Transcript Introduction - Computer Science

Mobile and Wireless Networks
Summer 2005
Wichita State University
Computer Science
Chin-Chih Chang
http://www.cs.wichita.edu/~chang
[email protected]
1
Overview of the Course
 Lecture
•
•
•
•
•
•
•
Introduction (06/06, 06/07, 06/08)
Wireless LANS and PANS (06/09, 06/10, 06/13)
Wireless WANS AND MANS (06/14, 06/15, 06/16)
Wireless Internet (06/17, 06/20, 06/21)
Ad Hoc Wireless Networks (06/22, 06/23, 06/24)
MAC Protocols for Ad Hoc Wireless Networks (07/06, 07/07, 07/08)
Transport Layer and Security Protocols for Ad Hoc Wireless Networks
(07/11, 07/12, 07/13)
• Hybrid Wireless Networks (07/14, 07/15, 07/18)
• Recent Advances in Wireless Networks (07/19, 07/20, 07/21)
 Lab
• J2ME
• Mobile Web Service
2
Chapter 1: Introduction
 Fundamentals
 Electromagnetic spectrum
 Radio propagation
mechanisms
 Characteristics of the wireless
channel
 Modulation techniques
 Multiple access techniques




Voice coding
Error control
Computer networks
IEEE 802 Networking
Standard
 Wireless networks and book
overview
3
Fundamentals
 A computer network is an interconnected
collection of autonomous computers.
 Networking Goals:
• Resource sharing - e.g., shared printer, shared files.
• Increased reliability - e.g., one failure does not cause
system failure.
• Economics - e.g., better price/performance ratio.
• Communication - e.g., e-mail.
4
Mobile communication
 Two aspects of mobility:
• User mobility: users communicate (wireless) “anytime, anywhere, with
anyone”
• Device portability: devices can be connected anytime, anywhere to the
network
 Wireless vs. mobile








