1Data Communications & NetworksSession 3 – Main ThemeData Encoding and TransmissionDr. Jean-Claude FranchittiNew York UniversityComputer Science DepartmentCourant Institute of Mathematical SciencesAdapted from course textbook resourcesComputer Networking: A Top-Down Approach, 5/ECopyright 1996-2009J.F. Kurose and K.W. Ross, All Rights Reserved222Data Encoding and TransmissionData Encoding and TransmissionAgenda11Session OverviewSession Overview33Summary and ConclusionSummary and Conclusion3What is the class about? Course description and syllabus:»http://www.nyu.edu/classes/jcf/g22.2262-001/»http://www.cs.nyu.edu/courses/spring10/G22.2262-001/index.html Textbooks:» Computer Networking: A Top-Down Approach (5thEdition)James F. Kurose, Keith W. RossAddison WesleyISBN-10: 0136079679, ISBN-13: 978-0136079675, 5th Edition (03/09)4Course Overview Computer Networks and the Internet Application Layer Fundamental Data Structures: queues, ring buffers, finite state machines Data Encoding and Transmission Local Area Networks and Data Link Control Wireless Communications Packet Switching OSI and Internet Protocol Architecture Congestion Control and Flow Control Methods Internet Protocols (IP, ARP, UDP, TCP) Network (packet) Routing Algorithms (OSPF, Distance Vector) IP Multicast Sockets5 Data Transmission and Encoding Concepts ADTs and Protocol Design Summary and ConclusionData Transmission and Encoding Session in Brief6Icons / Metaphors6Common RealizationInformationKnowledge/Competency PatternGovernanceAlignmentSolution Approach722Data Encoding and TransmissionData Encoding and TransmissionAgenda11Session OverviewSession Overview33Summary and ConclusionSummary and Conclusion8ADTs and Protocol DesignData Encoding and Transmission - RoadmapData Encoding and Transmission Concepts22Data Encoding and TransmissionData Encoding and Transmission9Simplified Data Communications Model10S(t) = A sin(2πft + Φ)11Terminology (1/3) Transmitter Receiver Medium Guided medium E.g., twisted pair, optical fiber Unguided medium E.g., air, water, vacuum12Terminology (2/3) Direct link No intermediate devices Point-to-point Direct link  Only 2 devices share link Multi-point More than two devices share the link13Terminology (3/3) Simplex One direction e.g., television  Half duplex Either direction, but only one way at a time e.g. police radio Flux duplex Both directions at the same time e.g., telephone14Analog and Digital Data Transmission Data  Entities that convey meaning Signals Electric or electromagnetic representations of data Transmission Communication of data by propagation and processing of signals15Data Analog Continuous values within some interval e.g., sound, video Digital Discrete values e.g., text, integers16Signals Means by which data are propagated Analog Continuously variable Various media e.g., wire, fiber optic, space Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz Digital Use two DC components17Data and Signals Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data Modem Can use digital signal to carry analog data Compact Disc audio18Analog Transmission Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance  Use amplifiers to boost signal Also amplifies noise19Digital Transmission Concerned with content Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal Extracts bit pattern Retransmits Attenuation is overcome Noise is not amplified20Advantages/Disadvantages of Digital Cheaper Less susceptible to noise Greater attenuation Pulses become rounded and smaller Leads to loss of information21Attenuation of Digital Signals22Interpreting Signals Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpreting of signals Signal to noise ratio Data rate Bandwidth23Encoding Schemes Non-return to Zero-Level (NRZ-L) Non-return to Zero Inverted (NRZI) Bipolar –AMI Pseudoternary Manchester Differential Manchester B8ZS HDB324Non-Return to Zero-Level (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 positive for the other This is NRZ-L25Non-Return 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 (low to high or high to low) denotes a binary 1 No transition denotes binary 0 An example of differential encoding26NRZ27Differential Encoding Data represented by changes rather than levels More reliable detection of transition rather than level In complex transmission layouts it is easy to lose sense of polarity28Summary of Encodings29NRZs Pros and Cons Pros Easy to engineer Make good use of bandwidth Cons DC component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission30Biphase Manchester Transition in middle of each bit period Transition serves as clock and data Low to high represents one High to low represents zero Used by IEEE 802.3 Differential Manchester Mid-bit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE 802.531Biphase Pros and Cons Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth Pros Synchronization on mid bit transition (self clocking) No dc component Error detection Absence of expected transition32Asynchronous/Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous33Asynchronous Data transmitted on character at a time 5 to 8 bits Timing