Research On Wavelength Assignment Strategy In Optical Network-on-chip

With the increasing demand for high performance computing and aggressive technology scaling has driven the trend of integrating hundreds to thousands of processing cores on a single die.Network-on-chip(No C)is becoming an important interconnect mode for the next generation of many-cores.As the electrical No C is restricted by the power consumption and cannot scale well in performance when the number of cores increases,optical network-onchip(ONo C)is proposed as a promising solution,has the advantages of ultra-high bandwidth,low latency,and distance-independent power dissipation.Compared with the electrical No C,a distinct feature of ONo C is that wavelength division multiplexing(WDM)technology is supported,multiple wavelengths of optical signal can be used for connections at the same time in one single waveguide.However,undesirable mode coupling in optical devices may cause crosstalk between different wavelengths.The crosstalk of multiple wavelengths accumulated in ONo C may cause the reduction of the communication reliability due to the deterioration of the optical signal-to-noise ratio(OSNR)in the network.Wavelength assignment(WA)is one of the key technologies in ONo C design,which has an important impact on network performance.This thesis aims to design an optimized WA strategy to improve the communication performance of ONo C,including reducing crosstalk,improving OSNR,and reducing communication latency.Since the physical communication path is fixed after the application is mapped onto ONo C,some crosstalk is inevitable when multiple communications execute simultaneously,this thesis proposes a crosstalk reduced wavelength assignment strategy in the most energyefficient case,aiming at minimizing the worst-case crosstalk of the application in ONo C under the constraint of wavelength conflict.Considering the characteristics of crosstalk and wavelengths spacing in ONo C,an optimization model of maximizing OSNR is established.By defining the overlapped communication path and optimizing the wavelength spacing between the wavelengths assigned to the overlapping paths,the accumulation of crosstalk is reduced.A wavelength assignment scheme based on ant colony algorithm(WA-ACO)is designed,which can automatically assign one wavelength to each pair of communication and find the solution with minimum crosstalk.The simulation results indicate that,compared to several fixed wavelength assignment methods,the proposed strategy WA-ACO improves the worst-case OSNR by at least 21.2% and 19.84% for two applications(pip and mwd),enhancing the ONo C communication reliability effectively.Although the scheme of reserving one wavelength for each pair of communication is the most energy-efficient,it fails to take full advantage of the high bandwidth brought by WDM technology in ONo C.Considering the problem of increasing crosstalk while reducing transmission time by using multi-wavelength to support parallel communication,this thesis designs a crosstalk and latency aware multi-wavelength assignment strategy(CLWA)at design time.By allocating a reasonable number of wavelength for each communication and optimizing the wavelength spacing,a balanced optimization of network reliability and communication latency can be realized.The crosstalk and latency analysis models are established to measure the performance of the proposed strategy.The intelligent ant colony algorithm and Pareto sorting strategy are combined to solve the multi-objective wavelength allocation problem.The simulation results show that the proposed strategy CLWA can effectively obtain a group of Pareto frontier solutions under the different number of wavelengths,which can improve OSNR while ensuring the communication latency,meet the reliable and low latency data transmission requirements.The heuristic algorithm usually needs an amount of iteration time,in order to meet the realtime response requirements of online decision-making,random and continuous allocation strategies are adopted because of the simple response,but it is easy to lead to a large number of adjacent wavelength channels to be assigned to the same physical path,resulting in increased crosstalk accumulation and reduced communication reliability.This thesis proposes a communication aware and reliable online wavelength assignment strategy,to make full use of the flexibility of reconfigurable ONo C and improve communication reliability.An overlapping graph model is established based on graph theory,restrict the communication pairs at both ends of the connecting edge cannot use the same wavelength at the same time to satisfy the wavelength conflict constraint.An overlapped subgraph partitioning algorithm is designed to dynamically determine the transmission waveguides of each pair of communication in the dual waveguide ONo C,so as to reduce the overlap of communications and reduce crosstalk sources.Moreover,this subgraph partitioning algorithm can be extended to use in ONo C with two or more connected waveguides.In each decision step,the locally optimal solution is selected instead of backtracking iteration,which can meet the demand for quick response in online operation.In order to further reduce crosstalk and improve communication reliability,the spacing of wavelengths used in communication is optimized in each subgraph,avoiding assign the adjacent wavelength channel to the overlapped communication.Compared with the random and first-fit strategies,the results show that our proposed scheme can improve the average OSNR by 11.89% and 16.81%,respectively.