In contrast with traditional SDH/SONET networks, where optical links are terminated by DXC equipment, regeneration is not granted unless special measures are taken. In the meantime, routing algorithms that are developed for infrastructures that include regeneration may not always guarantee optimum operation, as far as both physical and network performance is concerned [1]. In order to achieve this, Wavelength Routing Algorithms (WRAs) should account for Physical Layer Impairments (PLI) [2-5]. Many papers are involved in ways of incorporating a PLI constraint into the decision making of the routing engine, for the optimum path establishment, that would simultaneously achieve optimal utilization of resources and acceptable physical layer performance. A common consensus is that such an algorithm should be simple in terms of algorithmic steps and the PLI constraint should be accurate and fast to calculate or to retrieve [3,4]. Nevertheless the combined effect of the resource allocation and ‘degradation’ minimization has not been thoroughly investigated. In this paper, we expand on the outcomes of previously established results to investigate the interplay between the path establishment decision and the PLI constraint. As in the previously investigated work, the Q-factor is analytically calculated for each and every path but also for the channels that may be affected by the establishment of the new channel [2,3]. The Q-factor accounts for four-wave mixing (FWM) and cross-phase modulation (XPM) and Optical-Signal-to-Noise degradation through the analytical models of [6]. Cross-channel nonlinearities that are the main degradation source in WDM systems do not only accumulate with the span count but increase with the number of channels. Hence, WRAs that look for the shortest path do not necessarily guarantee good physical performance, so the complexity of the algorithms that check for the physical performance is increased. For that reason we investigate the performance of an algorithm where path establishment is based on the shortest generalized cost, i.e. the path that minimizes a pre-defined cost. At the same time the algorithm used here is the light version of the algorithm that only checks for the Q factor of the chosen channel at the end of the procedure just before establishing it [2]. Three different costs are defined: the length of the path, the ‘congestion’ of the path and the ‘noise’ of the channel in the path. The rationale behind this approach is the following. The traditional shortest path (SP) algorithm is used as reference. By minimizing the congestion of the path, the algorithm is indirectly seeking for low nonlinearities as well. Hence the light version of the algorithm can be utilized with very good results. By seeking the ‘shortest noise’ path the algorithms is looking for the simultaneous optimization of resources and noise and hence the light version of the algorithm is only required as only the path with the ‘best’ possible performance needs to be checked. The purpose of the paper is to present the algorithms and show how the PLI constraint can be incorporated into the path establishment without adding extra burden to the algorithm. ©OSA 1-55752-830-6
a1768_1.pdf JThA64.pdf
2. WRA and Simulator Assumptions For each wavelength find shortest ‘COST’ path and add it to candidate path set
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Fig. 1. a) The routing engine developed for PLI aware WRA b) the DT network as presented in [6] c) the block diagram of the physical layer path as used for the calculation of the Q – factor (OXC = Optical Cross Connect, SMF = Single Mode Fibre, DCF= Dispersion Compensating Fibre, and EDFA= Erbium Doped Fibre Amplifier.
In dynamic optical networks path selection between the available paths and wavelength assignment between the wavelengths of those path are two algorithmically separate procedures. The general flowchart of the simplified version of the WRA is shown in Fig. 1 a). In previous work it was discussed that the difference between the two algorithms is very small as far as the BP is concerned [4]. Assuming dynamic connection requests, the following description holds for the algorithm. Three different WRAs can be formed according to the path cost choice, all based on the Bellman-Ford algorithm. In the first case, for each of the wavelengths the engine looks for the ‘shortest-path’ (S-P). Then a set of available wavelengths is returned. One candidate lightpath is chosen based on the ‘best-fit’ criterion [1]. For the PLI blind case this is the path to be established. For the PLI aware case the engine will calculate the Q-factor for the candidate paths and blocks the request if the Q-factor is lower than a threshold, QA. Otherwise it probes the system with candidate lightpath and calculates the Q-factor of the affected existing connections. If this is lower than the threshold QB the lightpath in question is dropped. In the second case the decision is based on the path congestion (S-C). This is done on a link-by-link basis and the standard deviation of the occupancy of the link is used as cost. Note that the algorithm is approximately similar with the ‘shortest widest path’ [3] but not the same. However it is a fast algorithm and it is used here in order to provide a ‘hidden’ criterion for better physical performance. This means that it is expected that the least occupied path will be checked at the end and is less likely to be blocked due to insufficient Q-factor, while