Physical and Data Link Layer Kameswari Chebrolu Dept. of Electrical - - PowerPoint PPT Presentation

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Physical and Data Link Layer Kameswari Chebrolu Dept. of Electrical - - PowerPoint PPT Presentation

Aug 31, 2022 182 likes •295 views

Physical and Data Link Layer Kameswari Chebrolu Dept. of Electrical Engineering, IIT Kanpur Problem Statement Make two computers talk to each other Pictures courtesy Peterson & Davie Hosts Communication end-points PCs,

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SLIDE 1

Physical and Data Link Layer

Kameswari Chebrolu

  • Dept. of Electrical Engineering, IIT Kanpur
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SLIDE 2

Problem Statement

  • Make two computers talk to each other

Pictures courtesy Peterson & Davie

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SLIDE 3

Hosts

Communication end-points

– PCs, Workstations, PDAs, Cellphones, Servers

Pictures courtesy Google

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SLIDE 4

Interface Cards/Network Adaptor

Attach the host to the link

Pictures courtesy Google

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SLIDE 5

Links

Carry signals from one place to other place(s) Fiber Optics Co-axial Cat5-twisted pair

Pictures courtesy Google

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SLIDE 6

Characteristics of Links

  • Data Rate
  • Loss rate
  • Delay

Link Type Typical Bandwidths Distance Twisted Pair 10-100 Mbps 100m Coaxial Cable 10-100 Mbps 200-500m Fiber Optics 100-2400 Mbps 40kms

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SLIDE 7

Shannon's Theorem

  • Signals attenuate with distance
  • Get distorted due to noise, crosstalk, fading,

multi-path

  • Signal to Noise Ratio (SNR) measures these

effects

  • C = W log_2 (1 + S/N) bits/sec
  • Data over telephone line calculation
  • W = 3300Hz – 300Hz = 3000Hz; S/N = 1000

(30db); C ~ 30kbps

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SLIDE 8

Encoding

  • Physical media transmit Analog signals
  • Modulate/demodulate:

– Encode/decode binary data into signals – E.g. Non-return to Zero (NRZ)

  • 0 as low signal and 1 as high signal

Bits NRZ 1 1 1 1 1 1 1

Picture courtesy Peterson & Davie

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SLIDE 9

Problems with NRZ

  • Consecutive 1s and 0s

– Changes the average making it difficult to detect

signals (baseline wander)

– Clock Recovery

  • Sender's and receiver clocks have to be precisely

synchronized

  • Receiver derives the clock from the received signal vis

signal transition

  • Lesser number of transitions leads to clock drift
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SLIDE 10

Alternative Encodings

  • Non-return to Zero Inverted (NRZI)

– To encode a 1, make a transition – To encode a 0, stay at the current signal – Solves problem of consecutive 1's but not 0's

  • Manchester Encoding

– Transmits XOR of the NRZ encoded data and the

clock

  • 0 is encoded as low-to-high transition, 1 as high-to-low

transition

– Only 50% efficient

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SLIDE 11

Example

Bits NRZ Clock Manchester NRZI 1 1 1 1 1 1 1

Picture courtesy Peterson & Davie