Computer Networks
The Physical Layer
1
PHY
• Transmitting information on wires.
• How is information represented?
– Digital systems.
– Analog systems.
2
Signals and Systems
What is a signal?
What is a system?
3
Signals and Systems (cont’d)
• Signal: electro-magnetic wave carrying
information.
– Time varying function produced by physical
device (voltage, current, etc.).
• System: device (or collection thereof) or
process (algorithm) having signals as input
and output.
4
Signals and Systems (cont’d)
5
Signals and Systems (cont’d)
• Periodic signals:
– f(t+T) = f(t)
Period = T (seconds)
• Frequency = 1/ Period
– “cycles” / sec. = Hertz (Hz)
6
Analog Technology
• Analog devices maintain exact physical analog
of information.
– E.g., microphone: the voltage v(t) at the output
of the mic is proportional to the sound pressure
v(t)
7
Digital Technology
• It uses numbers to record and process
information
– Inside a computer, all information is represented
by numbers.
• Analog-to-digital conversion: ADC
• Digital-to-analog conversion: DAC
010001010
ADC
DAC
8
Digital Technology
• All signals (including multimedia) can be
encoded in digital form.
• Digital information does not get distorted
while being stored, copied or communicated.
9
Digital Communication Technology
• Early example: the telegraph (Morse code).
– Uses dots and dashes to transmit letters.
– It is digital even though uses electrical signals.
• The telephone has become digital.
• CDs and DVDs.
• Digital communication networks form the
Internet.
• The user is unaware that the signal is
encoded in digital form.
10
Two Levels are Sufficient
• Computers encode information using only two
levels: 0 and 1.
• A bit is a digit that can only assume the
values 0 and 1 (it is a binary digit).
• A word is a set of bits
– Example: ASCII standard for encoding text
• A = 1000001; B = 1000010; …
• A byte is a word with 8 bits.
11
Definitions
•
•
•
•
1 KB = 1 kilobyte = 1,000 bytes = 8,000 bits
1 MB = 1 megabyte = 1,000 KB
1 GB = 1 gigabyte = 1,000 MB
1 TB = 1 terabyte = 1,000 GB
•
•
•
•
1 Kb = 1 kilobit = 1,000 bits
1 Mb = 1 megabit = 1,000 Kb
1 Gb = 1 gigabit = 1,000 Mb
1 Tb = 1 terabit = 1,000 Gb
12
Digitization
• Digitization is the process that allows us to
convert analog to digital (implemented by ADC).
• Analog signals: x(t)
– Defined on continuum (e.g. time).
– Can take on any real value.
• Digital signals: q(n)
– Sequence of numbers (samples) defined by a
discrete set (e.g., integers).
13
Digitization - Example
Analog signal x(t)
Digitized signal q(n)
0.15
0.1
0.1
0.05
0.05
0
q(n)
0.15
x(t)
0.2
0
-0.05
-0.05
-0.1
-0.1
-0.15
-0.15
1.35
1.355
1.36
1.365
1.37
1.375
-0.2
1.35
1.355
1.36
1.365
1.37
1.375
14
Some Definitions
• Interval of time between two samples:
– Sampling Interval (T).
• Sampling frequency F=1/T.
• E.g.: if the sampling interval is 0.1 seconds,
then the sampling frequency is 1/0.1=10.
– Measured in samples/second or Hertz.
• Each sample is defined using a word of B
bits.
– E.g.: we may use 8 bits (1 byte) per sample.
15
Bit-rate
• Bit-rate = numbers of bits per second we
need to transmit
– For each second we transmit F=1/T samples.
– Each sample is defined with a word of B bits.
– Bit-rate = F*B.
• Example: if F is 10 samples/s and B=8, then
the bit rate is 80 bits/s.
16
Example of Digitization
Bit-rate=BF=16 bits/second
B=4 bits/sample
10101110010100110011010000110100
0
1
F=4 samples/second
2
Time (seconds)
17
Bit-rate - Example 1
• What is the bit-rate of digitized audio?
