Chapter 2 Physical Layer.docx

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Information Technology Management
ITM 301
Franklyn Prescod

Chapter 2 The Physical Layer21 The Theoretical Basis for Data Communication 212 BandwidthLimited Signals no transmission facility can transmit signals without losing some power in the processDistortion all transmission facilities diminish different Fourier components by different amountsfor wires the amplitudes are transmitted mostly undiminished from 0 up to some frequency measured in cyclessec or Hertz with all frequencies above this cutoff attenuated Bandwidth the width of the frequency range transmitted without being strongly attenuated The cutoff is not really sharp so the quoted bandwidth is usually from 0 to the frequency at which the received power has fallen by halfBandwidth is a physical property of the transmission medium that depends on the construction thickness and length of a wire or fiber Filters are used to further limit the bandwidth of a signal which allows more signals share a given region of spectrum which improves the overall efficiency The frequency range for some signals will not start at 0 but this doesnt matter because the bandwidth is still the width of the band of frequencies that are passed and the information carried depends only on the width and not on the starting and ending frequencies Baseband signals that run up from 0 to maximum frequency Passband signals that are shifted to occupy a higher range of frequencies wireless transmissions Digital Transmission the goal is to receive a signal with just enough fidelity to reconstruct the sequence of bits that was sent limiting the bandwidth limits the data rate even for perfect channels Analog Bandwidth a quantity measured in Hz Digital Bandwidth is the maximum data rate of a channel a quantity measured in bits per second That data rate is the end result of using the analog bandwidth of a physical channel for digital transmission22 Guided Transmission Media 221 Magnetic Media common way to transport data from one compute to another is to write them on magnetic tape or removable media recordable DVD physically transport the tape or disks to the destination machine and read them back effective bandwidth of a 1000 tapes in a box equaling 800 terabytes being delivered in the US in 24 hours by express mail is 6400 terabits86400 seconds 222 Twisted Pairsmost common transmission media is twisted pair Twister Pair consists of two insulated copper wires typically about 1mm thick The wires are twisted together in a helical form in which the twisting is done because two parallel wires constitute a fine antenna When the wires are twisted the waves from different twists cancel out so the wire radiates less effectively A signal is carried as the difference in voltage between the two wires in the pair which provides a better immunity to external noise Telephone System common application able to run several kilometers without amplification for longer distances the signal becomes too attenuated and repeaters are needed when many twisted pairs run in parallel for a substantial distance they are bundled together and encased in a protective sheath the pairs in the bundles would have interfered with each other if it were not for the twisting Twisted pairs can be used for transmitting analog or digital information Bandwidth depends on the thickness of the wire and the distance travelled but several megabitssecFullDuplex Links links that can be used in both directions at the same time HalfDuplex Links links that can be used in either direction but only one way at a time Simplex Links links that allow traffic in only one direction Shielding reduces the susceptibility to external interference and crosstalk with other nearby cables to meet demanding performance specifications LAN 100Mbps Ethernet uses two out of the four pairs one pair for each direction 1Gbps uses all four pairs in both directions simultaneously to reach higher speeds223 Coaxial CableCoaxial Cable consists of a stiff copper wire as the core surrounded by an insulating material The insulator is encased by a cylindrical conductor The outer conductor is covered in protective plastic sheath Two kinds50ohm cable commonly used when it is intended for digital transmission from the start75ohm cable commonly used for analog transmission and cable television It has better shielding and greater bandwidth than unshielded twisted pairs so it spans longer distances at higher speeds It provides noise immunity and offer up to GHz bandwidth Widely used within telephone systems for long distance but has now been replaced by fiber224 Power LinesPower Lines deliver electrical power to houses and electrical wiring within houses distributes the power to electrical outlets the data signal is superimposed on the lowfrequency power signal on the active or hot wire as both signals use the wiring at the same time the difficulty with using household electrical wiring for a network is that it is designed to distribute power signals which are different from data signals The electrical properties of the wiring vary from house to house and change as appliances are turned on and off which cause data signals to bounce around the wiring
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