imposing better load balancing to the system. Finally for the third system, the decision of the path is made according to the noise of the system (S-Noise as explained below). Minimization of the noise means that the lightpath has the best Q factor and hence although the shortest path (least resource utilization) is not directly guaranteed, good physical performance is only achieved when the length of the path is smaller than the transparent length of the system. Note that when the case of QA=0 is considered, the algorithms behave as PLI blind ones. In order to check the algorithms’ blocking probability, experiments are performed at the Deutsche Telecom (DT) Network as it was established in [6] and the graph is shown in Fig. 2, with N=17 nodes. Each link is considered to have 40 wavelengths. One type of service is considered, with constant lifetime of 1 time unit and inter-arrival times of the connection requests following a Poisson distribution with average inter-arrival time of 10 time units. To apply the algorithm for the DT network, an appropriate traffic forecast for 2007 is taken with 315 demands at 10 Gbit/s. No wavelength conversion, protection or regeneration is considered. To evaluate the Q-factor, an analytical model is used that provides a good trade-off between accuracy and upgradeability and was presented in [5]. Furthermore the physical characteristics of the links that are shown in Fig. 1c) are as follows: 40 km single mode fibre spans, each being fully dispersion compensated by 8km dispersion compensating with 50 GHz spacing and power per channel P=0 dBm. The Q-factor is given by Q≈RPs,M/[σ0+σ1]. Here it should be noted that as the power of the channels is kept at a similar level on a link by link basis so that the minimization of the denominator is equivalent to the optimization of the Q-factor. This is the noise that is σlink = ©OSA 1-55752-830-6
a1768_1.pdf JThA64.pdf
/[σ0+σ1]. The algorithm labeled S- Noise uses as cost this exact parameter. The σlink comprises factors from all the links of the path from all the associated FWM, XPM, ASE degradation and receiver noise effects. 3. Results Fig. 2 illustrates blocking probability (BP) results versus traffic scaling factor (in Erlangs) for the specific algorithms. In Fig. 2a) the traditional S-P algorithm for QA=0 and QA=10 is used and the BP is greatly affected by the PLI awareness [2]. For this case both the heavy and the light versions of the algorithm as presented in [2] and [3] are used in order to show that the difference between the two algorithms is not large. In Fig. 2b) the BP of the SC is shown. Although the performance of the PLI blind version is worse than the S-P one, the overall performance for the PLI aware WRA is very interesting, as although resources may be wasted for path allocation, physical performance is guaranteed. A similar conclusion was derived for the shortest widest path algorithm. Here however the algorithmic steps are less [2]. The most interesting performance results however are evident in Fig.3b). Here the remarkable point is that path establishment is performed according to the physical performance and hence, when the Q- factor is checked before the lightpath assignment, very few lightpaths are blocked. So although the PLI blind version of the algorithm is performing worst than its S-P counterpart the PLI aware version is slightly better. 4. Conclusions Three new WRA that account for PLI are suggested and applied on the DT network. The main conclusion stems from the fact that when path establishment is performed on a basis of a cost that accounts for PLI, the PLI aware WRA is relaxed as far as complexity is concerned. 5. Acknowledgement The authors would like to thank the IST NOBEL project for partially funding this work. 6. References [1] H. Zang, J. P. Jue, and B. Mukherjee, “A Review of Routing and Wavelength Assignment Approaches for Wavelength-Routed Optical WDM Networks,” Opt. Net. Mag., vol. 1, Jan. 2000. [2] C. T. Politi, V. Anagnostopoulos, C. Matrakidis, A. Stavdas; “Physical Layer Impairment Aware Routing Algorithms Based on Analytically Calculated Q-Factor”, OFG1, Anaheim , OFC/NFOEC March 2006. [3] C. T. Politi, C. Matrakidis, V. Anagnostopoulos, A. Stavdas; “Cross-Layer Routing Algorithms in a European Scale Network”, OFG1, Cannes France , ECOC 2006, [4] R. Cardillo, V. Curri, M. Mellia; “Considering transmission impairments in configuring wavelength routed optical networks”, OFG6, Anaheim , OFC/NFOEC March 2006, [5] H. Zang, J. P. Jue, and B. Mukherjee, “A Review of Routing and Wavelength Assignment Approaches for Wavelength-Routed Optical WDM Networks,” Opt. Net. Mag., vol. 1, Jan. 2000. [6] A. Stavdas, S. Sygletos, M. O'Mahoney, H. L. Lee, C. Matrakidis, A. Dupas, “IST-DAVID: concept presentation and physical layer modeling of the metropolitan area network” J. of Lightwave Technol. Vol. 21, pp. 372 – 383, Feb. 2003 [7] M. Gunkel, R. Leppla, M. Wade, A. Lord, D. Schupke, G. Lehmann, C. Fürst, S. Bodamer, B. Bollenz, H. Haunstein, H. Nakajima, J. Martensson: " A Cost Model for the WDM Layer”, Conference on Photonics in Switching 2006, Herakleion, Greece, October 2006
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Fig. 2 a) Blocking probability versus offered traffic load for the cases: a) S-P and QA=0, S-P and QA=10, S-C and QA=0 and S-C and QA=10 c) S-Noise and QA=0 and S-Noise and QA=10. QB is the same as the as QA in all cases.
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