–
–
–
–
Sampling rate: F= 44.1 KHz
Quantization with B=16 bits
Bit-rate = BF= 705.6 Kb/s
Example: 1 minute of uncompressed
stereo music takes more than 10 MB!
18
Bit-rate - Example 2
• What is the bit-rate of digitized speech?
– Sampling rate: F = 8 KHz
– Quantization with B = 16 bits
– Bit-rate = BF = 128 Kb/s
19
Data Transmission
• Analog and digital transmission.
– Example of analog data: voice and video.
– Example of digital data: character strings
• Use of codes to represent characters as
sequence of bits (e.g., ASCII).
• Historically, communication infrastructure for
analog transmission.
– Digital data needed to be converted: modems
(modulator-demodulator).
20
Digital Transmission
• Current trend: digital transmission.
–
Cost efficient: advances in digital circuitry.
(VLSI).
• Advantages:
– Data integrity: better noise immunity.
– Security: easier to integrate encryption
algorithms.
– Channel utilization: higher degree of
multiplexing (time-division mux’ing).
21
Guided Transmission Data
•
•
•
•
Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
22
Magnetic Media
• Examples?
• Advantages?
• Disadvantages?
23
Twisted Pair
•
•
•
•
•
•
Oldest but still very common.
Telephone system.
Cheap and effective for long ranges.
Bundles of twisted pairs.
Can transmit both analog and digital signals.
Bandwidth depends on thickness of wire and
distance traveled.
– Mb/s for few kilometers.
24
Twisted Pair
•
•
(a) Category 3 UTP.
(b) Category 5 UTP.
25
Twisted Pair
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci211752,00.html
26
Coaxial Cable
• Better performance than twisted pair, i.e.,
higher bandwidth and longer distances.
– Good noise immunity.
• But…
• Bandwidths close to 1GHz.
• Used widely in telephone networks for longer
distances; but gradually being replaced by
fiber.
• Used for CATV!
27
Coaxial Cable
28
Fiber Optics
• Optical transmission.
• Optical transmission system: light source,
medium, and detector.
• Pulse of light = “1”.
• No light = “0”.
• Transmission medium: ultra thin fiber of
glass.
• Detector: generates electrical pulse when
perceives light.
29
Transmitting Light
• (a) Three examples of a light ray from inside a
silica fiber impinging on the air/silica boundary at
different angles.
• (b) Light trapped by total internal reflection.
30
Fiber Cables
•
•
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
31
Fiber Optic Networks
• A fiber optic ring.
32
Fiber Optic Networks (2)
• A passive star connection in a fiber optics network.
33
Fiber versus Copper Wire
• Fiber can handle much higher bandwidths.
• Low attenuation: 50km without repeater.
• Unaffected by power surges/outages, and
interference.
• Fiber is thin and lightweight: easy to deploy
and add new capacity.
• Difficult to tap.
• But…
34
Fiber versus Copper (cont’d)
• Fiber can be damaged easily.
• Optical transmission is unidirectional, so need
2 fibers or 2 frequencies for 2-way
communication.
• Fiber and fiber interfaces is more expensive.
35
Public Switched Telephone
System
• Structure of the Telephone System
• The Politics of Telephones
• The Local Loop: Modems, ADSL and
Wireless
• Trunks and Multiplexing
• Switching
36
Structure of the Telephone
System
• (a) Fully-interconnected network.
• (b) Centralized switch.
• (c) Two-level hierarchy.
37
Structure of the Telephone
System (2)
• A typical circuit route for a medium-distance
call.
38
Major Components of the
Telephone System
• Local loops:
ƒ Connection from subscriber to end office.
• Trunks
ƒ Outgoing lines connecting offices.
ƒ Toll office:
ƒ Connects end offices.
• Switching offices
ƒ Where calls are moved from one trunk to
another.
39
PSTN
40
Local Loop
•
•
•
•
“Last mile”.
End office-subscriber connection.
Analog, twisted pair.
Traditionally, voice but it has been changing:
data transmission.
• To transmit data, conversion digital to analog:
modem.
• At phone office, data usually converted back
to digital for long-distance transmission over
trunks.
41
Transmission Impairments
• Problems that happen with signal as it propagates.
