Physical Layer[1]

Physical LayerData Encoding Schemes : 

Physical LayerData Encoding Schemes By Mr. Ammar Abbas

Encoding : 

Encoding Encoding is the process of transforming information from one format into another. The opposite operation is called decoding. Line Encoding is usually used for transmitting and receiving data. Type of data can be Analog or Digital Similarly the Encoding can involve digital or analog signals for transmitting and receiving data.

Introduction : 

Introduction Data are entities that convey meaning Signals are the electric or electromagnetic encoding of data Computer networks and data/voice communication systems transmit signals Data and signals can be analog or digital

Waveforms : 

Waveforms Time Analog Digital

Noises : 

Noises

Wave properties : 

Wave properties Amplitude: The “height” of the wave above (or below) a central point, often measured in volts (V) Frequency: The number of waves that pass a given point per second, measured in Hertz (Hz) Phase: Position of the waveform at a given time, measured in degrees of shift (o)

Amplitude : 

Amplitude

Frequency (I) : 

Frequency (I)

Frequency (II) : 

Frequency (II) The frequency is the number of times a signal makes a complete cycle within a given time frame. Spectrum - The range of frequencies that a signal spans from minimum to maximum. Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal.

Frequency (III) : 

Frequency (III) For example, consider an average voice: The average voice has a frequency range of roughly 300 Hz to 3100 Hz. The spectrum would thus be 300 - 3100 Hz The bandwidth would be 2800 Hz

Phase (I) : 

Phase (I)

Phase (II) : 

Phase (II) The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. A change in phase can be any number of angles between 0 and 360 degrees. Phase changes often occur on common angles, such as 45, 90, 135, etc.

Signal Strength : 

Signal Strength All signals experience loss (attenuation). Attenuation is denoted as a decibel (dB) loss. Decibel losses (and gains) are additive. dB = 10 log (P2/P1) 3-dB if the power of a signal drops to half 6-dB if … drops to a quarter

Data to Signal : 

Data to Signal

Analog data-analog signals : 

Analog data-analog signals

Slide 17: 

Amplitude Modulation Carrier wave Baseband signal Modulated wave Amplitude varying-frequency constant

Frequency Modulation DiagramVarying the frequency of the waves of a carrier in order to transmit analog or digital data. Frequency modulation (FM) is widely used in audio transmission, not only for its namesake FM radio, but for the audio channels in television : 

Frequency Modulation DiagramVarying the frequency of the waves of a carrier in order to transmit analog or digital data. Frequency modulation (FM) is widely used in audio transmission, not only for its namesake FM radio, but for the audio channels in television

Data Codes : 

Data Codes The set of all textual characters or symbols and their corresponding binary patterns is called a data code. There are two basic data code sets plus a third code set that has interesting characteristics: EBCDIC ASCII Unicode can code symbols in 110 languages

Line Encoding : 

Line Encoding The waveform pattern of voltage or current used to represent the 1s and 0s of a digital signal on a transmission link is called line encoding. The common types of line encoding are Unipolar, Polar, Bipolar and Manchester encoding.

Digital Encoding performance criteria's : 

Line coding should make it possible for the receiver to synchronize itself in time to the the received signal. If the synchronization is not ideal, then the signal to be decoded will not have optimal differences. The data loss or error will occur. (Solution self clocking) DC Component avoidance Efficient Band Width Bit error rate Digital Encoding performance criteria's

Unipolar Encoding : 

Unipolar Encoding Unipolar encoding has 2 voltage states, with one of the states being 0 volts. Since Unipolar line encoding has one of its states at 0 Volts, it is also called Return to Zero (RTZ). A common example of Unipolar line encoding is the TTL logic levels used in computers and digital logic.

Polar Encoding : 

Polar Encoding When the digital encoding is symmetrical--around 0 Volts--it is called a Polar Code. For example, the RS-232D interface uses Polar line encoding. The signal does not return to zero; it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return To Zero (NRZ). Polar line encoding is the simplest pattern that eliminates most of the residual DC problem.

Bipolar Line Encoding : 

Bipolar Line Encoding Bipolar line encoding has 3 voltage levels. A low or 0 is represented by a 0 Volt level and a 1 is represented by alternating polarity pulses. By alternating the polarity of the pulses for 1s, the residual DC component cancels.

Manchester Line Encoding : 

Manchester Line Encoding In Manchester Line Encoding, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism (and also as data): a low to high transition represents a 1 and a high to low transition represents a 0.

Digital data Encoding Schemes : 

Digital data Encoding Schemes

NRZ-L : 

NRZ-L Digital 1s are represented as one voltage (amplitude), while digital 0s are represented as another: Cheap to implement Check for voltage of each bit A long series of 1s or 0s produces a flat, unchanging voltage level (produces synchronization problems)

NRZI : 

NRZI Digital 1s are represented by a voltage change (high-to-low, or low-to-high), while 0s are represented as a continuation of the same voltage level: Even cheaper to implement (only check for changes) A long series of 0s produces a flat, unchanging voltage level

Manchester encoding : 

Manchester encoding Digital 1s are represented by a midway voltage change from low to high, while 0s are represented as midway voltage changes from high to low Hardware has to work twice as fast to detect changes Baud rate (number of signal changes) is twice bits per second rate

Differential Manchester : 

Differential Manchester Digital 0s are represented by a voltage change (high-to-low, or low-to-high) at the beginning of the bit as well as a midway voltage change, while 1s are represented as a continuation of the same voltage level at the beginning, followed by a midway voltage change

