chapter 2: the physical layermaccabe/classes/585/fall02/chap0...chapter 2 – p.13/74 topics...
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Chapter 2: The Physical LayerComputer Networks
Maccabe
Computer Science DepartmentThe University of New Mexico
August 2002
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.2/74
Theoretical Foundations
Fourier Analysis
Bandwidth-Limited Signals
The Maximum Data Rate of a Channel
Chapter 2 – p.3/74
Fourier Analysis
Any well behaved periodic function, g�
t�
, with periodT can be expressed as:
g
�
t
�
� 12
c∞
∑n �1
an sin
�
2πn f t� ∞
∑n �1
bn cos
�
2πn f t
�
Fundamental frequency
f � 1T
each (collective) term is called a harmonic
Chapter 2 – p.4/74
Fourier Analysis (2)
Solving for an, bn, and c
an
� 2T
T
0g
�
t�
sin�
2πn f t
�
dt
bn
� 2T
T
0g
�t
�
cos
�
2πn f t
�
dt
c � 2T
T
0g
�
t
�
dt
Chapter 2 – p.5/74
Example: Sending “b”
0 1 1 0 0 0 1 01
0 Time T
pretend that the pattern repeats:
an
� 1πn
�
cos
�
πn
�
4
�
� cos�
3πn
�
4
� �
cos
�
6πn
�
4
�
� cos
�
7πn
�
4
��
bn
� 1πn
�
sin
�
3πn�
4�
� sin
�
πn
�
4
� �
sin
�
7πn
�
4
�
� sin
�
6πn
�
4
��
c � 3
�
4
Chapter 2 – p.6/74
Sending “b” – One Harmonic
0 1 1 0 0 0 1 01
0 Time T
1
0
rms
ampl
itude
1 152 3 4 5 6 7 9 10111213 148
0.500.25
Harmonic number
1 harmonic
1
(a)
(b)
Chapter 2 – p.7/74
Sending “b” – Two Harmonics
0 1 1 0 0 0 1 01
0 Time T
rms
ampl
itude
1 152 3 4 5 6 7 9 10111213 148
0.500.25
Harmonic number(a)
1
0
2 harmonics
1 2(c)
Chapter 2 – p.8/74
Sending “b” – Four Harmonics
0 1 1 0 0 0 1 01
0 Time T
rms
ampl
itude
1 152 3 4 5 6 7 9 10111213 148
0.500.25
Harmonic number(a)
1
0
4 harmonics
1 2 3 4(d)
Chapter 2 – p.9/74
Sending “b” – Eight Harmonics
0 1 1 0 0 0 1 01
0 Time T
rms
ampl
itude
1 152 3 4 5 6 7 9 10111213 148
0.500.25
Harmonic number(a)
1
0Time
8 harmonics
1 2 3 4 5 6 7 8Harmonic number
(e)
Chapter 2 – p.10/74
Bandwidth-Limited Signals
Bandwidth – range of frequencies transmittedwithout significant power lossDistortion – different Fourier components may bediminished at different ratesFrequency of the highest harmonic
Desired bit rate of b bits/sec (e.g., 2400 bps)
T � 8
�
b sec (e.g., 3.33 milliseconds)
highest harmonic = 1/T or 1/(8/b) Hz (e.g., 1/(8/2400) =
300Hz)
Voice-grade line – filter at about 3100Hzlimits bps to 24,800
Chapter 2 – p.11/74
Data Rates and Harmonics
First # HarmonicsBps T(msec) Harmonic (Hz) Sent
300 26.67 37 80600 13.33 75 40
1200 6.67 150 202400 3.33 300 104800 1.67 600 59600 0.83 1200 2
19200 0.42 2400 138400 0.21 4800 0
Chapter 2 – p.12/74
Maximum Data Rate of a Channel
Nyquist, 1924: noiseless channellow-pass filter of bandwidth H
signal can be reconstructed using 2H samples per second
Nyquist’s theorem: maximum data rate � 2H log2V bps
(V is the number of discrete levels)
Shannon, 1948: random (thermodynamic) noisesignal-to-noise ratio S
�N
decibel: 10log10 S�
N
Shannon’s theorem:
maximum data rate � H log2
�
1
�
S
�
N
�
bps
3000-Hz, 30 dB implies 30,000 bps
Chapter 2 – p.13/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.14/74
Magnetic Media
Sneaker Net
Great bandwidth, poor latency
Chapter 2 – p.15/74
UTP (Unshielded Twisted Pair)
waves from different twists cancelmore twists per centimeter
less crosstalk
better quality over longer distanceCategories
Category Bandwidth
3 16 MHz
5 100 MHz
6 250 MHz
7 600 MHz
(a) (b)
(a) Category 3, (b) Category 5 Chapter 2 – p.16/74
Coax (Coaxial Cable)
1 GHz Bandwidth for modern cables
Copper core
Insulating material
Braided outer conductor
Protective plastic covering
Chapter 2 – p.17/74
Fiber Optics
Rant about growth rates
Basics
Transmission of light
Fiber Cables
Fiber Optic Networks
Fiber and Copper
Chapter 2 – p.18/74
Growth Rates, an asideOr, you think you have it good
Computing1981: 4.77 MHz (original IBM PC)
2001: 2 GHz
20x / decade!
