Data and Computer Comm. Stallings – Chapters 3-5 (Quick Overview) 3: Data Transmission 4: Guided and Wireless Transmission 5: Signal Encoding Techniques NOTE: Many figures and other materials in this presentation are borrowed from required and reference textbooks cited on the class web page. 1 QUICK OVERVIEW Chapter 3 Data Transmission 2 Terminology (1) Transmitter Receiver Medium Guided medium e.g. twisted pair, optical fiber Unguided medium e.g. air, water, vacuum 3 Terminology (2) Direct link No intermediate devices Point-to-point Direct link Only 2 devices share link Multi-point More than two devices share the link 4 Terminology (3) Simplex One direction e.g. television Half duplex Either direction, but only one way at a time e.g. police radio Full duplex Both directions at the same time e.g. telephone 5 Spectrum & Bandwidth Spectrum Range of frequencies contained in signal Absolute bandwidth Width of spectrum Effective bandwidth Often just bandwidth Narrow band of frequencies containing most of the energy DC component Component of zero frequency 6 Data Rate and Bandwidth Any transmission system has a limited band of frequencies Limits the data rate that can be carried 7 Data 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 (CD) audio 8 Analog Transmission Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance Use amplifiers to boost signal But also amplifies noise 9 Digital 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 amplified 10 Advantages of Digital Transmission Digital technology Low-cost LSI/VLSI technology Data integrity Longer distances over lower quality lines Capacity utilization High bandwidth links can be more economical High degree of MUXing easier with digital Security and privacy Encryption Integration Can treat analog and digital data similarly 11 Channel Capacity Data rate In bits per second (bps) Rate at which data can be communicated Bandwidth In cycles per second (Hertz) Constrained by transmitter and medium 12 QUICK OVERVIEW Chapter 4 Transmission Media 13 Overview Guided - wired Unguided - wireless Characteristics and quality determined by medium and signal For guided, medium is more important For unguided, bandwidth produced by antenna is more important Key concerns are data rate and distance 14 Design Factors Bandwidth Higher bandwidth enables higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation 15 Electromagnetic Spectrum 16 Guided Transmission Media Twisted Pair Cheap, widely deployed and used Easy to work with Limited data rate and range Coaxial cable Highly versatile medium Television distribution Long distance telephone transmission Short distance computer systems links Local area networks Optical fiber Greater capacity Smaller size & weight Lower attenuation EM isolation Greater repeater spacing Long-haul, metropolitan, and rural exchange trunks Subscriber loops LANs 17 Example: FO Ring with Active Repeaters 18 Wireless Transmission Unguided media Transmission and reception via antenna Directional Focused beam Careful alignment required Omnidirectional Signal spreads in all directions Can be received by many antennae 19 Terrestrial Microwave Parabolic dish Focused beam Line of sight Variety of applications Long-haul telecommunications services Voice and TV transmission Point-to-point links between buildings Higher frequencies give higher data rates 20 Satellite Microwave Point-to-point or multi-point comm. Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another Requires geo-stationary orbit Height of 35,784km Television distribution Long-distance telephone Private business networks 21 Example: Principal Satellite Bands 22 Broadcast Radio Omnidirectional Variety of applications FM radio UHF and VHF television Data (e.g. 802.11, Bluetooth) Line of sight Suffers from multipath interference Reflections 23 Infrared Point-to-point connectivity Transceivers modulate non-coherent infrared light Line of sight (or reflection) Blocked by walls Several areas of application Appliance remote control Data port 24 QUICK OVERVIEW Chapter 5 Data Encoding 25 Encoding Techniques Four basic categories of choice Digital data, digital signal Analog data, digital signals Digital data, analog signal Analog data, analog signal 26 Digital Data, Digital Signal Digital signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements 27 Terms (1) Unipolar All signal elements have same sign Polar One logic state represented by positive