1 / 83

The physical layer

The physical layer. Physical Layer. Sending raw bits across “the wire”. Issues: What’s being transmitted. Transmission medium. How it’s being transmitted. Signal. Signal: electro-magnetic wave carrying information. Time domain: signal as a function of time.

kueng
Download Presentation

The physical layer

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. The physical layer

  2. Physical Layer • Sending raw bits across “the wire”. • Issues: • What’s being transmitted. • Transmission medium. • How it’s being transmitted.

  3. Signal • Signal: electro-magnetic wave carrying information. • Time domain: signal as a function of time. • Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities. • Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).

  4. Time Domain • Periodic signals: • Same signal pattern repeats over time. • Example: sine wave • Amplitude (A) • Period (or frequency) (T = 1/f) • Phase(f)

  5. Frequency Domain • Signal consists of components of different frequencies. • Spectrum of signal: range of frequencies signal contains. • Absolute bandwidth: width of signal’s spectrum.

  6. Example: S(f) • Spectrum of S(f) extends from f1 to 3f1. • Bandwidth is 2f1. f 1 3 2

  7. Analog Technology • Analog devices maintain exact physical analog of information • E.g., microphone: the voltage at the output of the mic is proportional to the sound pressure • Early telephones were all analog • Problems with analog signals: • Difficult to store (e.g.: audio tapes, videotapes) • Must be processed by analog systems which often add distortion • Noise always adds to the signal

  8. Digital Technology • It use numbers to record and process information • Inside a computer, all information is represented by numbers • Analog-to-digital conversion: ADC • Digital-to-analog conversion: DAC • All signals (including multimedia) can be encoded in digital form • Digital information does not get distorted while being stored, copied or communicated

  9. Digital Communication Technology • Example: The telegraph (Morse code) • Uses dots and dashes to transmit letters • It is digital even though uses electrical signals • The telephone has become digital • CDs and DVDs • Digital communication networks form the Internet • The user is unaware that the signal is encoded in digital form

  10. 2 Levels Are Sufficient • Computers encode numbers using only two levels: 0 and 1 • A bit is a digit that can only assume the values 0 and 1 (it is a binary digit) • A word is a number formed by several bits • Example: ASCII standard for encoding text • A = 1000001; B = 1000010; … • A byte is a word with 8 bits

  11. Definitions • 1 byte = 8 bits • 1 KB = 1 kilobyte = 1,024 bytes = 8*1,024 bits • 210 = 1,024 is powr of 2 closest to 1,000. • [also 1,000 bytes] • 1 MB = 1 megabyte = 1,000 KB • 1 GB = 1 gigabyte = 1,000 MB • 1 TB = 1 terabyte = 1,000 GB

  12. Definitions (cont’d) • 1 Kb = 1 kilobit = 1,024 bits [also, 1,000 bits] • 1 Mb = 1 megabit = 1,000 Kb • 1 Gb = 1 gigabit = 1,000 Mb • 1 Tb = 1 terabit = 1,000 Gb

  13. Digitization • Digitization is the process that allows us to convert analog to digital (implemented by ADC) • Analog signals: x(t) • Defined on continuum (e.g. time) • Can take on any real value • Digital signals: q(n) • Sequence of numbers (samples) defined in a discrete set (e.g., integers)

  14. Digitization - Example Analog signal x(t) Digitized signal q(n) q(n) x(t)

  15. Some Definitions • Interval of time between two samples: • Sampling Interval (T) • Sampling frequency F=1/T • E.g.: if the sampling interval is 0.1 seconds, then the sampling frequency is 1/0.1=10 • Measured in samples/second or Hertz • Each sample is defined using a word of B bits • E.g.: we may use 8 bits (1 byte) per sample.

  16. Bit-rate • Bit-rate = numbers of bits per second we need to transmit • For each second we transmit F=1/T samples • Each sample is defined with a word of B bits • Bit-rate = F*B • Example: if F is 10 samples/s and B=8, then the bit rate is 80 bits/s

  17. Bit-rate=BF=16 bits/second B=4 bits/sample Example of Digitization 10101110010100110011010000110100 Time (seconds) 0 1 2 F=4 samples/second

  18. Bit-rate - Example 1 • What is the bit-rate of digitized audio? • Sampling rate: F= 44.1 KHz • Quantization with B=16 bits • Bit-rate = BF= 705.6 Kb/s • Example: 1 minute of uncompressed stereo music takes more than 10 MB!

