Physical Layer Lecture Progression Bottom-up through the layers: - - PowerPoint PPT Presentation

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Physical Layer Lecture Progression Bottom-up through the layers: - - PowerPoint PPT Presentation

Jan 22, 2023 458 likes •1.03k views

Physical Layer Lecture Progression Bottom-up through the layers: Application - HTTP, DNS, CDNs Transport - TCP, UDP Network - IP, NAT, BGP Link - Ethernet, 802.11 Physical - wires, fiber, wireless Followed by

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SLIDE 1

Physical Layer

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SLIDE 2

Lecture Progression

  • Bottom-up through the layers:
  • Followed by more detail on:
  • Quality of service, Security (VPN, SSL)

Computer Networks 2

Application - HTTP, DNS, CDNs Transport - TCP, UDP Network - IP, NAT, BGP Link

  • Ethernet, 802.11

Physical

  • wires, fiber, wireless
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SLIDE 3

Where we are in the Course

  • Beginning to work our way up starting with the

Physical layer

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Physical Link Network Transport Application

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SLIDE 4

Scope of the Physical Layer

  • Concerns how signals are used to transfer message

bits over a link

  • Wires etc. carry analog signals
  • We want to send digital bits

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…10110

10110… Signal

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SLIDE 5

Topics

1. Coding and Modulation schemes

  • Representing bits, noise

2. Properties of media

  • Wires, fiber optics, wireless, propagation
  • Bandwidth, attenuation, noise

3. Fundamental limits

  • Nyquist, Shannon

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SLIDE 6

Coding and Modulation

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SLIDE 7

Topic

  • How can we send information across a link?
  • This is the topic of coding and modulation
  • Modem (from modulator–demodulator)

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…10110

10110…

Signal

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SLIDE 8

A Simple Coding

  • Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0
  • This is called NRZ (Non-Return to Zero)

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Bits NRZ

1 1 1 1 1 1 1 +V

  • V
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SLIDE 9

A Simple Modulation (2)

  • Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0
  • This is called NRZ (Non-Return to Zero)

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Bits NRZ

1 1 1 1 1 1 1 +V

  • V
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SLIDE 10

A Simple Modulation (3)

  • Problems?
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SLIDE 11

Many Other Schemes

  • Can use more signal levels
  • E.g., 4 levels is 2 bits per symbol
  • Practical schemes are driven by engineering

considerations

  • E.g., clock recovery

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SLIDE 12

Clock Recovery

  • Um, how many zeros was that?
  • Receiver needs frequent signal transitions to decode bits
  • Several possible designs
  • E.g., Manchester coding and scrambling (§2.5.1)

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1 0 0 0 0 0 0 0 0 0 … 0

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SLIDE 13

Ideas?

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Answer 1: A Simple Coding

  • Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0
  • Then go back to 0V for a “Reset”
  • This is called RZ (Return to Zero)

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Bits RZ

1 1 1 1

  • V

+V

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SLIDE 15

Answer 2: Clock Recovery – 4B/5B

  • Map every 4 data bits into 5 code bits without long

runs of zeros

  • 0000  11110, 0001  01001, 1110  11100, …

1111  11101

  • Has at most 3 zeros in a row
  • Also invert signal level on a 1 to break up long runs of 1s

(called NRZI, §2.5.1)

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Answer 2: Clock Recovery – 4B/5B (2)

  • 4B/5B code for reference:
  • 000011110, 000101001, 111011100, …

111111101

  • Message bits: 1 1 1 1 0 0 0 0 0 0 0 1

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Coded Bits: Signal:

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SLIDE 17

Clock Recovery – 4B/5B (3)

  • 4B/5B code for reference:
  • 000011110, 000101001, 111011100, …

111111101

  • Message bits: 1 1 1 1 0 0 0 0 0 0 0 1

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Coded Bits: Signal: 1 1 1 0 1 1 1 1 1 0 0 1 0 0 1

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Modulation vs Coding

  • What we have seen so far is called coding
  • Signal is sent directly on a wire
  • These signals do not propagate well as RF
  • Need to send at higher frequencies
  • Modulation carries a signal by modulating a carrier
  • Baseband is signal pre-modulation
  • Keying is the digital form of modulation (equivalent to

coding but using modulation)

