Chapter 2 The Physical Layer

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• Chapter 2 The Physical Layer

The lowest layer of reference model. It defines
the mechanical, electrical, and timing interfaces
to the network.
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Bandwidth-Limited Signals

• A binary signal and its root-mean-square Fourier
  amplitudes.
• (b) (c) Successive approximations to the
  original signal.

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Bandwidth-Limited Signals (2)

• (d) (e) Successive approximations to the
  original signal.

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BANDWIDTH AND INFORMATION CAPACITY
Bandwidth is the span of frequencies within the
spectrum occupied by a signal and used by the
signal for conveying information. Carrying
information requires bandwidth.
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Noiseless Channel Nyquist Bit Rate
L is the number of signal levels used to
represent data.
Increasing the levels of a signal may reduce the
reliability of the system.
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Noisy Channel Shannon Capacity
The theoretical highest data rate for a noisy
channel
where capacity is in bits/second, bandwidth is in
hertz, and signal and noise powers are measured
in the same physical units, such as watts. Bits
are fundamental units of information.
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Using both limits

• The Shannon capacity gives us the upper limit
• the Nyquist formula tells us how many signal
  levels we need.

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Guided Transmission Data

• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics

WirelessTransmission

• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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Structure of the Telephone System
Public Switched Telephone System

• (a) Fully-interconnected network.
• (b) Centralized switch.
• (c) Two-level hierarchy.

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Signal Encoding Techniques

1. Digital data, digital signal
2. Analog data, digital signal
3. Digital data, analog signal
4. Analog data, analog signal

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Encoding Schemes
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NRZ
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Biphase

• Manchester
• Transition in middle of each bit period
• Transition serves as clock and data
• Low to high represents one
• High to low represents zero
• Used by IEEE 802.3
• Differential Manchester
• Mid-bit transition is clocking only
• Transition at start of a bit period represents
  zero
• No transition at start of a bit period represents
  one
• Note this is a differential encoding scheme
• Used by IEEE 802.5

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Manchester Encoding
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• TQ 6. The waveform of following figure belongs to
  a Manchester encoded binary data stream.
  Determine the beginning and end of bit periods
  (i.e., extract clock information) and give the
  data sequence.

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Modulation Rate
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Digital Data, Analog Signal

• Public telephone system
• 300Hz to 3400Hz
• Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PK)

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Modulation Techniques
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Modems (2)
Constellation Diagrams

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

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Modems (3)
(b)
(a)

• (a) V.32 for 9600 bps.
• (b) V32 bis for 14,400 bps.

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Multiplexing
Frequency Division MultiplexingWavelength
Division MultiplexingTime Division Multiplexing
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Simple Circuit Switched Network
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Packet Switching
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Basic Operation

• Data transmitted in small packets
• Typically 1000 octets (bytes)
• Longer messages split into series of packets
• Each packet contains a portion of user data plus
  some control information
• Control information
• Routing (addressing) information
• Packets are received, stored briefly (buffered)
  and past on to the next node
• Store and forward

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Advantages

• Line efficiency
• Single node to node link can be shared by many
  packets over time
• Packets queued and transmitted as fast as
  possible
• Data rate conversion
• Each station connects to the local node at its
  own speed
• Nodes buffer data if required to equalize rates
• Packets are accepted even when network is busy
• Delivery may slow down
• Priorities can be used