Communication1981: 56 kbps (ARPANET)
2001: 1 Gbps
125x / decade!!!
Lies, damn lies, and statistics . . .Communication has just become commodity
Parallel computing is the answer to the limit
Chapter 2 – p.19/74
Basics
light source, transmission medium, detector
refraction and reflectionTotal internal reflection.
Air/silica boundary
Light sourceSilica
Air
(a) (b)
β1 β2 β3
α1 α2 α3
single-mode (wave guide): one wavelength of light
multi-mode: multiple wavelengths
Chapter 2 – p.20/74
Transmission of Light
Attenuation in decibels � 10log10transmitted power
received power
0.80 0.9
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1.0 1.1 1.2 1.3Wavelength (microns)
0.85µ Band
1.30µ Band
1.55µ Band
Atte
nuat
ion
(dB
/km
)
1.4 1.5 1.6 1.7 1.8
chromatic dispersionlower the signalling rateshaped pulses (solitons) Chapter 2 – p.21/74
Fiber Cables
StructureJacket
(plastic) Core Cladding
Sheath Jacket
Cladding (glass)
Core (glass)
(a) (b)
core: 8–10 micronscladding: keep light in the corejacket: protects cladding
Connecting cablesFiber sockets (10-20% loss)Mechanical splices (10% loss)Fused splices (minimal loss)
Chapter 2 – p.22/74
Fiber Cables – LEDs and Semicon-ductor Lasers
Item LED Laser
Data rate Low HighFiber type Multimode Multimode or
single modeDistance Short LongLifetime Long ShortTemp sensitivity Minor SubstantialCost Low High
Chapter 2 – p.23/74
Fiber Optic Networks
Fiber taps are not so easy
passive and active taps
To/from computer
Direction of light propagation
Optical transmitter
(LED)
Signal regenerator (electrical)
Optical receiver
(photodiode)
Copper wire
Fiber
Optical fiber Interface
Computer
Detail of interface
Chapter 2 – p.24/74
Fiber Optic Networks–Star Network
Receiver
Transmitter
Each incoming fiber illuminates the whole star
Each outgoing fiber sees light from all the incoming fibers
Computer interfaces
Chapter 2 – p.25/74
Fiber and Copper
Advantages of fiberhigher bandwidth
fewer repeaters
lighter
thiner
more secure (doesn’t leak and is harder to tap)
Disadvantagesnew technology
one-way communication
interfaces are more expensive
Chapter 2 – p.26/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.27/74
Electromagnetic Spectrum
λ f � c100 102 104 106 108 1010 1012 1014 1016 1018 1020 1022 1024
Radio Microwave Infrared UV X-ray Gamma ray
f (Hz)
Visible light
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016
f (Hz)
Twisted pair
Coax
Satellite
TV
Terrestrial microwave
Fiber
optics
MaritimeAM
radioFM
radio
Band LF MF HF VHF UHF SHF EHF THF
Chapter 2 – p.28/74
Wide band
Most transmission use a narrow frequency band tooptimize reception
Frequency hopping spread spectrum
change frequencies hundreds of times per second
security
avoids multipath fading
Direct sequence spread spectrum
spread the signal over a wide frequency band
used in cell phones
Chapter 2 – p.29/74
Radio Waves
Low frequencylow bandwidth
penetrate buildings
follow curvature of earth
omnidirectional
Higher frequenciesbad for life forms
straight line
bounce off of obstacles
absorbed by rain
Chapter 2 – p.30/74
Radio Transmission
erehpsonoI
Earth's surface Earth's surface
(a) (b)
Ground wave
Chapter 2 – p.31/74
Politics
beauty contest
ISM: Industrial, Scientific, Medical
Freq.