  19. Bit-rate - Example 2 • What is the bit-rate of digitized speech? • Sampling rate: F = 8 KHz • Quantization with B = 16 bits • Bit-rate = BF = 128 Kb/s

  20. Bandwidth and Bit Rate • Bit rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal). • Example: 2Mbits/sec, or 2Mbps. • If data rate of signal is W bps, good representation achieved with 2*W Hz bandwidth. • Nyquist-Shannon sampling theorem: If a function x(t) contains no frequencies higher than B hertz, it is completely determined by giving its ordinates at a series of points spaced 1/(2B) seconds apart.

  21. Data Transmission • Analog and digital transmission. • Example of analog data: voice and video. • Example of digital data: character strings • Use of codes to represent characters as sequence of bits (e.g., ASCII). • Historically, communication infrastructure for analog transmission. • Digital data needed to be converted: modems (modulator-demodulator).

  22. Digital Transmission • Current trend: digital transmission. • Cost efficient: advances in digital circuitry (VLSI). • Advantages: • Data integrity: better noise immunity. • Security: easier to integrate encryption algorithms. • Channel utilization: higher degree of multiplexing (time-division mux’ing).

  23. The Theoretical Basis for Data Communication • Fourier Analysis • Any periodical signal can be decomposed as a sum of sinusoidal signals at frequencies which are multiple of the original frequency • We call those the “harmonics” • Bandwidth-Limited Signals • Not all harmonics pass through a channel • The result is a distortion in the shape of the signal • Maximum Data Rate of a Channel

  24. Bandwidth-Limited Signals A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

  25. Bandwidth-Limited Signals (2) (d) – (e) Successive approximations to the original signal.

  26. Bandwidth-Limited Signals (3) Relation between data rate and harmonics.

  27. Guided Transmission Data • Magnetic Media • Write the data on a storage system (eg. tapes or hard drive), carry them over physically • Twisted Pair • Coaxial Cable • Fiber Optics

  28. Twisted Pair • Category 3 UTP (unshielded twisted pair) • Possible bandwidth 16MHz, telephony systems, 10BASE-T Ethernet • Category 5 UTP – standard for Fast Ethernet • Up to 100MHz • since about 1988 – more twists, less crosstalk, better signal over longer distances • Category 6 UTP – standard for Gigabit Ethernet • Up to 250MHz (500MHz for 6a)

  29. Coaxial Cable • More expensive than twisted pair • High bandwidth and excellent noise immunity • Impedance is an important metric (50-75ohms) • It must be manufactured to exact specifications, not only an inner conductor wrapped in a shielding (as audio cables are) • Can transmit bandwidths way into the GHz.

  30. Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. -Not a mirror! That would lead to losses at every reflection!

  31. Single mode vs multi-mode • Multi-mode fiber: light reflected on various angles inside the fiber. • If the fiber is so narrow that it is only several wavelengths, the light can travel only in a single way, in a straight line, without bouncing. • The fiber acts like a wave guide • Called a single mode fiber • Smaller loss, more suitable for long distance transmission

  32. Transmission of Light through Fiber Attenuation of light through fiber in the infrared region.

  33. Fiber Cables -Core: 50 microns for multi-mode, 8-10 microns for single mode -Cladding: glass with a lower refraction index, to keep the light in the core -Connection: -connectors (plug in) – about 20% attenuation -mechanical splicing, tuned by an operator – 10% attenuation -fused (melted together) – almost no attenuation

  34. Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources.

  35. Fiber Optic Networks A fiber optic ring with active repeaters.

  36. Fiber Optic Networks (2) A passive star connection in a fiber optics network.

  37. Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared and Millimeter Waves • Lightwave Transmission

  38. Narrow-band vs spread spectrum • Spectrum • About 8 bits / Hz (using all the tricks in the book) • Narrowband: • Δf / f << 1 • Spread spectrum • Frequency hopping spread spectrum • Several times / sec, military communications, good resistance to multipath fading • Direct sequence spread spectrum DSSS: 802.11b, CDMA telephony, GPS, Galileo, ZigBee • Ultra-wide band • any radio technology having bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency

  39. The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

  40. Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.

  41. Politics of the Electromagnetic Spectrum The ISM bands in the United States (Industrial, Scientifical, Medical: also known as unlicenced bands)

  42. Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

  43. Communication Satellites • Geostationary Satellites • Medium-Earth Orbit Satellites • Low-Earth Orbit Satellites • Satellites versus Fiber

  44. Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

  45. Communication Satellites (2) The principal satellite bands.

  46. Communication Satellites (3) VSATs using a hub.

  47. Low-Earth Orbit SatellitesIridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth.

  48. Globalstar (a) Relaying in space. (b) Relaying on the ground.

  49. Traditional telephony

  50. Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching

More Related