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Passband Modulation (2)

  • Carrier is simply a signal oscillating at a desired

frequency:

  • We can modulate it by changing:
  • Amplitude, frequency, or phase

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SLIDE 20

Comparisons

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NRZ signal of bits Amplitude shift keying Frequency shift keying Phase shift keying

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SLIDE 21

Philosophical Takeaways

  • Everything is analog, even digital signals
  • Digital information is a discrete concept

represented in an analog physical medium ○ A printed book (analog) vs. ○ Words conveyed in the book (digital)

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SLIDE 22

Simple Link Model

  • We’ll end with an abstraction of a physical channel
  • Rate (or bandwidth, capacity, speed) in bits/second
  • Delay in seconds, related to length
  • Other important properties:
  • Whether the channel is broadcast, and its error rate

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Delay D, Rate R Message

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Message Latency

  • Latency is the delay to send a message over a link
  • Transmission delay: time to put M-bit message “on the wire”
  • Propagation delay: time for bits to propagate across the wire
  • Combining the two terms we have:

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Message Latency (2)

  • Latency is the delay to send a message over a link
  • Transmission delay: time to put M-bit message “on the wire”

T-delay = M (bits) / Rate (bits/sec) = M/R seconds

  • Propagation delay: time for bits to propagate across the wire

P-delay = Length / speed of signals = Length / ⅔c = D seconds

  • Combining the two terms we have: L = M/R + D

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Latency Examples

  • “Dialup” with a telephone modem:
  • D = 5 ms, R = 56 kbps, M = 1250 bytes
  • Broadband cross-country link:
  • D = 50 ms, R = 10 Mbps, M = 1250 bytes

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Remembering L = M/R + D

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Latency Examples (2)

  • “Dialup” with a telephone modem:
  • D = 5 ms, R = 56 kbps, M = 1250 bytes
  • L = (1250x8)/(56 x 103) sec + 5ms = 184 ms!
  • Broadband cross-country link:
  • D = 50 ms, R = 10 Mbps, M = 1250 bytes
  • L = (1250x8) / (10 x 106) sec + 50ms = 51 ms
  • A long link or a slow rate means high latency: One component

dominates

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Bandwidth-Delay Product

  • Messages take space on the wire!
  • The amount of data in flight is the bandwidth-delay

(BD) product

BD = R x D

  • Measure in bits, or in messages
  • Small for LANs, big for “long fat” pipes

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SLIDE 28

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Bandwidth-Delay Example

  • Fiber at home, cross-country

R=40 Mbps, D=50 ms

110101000010111010101001011

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SLIDE 29

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Bandwidth-Delay Example (2)

  • Fiber at home, cross-country

R=40 Mbps, D=50 ms BD = 40 x 106 x 50 x 10-3 bits = 2000 Kbit = 250 KB

  • That’s quite a lot of data in

the network”!

110101000010111010101001011

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SLIDE 30

Media

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SLIDE 31

https://www.merriam-webster.com/dictionary/media

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Types of Media

  • Media propagate signals that carry bits of

information

  • We’ll look at some common types:
  • Wires
  • Fiber (fiber optic cables)
  • Wireless

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Wires – Twisted Pair

  • Very common; used in LANs and telephone lines
  • Twists reduce radiated signal

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Category 5 UTP cable with four twisted pairs

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SLIDE 34

Wires – Coaxial Cable

  • Also common. Better shielding for better performance
  • Other kinds of wires too: e.g., electrical power (§2.2.4)

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SLIDE 35

Fiber

  • Long, thin, pure strands of glass
  • Enormous bandwidth (high speed) over long distances

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Light source (LED, laser) Photo- detector Light trapped by total internal reflection Optical fiber

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SLIDE 36

Fiber (2)

  • Two varieties: multi-mode (shorter links, cheaper)

and single-mode (up to ~100 km)

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Fiber bundle in a cable One fiber

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SLIDE 37

Signals over Fiber

  • Light propagates with very low loss in three very

wide frequency bands

  • Use a carrier to send information

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Wavelength (μm) Attenuation (dB/km)