Bandwidth
902 MHz
928 MHz
26 MHz
2.4 GHz
2.4835 GHz
5.735 GHz
5.860 GHz
. . . . . .
. . . . . .
83.5 MHz
125 MHz
Chapter 2 – p.32/74
Others
Infared – doesn’t penetrate wallsLightwave
old light signals
lasers
� �� �
� �� �
� �� �
� �
Laser beam misses the detector
Laser
Photodetector Region of turbulent seeing
Heat rising off the building
Chapter 2 – p.33/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.34/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.35/74
Public Switched Telephone Network
Structure
Politics (skip)
The Local Loop
Trunks and Multiplexing
Switching
Chapter 2 – p.36/74
Network Structure
(a) (b) (c)
minimize number of wires
add multiple levels
Chapter 2 – p.37/74
Typical Circuit
Telephone
End
office
Toll
office
Intermediate switching office(s)
Telephone
End
office
Toll
office
Local loop
Toll connecting
trunk
Very high bandwidth
intertoll trunks
Toll connecting
trunk
Local loop
local loops
trunks
switching offices
Chapter 2 – p.38/74
The Local Loop
Modems
(A)DSL
Wireless
Chapter 2 – p.39/74
Modems
Analog and digital transmission
Sine wave carrier
Baud
Phase shift keying
Limits
Chapter 2 – p.40/74
Analog and Digital Transmission
Toll office
Toll office
Toll office
End office
Codec
Codec
Modem bank
ISP 1
ISP 2
Digital line
Up to 10,000 local loops
High-bandwidth trunk (digital, fiber)
Medium-bandwidth trunk
(digital, fiber)
Computer
Modem
Local loop (analog, twisted pair)
modem – modulator, demodulatorcodec – coder, decoder
Chapter 2 – p.41/74
Modems – Sine Wave Carrier
0 1 0 1 1 0 0 1 0 0 1 0 0
(a)
(b)
(c)
(d)
Phase changes
(a) binary, (b) AM, (c) FM, (d) phase modulationChapter 2 – p.42/74
Baud and Symbols
Baud rate is the sampling rate
Baud is the time to read one symbol
When the number of symbols is 2, the baud rate isthe bit rate
Modern modems use large sets of symbols
Chapter 2 – p.43/74
Quadrature Phase Shift Keying
270
(a)
90
0 180
270
(b)
90
0
270
(c)
90
0 180
Constellation diagrams
Amplitude (distance from origin)
Phase
QAM: Quadrature Amplitude Modulation
Chapter 2 – p.44/74
Trellis Coded Modulation
270
(b)
90
270
(c)
90
0 180 0 180
add bits for error correction
V.32: 32 constellation points, 4 data bits, 1 parity bit
V.32bis: 6 data bits, 1 parity bit
Chapter 2 – p.45/74
Limits
Base sampling rate – 2400 baudStandard Data bits bps
V.32 4 9600V.32bis 6 14,400V.34 12 28,800V.34bis 14 33,600
Variationshandshake to determine line quality
compression35 kbps is the Shannon limit, 56 kbps?
