1 / 38

Physical Layer (I) Data Encoding Techniques

Advanced Computer Networks . Physical Layer (I) Data Encoding Techniques. Data Encoding Techniques. Digital Data, Analog Signals Analog Data, Digital Signals Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) Time Division Multiplexing (TDM)

kat
Download Presentation

Physical Layer (I) Data Encoding Techniques

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. Advanced Computer Networks Physical Layer (I)Data Encoding Techniques

  2. Data Encoding Techniques • Digital Data, Analog Signals • Analog Data, Digital Signals • Frequency Division Multiplexing (FDM) • Wave Division Multiplexing (WDM) • Time Division Multiplexing (TDM) • Pulse Code Modulation (PCM) • Delta Modulation (M) • Digital Data, Digital Signals Advanced Computer Networks Data Encoding

  3. Analog and Digital Transmissions Use of both analog and digital transmissions for a computer-to-computer call Conversion is done by the MODEM andCODEC Advanced Computer Networks Data Encoding

  4. Digital Data, Analog Signals • Digital data is encoded by modulating one of the threecarrier characteristics: • Amplitude • Frequency • Phase or • Combination of the characteristics Advanced Computer Networks Data Encoding

  5. Signal Modulation A binary signal Amplitude modulation Frequency modulation • Phase modulation Advanced Computer Networks Data Encoding

  6. Signal Modulation Frequency Modulation Advanced Computer Networks Data Encoding

  7. Signal Modulation • Phase Modulation Advanced Computer Networks Data Encoding

  8. Signal Modulation • Phase Modulation • Smaller phase shifts can represent more bits • QPSK (quadrature phase-shift keying ) represents two bits Advanced Computer Networks Data Encoding

  9. Signal Modulation Amplitude Modulation Advanced Computer Networks Data Encoding

  10. Signal Modulation • MODEMs use combination of modulation techniques to transmit more bits. • Multiple amplitude & Multiple phase shifts are combined to transmit several bits per symbol. • QPSK (Quadrature Phase Shift Keying)uses multiple phase shifts per symbol. • MODEMs use Quadrature Amplitude Modulation (QAM) technique. Advanced Computer Networks Data Encoding

  11. Signal Modulation • In QAM, constellation points are usually arranged in a square grid with equal vertical and horizontal spacing • Constellation points: a point specified by amplitude and phase • Number of points in grid is a power of 2 • 16-QAM, 64-QAM and 256-QAM are more common • Higher-order constellation transmits more bits/symbol • Higher-order QAM can deliver more data less reliably • Higher-order QAM is more susceptible to noise Advanced Computer Networks Data Encoding

  12. Constellation Diagrams (a) QPSK. (b) QAM-16. (c) QAM-64. QAM-64 V = 64 levels log2V = 6 bit/pulse Advanced Computer Networks Data Encoding

  13. Analog Data, Digital Signals • Digitization • Conversion of Analog data into Digital data • Digital data can then be transmitted using Digital signaling techniques (e.g., NRZL,…) • Digital data can then be converted to Analog signal • Analog to digital conversion is done using a CODEC Advanced Computer Networks Data Encoding

  14. Digitization Stages • Sampling of amplitude signals • PAM (Pulse Amplitude Modulation) sampler • Digitizing of the amplitude signals (Quantizer) • PCM (Pulse Code Modulation) pulses • Quantizing levels (256 quantization level needs 8 bits) • Encoding the stream of bits Advanced Computer Networks Data Encoding

  15. Analog Data, Digital Signals • Sampling Theorem? • In sampling, samples contain all information of original signal if • Signal is sampled at regular intervals • At a rate higher than twice the highest signal frequency • Voice analog data limited to below 4000Hz, thus, require 8000 sample per second (i.e., 125 microsec/sample) • Signal can be reconstructed from the samples using a low-pass filter Advanced Computer Networks Data Encoding

  16. PCM Stages Advanced Computer Networks Data Encoding

  17. Multiplexing • Multiplexing (general definition) is • Sharing a resource over time • A method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. (a) (b) A A A A Trunk group B B B B MUX MUX C C C C Advanced Computer Networks Data Encoding

  18. 4 users frequency time Frequency Division Multiplexing (FDM) • FDM • A single, large frequency band is assigned to the system and is shared among a group of users. • Means that the total bandwidth available to the system is divided into a series of non-overlapping frequency sub-bands that are then assigned to each communicating source and user Advanced Computer Networks Data Encoding

  19. Frequency Division Multiplexing • Transmitter 1 generates a signal in the frequency sub-band between 92.0 MHz and 92.2 MHz • Transmitter 2 in the sub-band between 92.2 MHz and 92.4 MHz, and • Transmitter 3 generates in the sub-band between 92.4 MHz and 92.6 MHz Advanced Computer Networks Data Encoding

  20. Frequency Division Multiplexing Advanced Computer Networks Data Encoding

  21. frequency time Time Division Multiplexing (TDM) TDM Combining a set of low-bit-rate streams, each with a fixed and pre-defined bit rate, into a single high-speed bit stream that can be transmitted over a single channel Advanced Computer Networks Data Encoding

  22. Time Division Multiplexing (TDM) Different data rate TDM Time slots are assigned to each source proportional to their data rate Advanced Computer Networks Data Encoding

