Physical layer Encoding data into signals Computer networks Girts - - PowerPoint PPT Presentation

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Physical layer Encoding data into signals Computer networks Girts - - PowerPoint PPT Presentation

Apr 04, 2023 176 likes •725 views

Physical layer Encoding data into signals Computer networks Girts Strazdins, gist@ntnu.no, NTNU i lesund, 2020 Outline 1. Digital and analog data 2. Signaling, symbols, bitrate 3. Digital signaling 4. Analog signaling 5. Sender and

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SLIDE 1

Physical layer

Encoding data into signals

Computer networks

Girts Strazdins, gist@ntnu.no, NTNU i Ålesund, 2020

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SLIDE 2

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 3

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 4

Data and signals

Communication channel Receiver

Data Received Data Signals Can be digital or analog Can be digital or analog

Sender

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SLIDE 5

Analog data

  • Continuous, without natural discrete boundaries
  • Examples: temperature, audio signal
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SLIDE 6

Digital data

  • Any data with naturally separate categories,
  • boundaries. Discrete data.
  • Examples: integer numbers, text.
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SLIDE 7

We focus on digital data

  • Computers operate with digital data only
  • In this course we focus only on digital data

transmission

Discretization 101001010101.......

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SLIDE 8

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 9

Transmitting data with signals

Sender Receiver

Data Received Data Signals In computer communication we use electromagnetic signals

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SLIDE 10

What can be signals?

Anything, as long as:

  • Signals can encode symbols – signs with a meaning
  • Sender and receiver agree on their meaning
  • These can be transmitted
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SLIDE 11

Example symbols

Sender Receiver

Data Received Data Symbols: black and white card

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SLIDE 12

Encoding 1 and 0

Sender Receiver

Data Received Data Symbols: black and white card We agree that white card means 0 and black card means 1

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SLIDE 13

1 bit per symbol

  • 2 possible symbols: 0 and 1.
  • We can transmit 1 bit of information with each

symbol

  • To transmit an 8-bit number, we need to show 8

symbols:

  • 12 in decimal

= 00001010 =

  • 27 in decimal

= 00011011 =

  • 254 in decimal = 11111110 =
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SLIDE 14

What if we have 4 cards?

How many bits per symbol?

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SLIDE 15

What if we have 4 cards?

4 = 22 possible symbols = 2 bits per symbol 00 01 10 11

  • To transmit an 8-bit number, we need to show 4

symbols:

  • 12 in decimal

= 00001010 = 00 00 10 10 =

  • 27 in decimal

= 00011011 = 00 01 10 11 =

  • 254 in decimal = 11111110 = 11 11 11 10 =
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SLIDE 16

Why not infinite number of symbols?

  • More symbols = harder:
  • Harder to signal them
  • Harder to separate them for the receiver
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SLIDE 17

Why not infinite number of symbols?

  • More symbols = harder:
  • Harder to signal them
  • Harder to separate them for the receiver
  • Example: separating these is easy:
  • How about these?
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SLIDE 18

Baudrate and bitrate

  • Baudrate = the rate (speed) at which symbols can

be shown, measured in symbols/second

  • Bitrate = how many bits per second the channel

can transmit (the speed of the channel)

  • Bitrate = baudrate x bitsPerSymbol
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SLIDE 19

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 20

Digital signaling

Symbols are distinct, grouped in separate categories. Examples:

  • Pieces of paper with distinct colors
  • Morse code (sound) ... --- ...
  • Distinct voltage levels in wires (+5V, 0V)
  • Digital signaling is easier over wires, harder with

wireless.

  • Examples of digital signaling: Ethernet and ISDN lines
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SLIDE 21

Transmitting data with signals

Sender Receiver

Data Received Data Signals In computer communication we use electromagnetic signals

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SLIDE 22

Voltage levels as digital signals

  • Easiest example: 0V means 0 while +3V means 1
  • We get 1 bit per symbol
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SLIDE 23

4 levels = 2 bits/symbol

If we can use 4 different voltage levels (-10V, -5V, +5V, +10V), we get 4 symbols = 2 bits per symbol:

  • -10V = 00
  • -5V = 01
  • +5V = 10
  • +10V = 11
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SLIDE 24

Goals for digital signaling

Ideally, we want the signaling to be:

  • Effective: high bitrate (speed)
  • Synchronized: same baudrate for sender and

receiver

  • Robust: noise should not confuse the receiver
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SLIDE 25

Sender and receiver synchronization

Receiver must sample (read) the symbols at the same rate at which they were sent (baudrate) Example of synchronization error:

Sender signals the symbols: time 0 1 Receiver samples symbols: 0 0 1 1

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SLIDE 26

Popular encoding types

  • 1. Absolute
  • 2. Differential
  • 3. Bi-phase
  • We observe these for learning purposes. More

advanced techniques used in practice.