By SVG: Sassospicco Raster: Alexwind, CC-BY-SA-3.0, via Wikimedia Commons

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SLIDE 38

Wireless

  • Sender radiates signal over a region
  • In many directions, unlike a wire, to potentially many

receivers

  • Nearby signals (same freq.) interfere at a receiver; need to

coordinate use

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SLIDE 39

Wireless Interference

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SLIDE 40

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WiFi WiFi

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SLIDE 41

Wireless (2)

  • Unlicensed (ISM) frequencies, e.g., WiFi, are widely

used for computer networking

802.11 b/g/n 802.11a/g/n

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SLIDE 42

Multipath (3)

  • Signals bounce off objects and take multiple paths
  • Some frequencies attenuated at receiver, varies with

location

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Wireless (4)

  • Various other effects too!
  • Wireless propagation is complex, depends on

environment

  • Some key effects are highly frequency dependent,
  • E.g., multipath at microwave frequencies

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SLIDE 44

Limits

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SLIDE 45

Topic

  • How rapidly can we send information over a link?
  • Nyquist limit (~1924)
  • Shannon capacity (1948)
  • Practical systems are devised to approach these

limits

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SLIDE 46

Key Channel Properties

  • The bandwidth (B), signal strength (S), and noise (N)
  • B (in hertz) limits the rate of transitions
  • S and N limit how many signal levels we can distinguish

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Bandwidth B Signal S, Noise N

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SLIDE 47

Nyquist Limit

  • The maximum symbol rate is 2B
  • Thus if there are V signal levels, ignoring noise, the

maximum bit rate is:

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R = 2B log2V bits/sec

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

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SLIDE 48

Claude Shannon (1916-2001)

  • Father of information theory
  • “A Mathematical Theory of

Communication”, 1948

  • Fundamental contributions

to digital computers, security, and communications

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Credit: Courtesy MIT Museum

Electromechanical mouse that “solves” mazes!

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Shannon Capacity

  • How many levels we can distinguish depends on S/N
  • Or SNR, the Signal-to-Noise Ratio
  • Note noise is random, hence some errors
  • SNR given on a log-scale in deciBels:
  • SNRdB = 10log10(S/N)

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1 2 3 N S+N

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Shannon Capacity (2)

  • Shannon limit is for capacity (C), the maximum

information carrying rate of the channel:

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C = B log2(1 + S/N) bits/sec

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Shannon Capacity Takeaways

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C = B log2(1 + S/N) bits/sec

  • There is some rate at which we can transmit data

without loss over a random channel

  • Assuming noise fixed, increasing the signal power

yields diminishing returns : (

  • Assuming signal is fixed, increasing bandwith

increases capacity linearly!

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Wired/Wireless Perspective (2)

  • Wires, and Fiber
  • Engineer link to have requisite SNR and B

→Can fix data rate

  • Wireless
  • Given B, but SNR varies greatly, e.g., up to 60 dB!

→Can’t design for worst case, must adapt data rate

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Engineer SNR for data rate Adapt data rate to SNR

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SLIDE 53

Putting it all together – DSL

  • DSL (Digital Subscriber Line, see §2.6.3) is widely

used for broadband; many variants offer 10s of Mbps

  • Reuses twisted pair telephone line to the home; it has up

to ~2 MHz of bandwidth but uses only the lowest ~4 kHz

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DSL (2)

  • DSL uses passband modulation (called OFDM)
  • Separate bands for upstream and downstream (larger)
  • Modulation varies both amplitude and phase (QAM)
  • High SNR, up to 15 bits/symbol, low SNR only 1 bit/symbol

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Upstream Downstream 26 – 138 kHz 0-4 kHz 143 kHz to 1.1 MHz Telephone Freq. Voice Up to 1 Mbps Up to 12 Mbps

ADSL2:

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Phy Layer Innovation Still Happening!

  • Backscatter “zero power” wireless
  • mm wave 30GHz+ radio equipment
  • Free space optical (FSO)
  • Cooperative interference management
  • Massive MIMO and beamforming
  • Powerline Networking
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All distilled to a simple link model

  • Rate (or bandwidth, capacity, speed) in bits/second
  • Delay in seconds, related to length
  • Other important properties:
  • Whether the channel is broadcast, and its error rate

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Delay D, Rate R Message