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Physical Layer
1
Analog vs. Digital

Analog: continuous values over time

Digital: discrete values with sharp change over time
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Analog vs. Digital

Can be used in three contexts: information, signal,
transmission
Digital
Analog
Text, integers,
binary strings
Voice, video
Signal
Square waves
Sine waves
Transmission
Use repeater to
boost signal
Use amplifier to
boost signal
Information
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All Information Encoded Digital




All information can be encoded in digital data format and
become a binary string
Digitizing analog data: sampling and quantization
We are focusing on digital data (binary strings) for the
purpose of this class
Benefits of everything going digital
- Digital processing, storage, transmission
- Zero distortion possible with digital storage & transmission (see later)
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Signal Decomposition

All signals can be decomposed into harmonic sine waves
=
+ 1.3 X
+ 0.56 X
+ 1.15 X
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Analog Signal vs. Digital Signal


Digital signal has a wide frequency spectrum
- Subject to strong attenuation and distortion
- Not good for long distance transmission
- Used for short distance transmission such as Ethernet
Analog signal is used for long distance transmission
- Need modulation technique (more later)
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Analog vs. Digital Transmission





Transmission: Communication of data by propagation and
processing of signals
Issue: signal distorted and attenuated over distance
Analog Transmission
- Use amplifiers to boost signal
- Amplify both signal and distortion
Digital Transmission
- Use repeaters to boost signal
• receives signal
• extracts bit pattern
• Retransmits
Benefits of digital transmission?
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Digital Signal
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Digital Signal



Digital signal
- Discrete voltage levels
Transmission is synchronous, i.e., a clock is used to
sample the signal.
- In general, the duration of one bit is equal to one or
two clock ticks
- Receiver’s clock must be synchronized with the
sender’s clock
Encoding can be done one bit at a time or in blocks of,
e.g., 4 or 8 bits.
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Encoding Example 1 :
Non-Return to Zero (NRZ)
0
1
0
0
0
1
1
0
1
.85
V
0
-.85


1 -> high signal; 0 -> low signal
Long sequences of 1’s or 0’s can cause problems:
- Sensitive to clock skew, i.e. hard to recover clock
- Difficult to interpret 0’s and 1’s
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Encoding Example 2:
Non-Return to Zero Inverted (NRZI)
0
1
0
0
0
1
1
0
1
.85
V
0
-.85


1 -> make transition; 0 -> signal stays the same
Solves the problem for long sequences of 1’s, but not
for 0’s.
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Encoding Example 3:
Ethernet Manchester Encoding
0
1
1
0
.85
V
0
-.85
.1s



Positive transition for 0, negative for 1
Transition every cycle communicates clock (but
need 2 transition times per bit)
DC balance has good electrical properties
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Analog Signal and Modulation
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Concepts with Sine Wave

Peak Amplitude (A)
- maximum strength of signal

Frequency (f) and Period (T)
- Hertz (Hz) or cycles per second
- T = 1/f

Phase ()
- Relative position in time

Wavelength ()
-  = vT
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Amplitude Shift Keying (ASK)
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
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Quadrature Amplitude Modulation (QAM)
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Medium
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Physical Media



Guided (twisted pair, fiber) vs. unguided (air, water, vacuum)
Simplex, half duplex, full duplex
Characteristics
- Bit Error Rate
- Data Rate (what is the difference between data rate & bandwidth?)
- Degradation with distance
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Transmission Channel
Considerations
Good

Bad
Every medium supports
transmission in a certain
frequency range.
- Outside this range, effects such
as attenuation, .. degrade the
signal too much

Transmission and receive
hardware will try to maximize
the useful bandwidth in this
frequency band.
Frequency
- Tradeoffs between cost, distance,
bit rate

As technology improves, these
parameters change, even for
the same wire.
- Thanks to our EE friends
Signal
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Capacity Limit

Nyquist Theorem: for a noiseless channel of
width H, the maximum capacity 2 x H baud rate.

Shannon’s theorem: for a noisy channel of and
bandwidth H and signal to noise ratio of S/N, the
maximum capability (bps) is H x log(1 + S/N)
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Copper Wire

Unshielded twisted pair
-

Two copper wires twisted - avoid antenna effect
Grouped into cables: multiple pairs with common sheath
Category 3 (voice grade) versus category 5
100 Mbit/s up to 100 m, 1 Mbit/s up to a few km
Cost: ~ 10cents/foot
Coax cables.
- One connector is placed inside the other connector
- Holds the signal in place and keeps out noise
- Gigabit up to a km

Signaling processing research pushes the capabilities
of a specific technology.
- E.g. modems, use of cat 5
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Light Transmission in Fiber
1.0
LEDs
Lasers
tens of THz
loss
(dB/km)
0.5
1.3
1.55
0.0
1000
1500 nm
(~200 Thz)
wavelength (nm)
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Fiber Types

Multimode fiber.
- 62.5 or 50 micron core carries multiple “modes”
- used at 1.3 microns, usually LED source
- subject to mode dispersion: different propagation
modes travel at different speeds
- typical limit: 1 Gbps at 100m

Single mode
-
8 micron core carries a single mode
used at 1.3 or 1.55 microns, usually laser diode source
typical limit: 1 Gbps at 10 km or more
still subject to chromatic dispersion
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Wireless Technologies


Great technology: no wires to install, convenient
mobility, ..
High attenuation limits distances.
- Wave propagates out as a sphere
- Signal strength reduces quickly (1/distance)3

High noise due to interference from other transmitters.
- Use MAC and other rules to limit interference
- Aggressive encoding techniques to make signal less sensitive
to noise


Other effects: multipath fading, security, ..
Ether has limited bandwidth
- Try to maximize its use
- Government oversight to control use
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The Frequency Spectrum is
crowded…
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