Browsing by Subject "Computer network protocols."

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Open Access
Multi-layer traffic engineering in optical networks under physical layer impairments
(2010) Şengezer, Namık
We study Traffic Engineering (TE) in Multiprotocol Label Switching (MPLS)/Wavelength Division Multiplexing (WDM) networks and propose a multi-layer TE method. MPLS provides powerful TE features for IP networks and is widely deployed in backbone networks. WDM can increase the transmission capacity of optical fibers to tremendous amounts, therefore it has been the dominant multiplexing technology used in the optical layer. The proposed multi-layer TE solution facilitates efficient use of network resources where the TE mechanisms in the MPLS and WDM layers coordinate. We consider a static WDM layer and available traffic expectation information. The TE problem arising in the considered scenario is the Virtual Topology Design (VTD) problem, which involves the decision of WDM lightpaths to be established, calculation of MPLS Label Switched Paths (LSPs) on the resulting virtual topology, and calculation of the routes and wavelengths in the physical topology that correspond to the lightpaths in the virtual topology. We assume a daily traffic pattern changing with the time of day and aim to design a static virtual topology that satisfies as much of the offered traffic as possible, over the whole day. In our proposed solution, the multi-layer VTD problem is solved by decomposing it into two sub-problems, each involving in a single layer. The decomposition approach is used in the thesis due to the huge computational burden of the combined solution for real-life networks. The sub-problem in the MPLS layer is the design of the lightpath topology and calculation of the LSP routes on this virtual topology. This problem is known to be NP-complete and finding its optimum solution is possible only for small networks. We propose a Tabu Search based heuristic method to solve two versions of this problem, resource oriented and performance oriented. Integer Linear Programming (ILP) relaxations are also developed for obtaining upper and lower bounds. We show that the gap between the produced solutions and the lower and upper bounds are around 10% and 7% for the resource and performance oriented problems, respectively. Since the actual traffic can show deviations from the expected values, we also developed an MPLS layer online TE method to compensate the instantaneous fluctuations of the traffic flows. In the proposed method, the LSPs are rerouted dynamically using a specially designed cost function. Our numerical studies show that using the designed cost function results in much lower blockings than using commonly used Widest Shortest Path First and Available Shortest Path First approaches in the literature. The corresponding sub-problem of the multi-layer VTD problem in the WDM layer is the Static Lightpath Establishment (SLE) problem. Along with the capacity and wavelength continuity constraints, we also consider the Bit Error Rate (BER) constraints due to physical layer impairments such as attenuation, polarization mode dispersion and switch crosstalk. This problem is NP-complete even without the BER constraints. We propose a heuristic solution method and develop an exact ILP formulation to evaluate the performance of the proposed method for small problem sizes. Our proposed method produces solutions close to the optimum solutions for the cases in which the ILP formulation could be solved to optimality. Then, these solution methods for the single layer sub-problems are combined in a multi-layer TE scheme to solve the VTD problem in both layers jointly. The proposed TE scheme considers the physical layer limitations and optical impairments. This TE scheme can be applied by keeping each layer’s information hidden from the other layer, but our simulations show that it can produce more effective and efficient solutions when the physical layer topology information is shared with the MPLS layer. We also investigate the effect of non-uniform optical components in terms of impairment characteristics. The numerical results show that more traffic can be routed when all the components in the network have moderate impairment characteristics, compared to the case in which some components have better and some have worse impairment characteristics.
Open Access
Performance analysis of the carrier-sense multiple access protocol for future generation wireless networks
(2013) Köseoğlu, Mehmet
Variants of the carrier-sense multiple access (CSMA) protocol has been employed in many communications protocols such as the IEEE 802.11 and Ethernet standards. CSMA based medium access control (MAC) mechanisms have been recently proposed for other communications scenarios such as sensor networks and acoustical underwater networks. Despite its widespread use, the performance of the CSMA protocol is not well-studied from the perspective of these newly encountered networking scenarios. We here investigate the performance of the CSMA protocol from the point of three different aspects: throughput in networks with large propagation delay, short-term fairness for delay sensitive applications in large networks and energy efficiency-throughput trade-off in networks with battery operated devices. Firstly, we investigate the performance of the CSMA protocol for channels with large propagation delay. Such channels are recently encountered in underwater acoustic networks and in terrestrial wireless networks covering larger areas. However, a mathematical model of CSMA performance in such networks is not known. We propose a semi-Markov model for a 2-node CSMA channel and then extend this model for arbitrary number of users. Using this model, we obtain the optimum symmetric probing rate that achieves the maximum network throughput as a function of the average propagation delay, ¯d, and the number of nodes sharing the channel, N. The proposed model predicts that the total capacity decreases with ¯d −1 as N goes to infinity when all nodes probe the channel at the optimum rate. The optimum probing rate for each node decreases with 1/N and the total optimum probing rate decreases faster than ¯d −1 as N goes to infinity. Secondly, we investigate whether the short-term fairness of a large CSMA network degrades with the network size and density. Our results suggest that (a) the throughput region that can be achieved within the acceptable limits of shortterm fairness reduces as the number of contending neighboring nodes increases for random regular conflict graphs, (b) short-term fair capacity weakly depends on the network size for a random regular conflict graph but a stronger dependence is observed for a grid topology. We also present related results from the statistical physics literature on long-range correlations in large systems and point out the relation between these results and short-term fairness of CSMA systems. Thirdly, we investigate the energy efficiency of a CSMA network proposing a model for the energy consumption of a node as a function of its throughput. We show that operating the CSMA network at a very high or at a very low throughput is energy inefficient because of increasing carrier-sensing and sleeping costs, respectively. Achieving a balance between these two opposite operating regimes, we derive the energy-optimum carrier-sensing rate and the energy-optimum throughput which maximize the number of transmitted bits for a given energy budget. For the single-hop case, we show that the energy-optimum total throughput increases as the number of nodes sharing the channel increases. For the multi-hop case, we show that the energy-optimum throughput decreases as the degree of the conflict graph of the network increases. For both cases, the energy-optimum throughput reduces as the power required for carrier-sensing increases. The energy-optimum throughput is also shown to be substantially lower than the maximum throughput and the gap increases as the degree of the conflict graph increases for multi-hop networks.