eliminate one local loop
V.90 56-kbps down stream, 33-kbps upstream
V.92 48-kbps down stream, 48-kbps upstream
Chapter 2 – p.46/74
DSL: Digital Subscriber Line
Remove the filtersBandwidth
50
40
20
30
10
00 1000 2000 3000 4000
Meters5000 6000
Mpbs
Goalsuse existing Cat-3 lines
not interfere with current phone uses
always on
much better than 56kbps
Chapter 2 – p.47/74
Techniques
POTS (Plain Old Telephone Service) – FrequencyMultiplexing
DMT (Discrete MultiTone)
Pow
er
Voice Upstream Downstream
256 4-kHz Channels
0 25 1100 kHz
Chapter 2 – p.48/74
DSL Equipment
DSLAM
Splitter
Codec
Splitter
Telephone
To ISP
ADSL modem
Ethernet
Computer
Telephone line
Telephone company end office Customer premises
Voice switch
NID
NID: Network Interface DeviceADSL Modem: 250 QAM modemsDSLAM: Digital Subscriber Line Access Multiplexer
Chapter 2 – p.49/74
Wireless Local Loop
ISPTelephone Network
IEEE 802.16 (Broadband wireless)36Gbps down 1 Gbs upPut the ISP on Sandia crest
good line of sight
no rain or tree leaves! Chapter 2 – p.50/74
Trunks and Multiplexing
Frequency division multiplexing
Wavelength division multiplexing
Time division multiplexing
aside on compression
SONET
Chapter 2 – p.51/74
Frequency Division Multiplexing
300 3100
Channel 3
Channel 2
Channel 1
1
1
1
Atte
nuat
ion
fact
or
64
Frequency (kHz)
(c)
Channel 1 Channel 3
Channel 2
68 72
60 64
Frequency (kHz)
(b)
Frequency (Hz)
(a)
68 72
60
(a) 3 signals, (b) 3 frequency shifted signals, (c)combined signal Chapter 2 – p.52/74
Wavelength Division Multiplexing
Spectrum on the shared fiber
Pow
er
λ
Fiber 4 spectrum
Pow
er
λ
Fiber 3 spectrum
Pow
er
λ
Fiber 2 spectrum
Pow
erλ
λ1
λ1+λ2+λ3+λ4
Fiber 1 spectrum
Pow
er
λ
Fiber 1λ2
Fiber 2λ3
Fiber 3Combiner Splitter
Long-haul shared fiber
λ4
λ2
λ4
λ1
λ3Fiber 4
Filter
FDM for optical
Chapter 2 – p.53/74
Time Division Multiplexing (T1)
Channel 1
Channel 2
Channel 3
Channel 4
Channel 24
193-bit frame (125 µsec)
7 Data bits per channel
per sample
Bit 1 is a framing code
Bit 8 is for signaling
0
1
codec 8000 samples / second (4000 Hz signals)125 µsec / samplePCM (Pulse Coded Modulation)codec is multiplexed between 24 analog lineseach analog line inserts 8 bits every 125 µsec7 � 8000 � 56 � 000 bps / channel1.544 Mbps aggregate
Chapter 2 – p.54/74
Framing
Framing bit is used to recover the sender’s clock
The framing bit is 010101010101010....
I.e., 4000 Hz
Filtered for analog customer (3100 Hz filter)
Not likely for digital customers
Chapter 2 – p.55/74
Multiple T1s
6 5 4 3 2 1 05 1
4 0
6 2
7 3
6:17:14:1
4 T1 streams in
1 T2 stream out
6.312 Mbps
T2
1.544 Mbps
T1
44.736 Mbps
T3
274.176 Mbps
T4
7 T2 streams in 6 T3 streams in
Bitwise multiplexing4 to 1; 7 to 1; and 6 to 1Overhead added at each step for framing andrecoveryT2 and T4 are only used inside the phone company
Chapter 2 – p.56/74
Compression
15
10
5
01 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1
TimeSampling interval
Dig
itiza
tion
leve
ls
Bit stream sent
Consecutive samples always differ by +–1
Signal changed too rapidly for encoding to keep up
differential modulation (send change to value, ratherthan value)delta modulation (shown) 1 or � 1predictive encoding: extrapolate from earlier valuesand then send the change to this extrapolation
Chapter 2 – p.57/74
SONET (Synchronous OpticalNETwork)
Sonet frame (125 µsec)
Sonet frame (125 µsec)
9 Rows
. . .
. . .
