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Lecture 3

Lecture 3. Transmission basics Chapter 3, pages 75-96. Dave Novak School of Business University of Vermont Sources: 1) Network+ Guide to Networks, Dean 2013 2) Comer, Computer Networks and Internets, 2004 3) Other sources cited within the lecture slides.

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Lecture 3

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  1. Lecture 3 • Transmission basics • Chapter 3, pages 75-96 Dave Novak School of Business University of Vermont Sources: 1) Network+ Guide to Networks, Dean 2013 2) Comer, Computer Networks and Internets, 2004 3) Other sources cited within the lecture slides

  2. Objectives • Transmission basics • Analog –vs- Digital • Simple transmission RS232 • Line coding • Modulation • AM, FM, Phase Shift • Multiplexing • FDM, TDM, WDM • Broadband –vs- Baseband

  3. Terminology • Data – some entity that has meaning or conveys information • Signal – an electromagnetic representation of data • Transmission – moving data from source to destination through the use of signals

  4. Terminology • Communication channel – any pathway over which information is transmitted • Can be a physical wire, radio wave, or any radiated source of energy (even if it has no physical presence) • Transmitted information has a source and a destination

  5. Background • Electromagnetic radiation is the basis for all data transmission • Electromagnetic radiation propagates (spreads, disseminates) along different media (copper wire, fiber, etc.) and in free space (air)

  6. Background

  7. Background • Different parts of the electromagnetic frequency spectrum can be used for data transmission depending on the medium used and the communications standards being followed • The specific media properties affect • Bandwidth • Attenuation • Noise • Distortion

  8. Background • Bandwidth – range of frequencies occupied or used by a carrier wave • Attenuation – strength of signal decreases as it propagates • Noise – unwanted electromagnetic energy that degrades the signal (crosstalk, background interference) • Distortion – original shape or characteristic of waveform is altered

  9. Types of Signals: Analog & Digital • Two basic types of signals • 1) Analog • 2) Digital

  10. Analog Signals • Characterized by data whose value varies over a continuous range • Temperature values at a certain location can assume an infinite number of values over time • Examples of analog data • Video • Audio • Historically telephony networks

  11. Digital Signals • Characterized by data whose value is limited to a finite set of values • Examples of digital data • Text: printed English language (26 letters, 10 numbers, space, and punctuation) • Morse code (binary example – either a dot or a dash)

  12. Analog versus digital How does EMI impact each type of signal?

  13. Analog Signals • Analog signals can be represented by a waveform diagram • Electromagnetic waves have four propertiesthat are all important with respect to transmitting signals • 1) Amplitude • 2) Frequency • 3) Wavelength • 4) Phase

  14. Electromagnetic Waveform Diagram

  15. Electromagnetic waveform • Amplitude – A measure of waveform strength at a given point in time Amplitude at 0.25 sec = 5 volts Amplitude at 0.5 sec = 0 Amplitude at 0.75 sec = -5 volts

  16. Electromagnetic waveform • Frequency - # of times a wave cycles from high amplitude to low amplitude and back again measured in cycles/sec (Hertz, Hz) • Frequency = 1 cycle / sec = 1 Hz • 1 MHz cable can transfer (106) or 1,000,000 wave cycles of current in 1 sec (wave peak to wave peak) • 100 MHz can transfer 100,000,000 wave cycles / sec

  17. Electromagnetic waveform • Wavelength – the distance between corresponding points on a wave cycle Wavelength is generally expressed in some variant of meters or feet Wavelength is inversely proportional to frequency - high frequency implies short wavelength

  18. Electromagnetic waveform • Phase – the progress of a wave over time in relationship to a fixed point Example: two separate waves with same amplitude and frequency starting at different points in time Phases are 90 degrees apart Often hear phase difference or phase offset discussed

  19. Analog versus digital • Digital signals can be regenerated using repeaters and active hubs • Cleaned up to prevent the accumulation of noise and distortion • Allows signal to be transmitted over greater distances

  20. Analog versus digital • What happens to analog signals over distances even if they are amplified? Can you reconstruct the original signal?

  21. Analog versus digital • Analog: one-to-one relationship between how data are captured and recorded and how data are reproduced • For example: microphone and speaker • Capture sound as a stream of electrical fluctuations  pass through an amplifier  to speaker (no alteration) Source: http://www.informit.com/library/content.aspx?b=Planet_Broadband&seqNum=14

  22. Analog versus digital • Digital: Reproduces text, pictures, sound, etc by sampling original output as high speed and assigning numeric code to represent the original (1s and 0s) • Pass code through network to analog converter that turns code back into electrical fluctuations Source: http://www.informit.com/library/content.aspx?b=Planet_Broadband&seqNum=14

  23. Analog to digital conversion

  24. Data / signal combinations • Source: http://upload.wikimedia.org/wikipedia/commons/thumb/8/84/A-D-A_Flow.svg/981px-A-D-A_Flow.svg.png

  25. Data / signal combinations • Digital signals represent data with sequence of voltage pulses • Digital data / digital signal • Analog data / digital signal • Analog signals represent data with continuously varying electromagnetic wave • Digital data / analog signal • Analog data / analog signal • Source: Tseng, http://www.cs.sunysb.edu/~jgao/CSE370-spring06/lecture2.pdf

  26. Data / signal combinations • Source: http://dev.epubbud.com/uploads/6/7/6/6767539/images/Computer_Networks___Andrew_Tanenbaum__Fourth_Edition__chm/02fig23.gif

