Journal of Emerging Technologies in Web Intelligence Contents

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