  23. Statistical TDM • Statistical Multiplexing • Is similar to dynamic bandwidth allocation (DBA) • A communication channel is divided into an arbitrary number of variable bit-rate channels • The link sharing is adapted to traffic demands over each channel. • Statistical multiplexing improves channel utilization • Multiplexer visits incoming channels for information transmission (if they have) in a predefined order. • Each source is assigned the portion of the channel it requires. Advanced Computer Networks Data Encoding

  24. Wavelength Division Multiplexing • In fiber-optic communications, WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths. • Wavelength and frequency are tied together through a simple directly inverse relationship • Therefore, WDM and FDM actually describe the same concept Advanced Computer Networks Data Encoding

  25. Wavelength Division Multiplexing • Separating a beam of light into its colors • WDM with couplers & filters • WDM with coupler & demultiplexer Advanced Computer Networks Data Encoding

  26. Wavelength Division Multiplexing Wavelength division multiplexing. Advanced Computer Networks Data Encoding

  27. Delta Modulation (M) • The basic idea is to approximate the derivation of analog signal rather than its amplitude • The analog data is approximated by a staircase function that moves up or down () by one quantization level at each sampling time • Analog signal is approximated with a series of previous signal levels • The approximated signal is compared to the original signal • Increase (upstair) is represented as 1 and decrease (downstair) as 0 • In delta modulation, the transmitted data is reduced to a 1-bit data stream • For cases, where quality is not of primary importance Advanced Computer Networks Data Encoding

  28. Delta Modulation Advanced Computer Networks Data Encoding

  29. Digital Data, Digital Signals • Terms Unipolar All signal elements have same sign (+V1, +V2) Polar One logic state represented by positive voltage the other by negative voltage (V) Data rate Rate of data transmission in bits per second Duration (or length) of a bit Time taken for transmitter to emit the bit Modulation rate Rate at which the signal level changes (in baud = signal elements/sec) Advanced Computer Networks Data Encoding

  30. NRZ ( Non-Return-to-Zero) Codes NRZL ( Non-Return-to-Zero-Level) • Uses two different voltage levels (one positive and one negative) as the signal elements for the two binary digits (0,1). • Bipolar (V) The voltage is constant during the bit interval (no middle transition). 1 negative voltage 0  positive voltage Advanced Computer Networks Data Encoding

  31. NRZ ( Non-Return-to-Zero) Codes NRZI ( Non-Return-to-Zero-Invert on ones) The voltage is constant during the bit interval. NRZI is a differential encodingscheme(i.e., the signal is decoded by comparing the polarity of adjacent signal elements.) 1Signal transitionat the beginning of the bit time (either a low-to-high or a high-to-low transition)  0 no signal transition at the beginning of the bit time Advanced Computer Networks Data Encoding

  32. Bi–Phase Codes Bi-phase codes • Require at least one transition per bit • Transition in middle of each bit period • Transition serves as clock and data • The maximum modulation rate is twice that of NRZ Advantages • Synchronization • Error detection • The absence of an expected transition • ? Advanced Computer Networks Data Encoding

  33. Manchester Encoding • A biphase encoding Mechansim • There is always a mid-bit transition {which is used as a clocking mechanism}. • The direction of the mid-bit transition represents the digital data. Some textbooks disagree on this definition!! 1 low-to-hightransition 0 high-to-low transition Advanced Computer Networks Data Encoding

  34. Manchester Encoding • Transition serves as clock and data • Modulation rate twice than that of NRZ, implying a greater bandwidth • Used in 802.3 standard (Ethernet) • A transition may occur at the beginning of the bit interval Advanced Computer Networks Data Encoding

  35. Differential Manchester Encoding • Mid-bit transition is ONLY for Synchronization (clocking) • Differential Manchester is both differential and bi-phase. • Used in 802.5 standard (Token ring) • Modulation rate for Manchester and Differential Manchester is twicethe data rate 1 absence of transition at the beginning of the bit interval 0 presence of transitionat the beginning of the bit interval Advanced Computer Networks Data Encoding

  36. BiPolar-AMI Encoding Bipolar Alternate Mark Inversion • A multilevel binary approach • A 0 is represented by a lack of pulse • A 1 is represented by a positive or a negative pulse • The binary 1 pulses must alternate in polarity • Each 1 introduces a transition that can be used for synchronization • Error detection is possible for a single added or lost pulse. • Long runs of 0's don't allow synchronization. Advanced Computer Networks Data Encoding

  37. Data Encoding Summary • Digital Data, Analog Signals [modem] • Three forms of modulation (amplitude, frequency and phase) used in combination to increase the data rate. • Constellation diagrams (QPSK and QAM) • Digital Data, Digital Signals [wired LANs] • Tradeoffs between self clocking and required frequency. • Biphase, differential, NRZL, NRZI, Manchester, differential Manchester, bipolar. Advanced Computer Networks Data Encoding

  38. Data Encoding Summary • Analog Data, Digital Signals [codec] • Multiplexing Detour: • Frequency Division Multiplexing (FDM) • Wave Division Multiplexing (WDM) [fiber] • Time Division Multiplexing (TDM) • Statistical TDM (Concentrator) • PCM Stages (PAM, quantizier, encoder) • Delta Modulation Advanced Computer Networks Data Encoding

More Related