Journal of Emerging Technologies in Web Intelligence Contents

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144 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010transmission distance is significantly greater than theelectronic ones, signal need not to be regenerated inoptical networks. Optical fiber has very less weight incomparison to electronic MINs. Thus Optical networksgive the combination of high bandwidth and low latency.TABLE IICOMARISON OF ELECTRONIC AND OPTICAL NETWORKSCharacteristicsElectronicMultistageNetwotksOpticalMultistageNetworksSpeed Less HighEnergyElectricity LightTransmittedBandwidth Used for lessbandwidthapplicationsUsed for highbandwidthapplicationsLatency High LessError Probability High LessWeight More LessCost Less MoreSwitching Packet Switching CircuitSwitchingPathProvide Multi path Provide singlefrom source to path fromdestination. source todestinationComplexity More Complex Less ComplexStructureconsidered2-dimensional 3-dimensionalIV. PROBLEMS IN OPTICAL NETWORKSDue to the difference in speeds of the electronic andoptical switching elements and the nature of opticalsignals, optical MINs also hold their own challenges.A. Path Dependent LossPath dependent loss means that optical signalsbecome weak after passing through an optical path. In alarge MIN, a big part of the path-dependent loss isdirectly proportional to the number of couplers that theoptical path passes through [16]. Hence, it depends on thearchitecture used and its network size. Hence, if theoptical signal has to pass through more no of stages orswitches the path dependent loss will be more.B. Optical CrosstalkOptical crosstalk occurs when two signal channelsinteract with each other. There are two ways in whichoptical paths can interact in a switching network. Thechannels carrying the signals could cross each other.Alternatively; two paths sharing a switch couldexperience some undesired coupling from one path toanother within a switch. For example, assume that thetwo inputs are y and z, respectively, the two outputs willhave ly+lxz and lz+lxy, respectively, where l is path lossand x is signal crosstalk in a switch. Using the best devicex=35 dB and l=0.25 dB. For more practically availabledevices, it is more likely that x=20 dB and l=1 dB [5].Hence, when a signal passes many switches, the inputsignal will be distorted at the output due to the loss andcrosstalk introduced on the path.Crosstalk problem is more dangerous than the pathdependentloss problem with current optical technology.Thus, switch crosstalk is the most significant factor thatreduces the signal-to-noise ratio and limits the size of anetwork. Luckily, ensuring that a switch is not used bytwo input signals simultaneously can eliminate first-ordercrosstalk. Once the major source of crosstalk disappears,crosstalk in an optical MIN will have a very small effecton the signal-to-noise ratio and thus a large optical MINcan be built and effectively used in parallel computingsystems.V. APPROACHES TO SOLVE CROSSTALKA. Space Domain ApproachOne way to solve crosstalk problem is a spacedomain approach, where a MIN is duplicated andcombined to avoid crosstalk [8]. The number of switchesrequired for the same connectivity in networks with spacedomain approach is slightly larger than twice that for theregular network. This approach uses more than doublethe original network hardware to achieve the same. Thusfor the same permutation the hardware or we can say theno of switches will be double. Thus cost will be morewith the networks using space domain approach. In allthe four cases only one input and only one output isactive at a given time so that no cross talk occurs. Withthe space domain approach, extra switching elements(SEs) and links are used to ensure that at most one inputand one output of every SE will be used at any giventime.Figure 3. Crosstalk avoidance using space domain approachB. Time Domain ApproachAnother way to solve the problem of crosstalk is thetime domain approach [3]. With the time domainapproach, the same objective is achieved by treatingcrosstalk as a conflict; that is, two connections will beestablished at different times if they use the same SE.Whereas we want to distribute the messages to be sent tothe network into several groups, a method is used to find© 2010 ACADEMY PUBLISHER
JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010 145out which messages should not be in the same groupbecause they will cause crosstalk in the network. A set ofconnections is partitioned into several subsets such thatthe connections in each subset can be establishedsimultaneously in a network. There is no crosstalk inthese subsections. This approach makes importance inoptical MINs for various reasons. First, most of themultiprocessors use electronic processors and opticalMINs. There is a big mismatch between the slowprocessing speed in processors and the highcommunication speed in networks carrying opticalsignals [15]. Second, there is a mismatch between therouting control and the fast signal transmission speed. Toavoid crosstalk, the TDM approach is used, where the setof messages are partitioned into several groups such thatthe messages in each group can be sent simultaneouslythrough the network without any crosstalk.VI. METHODS FOR MESSAGE PARTITIONING INTDM APPROACHA. Window methodWindow method is the method that is used to find themessages that are not in the same group because it causescrosstalk in the network. If we consider the network ofsize N x N, there are N source and N destination address.Combining source and its destination address formscombination matrix. From this, optical window size is M-1 where M = log 2 N and N is size of network. In windowmethod, the number of windows is equal to the number ofstages [11].After finding conflicts using window method,conflict graph is generated shown in figure. The numberof nodes is the size of the network. The nodes that arehaving conflict are connected through edge. Degree ofeach message is the number of conflicts to the othermessage. Conflict graph is shown in figure 4.Figure 4. Conflict graphThe conflict matrix is a square matrix with N x Nentry, it consists of the output of the window method, asshown in figure 5. The definition of Conflict Matrix is thematrix M ij with size N x N. N is the size of the network.B. Improved window methodIn this method the first window is eliminated for thiswe make the conflict matrix initialized to 0, here Numberof windows is M-1. It takes less time to find conflictsthan the windows method. Therefore, it is calledimproved windows method [11,12].Figure 5. Conflict matrix© 2010 ACADEMY PUBLISHER
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