The role of FDLS in scheduling in OBS networks

Scheduling in a node plays an important role in improving the efficiency of exploiting the bandwidth of optical burst switched networks. This article focuses on analyzing the role of FDL buffer in scheduling and evaluating its performances basing on the simulation results on NS2.
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The role of FDLS in scheduling in OBS networksJOURNAL OF SCIENCE, Hue University, Vol. 69, No. 6, 2011THE ROLE OF FDLS IN SCHEDULING IN OBS NETWORKSVo Viet Minh Nhat1 and Nguyen Hong Quoc21Faculty of Hospitality and Tourism, Hue University2College of Pedagogy, Hue UniversityAbstract. Scheduling in a node plays an important role in improving the efficiency ofexploiting the bandwidth of optical burst switched networks. However, a burst cannot bescheduled and will be dropped if resources, including a wavelength channel and a positionof scheduling an arriving burst on the channel, are not available. A solution to this problemis to use an FDL buffer to delay the appearance of the arriving burst at output, by changingthe position of scheduling the arriving burst on the selected available wavelength channel,hopefully there exists an available resource at the delayed output time. This article focuseson analyzing the role of FDL buffer in scheduling and evaluating its performances basingon the simulation results on NS2.Keywords: OBS, Scheduling, FDL buffer.1IntroductionTo satisfy the increasing demands for bandwidth in backbone networks such as theInternet, many models of optical network have been studied and proposed. In thesemodels, wavelength routing (WR) networks are relatively easy to be installed, butdifficult to handle on variable increment flows and frequently changed-status links.Optical packet switching (OPS) networks are obviously the next choice to overcome theabove disadvantages, but they require more advanced technologies such as opticalbuffer and optical packet switching fabric which are currently unavailable. A newapproach is the model of optical burst switching (OBS) which combines the advantagesof wavelength routing and optical packet switching networks.Fig. 1. The offset time is calculated at least equal to the total of processing and transmissiontime from source to destination75From the first proposal of C. Qiao and M. Yoo [1][2], OBS models quickly getmuch attention from both academic and industrial communities [3][4]. Within an OBSnetwork, an ingress node will gather incoming data (e.g. IP packets) into bursts and eachburst is then sent out after its control packet (burst header packet, BHP). The controlpacket is transmitted on one channel (called the control channel) separated from thechannels carrying burst data (called the data channel) and it was sent before its burstdata an offset time (Fig. 1). At the intermediate (core) nodes (from source todestination), the control packet will reserve the necessary resources for its data burstcoming later. When arriving at the egress node, the original data will be retrieved fromthe data burst and sent to their destination.Scheduling is considered one of the important operations in OBS networks.When a burst arrives at a node, depending on the burst destination, the correspondingresources at output, including the wavelength which carries the arriving burst and theposition (timeslot) on which the burst wants to be scheduled, will be provided.However, if there is no resource available at the output, for example if the wavelengthcarrying the burst is unavailable, or there is no position to schedule the burst, or bothwavelength and position are unavailable, the burst will be dropped. In this case, it iscalled a burst blocking in scheduling.Similar to the methods of contention resolution presented in [5], a solution is touse fiber delay links (FDLs), meaning that a blocked arriving burst is put into a FDLbuffer (including one or several of fiber delay links) to delay its output (changing theposition of scheduling the burst on an output wavelength), with the hope that anotheravailable position on the same output wavelength is found after the delay. Anotherapproach is to use a wavelength converter to change the wavelength carrying the currentburst to another available wavelength at output. The last solution is deflection routing;this means that the output port of the arriving burst is changed so that the resources,including the wavelength and the position of scheduling the burst, are available. Thisarticle focuses on the approach that uses the FDL buffer.The structure of the article is as follows: Section 2 describes the architectures ofOBS networks with the use of FDL buffer. Based on these architectures, the algorithmsof scheduling integrating with FDLs are presented in Section 3. Through the simulationresults on the NS2-obs0.9a package presented in Section 4, the effectiveness of usingFDL in scheduling on OBS networks will be analyzed.2Architecture of OBS node integrated with FDLAs shown in [8], there are two architectures of FDL buffer proposed for an OBS corenode: the architecture of FDL located at input (input-buffer) as shown in Fig. 1a and thearchitecture of FDL arranged in output (output-buffer) as shown in Fig. 1b.76(a)(b)Fig. 2. Architecture of an OBS core node with FDLs arranged at input (a) or output (b) [6]In the architecture of FDL located at input, n FDLs corresponding to n datachannels (wavelengths) at an input port are used in order to avoid any contention thatmay occur when a lot of arriving bursts dispute on the same FDL at the same time. FDL77buffers can be fixed (including FDLs having the same length), dynamic (includingFDLs having different lengths) or hybrid (combination of fixed and dynamic). Thedelay of a burst may be feed forward, this means that the burst is delayed only once by aFDL, or loop back, it means that the burst is delayed repeatedly on the same FDL.The architecture of FDL buffer allocated at output is similar to the one of inputFDL buffer, with the only difference being its placement (at output).3Algorithms of scheduling in combination with FDL bufferBased on the architectures of OBS core node integrating with FDLs, the algorithms ofscheduling in combination ...