Transcript Basic Concepts - Mahmoud Youssef

Physical Layer Propagation
Chapter 3
Copyright 2003 Prentice-Hall
Panko’s Business Data Networks and Telecommunications, 4th edition
Perspective

The Physical Layer

Chapter 2 – Standards
 Standards above the physical layer

Chapter 3 – The Physical Layer
 Real connections between machines
 No messages
 Propagation effects
2
Figure 3.1: Signal and Propagation
Received Signal
(Attenuated &
Distorted)
Transmitted Signal
Propagation
Transmission Medium
Sender
Receiver
3
Figure 3.2: Analog and Binary Data
Analog Data
Binary Data
1101011000011100101
Smoothly changing
among an infinite
number of states
(voltage levels, etc.)
Abrupt Transitions
among two states
(voltage levels, etc.)
4
Quiz

Which is Analog? Which is Binary?
Gender
Thermometer
Clock
On/Off
Switch
5
Figure 3.3: Binary Data and Binary Signal
15 Volts
(0)
Clock Cycle
0
0
0 Volts
1
There are two states (in this case,
voltage levels). One, (high)
represents a 0. The other (low)
represents a 1. State is held
constant within each clock cycle.
Can jump abruptly at the end of
each cycle. One bit is sent per
clock cycle.
Transmitted
Signal
-15 Volts
(1)
6
Figure 3.4: Binary Data and Digital Signal
11
11
10
10
01
01
Client PC
00
01
00
Server
In digital transmission, there are few states (in this case, four).
Binary transmission, in which there are two states, is a special case
of digital transmission.
With four states, two information bits can be sent per clock cycle.
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Quiz

Which is Analog? Which is Digital?
On/Off
Switch
Number
Of
Fingers
Calendar
Clock
Audio CD
8
Figure 3.4: Binary Data and Digital Signal
11
11
10
Baud Rate =
# of Clock Cycles/Second
10
01
01
Client PC
00
01
00
Server
Suppose that the clock cycle is 1/10,000 second.
Then the baud rate is 10,000 baud (10 kbaud).
The bit rate will be 20 kbps (two bits/clock cycle
times 10,000 clock cycles per second).
(The bit rate gives the number of information bits per second.)
9
Bit Rate versus Baud Rate
Number of
Possible States
Bits per Clock
Cycle
If a Baud Rate is 1200 Baud,
Bit Rate is
2 (Binary)
1
1,200 bps
4
2
2,400 bps
8
3
3,600 bps
16
4
4,800 bps
Each Doubling of States Gives One More Bit per Clock Cycle
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Perspective

Analog Data



Smooth changes among an infinite number of
states—like hands going around an analog clock
Digital Data

Few states

In a digital clock, each position can be in one of
ten states (the digits 0 through 9)
Binary Data

Two states (a special case of digital)
11
Figure 3.5: Using a Modem to Send Binary
Data Over an Analog Transmission Line
Modulated Analog
Signal
Binary Data
Telephone
1010010101
PSTN
Modem
Computer
Amplitude (Loudness or Intensity) Modulation
1
0
1
1
12
Figure 3.6: Sending Analog Data Over a
Digital Line
Encoding
110010101
Digital
Transmission
Line
Analog
Data
100001101
Decoding
Many Time Periods
So Fairly Smooth
13
Figure 3.7: 4-Pair Unshielded Twisted Pair
Cable with RJ-45 Connector
Single Twisted Pair
Four pairs (each pair is twisted) are
enclosed in a jacket.
Jacket
14
Figure 3.7: 4-Pair Unshielded Twisted Pair
Cable with RJ-45 Connector
The cord terminates in an 8-pin
RJ-45 connector, which plugs
into an RJ-45 jack in the NIC,
hub, or switch.
Pin 1 on this side
No metal
Shielding around
4 pairs
RJ-45
Connector
RJ-45
Jack
15
Figure 3.7: 4-Pair Unshielded Twisted
Pair Cable with RJ-45 Connector
UTP Cord
RJ-45
Connector
16
Figure 3.7: 4-Pair Unshielded Twisted
Pair Cable with RJ-45 Connector
With
RJ-45
Connector
Pen
4 Pairs
Separated
17
Figure 3.8: Noise and Attenuation
Power
Noise Floor
(average)
Signal
Noise
Distance
18
Figure 3.8: Noise and Attenuation
Power
Signal
Noise Spike
Damage
Noise
Noise Floor
(average)
Distance
19
Figure 3.8: Noise and Attenuation
SNR = Signal Power / Noise Power
If SNR is high, noise errors are rare
As signal travels, it attenuates,
and noise errors increase
Power
Signal
Signalto-Noise
Ratio
(SNR)
Noise Floor
(average)
Noise
Distance
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Noise and Attenuation

The TIA/EIA-568 standard recommends that
UTP runs be kept to 100 meters

If this distance limit is observed, problems with
noise and attenuation usually are minor

Low-tech solution, but it works well.
21
Figure 3.9: Twisting Wire Paris to Reduce
Electromagnetic Interference (EMI)
Interference
Twisted
Wire
Interference On the Two Halves of a Twist Cancels Out
22
Figure 3.10: Crosstalk Electromagnetic Interference
(EMI) and Terminal Crosstalk Interference
Untwisted
at Ends
Signal
Crosstalk Interference
Terminal Crosstalk
Interference
23
Figure 3.10: Crosstalk Electromagnetic Interference
(EMI) and Terminal Crosstalk Interference

EMI is any interference from outside

Twisting each pair reduces EMI.

