1 / 58

Last Week

ITC242 – Introduction to Data Communications Week 9 Topic 14 Data Transmission Topic 15 Data Communication Fundamentals Reading 3. Last Week. Reading 2 – Wide Area and Large-Scale Networks Describe the basic concepts associated with wide area networks

illiana-bentley
Télécharger la présentation

Last Week

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ITC242 – Introduction to Data CommunicationsWeek 9Topic 14 Data TransmissionTopic 15 Data Communication FundamentalsReading 3

  2. Last Week Reading 2 – Wide Area and Large-Scale Networks • Describe the basic concepts associated with wide area networks • Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

  3. Topic 14 – Data Transmission Learning Objectives • Describe the difference between analogue and digital signals • Discuss the various transmission impairments that affect signal quality and transfer rate • Use Shannon’s formula to calculate the capacity of a channel

  4. Electromagnetic Signals • Analog Signal • signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal • Digital Signal • signal intensity maintains a constant level for some period of time and then changes to another constant level

  5. Analog Sine Wave

  6. Digital Square Wave

  7. Signal Characteristics • Peak Amplitude (A) • Maximum signal value, measured in volts • Frequency (f) • Repetition rate • Measured in cycles per second or Hertz (Hz) • Period (T) • Amount of time it takes for one repetition, T=1/f • Phase () • Relative position in time, measured in degrees

  8. Frequency Domain Concepts • Spectrum of a signal is the range of frequencies that it contains • Absolute bandwidth of a signal is the width of the spectrum • Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found • The greater the bandwidth, the higher the information-carrying capacity of the signal

  9. Bandwidth • Width of the spectrum of frequencies that can be transmitted • if spectrum=300 to 3400Hz, bandwidth=3100Hz • Greater bandwidth leads to greater costs • Limited bandwidth leads to distortion

  10. Voice/Audio Analog Signals • Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage • Human voice has frequency components ranging from 20Hz to 20kHz • For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz • Actual bandwidth used is 4kHz to isolate audio signal from adjacent bandwidths.

  11. Voice Bandwidth

  12. Digital Text Signals • Transmission of electronic pulses representing the binary digits 1 and 0 • How do we represent letters, numbers, characters in binary form? • Earliest example: Morse code (dots and dashes) • Most common current forms: ASCII, UTF

  13. e.g. ASCII Character Set Control Characters

  14. e.g. ASCII Character Set Printable Characters

  15. Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate • bandwidth • physical environment • number of receivers • impairments

  16. Impairments and Capacity • Impairments exist in all forms of data transmission • Analog signal impairments result in random modifications that impair signal quality • Digital signal impairments result in bit errors (1s and 0s transposed)

  17. Transmission Impairments:Guided Media • Attenuation • loss of signal strength over distance • Attenuation Distortion • different losses at different frequencies • Delay Distortion • different speeds for different frequencies • Noise • distortions of signal caused by interference

  18. Transmission Impairments:Unguided (Wireless) Media • Free-Space Loss • Signals disperse with distance • Atmospheric Absorption • Water vapor and oxygen contribute to signal loss • Multipath • Obstacles reflect signal creating multiple copies • Refraction • Thermal Noise

  19. Types of Noise • Thermal (aka “white noise”) • Uniformly distributed, cannot be eliminated • Intermodulation • When different frequencies collide (creating “harmonics”) • Crosstalk • Overlap of signals • Impulse noise • Irregular spikes, less predictable

  20. Channel Capacity • The rate at which data can be transmitted over a given path, under given conditions • Four concepts • Data rate • Bandwidth • Noise • Error rate

  21. Shannon’s Equation • The signal to noise ratio (SNR) sets the upper bound on the achievable data rate. • Shannon’s equation provides a theoretical maximum channel capacity given SNR. • C = B log2 (1 + SNR) • B = Bandwidth of channel in Hertz • C= Channel capacity in bps • SNR = Signal-to-noise ratio

  22. Shannon’s Equation • What is the capacity of a signal operating at 5kHz with a SNR of 3? C = B log2 (1 + SNR) C = 5000 Hz x log2(1+3) C = 5000 Hz x log24 C = 5000 Hz x 2 C = 10000 bps

  23. Review • Describe the difference between analogue and digital signals • Transmission impairments – attenuation and noise affect signal quality • Shannon’s formula provides a theoretical estimate of maximum channel capacity

  24. Topic 15 – Data Communication Fundamentals Learning Objectives • Explain the difference between analogue and digital transmission • Describe digital and analogue encoding techniques for the transmission of data • Explain the difference between asynchronous and synchronous transmission and when each technique is used • Describe the process of error detection