• Attenuation: loss of energy as signal propagates.
– Different frequencies suffer different attenuation.
– Different Fourier components attenuated by different
amount.
• Distortion: different Fourier components shifted in
time.
• Noise: unwanted energy from other sources.
– E.g., thermal noise: unavoidable random motion of
electrons in wire.
42
Modulation
• Signal with wide range of frequencies is
undesirable.
• Square waves exhibit wide frequency range.
• To avoid that, AC signaling is used.
– Sine wave “carrier” to carry information.
• Modulation:
– Information is encoded in the carrier by
varying either amplitude, frequency, or
phase.
43
Modulation: Examples
Binary signal
Amplitude
modulation
Frequency
modulation
Phase
modulation
44
Modem
• Modulator-demodulator.
• Modulates digital signal at the source and
demodulates received signal at the
destination.
• How to transmit faster?
– Nyquist says that capacity is achieved at
2*H*log2V.
– So there is no point sampling faster than 2*H.
– But, can try to send more bits per sample.
45
Baud Rate
• Baud rate = symbols/sec.
• Data rate = bits/sec.
• If 2 voltage levels are used, then
– 1 symbol=1bit.
– Baud rate = bit rate.
• But, if can encode more than 1 bit in a symbol…
– E.g., if voltages 0, 1, 2, and 3, every symbol consists of
2 bits.
– Thus, 2400 baud line corresponds to 4800 bps.
– The same thing for 4 different frequencies: QPSK.
46
Bandwidth, Baud- and Bit Rates
• Bandwidth: physical property of medium.
– Range of frequencies transmitted with
adequate quality.
– Measured in Hz.
• Baud rate is number of samples/sec or
symbols/sec.
• Modulation technique determines number of
bits/symbol: symbols/sec * bits/symbol.
• Modern modems transmit several bits/symbol
frequently combining multiple modulation
schemes.
47
Full Duplex, Half Duplex, Simplex
• Full duplex: traffic in both directions
simultaneously.
• Half duplex: traffic in both directions but 1
direction at a time.
• Simplex: traffic allowed only one way.
• Examples?
48
What’s next?
•
•
•
•
Modems were getting faster, e.g., 56Kbps.
But, demand for faster access was growing!
CATV and satellite as competitors.
Phone company’s response: DSL.
– “Broadband” access.
– ADSL: asymmetric digital subscriber line.
– When you subscribe to DSL service, you are
connected to the local office without the filter
to frequencies below 300Hz and above
3400Hz.
– Physical limitation still exists and depends on
thickness, length, etc.
49
Digital Subscriber Lines
• Bandwidth versus distanced over category 3
UTP for DSL.
50
Digital Subscriber Lines (2)
• Operation of ADSL using discrete multitone
modulation.
Available 1.1MHz local loop spectrum divided into 256 channels
(4.3KHz each).
51
ADSL
• Typically, 32 channels for upstream and the
rest for downstream traffic.
• Usually, 512 Kbps downstream and 64 Kbps
upstream (standard) and 1 Mbps downstream
and 256 Kbps upstream (premium).
• Within each channel, modulation scheme is
used (sampling at 4000 baud).
52
Typical ADSL Setup
• A typical ADSL equipment configuration.
53
Wireless Local Loop
• Last mile is wireless.
• Why?
• Historically: local telcos had monopoly for
local telephone service.
– In the mid 1990’s market open to competition,
e.g., long distance carriers.
– Cheaper alternative to stringing cables to
customers is using a wireless local loop.
• Mobile telephony?
• “Fixed” wireless.
54
Wireless Local Loops
• Architecture of an LMDS system.
Tower with multiple highly directional
antennae; but small range (2-5Km).
55
Trunking and Multiplexing
56
Trunking
• Deployment of high-bandwidth pipes.
– Current and future demand.
– Switching offices higher in the PSTN
hierarchy.
• Multiplexing: ability to send a number of
conversations simultaneously over the same
pipe.
• Multiplexing schemes:
– Frequency Division Multiplexing (FDM).
– Time Division Multiplexing (TDM).
