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

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

Jan 06, 2024 632 likes •1.08k views

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

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Physical Layer

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Lecture Progression

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

Computer Networks 2

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

  • Ethernet, 802.11

Physical

  • wires, fjber, wireless
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Where we are in the Course

  • Beginning to work our way up startjng with the

Physical layer

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Physical Link Network Transport Applicatjon

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T

  • pics

1. Coding and Modulatjon schemes

  • Representjng bits, noise

2. Propertjes of media

  • Wires, fjber optjcs, wireless, propagatjon
  • Bandwidth, atuenuatjon, noise

3. Fundamental limits

  • Nyquist, Shannon

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Coding and Modulation

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T

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

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…10110

10110…

Signal

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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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A Simple Modulation (3)

  • Problems?
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Many Other Schemes

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

consideratjons

  • E.g., clock recovery

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Clock Recovery

  • Um, how many zeros was that?
  • Receiver needs frequent signal transitjons 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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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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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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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
  • Modulatjon carries a signal by modulatjng a carrier
  • Baseband is signal pre-modulatjon
  • Keying is the digital form of modulatjon (equivalent to

coding but using modulatjon)

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Comparisons

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

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Philosophical T akeaways

  • Everything is analog, even digital signals
  • Digital informatjon 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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Simple Link Model

  • We’ll end with an abstractjon of a physical channel
  • Rate (or bandwidth, capacity, speed) in bits/second
  • Delay in seconds, related to length
  • Other important propertjes:
  • 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: tjme to put M-bit message “on the wire”
  • Propagatjon delay: tjme 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: tjme to put M-bit message “on the wire”

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

  • Propagatjon delay: tjme 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 fmight 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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Media

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T ypes of Media

  • Media propagate signals that carry bits of

informatjon

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

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Wires – T wisted 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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Wires – Coaxial Cable

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

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Fiber (2)

  • Two varietjes: multj-mode (shorter links, cheaper)

and single-mode (up to ~100 km)

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Fiber bundle in a cable One fjber

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Multipath (3)

  • Signals bounce ofg objects and take multjple paths
  • Some frequencies atuenuated at receiver, varies with

locatjon

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Wireless (4)

  • Various other efgects too!
  • Wireless propagatjon is complex, depends on

environment

  • Some key efgects are highly frequency dependent,
  • E.g., multjpath at microwave frequencies

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