  27. Data / signal combinations • Different types of data may be digital or analog by nature • Different types of networks use different types of signals • A wired LAN using Ethernet uses baseband, digital signaling • A WiFi LAN uses broadband, analog signaling • Potential benefits and drawbacks associated with different types of signals

  28. Analog to digital conversion • For example • 1) Digital signals can be reproduced EXACTLY – without disruption or degradation • 2) Digital signature processors find patterns in signals and uses those patterns to compress duplicate information – dramatically reduces bandwidth requirements Source: http://www.informit.com/library/content.aspx?b=Planet_Broadband&seqNum=14

  29. Analog to digital conversion • Convert commonly occurring analog data / information such as voice and video to digital data for transmission over a either analog/digital network • We can transmit digital data • Faster • Cheaper • With fewer errors

  30. How do computers communicate? • At a very basic level computers use binary digits (bits) to represent information • Bits are transmitted over some medium • Electrical current over copper cable • Pulses of light over fiber optic cable

  31. How do computers communicate? • How can information be represented by electrical signals? • Can be generally explained via local asynchronous communication (RS-232) • Example: electrical voltage over copper wires

  32. How do computers communicate? • Simple electronic communication systems • Electric current to encode data • For example: Negative voltage represents a 1 and positive voltage represents a 0 • Transmit a “1” by transmitting negative voltage over a copper wire

  33. Local asynchronous communication • RS-232 (EIA) emphasizes need for standards and illustrates how they are used in networking • Most widely accepted way to transfer characters across copper wires • Defines serial, asynchronous communication • Voltage ranges from ± 15 volts • Distance limited to 50 ft • Characters consist of 7 bits

  34. Local asynchronous communication • Limitations of hardware • Electronic devices cannot produce an exact voltage or change from one voltage to another instantly Wires are not perfect conduits Signal loses energy as it travels Takes time to change voltage

  35. Local asynchronous communication • Transmission hardware is typically rated in baud which is the signaling rate at which data are sent through a channel measured in transactions per second • Simple RS-232 scheme baud rate is equal to the bit rate as one bit is transferred per signal transition • 9600 baud = 9600 bps • Not true for more complex coding schemes

  36. Local asynchronous communication • Bit rate –vs- baud rate – they are directly related to one another • Bit rate – number bits transmitted per sec • Baud rate – number of signaling elements per sec • Depending on the signaling level or modulation technique, more than one bit can be transferred per sec • Bit rate = baud/sec x # of bits/baud

  37. Modems • Hardware circuit that accepts sequence of data bits and applies modulation to a carrier wave is called a modulator • Hardware circuit that accepts a modulated carrier wave and recreates the sequence of bits used to modulate the carrier is called a demodulator • To support full duplex transmission, eachsystem needs both – these are combined into a single device called a modem

  38. Modems • Different types of modems including RF (wireless) and optical fiber modems • Most familiar with 2-wire dialup modems • Half duplex – take turns sending info • Use a carrier that is an audible tone to mimic a telephone • Note that the term modem is not limited to the dialup device • Modern modems use a combination of modulation techniques to transmit multiple bits per baud

  39. Some Basic Issues • Why don’t we just transmit 1’s and 0’s? • Desire to transmit large amounts of data over long distances at really high speeds • Multiple conversion processes as different types of data travel over different physical networks (for example sending analog data over a digital network) • Transmission errors • Increase bit transfer rate

  40. Advanced transmission concepts • Line Encoding • Modulation • Multiplexing

  41. Line encoding • Different lineencoding schemes are used to transmit digital data using a digital signal • Improve bit-rate • Decrease bit-error rates • Digital data / digital signals • Encoding schemes can vary by • Layer 1 and 2 standards (which also impact media and distance)

  42. Line Encoding

  43. Modulation • Encode digital data onto a continuous analog carrier wave by modulating (altering one or more properties of the carrier wave) the signal • Digital data using analog signals • 1) Frequency Modulation • 2) Amplitude Modulation • 3) Phase-shift Modulation • Analog data using digital signals • 4) Pulse Code Modulation

  44. Digital data / analog signal a) Digital (binary) signal being represented b) Amplitude modulation (AM) c) Frequency modulation (FM) d) Phase-Shift modulation (PSM) Source: http://computing.dcu.ie/~humphrys/Notes/Networks/tanenbaum/2-24.jpg

  45. Analog data / digital signal • Most common technique for encoding analog data using digital signals is Pulse Code Modulation (PCM)

  46. Multiplexing • Technique that allows multiple signals to be transmitted simultaneously over a single medium • Medium is separated into multiple channels or subchannels • This can be done virtually or physically • Individual signals from different sources can be combined into a single complex signal and then the separate signals are recovered at the receiving end • Multiplexing depends on signal type (analog / digital) and medium

  47. Basic concept of multiplexing Image Source: http://www.cadvision.com/blanchas/Intro2dcRev2/page96.html

  48. Multiplexing • Frequency Division Multiplexing (FDM) • Wave Division Multiplexing (WDM) • Same concept as FDM but applied to fiber where optical signals are used • Time Division Multiplexing (TDM)

  49. Frequency Division Multiplexing (FDM) • Inherently an analog technology • Uses different frequency ranges over single medium • Total bandwidth is divided into subchannels consisting of smaller segments of available bandwidth • Carrier wave used by each sender/receiver pair operates within a unique frequency band to avoid interfering with other transmissions

  50. Frequency Division Multiplexing (FDM) Source: http://ecee.colorado.edu/~ecen4242/adsl/adsltechnology_files/multip10.gif

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