57
The Multiplexing Problem
frequency
Shared channel
(how to divide resource among multiple recipients?)
time
Analogy: a highway shared by many users
58
Frequency-Division
Multiplexing
frequency
user 1
user 2
user 3
user 4
guard-band
time
Analogy: a highway has multiple lanes
59
Time-Division Multiplexing
frequency
user 1
user 2
user 3 user 4
user 1
user 2
guard-band
time
Requirement: precise time coordination
60
Frequency-Time-Division
frequency
time-slot (usually of the same size)
time
61
Frequency Division
Multiplexing
• (a) The original bandwidths.
• (b) The bandwidths raised in frequency.
• (c) The multiplexed channel.
62
FDM versus TDM
• FDM requires analog circuitry.
• TDM can be done entirely using digital
electronics.
• But TDM can only be used for digital data.
– Analog signals from local loops need to be
digitized (at the local office).
– At end office, all individual local loops arrived,
are digitzed, and multiplexed.
63
TDM Multiplexing
64
PCM
• Pulse Code
65
PCM
• Pulse Code Modulation:
– Digitization of voice channels.
– Sampling frequency…
• If voice signal peaks at 4KHz, what’s the
sampling frequency?
• Nyquist: 8000 samples/sec, or 125
microsec/sample.
• Each sample is 8 bits (7 for data and 1 for
control).
• Data rate: 7*8000 = 56Kbps of data and 8Kbps
of signaling (per channel).
• No world-wide standard for PCM.
• In the US and Japan: T1 (technically DS1).
66
T1
• The T1 carrier (1.544 Mbps).
T1: 24 multiplexed voice channels: 1.544 Mbps.
67
T2 and Beyond…
• Multiplexing T1 streams into higher carriers.
68
SONET/SDH
• SONET and SDH multiplex rates.
SONET: Synchronous Optical NETwork.
SDH: Sync Digital Hierarchy.
Optical TDM for
fiber transmission
69
Switching
70
Circuit- and Packet Switching
• (a) Circuit switching.
• (b) Packet switching.
71
Switching
Circuit-
Message-
Packet Switching
72
Packet Switching
73
Wireless Transmission
74
Wireless Transmission
• Electron movement: electromagnetic waves
that propagate through space.
T
R
75
Propagation
• Maximum speed: speed of light, c, 3*108 m/s.
• In vacuum, all EM waves travel at the same
speed c.
• Otherwise, propagation speed is function of
frequency (c = λ * f), where f is frequency
(Hz) and λ is wavelength (m).
76
The Electromagnetic Spectrum
• The electromagnetic spectrum and its uses
for communication.
77
Radio Transmission
~1Km
• (a) In the VLF, LF, and MF bands, radio waves follow
the curvature of the earth. E.g., AM radio uses MF.
• (b) In the HF and VHF bands, they bounce off the
ionosphere. E.g., Hams and military.
78
Microwave Transmission
• Above 100MHz.
• Waves travel in straight lines.
• Directionality.
– Better quality.
– Space Division Multiple Access.
– But, antennas need to be aligned, do not go
through buildings, multi-path fading, etc.
• Before fiber, microwave transmission
dominated long-distance telephone
transmission.
79
Politics of the Electromagnetic
Spectrum
• Need agreements to regulate access.
– International and national.
• Local governments allocate spectrum for radio (AM
and FM), TV, mobile phones, emergency services,
etc.
– In the US, FCC.
• World-wide, ITU-R tries to coordinate allocation so
devices work everywhere.
• Separate frequency band that is unregulated.
– ISM: Industrial, Scientific, and Medical.
– Household devices, wireless phones, remote controls,
etc.
80
Spread Spectrum
• Narrow frequency band -> good reception (power,
bandwidth).
• But in some cases, wide band is used, aka, spread
spectrum.
– Modulate signal to increase bandwidth of signal to
be transmitted.
• 2 variations:
– Frequency Hopping (FH).
• Transmitter hops frequencies
– Direct Sequence (DS).
• Use spreading code to convert each bit of the original
signal into multiple bits.