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SLIDE 27

Absolute encoding

  • Each symbol is a specific voltage level
  • Example: Non-Return-to-Zero (NRZ), used in RS-232

(serial port communication)

  • Effectiveness: +
  • Synchronization: -
  • Robustness: -
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SLIDE 28

Differential encoding

  • One symbol (1) changes the voltage, the other

symbol (0) keeps the same voltage level

  • Example: Non-Return-to-Zero-Inverted (NRZI), used

in CDs

  • Effectiveness: +
  • Synchronization: -
  • Robustness: +/-
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SLIDE 29

Bi-phase encoding

  • Each symbol is a sequence of two voltage levels.
  • For example: 0 is high-to-low voltage change, while

1 is low-to-high change

  • Example: Manchester encoding, used in 10Mb/s

Ethernet

  • Effectiveness: -
  • Synchronization: +
  • Robustness: +
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SLIDE 30

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 31

Analog signaling

  • Idea: take an analog carrier wave, modulate it with

the signal (data)

  • Modulation: change properties of the carrier signal,

such as:

  • Amplitude
  • Frequency
  • Phase
  • Several properties can be combined (modern

techniques do that)

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SLIDE 32

Amplitude Shift keying (ASK)

  • Note: term Amplitude Modulation (AM) is used for

analog data. ASK is used for digital data.

  • Idea: use distinct amplitude levels of the signal.
  • Informally: volume, intensity, strength of the signal
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SLIDE 33

Frequency Shift Keying (FSK)

  • Idea: use distinct frequency waves in the signal
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SLIDE 34

Phase Shift Keying (PSK)

  • Idea: change the phase of the signal
  • Informally: move the signal to the left or right
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SLIDE 35

Limitations

  • We can’t increase the amplitudes and frequencies
  • indefinitely. It will be hard to manufacture

equipment.

  • We can’t increase the number of levels (symbols)

indefinitely: noise will create confusion.

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SLIDE 36

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 37

Sender and receiver synchronization

Receiver must sample (read) the symbols at the same rate at which they were sent (baudrate) Example of synchronization error:

Sender signals the symbols: time 0 1 Receiver samples symbols: 0 0 1 1

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SLIDE 38

Synchronization principles

  • Without synchronization the transmission is

worthless (can even be harmful - misinformation)

  • Main sources of challenge:
  • High speed of transmission = many symbols/second
  • Long periods with no change (or “silence”) – receiver

starts to wonder - what is going on?

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SLIDE 39

Main synchronization approaches

  • In practice, different approaches.
  • The main synchronization types:

a) Synchronization signal on a separate line b) Synchronization signal built into the line c) Start and stop signal before and after each message, short messages (low chance of de-synchronization within one message)

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SLIDE 40

Outline

  • 1. Digital and analog data
  • 2. Signaling, symbols, bitrate
  • 3. Digital signaling
  • 4. Analog signaling
  • 5. Sender and receiver synchronization
  • 6. Physical media
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SLIDE 41

Physical media

  • Media – tools and environment allowing to

transmit signals, the channel of communication.

  • Popular physical media for computer networks:
  • Coaxial copper cables (somewhat outdated)
  • Twisted-pair cables
  • Fiber optic cables
  • Wireless communication
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SLIDE 42

Coaxial cable

Was more actively used in early days

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SLIDE 43

Twisted pair cable

  • Shielded

Unshielded

RJ 45

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SLIDE 44
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SLIDE 45

Fiber-optic cables

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SLIDE 46

How the fiber works

Watch this video to understand how fiber optic cables are made and how they work: https://www.youtube.com/watch?v=0MwMkBET_5I

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SLIDE 47

Wireless media

Digital data sent with analog signals

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SLIDE 48

Wireless links

How far can you transmit?

Wireless routers or bridges Cellular networks (3G, 4G, …)

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SLIDE 49

Wireless links

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SLIDE 50

How many km did you say?

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SLIDE 51

Things to remember

  • We transmit symbols at a specific rate (baudrate)
  • The overall transmission speed (bitrate) depends
  • n baudrate and bits per symbol
  • Digital data can be encoded in both analog and

digital signals

  • We strive for efficient, robust and synchronized

signaling (encoding)

  • Different physical media are available, the most

popular nowadays are twisted-pair cables, fiber

  • ptic cables and wireless media
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SLIDE 52

Further reading

Topic Chapters in Kurose’s book Chapters in Hallsteinsen’s book Analog and digital data, signals, symbols, baudrate, bitrate, synchronization

  • See Data Encoding

Techniquest, Digital Modulation Techniques in Tutorialspoint https://www.tutorialspoin t.com/digital_communica tion/digital_communicati

  • n_data_encoding_techni

ques.htm 6.7 Analoge og digitale data og signaler Physical media 1.2.2 Physical media 6.6 Medier

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SLIDE 53

Image sources

  • https://www.instructables.com/id/Oscilloscope-How-To/
  • http://units.folder101.com/cisco/sem1/Notes/ch7-

technologies/encoding.htm

  • https://www.tmatlantic.com/encyclopedia/index.php?ELEM

ENT_ID=10420

  • https://www.tutorialspoint.com/digital_communication/digi

tal_communication_frequency_shift_keying.htm

  • https://www.tutorialspoint.com/digital_communication/digi

tal_communication_phase_shift_keying.htm

  • http://nowiknow.com/a-chicken-tax-policy/
  • https://www.bettycrocker.com/recipes/preparation/slow-

cooker-recipes/slow-cooker-chicken-recipes