chapter 2 the physical layer 1 the lowest layer of reference model. it defines the mechanical,...
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Chapter 2 The Physical Layer
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The lowest layer of reference model. It defines the mechanical, electrical, and timing interfaces to the network.
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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.
NOISELESS CHANNEL: NYQUIST BIT RATE
Lbandwidthratebit 2log2
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L is the number of signal levels used to represent data.
Increasing the levels of a signal may reduce the reliability of the system.
NOISY CHANNEL: SHANNON CAPACITY
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)1(log
)power Noise
power signal1(log
2
2
N
SBC
bandwidthCapacity
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.
The theoretical highest data rate for a noisy channel
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
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WIRELESSTRANSMISSION
• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission
TWISTED PAIRS
Category 5e UTP cable with four twisted pairs 7
COAXIAL CABLE
A coaxial cable 8
POWER LINES
A network that uses household electrical wiring. 9
FIBER CABLES
(a) Side view of a single fiber.(b) End view of a sheath with three fibers.
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DIGITAL SUBSCRIBER LINES (3)
A typical ADSL equipment configuration.
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FIBER TO THE HOME
Passive optical network for Fiber To The Home. 12
MODEMS
(a) A binary signal (b) Amplitude modulation
(c) Frequency modulation(d) Phase modulation
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SIGNAL ENCODING TECHNIQUES
Digital data, digital signal(Ethernet)
Analog data, digital signal(PCM)Digital data, analog signal(ADSL)Analog data, analog signal(phone)
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LINE CODING SCHEMES
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LINE CODES
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POLAR (+ AND – VOLTAGES) NRZ
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BIPHASE Manchester
Transition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3
Differential ManchesterMid-bit transition is clocking onlyTransition at start of a bit period represents zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5
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MANCHESTER ENCODING
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4.2
0
Effect of lack of synchronization
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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BIPOLAR (+, 0, - VOLTAGES) SCHEMES
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Alternate Mark Inversion (AMI): 1s are represented by alternating positive and negative voltages.
SIMPLE CIRCUIT SWITCHED NETWORK
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PACKET SWITCHED NETWORK
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