Physical Layer and Digital Transmission

PHYSICAL LAYER Physical Layer  actually interacts with the transmission medium Physical Layer  Bits enclosed into frames that comes from DLL is converted to Singals  Decides on the direction of the data flow  Decides on the number of logical channels for transporting data coming from different sources Physical Layer  Multiplexing - dividing link into logical channels  Switching  Circuit switching – allows two nodes to have a dedicated link  Packet switching  Message switching Physical Layer Signals  To be transmitted, data must be transformed to electromagnetic signals  Two types of Signals Data Analog Digital Analog Digital Physical Layer - Signals  Two forms of signals  Periodic  Period – completes a pattern within a measurable time frame  Cycle – completion of one full pattern  Aperiodic – it changes without exhibiting a pattern or cycle that repeats over time  Both analog and digital signals could be periodic or aperiodic  In data communications, periodic analog and aperiodic digital signals are commonly used. Physical Layer - Signals  Two classifications of Analog signals  Simple – cannot be decomposed into simpler signals (sine wave); consistent and continuous S(t) = A sin (2π ft + ø) Peak Amplitude – absolute value of its highest intensity proportional to the energy it carries Period – amount of time a signal needs to complete one cycle Frequency – number of periods in one second; rate of change with respect to time Physical Layer - Signals  Two classifications of Analog signals  Simple Phase – describes the position of the waveform relative to time zero, measured in degrees/radians Physical Layer - Signals  Two classifications of Analog signals  Simple time-domain plot Physical Layer - Signals  Two classifications of Analog signals  Simple time-domain frequency-domain Physical Layer - Signals  Two classifications of Analog signals  Composite – composed of multiple sine waves  A single-frequency sine wave is not useful thus we need to change one or more of its characteristics to make it useful, making it a composite signal made of many frequencies  Fourier Analysis – any composite signal can be represented as a combination of sine waves with different frequencies, phases and amplitudes. Physical Layer - Signals  Two classifications of Analog signals  Composite Physical Layer - Signals  Two classifications of Analog signals  Composite Physical Layer - Signals  Frequency Spectrum description of a signal using the frequency domain and containing all its components Physical Layer - Signals  When we pass composite signals at one end of the transmission medium, we may not receive the same at the other end because of medium’s characteristic of blocking or weakening some frequencies. Physical Layer - Signals  Bandwidth – range of frequencies that a medium can pass If the BW of a medium does not match the spectrum of a signal. Some of the frequencies are lost. Physical Layer - Signals  Digital Signal can represent analog through (+) voltage and (-) voltage. Period ~ bit interval Frequency ~ bit rate Physical Layer - Signals  Digital Signal bit interval – time required to send one single bit bit rate – number of bit interval per second. bit interval = 1 / bit rate Physical Layer - Signals  A digital signal is a composite signal having an infinite number of frequencies, hence, the BW is infinite To send n bps through an analog channel using the above approximation, we need BW ≥ n / 2 Physical Layer - Signals  Analog BW is measured in hertz and Digital BW is measured in bps Physical Layer - Signals  Two types of channel or link according to freq.  Low-pass  Band-pass Physical Layer - Signals  Digital Transmission a digital signal theoretically needs a BW between 0 and infinity, thus a low pass channel is needed  Analog Transmission analog signal requires a band-pass channel ***A band-pass channel is more available than low pass, thus, the BW of the medium can be divided into several band-pass channel to carry several analog signals. Physical Layer - Signals  Data Rate Limits  1. 2. 3. Data rate depends on three factors: BW available Level of signals we can use Quality of the channel (level of noise)  Calculation of Data Rate  Nyquist Bit Rate (Noiseless channel) Bit rate = 2 x BW x log2L (bps) BW – of the channel L – no. of signal levels used to represent data Physical Layer - Signals  Calculation of Data Rate  Shannon Capacity (Noisy channel) Capacity = BW x log2(1 + SNR) SNR = P signal / P noise (bps) Physical Layer - Signals  Transmission Impairment Physical Layer - Signals  Attenuation  Loss of energy  When a signal travels through a medium, it loses some of its energy in overcoming the resistance of the medium Physical Layer - Signals  Attenuation  Decibel (dB) – measures the relative strengths of the two signals or a signal at two different points. (-) dB = attenuated (+) dB = amplified dB = 10 log (P2 / P1) Physical Layer - Signals  Distortion  Signal changes form or shape  Occurs in composite signal because each frequency component has its own propagation speed through a medium Physical Layer - Signals  Noise  Corrupts the signal  Types of Noise o Thermal noise – random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter o Induced noise – from sources such as motors and appliances which acts as transmitter and the transmission medium as the receiver. o Crosstalk – effect of one wire on the other o Impulse noise – spike (a signal with high energy in a very short period of time) that comes from power lines, lightning, etc. Physical Layer - Signals  Noise Physical Layer - Signals  Measurements Used in Data Communications  Throughput – measurement of how fast data can pass through an entity; number of bits that can pass in one second.  Propagation Speed – distance a bit can travel through a medium in one second (vacuum, air)  Propagation Time – time required for a signal to travel from one point of transmission medium to another. Time = distance / speed Physical Layer - Signals  Measurements Used in Data Communications  Wavelength – distance a simple signal can travel in one period wavelength = propagation speed x period Digital Transmission  Line Coding - process of converting binary data to a digital signal Data Text Numbers Audio Video  Characteristics of Line Coding     Signal level vs Data level Pulse rate vs bit rate DC components Self-synchronization Digital Transmission - Line Coding  Signal level vs Data Level Signal level – no. of values allowed in a particular signal Data level – no. of values used allowed in a particular signal Digital Transmission - Line Coding  Pulse Rate vs Bit Rate Pulse rate – number of pulses per second (Pulse – min. amount of time required to transmit a symbol) Bit rate – number of bits per second Bit rate = Pulse rate x log2 L (L – data level) **If pulse carries more than 1 bit, then bit rate > pulse rate Digital Transmission - Line Coding  DC Components  DC component is sometimes unnecessary in a system especially when passing through a transformer because the signal can be distorted and may create errors. Also, this component consumes extra energy and found to be useless. Digital Transmission - Line Coding  Self-Synchronization  Sender’s bit intervals must correspond exactly to the sender’s bit intervals  A self-synchronizing digital signal includes timing information in the data being transmitted and this is done if there are transitions in the signal that alert the receiver to the beginning, middle or end of the pulse. Digital Transmission - Line Coding  Self-Synchronization Digital Transmission - Line Coding  Line Coding Schemes Digital Transmission - Line Coding  Line Coding – Unipolar  Uses only one polarity which is assigned as binary 1. The binary zero is represented by zero voltage.  Its constraints is the dc component and lack of synchronization. Digital Transmission - Line Coding  Line Coding – Polar  Uses two voltage levels, one positive and one negative.  Average voltage level on the line is decreased and the dc component problem is alleviated Digital Transmission - Line Coding  Line Coding – Polar  Non-return to Zero (NRZ) value of the signal is always either positive or negative NRZ-L (NRZ-level) level of the signal depends on the state of the bit (+) Voltage – 0 bit (-) Voltage – 1 bit NRZ-I (NRZ-Invert) signal is inverted if a 1 is encountered Digital Transmission - Line Coding  Line Coding – Polar  Return to Zero (RZ) uses three values: positive, negative & zero. signal changes not between bits but during each bit bit-1: positive to zero bit-0: negative to zero  It requires two signal changes to encode 1 bit which occupies more bandwidth Digital Transmission - Line Coding  Line Coding – Polar  Manchester uses an inversion at the middle of each bit interval for both synchronization and bit representation binary 1: negative to positive binary 0: positive to negative Digital Transmission - Line Coding  Line Coding – Polar  Differential Manchester inversion at the middle of the bit interval is used for synchronization but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit binary 1: no transition binary 0: transition Digital Transmission - Line Coding  Line Coding – Bipolar Uses three voltage levels: positive, negative and zero Binary 0: zero level Binary 1: alternating positive and negative  Alternate Mark Inversion (AMI) alternate 1 inversion  Bipolar n-Zero substitution (BnZS) wherever n consecutive zeros occur, some of the bits in these n bits become positive or negative Digital Transmission - Line Coding  Line Coding – 2B1Q - two binary, one quarternary - Use four voltage levels Digital Transmission - Line Coding  Line Coding – MLT-3 Multi-line transmission, three level (MLT-3) Use three levels of signals (+1, 0 and – 1) Signal transitions from one level to the next at the beginning of 1 bit; there is no transition aty the beginning of 0 bit. Digital Transmission  Block Coding Used to improve the performance of line coding Employs redundancy to ensure synchronization. Redundancy is also used to detect errors. Digital Transmission  Block Coding Procedure 1. Division – sequence of bits is divided into groups of m bits 2. Substitution – substitute an m-bit code for an nbit group 3. Line Coding Digital Transmission  Block Coding Schemes  4B/5B - every 4 bits of data is encoded into a 5-bit code - no more than one leading 0 and no more than two trailing 0s. (24 bit codes  25 bit codes) Digital Transmission  Block Coding Schemes  8B/10B - group of 8 bits of data is substituted by a 10 bit code ( 28 bit code  210 bit code) - provides more error detection capability  8B/6T - substitute an 8 bit group with a six symbol code - each symbol is ternary (one of three levels) - consumes less bandwidth (28  36) Digital Transmission  Sampling  Process of converting analog signals into discrete valued signals at a fixed interval  Avoids the use of amplifier in the transmission of analog signal which creates distortion & noise  Types of A/D conversion method  Pulse Amplitude Modulation (PAM) - uses sample and hold Types of A/D conversion method Digital Transmission - Sampling  Pulse Code Modulation (PCM) - modifies the pulses created by PAM to create a digital signal - uses quantization (assigning integral values to sampled instances) PAM  quantization binary encoding line coding Types of A/D conversion method  Pulse Code Modulation (PCM) Digital Transmission - Sampling  Nyquist Theorem - sampling rate ≥ 2 fhighest Digital Transmission - Sampling (samples per second) Bit rate = sampling rate x no. of bits sample A signal is sampled. Each sample requires at least 12 levels (+0 to +5 and 0 to -5). How many bits should be sent for each sample? We need 4 bits; 1 bit for the sign, 3 bits for the value