Data transmission in computer networks

Why keep beckon with a really low frequency follow the Earth s surface? Why are they non used for informations transmittal in computing simple machine webs?AnswersRadio moving ridges which are down the stairs 2 MHz follow the land. Following are the ground why its happened,DiffractionCurrent induced in the Earth s surface, which slows moving ridges near the Earth, and because of this moving ridge forepart tilt downward.These can be about ground that why Low frequence are non used for informations transmittal in computing machine webs.Low frequences & A agrave lower information rates ( Nyquist/Shannon )As more power is needed to convey and have low frequences, for this big aerial are needed.Penetration of humiliate frequences to stuff is high.Why does the ITU-R lone regulate lower frequences ( up to some hundred GHz ) and non higher frequences ( in the THz scope ) ?AnswersTHz scope frequences, such(prenominal) as infrared, seeable visible radiation, do nt interfere with othe r transmittal because ther are are blocked easy when obstructions are nowadayss.So merely the banal safety ordinances applied to the systems.Its non easy to bring forth the higer freuencies when the wireless systems stay beyond the 100 GHz.What are the different attacks in reulation sing nomadic echo systems in Europe and the US? What are the effects?AnswersEuropean attackClassical attack which was that, standardisation and regulate before any merchandises were available.ETSI which is founded by EU authoritiess to harmonise depicted object ordinances.All states had to obey or follow the criterions set by ETSI.American ApproachCompanies develop the systems, so market success attempt to coerce the criterion, or companies itself try to standardise.FCC is the illustration.Affects of different ApproachsSome criterions failed and some succeeded merely in Europe, HIPERLAN 1 ( wholly failed ) , and ISDN ( Succeeded ) . Some become the worldwide ( GSM ) .US attack is better for the most o f the systems, i.e. initial merchandises, and after it, standardized. WLAN 802.11 is a honorable illustration for the USA attack. But criterions about nomadic market which are good established in Europe but its non even known in US. i.e. free roaming, MMS, GPRS roaming.Why is the international handiness of the same ISM bands of import?AnswersInternational handiness of same ISM set is of import because ofInteroperability.big plenty market to do the consumer devices cheap.What are the chief jobs of aim extension? Why do wireless moving ridges non ever follow a consecutive line? Why is reflexion both utile and harmful?AnswersFollowing are the ground which cause the signal extension,AttenuationDispersingDiffractionContemplationRefractionIn the presence of big edifices, without multipath extension or contemplation of signal we ca nt have radio signal. Inter symbol intervention ( ISI ) is due to multipath extension.Why, typically, is digital passage non plenty for wireless transmittal ? What are general ends for digital transition? What are typical strategies?AnswersFDM is used worldwide for dividing different systems. So all the wireless system must utilize linear transition for modulating the digital signal onto a pallbearer frequence. Most of the wireless Stationss want to convey at the same clip, because of this ground all the signal should be transition onto different bearer frequences.Other ground behind digital transition,Antenna and medium features.Features for digital transition areSpectral efficiencyPower efficiencyRobustness.Typical strategies areAskPSKFSKWhat are the chief benefits of a spread spectrum system? How can distributing be achieved? How can DSSS systems profit from multi-path extension?AnswersBenefits of a Spread Spectrum SystemRobust against interventionBuilt-in securityFooting for CDMA engineeringsIf the signal degree is low plenty so it can be used as background of bing systems.By Chiping sequence or frequence hopping ( XORing ) a spot, we can accomplish Spreading. Guard infinites are now the perpendicularity of the splintering sequences or skiping forms. Higher Orthogonality- & A agrave Lower the correlativity of spread signals, or lower the hit chance of frequence skiping systems.To recombining signals going along different waies, DSSS system typically use pitch receiving systems.What limits the figure of coincident users in a TDM/FDM system compared to a CDM system? What happens to the transmittal quality of connexions if the burden gets higher in a cell, i.e. , how does an extra user influence the other users in the cell?AnswersMentionsJochen Schiller, Mobile Communication , 2nd ed. , Addison-Wesley