81
Infrared Transmission
• Short range (e.g., remote controls).
• Directional, cheap.
• But, do not pass through obstacles.
82
Lightwave Transmission
• Unguided optical transmission.
• E.g., laser communication between two
buildings for LAN interconnection.
• High bandwidth, low cost.
• Unidirectionality.
• Weather is a major problem (e.g., rain,
convection currents).
83
Communication Satellites
• Weather balloons.
• The moon.
• Artificial satellites:
–Geostationary.
–Medium-Earth Orbit.
–Low-Earth Orbit.
84
Satellite Communications
SAT
ground stations
85
Satellite Communications
• Satellite-based antenna(e) in stable
orbit above earth.
• Two or more (earth) stations
communicate via one or more
satellites serving as relay(s) in space.
• Uplink: earth->satellite.
• Downlink: satellite->earth.
• Transponder: satellite electronics
converting uplink signal to downlink.
86
Orbits
• Shape: circular, elliptical.
• Plane: equatorial, polar.
• Altitude: geostationary (GEO), medium earth
(MEO), low earth (LEO).
87
Communication Satellites
88
GEOs
• High-flying satellites.
• Orbit at 35,863 Km above earth and rotates in
equatorial plane.
• Many GEO satellites up there!
89
GEO: Plus’s and minus’s
• Plus’s:
– Stationarity: no frequency changes due to movement.
– Tracking by earth stations simplified.
– At that altitude, provides good coverage of the earth.
• Minus’s:
–
–
–
–
Weakening of signal.
Polar regions poorly served.
Delay!
Spectral waste for point-to-point communications.
90
Principal Satellite Bands
. Downlink frequencies interfere with microwave.
. Internationally-agreed frequency bands.
91
LEO Satellites
• Circular or slightly eliptical orbit under 2,000
Km.
• Orbit period: 1.5 to 2 hours.
• Coverage diameter: 8,000 Km.
• RTT propagation delay < 20ms (compared to
> 300ms for GEOs).
• Subject to large frequency changes and
gradual orbit deterioration.
92
LEO Constellations
• Advantages over GEOs:
– Lower delay, stronger signal, more localized
coverage.
• But, for broad coverage, many satellites
needed.
• Example: Iridium (66 satellites).
93
LEOs
constellation
SAT
SAT
SAT
ground stations
94
Low-Earth Orbit Satellites
Iridium
(a)
(b)
• (a) The Iridium satellites from six necklaces around the
earth.
• (b) 1628 moving cells cover the earth.
95
In Summary…
• GEOs
– Long delay - 250-300 ms.
• LEOs
– Relatively low delay - 40 - 200 ms.
– Large variations in delay - multiple hops/route
changes, relative motion of satellites, queuing.
96
Satellite Data Rates
• Satellite has 12-20 transponders, each
ranging from 36-50 Mbps.
•
•
•
•
T1: 1.54 Mbps.
T2: 6.312 Mbps.
T3: 44.736 Mbps.
T4: 274.176 Mbps.
97
The Mobile Telephone System
• First-Generation Mobile Phones:
Analog Voice
• Second-Generation Mobile Phones:
Digital Voice
• Third-Generation Mobile Phones:
Digital Voice and Data
98
The “Cell” Concept
• (a) Frequencies not reused in adjacent cells.
• (b) To add more users, smaller cells.
99
Mobile Phone System Structure
•
•
•
•
Hierarchy.
Base station.
Mobile Switching Center (MSC).
MSCs connected through PSTN.
100
Handoffs
• As mobile phones move, they switch cells,
and thus base stations.
• Soft versus hard handoffs.
– Two base stations while handoff is in
progress.
– Hard handoff.
• Roaming.
101
Cable Television
102
Community Antenna Television
• An early cable television system.
103
Internet over Cable
• Cable television
104
DSL
• The fixed telephone system.
105
ADSL versus Internet over Cable
• Both uses fiber in the backbone.
• ADSL uses twisted pair and IoC uses coax on
the edge.
• Coax has higher capacity but shared with TV.
• IoC’s capacity is unpredicatble as it depends
on how many users/traffic.
106