  25. Data Communication Components • Data • Analog: Continuous value data (sound, light, temperature) • Digital: Discrete value (text, integers, symbols) • Signal • Analog: Continuously varying electromagnetic wave • Digital: Series of voltage pulses (square wave) • Transmission • Analog: Works the same for analog or digital signals • Digital: Used only with digital signals

  26. Analog DataSignal Options • Analog data to analog signal • Inexpensive, easy conversion (eg telephone) • Data may be shifted to a different part of the available spectrum (multiplexing) • Used in traditional analog telephony • Analog data to digital signal • Requires a codec (encoder/decoder) • Provides benefits of digital transmission (e.g Audio CD)

  27. Digital DataSignal Options • Digital data to analog signal • Requires modem (modulator/demodulator) • Allows use of PSTN to send data • Necessary when analog transmission is used • Digital data to digital signal • Requires CSU/DSU (channel service unit/data service unit) • Less expensive when large amounts of data are involved • More reliable because no conversion is involved

  28. Transmission Choices • Analog transmission • only transmits analog signals, without regard to data content • attenuation overcome with amplifiers • signal is not evaluated or regenerated • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers • switching equipment evaluates and regenerates signal

  29. Advantages of Digital Transmission • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression

  30. Why Use Analog Transmission? • Already in place • Significantly less expensive • Lower attenuation rates • Sufficient for transmission of voice signals

  31. Analog Encoding of Digital Data • Data encoding and decoding technique to represent data using the properties of analog waves • Modulation: the conversion of digital signals to analog form • Demodulation: the conversion of analog data signals back to digital form

  32. Methods of Modulation • Amplitude modulation (AM) or amplitude shift keying (ASK) • Frequency modulation (FM) or frequency shift keying (FSK) • Phase modulation or phase shift keying (PSK)

  33. Amplitude Shift Keying (ASK) • In radio transmission, known as amplitude modulation (AM) • The amplitude (or height) of the sine wave varies to transmit the ones and zeros • Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

  34. ASK Illustration 1 0 0 1

  35. Frequency Shift Keying (FSK) • In radio transmission, known as frequency modulation (FM) • Frequency of the carrier wave varies in accordance with the signal to be sent • Signal transmitted at constant amplitude • More resistant to noise than ASK • Less attractive because it requires more analog bandwidth than ASK

  36. FSK Illustration 1 1 0 1

  37. Phase Shift Keying (PSK) • Also known as phase modulation (PM) • Frequency and amplitude of the carrier signal are kept constant • The carrier signal is shifted in phase according to the input data stream • Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

  38. PSK Illustration 0 0 1 1

  39. Modulation Example Asymmetric Digital Subscriber Line (ADSL) • Telephone exchange can provide support for a number of ISPs, • At the exchange a combined data/voice signal is transmitted over a subscriber line • At subscriber’s site, twisted pair is split and routed to both a PC and a telephone • At the PC, an ADSL modem demodulates the data signal for the PC. • At the telephone, a microfilter passes the 4-kHz voice signal. • The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques

  40. DSL Modem Layout

  41. Digital Encoding of Analog Data • Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form • Best-known technique for voice digitization is pulse-code modulation (PCM) • The sampling theorem: Exact reconstruction of a continuous-time baseband signal from its samples is possible if the signal is bandlimited and the sampling frequency is greater than twice the signal bandwidth • Good-quality voice transmission can be achieved with a data rate of 8 kbps • Some videoconference products support data rates as low as 64 kbps

  42. Converting Samples to Bits • Quantizing • Similar concept to pixelization • Breaks wave into pieces, assigns a value in a particular range • 8-bit range allows for 256 possible sample levels • More bits means greater detail, fewer bits means less detail

  43. Converting Samples to Bits

  44. Codec • Coder/Decoder • Converts analog signals into a digital form and converts it back to analog signals • Where do we find codecs? • Sound cards • Scanners • Voice mail • Video capture/conferencing

  45. Digital Encodingof Digital Data • Most common, easiest method is different voltage levels for the two binary digits • Typically, negative=1 and positive=0 • Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the value during a bit time is a level voltage

  46. Differential NRZ • Differential version is NRZI (NRZ, invert on ones) • A bit time: a constant-voltage pulse • A signal transition at the beginning of the bit time: Change=1, no change=0 • Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level

  47. Problems With NRZ • Difficult to determine where one bit ends and the next begins • In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses • Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

  48. Biphase Alternatives to NRZ • Require at least one transition per bit time, and may even have two • Modulation rate is greater, so bandwidth requirements are higher • Advantages • Synchronization due to predictable transitions • Error detection based on absence of a transition

More Related