CMPE 150 Winter 2009 Lecture 5 Januaryy 20 2009 P E Mantey CMPE 150 Introduction to Computer Networks Instructor Patrick Mantey mantey soe ucsc edu htt http www soe ucsc edu mantey d t Office Engr 2 Room 595J Office hours Tuesday 3 5 PM TA Anselm Kia akia soe ucsc edu Web site http www soe ucsc edu classes cmpe150 Winter09 Text Tannenbaum Computer p Networks 4th edition available in bookstore etc Syllabus Today s Agenda Physical Layer Finish tour of Data Communications Link Layer Signaling Timing Encoding E Error d detection t ti Error correction Topics terms Full duplex half duplex simplex Local subscriber loop p vs trunk backbone Alternatives to local subscriber loop WiMax 802 16 Multiplexing TDM FDM FDM via modulation on a carrier Baseband no modulation Baud vs bps Frequency Division Multiplexing a The original bandwidths b The Th b bandwidths d id h raised i d in i frequency f b The multiplexed channel DSL FDM Fig 2 2 28 28 ADSL with multi multi tone tone channels Cable TV Spectrum Allocation Wavelength Division Multiplexing division multiplexing Time Division Multiplexing The T1 carrier 1 544 Mbps Time Division Multiplexing 2 Delta modulation Time Division Multiplexing 3 M lti l i T1 streams Multiplexing t iinto t hi higher h carriers i Time Division Multiplexing 4 Two back to back SONET frames Time Division Multiplexing 4 SONET and SDH multiplex rates rates Spread p Spectrum p Frequency Hopping Direct Sequence Circuit Switching a Circuit switching switching b Packet switching Message Switching a Circuit switching b Message switching c Packet switching Packet Switching A comparison of circuit switched and packet packetswitched networks The Mobile Telephone System First Generation Mobile Phones Analog Voice Second Generation S dG ti M Mobile bil Phones Ph Digital Voice Third Generation Mobile Phones Digital Voice and Data Advanced Mobile Phone System a Frequencies are not reused in adjacent cells cells b To add more users smaller cells can be used Channel Categories The 832 channels are divided into four categories 1 2 2 3 4 C t l base Control b to t mobile bil to t manage the th system t Paging base to mobile to alert users to calls for them Access bidirectional for call setup and channel assignment Data bidirectional for voice fax or data D AMPS Digital Advanced Mobile Phone System a A D AMPS channel with three users b A D AMPS channel with six users GSM Global System for Mobile Communications GSM uses 124 frequency channels each of which uses an eight slot TDM system GSM 2 A portion of the GSM framing structure structure CDMA Code Division Multiple Access a Binary chip sequences for four stations b Bipolar chip sequences c Six examples of transmissions d Recovery of station C s signal Third Generation Mobile Phones Digital Voice and Data Basic services an IMT 2000 network should provide High quality voice transmission Messaging replace e mail fax SMS chat etc Multimedia music videos films TV etc Internet access web surfing w multimedia Cable Television Community C it Antenna A t Television T l i i Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable C bl Community Antenna Television An early cable television system system Internet over Cable Cable television Internet over POTS The fixed telephone system Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access Cable Modems Typical details of the upstream and downstream channels in North America Link Layer Chapter 3 of Tannenbaum Text Terms Definitions Term Data Element a single binary 1 0 Data Rate t data rate d t elements l t gett transmitted g Element Signal part of signal that occupies shortest interval of signaling code Signaling or Modulation rate rate signal elements are t transmitted itt d Units Bit Bits second bps Digital a voltage pulse of constant amplitude p Analog a pulse of constant freq amp or phase Baud signal elements sec Terms Unipolar Polar O e logic One og c state represented ep ese ted by pos positive t e voltage o tage the other by negative voltage Data rate All signal elements have same sign Rate of data transmission in bits per second Duration or length of a bit Time taken for transmitter to emit the bit Terms Modulation rate Rate at which the signal level changes M Measured d in i baud b d signal i l elements l t per second d Mark and Space Binary 1 and Binary 0 respectively Interpreting Signals Need to know Timing of bits when they start and end Signal levels Factors affecting successful interpreting of signals Signal to noise ratio S N or SNR Data rate Bandwidth Comparison of Encoding Schemes Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer providing isolation Concentrate power in the middle of the bandwidth Clocking g Synchronizing transmitter and receiver External clock vs vs Sync mechanism based on signal Comparison of Encoding Schemes Error detection Can be built in to signal encoding Signal interference and noise immunity Some codes are better than others Cost C t and d complexity l it Higher signal rate thus data rate lead to higher costs t Some codes require signal rate greater than data rate t Encoding Schemes Nonreturn to Zero Level NRZ L Nonreturn to Zero Inverted NRZI Bipolar AMI AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 Spectrum Normalized Frequency Stallings Fig 5 3 Nonreturn to Zero Level Zero Level NRZ L NRZ L Two different voltages for 0 and 1 bits Voltage constant during bit interval no transition I e no return to zero voltage e g Absence of voltage for zero constant positive voltage for one More often negative voltage for one value and d positive iti ffor th the other th This s iss NRZ L Nonreturn to Zero Inverted Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time Transition T iti l low tto high hi h or hi high h tto llow denotes a binary 1 No transition denotes binary 0 An example of differential encoding NRZ Ref Stallings Fig 5 2 NRZI used by USB http www interfacebus com Design Connector USB html Differential Encoding Data represented by changes rather than levels More M reliable li bl d detection t ti off ttransition iti rather th than level In complex transmission layouts it is easy to lose sense of polarity NRZ pros and cons Pros Easy to engineer Make good use of bandwidth Cons dc component p Lack of synchronization
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