Alternate Mark Inversion : 

Alternate Mark Inversion

Bipolar-AMI : 

Bipolar-AMI Three voltage level 0 transmitted as 0 voltage 1 transmitted as either +1 or –1 voltage Alternating between the two Disadvantages Long string of 0s Hardware capable to recognize + & - voltages

Slide 33: 

Manchester Transition in middle of each bit period Transition serves as clock and data Low to high represents one High to low represents zero Used by IEEE 802.3 (Ethernet) Differential Manchester Mid-bit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE 802.5 (Token Ring)

Data Encoding Methods : 

Data Encoding Methods

Bipolar with 8 Zero Substitution(B8ZS) : 

Bipolar with 8 Zero Substitution(B8ZS) This coding method takes care of the self- synchronization problem by breaking the alternation rule when it comes across a sequence of eight consecutive 0s This rule puts 1s in place of the fourth and fifth 0s and in places of the seventh and eighth 0s The first substitute incorrectly has the polarity of the previous 1, and the third substitute incorrectly has the polarity of the second substitute 1 The receiver recognizes an intentional violation and concludes that there is a sequence of eight 0s

B8ZS Eample : 

B8ZS Eample

Multi-Level Transmit MLT-3MLT-3 cycles through the voltage levels -1, 0, +1, and 0. It moves to the next state to transmit a 1 bit, and stays in the same state to transmit a 0 bit. : 

Multi-Level Transmit MLT-3MLT-3 cycles through the voltage levels -1, 0, +1, and 0. It moves to the next state to transmit a 1 bit, and stays in the same state to transmit a 0 bit.

4B/5B Encoding : 

4B/5B Encoding Every 4-bit pattern is assigned a 5-bit code Instead of the 4-bits, the 5-bit code is transmitted The 5-bit code is picked such that there are at least two transitions in every 5-bit code Therefore, an encoded stream will never contain more than 3 zeros in a row This helps synchronization and leads to higher data transfer rates Used in Fast Ethernet and FDDI Gigabit Ethernet uses 8B/10B Encoding, based on the same principle, where an 8-bit pattern is assigned a 10-bit code

4B/5B Digital Encoding : 

4B/5B Digital Encoding Encoding technique that converts four bits of data into five-bit quantities. The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes. The five-bit code is then transmitted using an NRZ-I encoded signal.

2 Binary 1 Quaternary(2B1Q) : 

2 Binary 1 Quaternary(2B1Q) 2B1Q uses four distinct signaling levels, with data represented in two-bit units 2B1Q represents the distinction between bits per second and baud rate 2B1Q has been implemented in broadband technologies such as ISDN, SDSL and HDSL

Basic digital communications system : 

Basic digital communications system Modulator Demodulator Transmission Channel Input transducer Transmitter Receiver Output transducer Carrier EM waves (modulated signal) EM waves (modulated signal) Analog signal analog signal A/D converter Digital signal Error correction coding Error detection/ correction D/A converter digital signal

Slide 44: 

Some Types of Digital Modulation Amplitude Shift Keying (ASK) The most basic (binary) form of ASK involves the process of switching the carrier either on or off, in correspondence to a sequence of digital pulses that constitute the information signal. One binary digit is represented by the presence of a carrier, the other binary digit is represented by the absence of a carrier. Frequency remains fixed Frequency Shift Keying (FSK) The most basic (binary) form of FSK involves the process of varying the frequency of a carrier wave by choosing one of two frequencies (binary FSK) in correspondence to a sequence of digital pulses that constitute the information signal. Two binary digits are represented by two frequencies around the carrier frequency. Amplitude remains fixed Phase Shift Keying (PSK) Another form of digital modulation technique which we will not discuss

Amplitude Shift Keying : 

Amplitude Shift Keying One amplitude encodes a 0 while another amplitude encodes a 1 (amplitude modulation).

Frequency Shift Keying : 

Frequency Shift Keying One frequency encodes a 0 while another frequency encodes a 1 (frequency modulation).

Phase Shift Keying : 

Phase Shift Keying One phase change encodes a 0 while another phase change encodes a 1 (phase modulation).

Quadrature phase modulation : 

Quadrature phase modulation Four different phase angles are used, namely: 45 degrees 135 degrees 225 degrees 315 degrees

Constellation Diagram : 

Constellation Diagram A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. It displays the signal as a two-dimensional scatter diagram in the complex plane at symbol sampling instants.

BPSK and QPSK Constellation Diagram : 

BPSK and QPSK Constellation Diagram

QAM : 

QAM

Quadrature Amplitude Modulation : 

Quadrature Amplitude Modulation In this technology, 12 different phases are combined with two different amplitudes. Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations. With 16 signal combinations, each baud equals 4 bits of information.

How do you send more data : 

How do you send more data Manipulate one or more of the main three properties (amplitude, frequency, or phase) to denote multiple bits The most common (because it’s cheaper) is amplitude, or frequency Baud rate vs. Bit rate, putting more bits in a baud Shannon’s Law allows you to calculate the maximum data transfer rate

Pulse Code Modulation (I) : 

Pulse Code Modulation (I) The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values. Used by telephone systems.

Pulse Code Modulation (II) : 

Pulse Code Modulation (II) How fast do you have to sample an input source to get a fairly accurate representation? Nyquist says 2 x bandwidth Thus, to digitize the human voice (4000 Hz), you need to sample at 8000 sample per second

Delta Modulation : 

Delta Modulation An analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop.