lect2 physical

Lecture 2 – Physical Layer Communications

Wenye Wang

Wenye Wang (NC State University) ECE 592-017 - Communication Systems for Electric Power Systems 1 / 35

Overview of the Communication Basics

1 Physical Layer Communications: Ch 1.1, Ch 2.2, Ch 3.1, Ch 3.2What are the options of communication links?What are the solutions to imperfect links?

2 Concepts of Communications and Networking Protocols

3 Serial Communication Basics

4 Ethernet Basics

5 Wireless Local Area Networks

6 Wide Area Networks: Wired and Wireless

7 IP, UDP and TCP

8 Internet and computer communications

9 Communication System Performance

10 Communications in Smart Grid






4 Ethernet Basics



7 IP, UDP and TCP





Revisit Utility Communications






4 Ethernet Basics



7 IP, UDP and TCP





The Physical Layer – Lowest of OSI

Provides a bit pipe between transmitter/receiver pairs

More communications than networking

What we need to know? basic techniques and principles

Transmission MediumGuided medium, like copper wire, coax, fiberUnguided medium, radio, satellite, etcSingle underlying phenomenon – Electromagnetic waves






Transmission MediumGuided medium, like copper wire, coax, fiberUnguided medium, radio, satellite, etc

Single underlying phenomenon – Electromagnetic waves






Transmission MediumGuided medium, like copper wire, coax, fiberUnguided medium, radio, satellite, etcSingle underlying phenomenon – Electromagnetic waves


What are Electromagnetic Waves?

“Charged particles-such as electrons and protons-createlectromagnetic fields when they move, and these fields transport thetype of energy we call electromagnetic radiation, or light.”

Two important ways for energy transportationMechanical waves: caused by a disturbance or vibration in matter,whether solid, gas, liquid, or plasma.Medium: Matter that waves are traveling through is called amediumElectromagnetic waves: Electromagnetic waves differ frommechanical waves in that they do not require a medium topropagate. This means that electromagnetic waves can travel notonly through air and solid materials, but also through the vacuumof space.




Two important ways for energy transportation

Mechanical waves: caused by a disturbance or vibration in matter,whether solid, gas, liquid, or plasma.Medium: Matter that waves are traveling through is called amediumElectromagnetic waves: Electromagnetic waves differ frommechanical waves in that they do not require a medium topropagate. This means that electromagnetic waves can travel notonly through air and solid materials, but also through the vacuumof space.




Two important ways for energy transportationMechanical waves: caused by a disturbance or vibration in matter,whether solid, gas, liquid, or plasma.

Medium: Matter that waves are traveling through is called amediumElectromagnetic waves: Electromagnetic waves differ frommechanical waves in that they do not require a medium topropagate. This means that electromagnetic waves can travel notonly through air and solid materials, but also through the vacuumof space.




Two important ways for energy transportationMechanical waves: caused by a disturbance or vibration in matter,whether solid, gas, liquid, or plasma.Medium: Matter that waves are traveling through is called amedium

Electromagnetic waves: Electromagnetic waves differ frommechanical waves in that they do not require a medium topropagate. This means that electromagnetic waves can travel notonly through air and solid materials, but also through the vacuumof space.




Two important ways for energy transportationMechanical waves: caused by a disturbance or vibration in matter,whether solid, gas, liquid, or plasma.Medium: Matter that waves are traveling through is called amediumElectromagnetic waves: Electromagnetic waves differ frommechanical waves in that they do not require a medium topropagate. This means that electromagnetic waves can travel notonly through air and solid materials, but also through the vacuumof space.


Description of Electromagnetic Energy

Frequency f

The number of oscillations per second of a waveOne wave or cycle per second is called a Hertz (Hz), afterHeinrich Hertz who established the existence of radio waves

Wavelength λ

Electromagnetic waves have crests and troughs similar to those ofocean waves. The distance between crests is the wavelength.Relationship between f and λ: λ× f = c, where c is the speed oflight.



Frequency fThe number of oscillations per second of a wave

One wave or cycle per second is called a Hertz (Hz), afterHeinrich Hertz who established the existence of radio waves

Wavelength λ




Frequency fThe number of oscillations per second of a waveOne wave or cycle per second is called a Hertz (Hz), afterHeinrich Hertz who established the existence of radio waves

Wavelength λ





Wavelength λ

Electromagnetic waves have crests and troughs similar to those ofocean waves. The distance between crests is the wavelength.

Relationship between f and λ: λ× f = c, where c is the speed oflight.




Wavelength λ



Digital or Analog

Digital – concept Information can be analog or digital

EM waves – analog by definition

Analog EM signal can be made to transfer digital data

“Digital interpretation of analog signal representing digitalrepresentation of analog data”.


The Electromagnetic Spectrum

visible light: 400-700 nm, (4.3 7.5× 1014)


Guided Medium: Coaxial Cable

Coaxial cabletwo concentric copper conductorsbidirectionalbaseband: (1) Single channel on cable and (2) Legacy Ethernet


Guided Medium: Fiber

Fiberglass fiber carrying light pulses,each pulse a bithigh-speed point-to-pointtransmission (e.g., 5 Gps)low error rate: repeaters spacedfar apart ; immune toelectromagnetic noise


Unguided Medium – Radio

Features

no physical “wire”bidirectionalpropagation environment effects: reflection; obstruction byobjects; and interference



Featuresno physical “wire”

bidirectionalpropagation environment effects: reflection; obstruction byobjects; and interference



Featuresno physical “wire”bidirectionalpropagation environment effects: reflection; obstruction byobjects; and interference






4 Ethernet Basics



7 IP, UDP and TCP





How to Delivery Messages Reliably?

FunctionsBreaking up the bit stream into framesProvide a well-defined service interface to the network layer

Deal with transmission errors (Error control)Use frames to encapsulate packets from upper layer.Regulate the flow of data so that slow receivers are not swampedby fast senders (Flow control).With or without errors?



FunctionsBreaking up the bit stream into framesProvide a well-defined service interface to the network layerDeal with transmission errors (Error control)Use frames to encapsulate packets from upper layer.

Regulate the flow of data so that slow receivers are not swampedby fast senders (Flow control).With or without errors?



FunctionsBreaking up the bit stream into framesProvide a well-defined service interface to the network layerDeal with transmission errors (Error control)Use frames to encapsulate packets from upper layer.Regulate the flow of data so that slow receivers are not swampedby fast senders (Flow control).

With or without errors?



FunctionsBreaking up the bit stream into framesProvide a well-defined service interface to the network layerDeal with transmission errors (Error control)Use frames to encapsulate packets from upper layer.Regulate the flow of data so that slow receivers are not swampedby fast senders (Flow control).With or without errors?


Framing – Breaking up the Bit Stream into Frames

Byte countFlag bytes with byte stuffingFlag bits with bit stuffing


A Simple Model-Binary Symmetric Channels


Why Use Error Control?


Single Parity Check

(Even) Parity Check

append a single bit to the data. If the number of 1 bits in thecodeword is even, a bit 0 is appended.E.g., 1011010→ 10110100Can it detect single errors?


Single Parity Check

(Even) Parity Checkappend a single bit to the data. If the number of 1 bits in thecodeword is even, a bit 0 is appended.

E.g., 1011010→ 10110100Can it detect single errors?


Single Parity Check

(Even) Parity Checkappend a single bit to the data. If the number of 1 bits in thecodeword is even, a bit 0 is appended.E.g., 1011010→ 10110100

Can it detect single errors?


Single Parity Check

(Even) Parity Checkappend a single bit to the data. If the number of 1 bits in thecodeword is even, a bit 0 is appended.E.g., 1011010→ 10110100Can it detect single errors?


Two-dimensional Parity Check

A two-dimensional array with oneparity check for each row and onefor each

Parity bits:

ci = si1 ⊕ si2 ⊕ si3 · · · sik

rj = s1j ⊕ s2j ⊕ s3j · · · snj

Detect all single, double errors?

Detect triple errors and quadrupleerrors?


Cyclic Redundancy Check (CRC)

Polynomial code

Bit string is represented by a polynomial with coefficients of 0 and1 only. E.g., 10011A k -bit frame is regarded as a polynomial with k terms, rangingfrom xk−1 to x0 . It is called a polynomial of degree (k − 1).If the data bits are denoted as mK−1,mK−2, · · · ,m1,m0, thepolynomial M(x) representation of the string with coefficients is

M(x) = mK−1xK−1 + mK−2xk−2 + · · · ,+m1x + m0



Polynomial codeBit string is represented by a polynomial with coefficients of 0 and1 only. E.g., 10011

A k -bit frame is regarded as a polynomial with k terms, rangingfrom xk−1 to x0 . It is called a polynomial of degree (k − 1).If the data bits are denoted as mK−1,mK−2, · · · ,m1,m0, thepolynomial M(x) representation of the string with coefficients is

M(x) = mK−1xK−1 + mK−2xk−2 + · · · ,+m1x + m0



Polynomial codeBit string is represented by a polynomial with coefficients of 0 and1 only. E.g., 10011A k -bit frame is regarded as a polynomial with k terms, rangingfrom xk−1 to x0 . It is called a polynomial of degree (k − 1).

If the data bits are denoted as mK−1,mK−2, · · · ,m1,m0, thepolynomial M(x) representation of the string with coefficients is

M(x) = mK−1xK−1 + mK−2xk−2 + · · · ,+m1x + m0



Polynomial codeBit string is represented by a polynomial with coefficients of 0 and1 only. E.g., 10011A k -bit frame is regarded as a polynomial with k terms, rangingfrom xk−1 to x0 . It is called a polynomial of degree (k − 1).If the data bits are denoted as mK−1,mK−2, · · · ,m1,m0, thepolynomial M(x) representation of the string with coefficients is

M(x) = mK−1xK−1 + mK−2xk−2 + · · · ,+m1x + m0


CRC Polynomial

Step 1: Both the sender and receiver agree upon a generatorpolynomial, G(x) of degree r . E.g, x2 + 1.

Step 2: Let r be the degree of G(x), append r zero bits to thelow-order end of the frame, that is x r M(x).

Step 3: Divide the G(x) into x r M(x) and obtain the remainderRemainder [x r M(x)

G(x) ].

Step 4: The CRC polynomial T (x) is obtained by

T (x) = x r M(x)− Remainder [x r M(x)

G(x)]


Error Detection with CRC

Divisible (Detectable) or not divisible (not detectable)

T (x) is divisible by G(x), so the remainder should be zero forcorrect transmissionAssume there is an error occurs, so the received bit string isT (x) + E(x). If no errors occur, then E(x) = 0.



Divisible (Detectable) or not divisible (not detectable)T (x) is divisible by G(x), so the remainder should be zero forcorrect transmission

Assume there is an error occurs, so the received bit string isT (x) + E(x). If no errors occur, then E(x) = 0.



Divisible (Detectable) or not divisible (not detectable)T (x) is divisible by G(x), so the remainder should be zero forcorrect transmissionAssume there is an error occurs, so the received bit string isT (x) + E(x). If no errors occur, then E(x) = 0.


Standards

XPX-16g(x) = 1 + x2 + x15 + x16

CRC-ITU-Tg(x) = 1 + x5 + x12 + x16


Example

CRC Coding and Detection

Consider the following generator polynomials to be used in a CRCscheme: G1(x) = x5 + x3 + x2 + 1 and G2(x) = x7 + x5 + x + 1.While transmitting a CRC encoded message, the followingpolynomial is received.

T1(x) = x7 + x5 + x4 + x2

Is T1(x) detectable with G1(x) and G2(x), or can any errors bedetected or not?If G1(x) is used as CRC, and another polynomial is received

T2(x) = x6 + x5 + x4 + x2 + x + 1

Assume T1(x) and T2(x) are the results of the same message andthere is no error in T1(x). What is the error in T2(x)? How tocorrect T2(x) to obtain T (x)?


Example

CRC Coding and DetectionConsider the following generator polynomials to be used in a CRCscheme: G1(x) = x5 + x3 + x2 + 1 and G2(x) = x7 + x5 + x + 1.While transmitting a CRC encoded message, the followingpolynomial is received.

T1(x) = x7 + x5 + x4 + x2


T2(x) = x6 + x5 + x4 + x2 + x + 1



Example


T1(x) = x7 + x5 + x4 + x2

Is T1(x) detectable with G1(x) and G2(x), or can any errors bedetected or not?

If G1(x) is used as CRC, and another polynomial is received

T2(x) = x6 + x5 + x4 + x2 + x + 1



Example


T1(x) = x7 + x5 + x4 + x2


T2(x) = x6 + x5 + x4 + x2 + x + 1



Summary






4 Ethernet Basics



7 IP, UDP and TCP





lect2 physical

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