87 Columns
3 Columns for overhead
SPESection overhead
Line overhead
Path overhead
STS-1 Synchronous Transport Signal-1SPE – Synchronous Payload Envelope
Chapter 2 – p.58/74
SONET Rates
Gross – signalling rateSPE – excludes line and selection overheadUser – excludes path overhead
Chapter 2 – p.59/74
Switching
(a)
(b)
Switching office
Physical (copper) connection set up when call is made
Packets queued for subsequent transmission
Computer
Computer
(a) circuit switched, (b) packet switchedChapter 2 – p.60/74
Message Switching
Data
Pkt 1
Pkt 2
Pkt 3
Pkt 1
Pkt 2
Pkt 3
Pkt 1
Pkt 2
Pkt 3
A B C D
Msg
Msg
Msg
A B C DA B C D
Propagation delay
Queuing delay
Call request signal
Time spent
hunting for an
outgoing trunk
Call accept signal
AB trunk
BC trunk
CD trunk
(a) (b) (c)
Tim
e
(a) circuit, (b) message, and (c) packet switchingstore-and-forward
Chapter 2 – p.61/74
Packet v Circuit Switching
Chapter 2 – p.62/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.63/74
Mobile Telephone System
First-Generation: Analog Voice
Second-Generation: Digital Voice (skip)
Third-Generation: Digital Voice and Data (skip)
Chapter 2 – p.64/74
History
Push-to-talk – 1946
Improved Mobile Telephone System (IMTS) – 1960
still hilltop transmission
23 channels – individual conversations
2 frequencies – send and receive
Advanced Mobile Phone System (AMPS) – 1982
cells
typically 10-20 km across
Chapter 2 – p.65/74
Cells
G
F
A
B
C
D
E
G
F
A
B
C
D
E
G
F
A
B
C
D
E
(a) (b)
Frequency sets have a buffer of two cellsMicro-cells are added to respond to more usersTemporary micro-cells for events
Chapter 2 – p.66/74
Mobility
MTSO – Mobile Telephone Switching Office
AKA, MSC – Mobile Switching Center
At any time each phone is in one cell
Switching center controls handoff
Handof takes about 300msec
Hard handoff – connection gets dropped
Chapter 2 – p.67/74
More Details: Channels
832 full duplex channelsEach channel is 30 kHz wide (frequency divisionmultiplexing)Transmission channels 824–849 MHzReceive channels 869–894 MHzSignal properties:
straight (requires line of sight)
bounce off buildings and ground (multipath fading)
absorbed by plants21 channels reserved for control45 voice channels (to avoid re-using channels usedin nearby cells)
Chapter 2 – p.68/74
More Details: Call Management
32-bit serial number10-digit phone number (encoded in 34 bits)Startup sequence
Scan 21 channels to find strongest base station
Broadcast serial number and phone number
Base station reports to MTSO
Phone re-registers about every 15 minutesOn “send” phone send number and identifier(retransmit on collision)Idle phones listen for pages, when an incoming callis accepted, the phone switches to the appropriatechannel
Chapter 2 – p.69/74
Topics
Theoretical Foundations
Guided Transmission
Wireless Transmission
Communication Satellites
Public Switched Telephone Network
Mobile Telephone System
Cable Television
Chapter 2 – p.70/74
Community Antenna
Tap Coaxial cable
Drop cable
Headend
Antenna for picking up distant signals
Chapter 2 – p.71/74
Internet over Cable
Copper twisted pair
Switch
Toll office
Head- end
High-bandwidth fiber trunk
End office
Local loop
(a)
(b)
House
High-bandwidth fiber trunk
Coaxial cable
House
Tap
Fiber node
Fiber
Fiber
Chapter 2 – p.72/74
Spectrum Allocation
0 108
TV TV Downstream data
Downstream frequencies
Ups
trea
m
data
Ups
trea
m
freq
uenc
ies
FM
550 750 MHz
5 42 54 88
Downstream analogEach channel is 6 MHz
With QAM-64 we get 6 � 6 Mbps (or 27 Mbps without
overhead)
Upstream uses QSPK, only 2 bits per baud
Chapter 2 – p.73/74
Cable Modems
Initializationupstream and downstream channels
minislot (about 8 bytes) for requesting bandwidth
ringing to determine distance from head-endContention for upstreamAll data packets are encrypted
Chapter 2 – p.74/74