voltage, the other by negative voltage Data rate Rate of data transmission in bits per sec (bps) Duration or length of a bit Time taken for transmitter to emit the bit 28 Terms (2) Modulation rate Rate at which the signal level changes Measured in units of baud baud = signal elements per second Mark and Space Binary 1 and Binary 0, respectively 29 Interpreting Signals Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpreting of signals SNR Data rate Bandwidth 30 Comparison of Encoding Schemes (1) Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of DC component an advantage, as allows AC coupling via transformer which provides isolation Concentrate power in the middle of the bandwidth Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on signal 31 Comparison of Encoding Schemes (2) Error detection Can be built into signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher signal rate (and thus data rate) leads to higher costs Some codes require signal rate greater than data rate 32 Encoding Schemes Simple NRZ Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Multilevel binary (>2 signal levels) Bipolar-AMI (alternate mark inversion) Pseudoternary Biphase Manchester Differential Manchester Scrambling techniques Bipolar with 8-zeros substitution (B8ZS) High-density bipolar-3 zeros (HDB3) 33 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 one value, constant positive voltage for the other (e.g. EEL3701C) Usually in data communication, negative voltage is used for one value and positive for other value For our purposes, 0 = high level, 1 = low level This scheme is NRZ-L 34 NRZI NRZ inverted (NRZI) NRZ is 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 (either low-to-high or high-to-low) denotes a binary 1 No transition denotes a binary 0 An example of differential encoding 35 NRZ 36 Differential Encoding Info. transmitted is represented in terms of changes between successive signal elements Rather than by signal elements themselves More reliable detection of transition rather than level May be more reliable to detect transition in presence of noise In complex transmission layouts, it is easy to lose sense of polarity of signal e.g. leads from attached device to multidrop twisted-pair line may be accidentally attached inverted problem with NRZ-L, but not with differential encoding 37 NRZ pros and cons Pros Easy to engineer Makes good use of bandwidth Cons DC component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission 38 Multilevel Binary (1) Uses more than two levels Bipolar-AMI (alternate mark inversion) Binary 0 represented by no line signal Binary 1 represented by positive or negative pulse Successive Binary-1 pulses alternate in polarity Advantages No loss of sync if long string of ones (zeros still a problem) No net DC component Lower bandwidth than with NRZ (see Fig. 5.3) Simple means of error detection 39 Multilevel Binary (2) Pseudoternary Similar to bipolar-AMI but opposite Binary 1 represented by absence of line signal Binary 0 represented by alternating positive and negative pulses Same advantages as bipolar-AMI Of course, opposite regarding long strings No loss of sync if long string of zeros Long string of ones still a problem 40 Bipolar-AMI and Pseudoternary 41 Tradeoff for Multilevel Binary Not as efficient a code as NRZ Each signal element only represents one bit In a 3-level system, theoretically we could represent log23 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Analysis shows multilevel requires ~3dB more signal power for same probability of bit error Equivalently, at a given SNR, BER for NRZ is significantly less than BER for multilevel binary 42 Biphase Manchester Transition in middle of each bit period Transition serves as both clock and data Low-to-high represents Binary 1 High-to-low represents Binary 0 e.g. Used by IEEE 802.3 CSMA/CD (Ethernet) Differential Manchester Mid-bit transition is for clocking only Transition at start of bit period represents Binary 0 No transition at start of bit period represents Binary 1 Yes, it is another differential encoding scheme! e.g. Used by IEEE 802.5 Token Ring 43 Example Mid-bit transition serves as both clock and data 0 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 0 0 1 1 1 1 Mid-bit transition for clock, start transition for data 44 Modulation Rate 45 Biphase Tradeoffs Cons At least one transition per bit time and possibly two Thus, maximum modulation rate is