Examples
stationary (wired and fixed) computer
notebook in a hotel
wireless LANs in historic buildings
Personal Digital Assistant (PDA)
 The demand for mobile communication creates the need for
integration of wireless networks into existing fixed networks:
• Local area networks: standardization of IEEE 802.11,
ETSI (European Telecommunications Standards Institute) (HIPERLAN combined technology for broadband cellular short-range communications
and wireless Local Area Networks (LANs) )
• Internet: Mobile IP extension of the Internet Protocol IP
• Wide area networks: e.g., internetworking of GSM and ISDN
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
6
Electromagnetic spectrum
twisted
pair
1 Mm
300 Hz
ELF
coax cable
10 km
30 kHz
VF VLF
LF
optical transmission
100 m
3 MHz
MF
HF
1m
300 MHz
VHF
UHF
ELF = Extremely Low Frequency (30 ~ 300 Hz)
VF = Voice Frequency (300 ~ 3000 Hz)
VLF = Very Low Frequency (3 ~ 30 KHz)
LF = Low Frequency (30 ~ 300 KHz)
MF = Medium Frequency (300 ~ 3000 KHz)
HF = High Frequency (3 ~ 30 MHz)
VHF = Very High Frequency (30 ~ 3000 MHz)
10 mm
30 GHz
SHF
EHF
100 m
3 THz
1 m
300 THz
infrared
visible light UV
UHF = Ultra High Frequency (300 MHz ~ 3GHz)
SHF = Super High Frequency (3 ~ 30 GHz)
EHF = Extremely High Frequency (30 ~ 300GHz)
Infrared (300 GHz ~ 400 THz)
Visible Light (400 THz ~ 900 THz)
UV = Ultraviolet Light (900 THz ~ 1016 Hz)
X-ray (1016 ~ 1022 Hz)
Gamma ray (1022 Hz ~)
Frequency and wave length:  = c/f
wave length , speed of light c  3x108m/s, frequency f
7
Electromagnetic spectrum
 The Electromagnetic spectrum is used for information
transmission by modulating the amplitude, frequency, or phase
of the waves.
 VLF, LF, and MF are called as ground waves.
• Transmission range up to a hundred kilometers
• Used for AM radio broadcasting
 HF and VHF
• The sky wave may get reflected several times between the Earth and the
ionosphere.
• Used by amateur ham radio operators and for military communication.
 VHF-/UHF-ranges for mobile radio
• simple, small antenna for cars
• deterministic propagation characteristics, reliable connections
8
Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
9
Electromagnetic spectrum
 SHF and higher for directed radio links, satellite communication
•
•
•
•
•
small antenna, focusing
Microwave transmissions travel in straight lines.
High signal-to-noise ratio (SNR)
Line-of-sight alignment is required.
large bandwidth available
 Wireless LANs use frequencies in UHF to SHF spectrum
• some systems planned up to EHF
• limitations due to absorption by water and oxygen molecules (resonance
frequencies)
– weather dependent fading, signal loss caused by heavy rainfall etc.
 Infrared waves and waves in the EHF band are used for short-range
communication.
• Widely used in television, VCR, stereo remote controls
 Visible light
• Used in the optical fiber
• Laser can be used to connect LANs on two buildings but can travel limited
distance and cannot penetrate through rain or thick fog.
10
Spectrum Allocation
 Spectrum allocation methods:
• Comparative binding (beauty contest) requires each carrier to
explain why its proposal serves the public interest best.
• Lottery system
• Auction
 The other option of allocating frequencies is not to allocate them.
 ITU (International Union Radiocommunication) has designated
ISM (industrial, scientific, medical) bands as open bands:
• Frequencies are not allocated but restrained in a short range.
• These bands usually used by wireless LANs and PANs are
around the 2.4 GHz band.
• Parts of the 900 MHz and 5 GHz bands are also available for
unlicensed usage.
11
Spectrum Allocation
Cellular
Phones
Cordless
Phones
Wireless
LANs
Others
Europe
USA
Japan
GSM 450-457, 479486/460-467,489496, 890-915/935960,
1710-1785/18051880
UMTS (FDD) 19201980, 2110-2190
UMTS (TDD) 19001920, 2020-2025
CT1+ 885-887, 930932
CT2
864-868
DECT
1880-1900
IEEE 802.11
2400-2483
HIPERLAN 2
5150-5350, 54705725
RF-Control
27, 128, 418, 433,
868
AMPS, TDMA, CDMA
824-849,
869-894
TDMA, CDMA, GSM
1850-1910,
1930-1990
PDC
810-826,
940-956,
1429-1465,
1477-1513
PACS 1850-1910, 19301990
PACS-UB 1910-1930
PHS
1895-1918
JCT
254-380
902-928
IEEE 802.11
2400-2483
5150-5350, 5725-5825
IEEE 802.11
2471-2497
5150-5250
RF-Control
315, 915
RF-Control
426, 868
ITU-R holds auctions for new frequencies, manages frequency bands
12
worldwide (WRC, World Radio Conferences)
Signals




physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
• continuous time/discrete time
• continuous values/discrete values
• analog signal = continuous time and continuous values
• digital signal = discrete time and discrete values
 signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift 
• sine wave as special periodic signal for a carrier:
s(t) = At sin(2  ft t + t)
13
Fourier representation of periodic signals
 Periodic signals can be represented by Fourier series.


1
g (t )  c   an sin( 2nft)   bn cos( 2nft)
2
n 1
n 1
1
1
0
0
t
ideal periodic signal
t
real composition
(based on harmonics)
14
Signals
 Different representations of signals
• amplitude (amplitude domain)
• frequency spectrum (frequency domain)
• phase state diagram (amplitude M and phase  in polar coordinates)
 Composed (multiple frequencies) signals transferred into frequency domain
using Fourier transformation
 Digital signals need
• infinite frequencies for perfect transmission (Fourier equation)
• modulation with a carrier frequency for transmission (analog signal!)
Q = M sin 
A [V]
A [V]
t[s]