Signals in adjacent pairs interfere with one
another (crosstalk interference).

Crosstalk interference is worst at the ends,
where the wires are untwisted. This is terminal
crosstalk interference.

Solution: buy good wire; untwist wires for
connector no more than 1.25 cm (0.5 in)
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Figure 3.11: Serial versus Parallel
Transmission
Serial
Transmission
(1 bit per clock cycle)
Parallel Transmission
(1 bit per clock cycle
per wire pair)
4 bits per clock cycle
on 4 pairs
25
Figure 3.11: Serial versus Parallel
Transmission

Serial Transmission: One Bit Per Clock Cycle

Parallel Transmission with N wire pairs: N bits
per clock cycle


Not limited to four wire pairs
Parallel transmission is faster than serial
transmission
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Figure 3.12: Optical Fiber Cabling
Cladding
Light
Source
(LED or
Laser)
Core
Light
Ray
Reflection at Core/Cladding Boundary
27
Figure 3.13: Wavelength Division
Multiplexing (WDM) in Optical Fiber
Light
Source 1
Optical Fiber Core
Light
Source 2
Multiple Light Sources Transmit on Different Wavelengths
Each Carries a Separate Signal
More Capacity Per Fiber
28
Figure 3.14:Full-Duplex Optical Fiber Cord
SC, ST, or other
connector
Fiber
Switch
Fiber
Router
A pair of fibers is needed for full-duplex (simultaneous 2-way) transmission.
Each carries a signal in only one direction.
29
Optical Fiber Cabling
Two fiber cords for
full-duplex (twoway) transmission
ST
Connectors
(Popular)
SC
Connectors
(Recommended)
30
Figure 3.15: Multimode & Single-Mode
Fiber
Graded Index of Refraction
(Decreasing from Center)
Light
Source
Cladding
Modes
Core
Multimode Fiber
Light travels faster at the edges to speed modes going the farthest
31
Figure 3.15: Multimode & Single-Mode
Fiber
Single Mode
Cladding
Core
Light
Source
Single Mode Fiber
Core is so thin that only one mode can propagate.
No modal dispersion, so can span long distances without distortion.
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Figure 3.16: Omnidirectional and Dish
Antennas
Omnidirectional Antenna
Dish Antenna
No need to point
to sender or receiver
Rapid attenuation
with distance
Concentrates incoming
and outgoing signals
Signals can travel far
33
Figure 3.17: Radio Wave
Wavelength
Amplitude
Frequency
Measured in Hz (Cycles per Second)
2 cycles in one second so 2 Hz
Wavelength * Frequency = Speed of Propagation
34
Figure 3.18: The Frequency Spectrum,
Service Bands, and Channels
Frequency
Spectrum
(0 Hz
to infinity)
Channel 5
A service band has a specific
purpose, such as FM radio or
cellular telephony.
Channel 4
Service
Band
Channel 3
Channel 2
Channel 1
0 Hz
Service bands are divided into
channels. Signals sent in
different channels do not
interfere with one
another.
Channels with wider bandwidths
can carry signals faster.
35
Figure 3.18: The Frequency Spectrum,
Service Bands, and Channels

Shannon’s Law

Here
W = B * Log2 (1 + S/N)

W = maximum possible speed in channel

B = bandwidth (highest frequency minus lowest
frequency)

S/N = signal to noise ratio

Wide bandwidth (broadband) gives high speed

Small bandwidth (narrow band) gives low speeds
36
Figure 3.19: Wireless Propagation
Problems
Inverse Square
Law Attenuation
Laptop
Comm. Tower
Very Rapid Attenuation with Distance
Compared to Wires and Fiber
37
Figure 3.19: Wireless Propagation
Problems
Shadow
Zone
Laptop
Comm. Tower
No
Signal
38
Figure 3.19: Wireless Propagation
Problems
Multipath
Interference
Laptop
Comm. Tower
Signals Arriving by Different Paths
May Cancel Out
39
Figure 3.20: Major Topologies

A network technology’s topology is how stations and
switches are connected physically
Point-to-Point
The Simplest Topology
40
Figure 3.20: Major Topologies
Star (Modern Ethernet)
Extended Star or Hierarchy
(Modern Ethernet)
Root
Only one possible
path between two
stations
41
Figure 3.20: Major Topologies
Mesh (Routers, Frame Relay, ATM)
A
Path
ABD
B
C
D
Multiple alternative
paths between two
stations
Path
ACD
42
Figure 3.20: Major Topologies
Ring (802.5, FDDI, SONET/SDH)
Only one possible
path between two
stations
43
Figure 3.20: Major Topologies
Daisy Chain Bus
(Ethernet 10Base2)
Multidrop Line Bus
(Ethernet 10Base5)
Transmit
Transmit
Only one possible path
between two stations
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