Physical Layer (I) Data Encoding Techniques Advanced Computer Networks

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 Encoding2

Analog and Digital Transmissions Advanced Computer Networks Data Encoding3 Use of both analog and digital transmissions for a computer-to- computer call Conversion is done by the MODEM and CODEC

Digital Data, Analog Signals Digital data is encoded by modulating one of the three carrier characteristics: ◦ A mplitude ◦ Frequency ◦ Phase or ◦ Combination of the characteristics Advanced Computer Networks Data Encoding4

Signal Modulation Advanced Computer Networks Data Encoding5 A binary signal A binary signal Frequencymodulation Amplitude modulation Phase modulation

Signal Modulation Advanced Computer Networks Data Encoding6 Frequency Modulation

Signal Modulation Advanced Computer Networks Data Encoding7 Phase Modulation

Signal Modulation Advanced Computer Networks Data Encoding8 Phase Modulation Smaller phase shifts can represent more bits QPSK (quadrature phase-shift keying ) represents two bits

Signal Modulation Advanced Computer Networks Data Encoding9 Amplitude Modulation

Signal Modulation combination 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 Encoding10

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 Encoding11

Constellation Diagrams Advanced Computer Networks Data Encoding12 (a) QPSK. (b) QAM-16. (c) QAM-64. (a) QPSK. (b) QAM-16. (c) QAM-64. QAM-64 V = 64 levels log 2 V = 6 bit/pulse

Analog Data, Digital Signals Advanced Computer Networks Data Encoding13 Digitization Conversion of Analog data into Digital data Digital data can then be transmitted using Digital signaaling techniques (e.g., NRZL,…) Digital data can then be converted to Analog signal Analog to digital conversion is done using a CODEC

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 Encoding14

Analog Data, Digital Signals Advanced Computer Networks Data Encoding15 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 (i.e., 125 microsec/sample) 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

PCM Stages Advanced Computer Networks Data Encoding16

Multiplexing Advanced Computer Networks Data Encoding17 MUX (a) (b) Trunk group AA A A B BB B C C C C Multiplexing (general definition) is Sharing a resource over time Sharing a resource over time A A method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium.shared medium

Frequency Division Multiplexing (FDM) Frequency Division Multiplexing (FDM) Advanced Computer Networks Data Encoding18 frequency time 4 users 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

Frequency Division Multiplexing Advanced Computer Networks Data Encoding19 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

Frequency Division Multiplexing Advanced Computer Networks Data Encoding20

Time Division Multiplexing (TDM) Advanced Computer Networks Data Encoding21 frequency time 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

Time Division Multiplexing (TDM) Advanced Computer Networks Data Encoding22 Different data rate TDM Time slots are assigned to each source proportional to their data rate

Statistical TDM Advanced Computer Networks Data Encoding23 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.

Wavelength Division Multiplexing Advanced Computer Networks Data Encoding24 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. 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

Wavelength Division Multiplexing Advanced Computer Networks Data Encoding25 Separating a beam of light into its colors WDM with couplers & filters WDM with couplers & filters WDM with coupler & demultiplexer WDM with coupler & demultiplexer

Wavelength Division Multiplexing Advanced Computer Networks Data Encoding26 Wavelength division multiplexing.

Delta Modulation (  M) Advanced Computer Networks Data Encoding27 derivation  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

Delta Modulation Advanced Computer Networks Data Encoding28

Digital Data, Digital Signals TermsUnipolar All signal elements have same sign (+V 1, +V 2 )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 Encoding29

NRZ ( Non-Return-to-Zero) Codes NRZL ( N on- R eturn-to- Z ero- L evel) 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). Advanced Computer Networks Data Encoding30 1  negative voltage 0  positive voltage

NRZ ( Non-Return-to-Zero) Codes Advanced Computer Networks Data Encoding31 NRZI ( N on- R eturn-to- Z ero- I nvert on ones) The voltage is constant during the bit interval. NRZI is a differential encoding scheme (i.e., the signal is decoded by comparing the polarity of adjacent signal elements.) 1  Signal transition at 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

Bi–Phase Codes Advanced Computer Networks Data Encoding32 Bi-phase codes Require at least one transition per bit Require at least one transition per bit Transition in middle of each bit period Transition in middle of each bit period Transition serves as clock and data Transition serves as clock and data The maximum modulation rate is twice that of NRZ The maximum modulation rate is twice that of NRZAdvantages Synchronization Synchronization Error detection Error detection The absence of an expected transition ?

Manchester Encoding Advanced Computer Networks Data Encoding33  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. 1  low-to-high transition 0  high-to-low transition Some textbooks disagree on this definition!!

Manchester Encoding Advanced Computer Networks Data Encoding34 Transition serves as clock and data Modulation rate twice than that of NRZ, implying a greater bandwidth Used in standard (Ethernet) Used in standard (Ethernet) A transition may occur at the beginning of the bit interval A transition may occur at the beginning of the bit interval

Differential Manchester Encoding Advanced Computer Networks Data Encoding35  Mid-bit transition is ONLY for Synchronization (clocking)  Differential Manchester is both differential and bi-phase.  Used in standard (Token ring)  Modulation rate for Manchester and Differential Manchester is twice the data rate 1  absence of transition at the beginning of the bit interval 0  presence of transition at the beginning of the bit interval

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 Encoding36

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 Encoding37

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 Encoding38