twice NRZ ⇒ Requires more bandwidth Pros Synchronization on mid-bit transition These are self-clocking codes No DC component Error detection Absence of expected transition Noise would have to invert both signal before and after expected transition to cause undetected error 46 Scrambling Techniques Biphase not well suited to long-distance applications due to high ratio of signaling rate to data rate Alternative is some type of scrambling scheme Replace sequences that would otherwise produce constant voltage (e.g. string of 0s in bipolar-AMI) Filling sequence Must produce enough transitions to sync Must be recognized by receiver and replace with original data Same length as original Advantages? No DC component No long sequences of zero-level line signal No reduction in data rate Error detection capability 47 B8ZS Bipolar with 8-Zeros Substitution (B8ZS) code Common in North America e.g. line coding option for 64kbps channels on 1.544Mbps T1 links Based on bipolar-AMI Basic operation If octet of all zeros and last voltage pulse preceding was positive, encode as: 0 0 0 + − 0 − + If octet of all zeros and last voltage pulse preceding was negative, encode as: 0 0 0 − + 0 + − Causes two violations of AMI code Since + 0…0 + and − 0…0 − not permitted These are unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros 48 B8ZS Example 000-+0+- 49 HDB3 High-Density Bipolar-3 Zeros (HDB3) code Common in Europe and Japan Also based on bipolar-AMI String of 4 zeros is replaced with sequences of 1 or 2 pulses In each case, 4th zero replaced with code violation For successive violations, if last violation negative then next violation positive and vice-versa so no DC component # of Bipolar Pulses Since Last Substitution Polarity of Preceding Pulse Odd − 000− +00+ + 000+ −00− Even Substitution Rules 50 B8ZS and HDB3 Odd: 0 0 0 − Even: + 0 0 + (assuming odd # of 1s or pulses since last substitution) Even: − 0 0 − 51 Other Encoding Schemes 4B/5B Used in FDDI, 100BASE-FX, 100BASE-T Every 4 bits of data encoded in a 5-bit code word These 5-bit codes are selected to have no more than one leading 0 and no more than two trailing 0s Thus, never get more than three consecutive 0s Resulting 5-bit codes are transmitted using NRZI Achieves 80% efficiency 8B/10B Used in 1000BASE-T, 1000BASE-SX, 1000BASE-LX 8-bit data stream encoded as 10-bit code words 52 Digital Data, Analog Signal 3 basic encoding or modulation techniques Amplitude-shift keying (ASK) Frequency-shift keying (FSK) Phase-shift keying (PSK) In all cases, resulting signal occupies a bandwidth centered on a carrier frequency 53 Modulation of Analog Signals for Digital Data 54 Example: Full-Duplex FSK Transmission on Voice-Grade Line fc = 1170Hz fc = 2125Hz From specification for Bell System 108 series modems 55 Analog Data, Digital Signal Digitization Conversion of analog data into digital data Digital data can then be transmitted using NRZ-L Digital data can then be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation Example Voice data limited to below 4000Hz Require at least 8000 samples per second If 7-bit resolution PCM (i.e. 128 levels), 8000 x 7 = 56 kb/s 56 Analog Data, Analog Signals Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency-division muxing (Chp. 8) Types of modulation Amplitude (AM) Frequency (FM) Phase (PM) 57 Example AM FM PM 58 Spread Spectrum Analog or digital data Analog signal Spread data over wide bandwidth Makes jamming and interception harder Frequency hoping (FHSS) Signal broadcast over seemingly random series of frequencies Direct Sequence (DSSS) Each bit is represented by multiple bits in transmitted signal Known as chipping code, spreads signal across a wider frequency band in direct proportion to # of bits used e.g. 10-bit chipping code spreads signal across frequency band 10x greater than 1-bit code 59 Example of DSSS 60 Odds and Ends What is “broadband” ? Sometimes referred to as a wideband transmission, broadband refers to telecommunications that provide a variety of channels of data over a single communication medium (wire). Two of the more commonly found and used broadband technologies are cable and DSL broadband. What is “baseband” ? Transmission of signals without modulation. In a baseband LAN (e.g. 100BASE-T), digital signals (1s and 0s) are inserted directly onto the cable as voltage pulses. The entire spectrum of the cable is consumed by the signal. This scheme does not allow FDM. 61