I= M cos 

f [Hz]
15
Antennas: isotropic radiator
 Radiation and reception of electromagnetic waves, coupling
of wires to space for radio transmission
 Isotropic radiator: equal radiation in all directions (three
dimensional) - only a theoretical reference antenna
 Real antennas always have directive effects (vertically and/or
horizontally)
 Radiation pattern: measurement of radiation around an
antenna
y
z
z
y
x
x
ideal
isotropic
radiator
16
Antennas: simple dipoles
 Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on
car roofs or /2 as Hertzian dipole
 shape of antenna proportional to wavelength
 Example: Radiation pattern of a simple Hertzian dipole
 Gain: maximum power in the direction of the main lobe compared to the
power of an isotropic radiator (with the same average power)
/4
y
/2
y
x
side view (xy-plane)
z
z
side view (yz-plane)
simple
dipole
x
top view (xz-plane)
17
Antennas: directed and sectorized
 Often used for microwave connections or base stations for mobile
phones (e.g., radio coverage of a valley)
y
y
z
x
z
side view (xy-plane)
x
side view (yz-plane)
top view (xz-plane)
z
z
x
x
top view, 3 sector
directed
antenna
top view, 6 sector
sectorized
antenna
18
Antennas: diversity
 Grouping of 2 or more antennas: multi-element antenna arrays
 Antenna diversity
• switched diversity, selection diversity
• receiver chooses antenna with largest output
• diversity combining
• combine output power to produce gain
• cophasing needed to avoid cancellation
/2
/4
/2
+
/4
/2
/2
+
ground plane
19
Signal propagation ranges
 Transmission range
• communication possible
• low error rate
 Detection range
• detection of the signal
possible
• no communication
possible
 Interference range
• signal may not be
detected
• signal adds to the
background noise
sender
transmission
distance
detection
interference
20
Radio propagation
 Radio waves can be propagated and receiving power is influenced
in different ways:
•
•
•
•
•
•
Direct transmission (path loss, fading dependent on frequency)
Reflection at large obstacles
Refraction through different media
Scattering at small obstacles
Diffraction at edges
shadowing
 Propagation in free space is always like light (straight line).
 Receiving power proportional to 1/d² (d = distance between sender
and receiver)
shadowing
reflection
refraction
scattering
diffraction
21
Radio propagation - example
22
Characteristics of the Wireless Channel
 Path loss: the ratio of the power of the transmitted signal to the
power of the same signal received by the receiver.
• Free space model: Assume there is only a direct-path between the transmitter
and the receiver.
• Two-way model: Assume there is a light-of-sight path and the other path
through reflection, refraction, or scattering between the transmitter and the
receiver
• Isotropic antennas (in which the power of the transmitted signal is the same
in all direction): The receiving power varies inversely to the distance of
power of 2 to 5.
 Fading: fluctuations in signal strength when received at the
receiver.
• Fast fading/small-scale fading: rapid fluctuations in the amplitude, phase, or
multipath delays.
• Slow fading/large-scale fading (shadow fading): objects that absorb the
23
transmissions lie between the transmitter and receiver.
Characteristics of the Wireless Channel
 Measures used for countering the effects of fading are diversity
and adaptive modulation.
• Diversity modulation:
• Time diversity: spread the data over time.
• Frequency diversity: spread the transmission over frequencies. Example:
the direct sequence spread spectrum and the frequency hopping spread
spectrum.
• Space diversity: use different physical transmission paths. An antenna
array could be used.
• Adaptive modulation: the transmitter adjusts the transmission based on the
feedback from the receiver.
• Complex to implement
24
Characteristics of the Wireless Channel
 Interference
• Adjacent channel interference: interfered by signals in nearby frequencies.
Solved by the guard bands.
• Co-channel interference: narrow-band interference due to other systems
using the same frequency. Solved by multiuser detection machenisms,
directional antennas, and dynamic channel allocation methods.
• Inter-symbol interference: distortion in the received signal caused by the
temporal spreading and the consequent (neighbor) overlapping of individual
pulses in the signal. Solved by adaptive equalization that involves
mechanisms for gathering the dispersed symbol energy into its original time
interval.
 Doppler Shift
• The change/shift in the frequency of the received signal when the transmitter
and the receiver are mobile to each other.
• Moving towards each other, the frequency will be higher; two moving away,
the frequency will be lower.
25
Multipath propagation
 Signal can take many different paths between sender and receiver
due to reflection, scattering, diffraction.
 Time dispersion: signal is dispersed over time
 interference with “neighbor” symbols, Inter Symbol
Interference (ISI)
 The signal reaches a receiver directly and phase shifted
 distorted signal depending on the phases of the different parts
multipath
LOS pulses pulses
signal at sender
signal at receiver
26
Characteristics of the Wireless Channel
 Transmission Rate Constraints
• The number of times of signal changes is called the baud rate. Bit rate =
baud rate x bits per signal
• Nyquist’s Theorem for noiseless channel:
• If the signal has L discrete levels over a transmission medium of
bandwidth B , the maximum data rate C = 2B log2 L bits/sec
• Example: a noiseless 3-kHz channel cannot transmit binary signals at a
rate exceeding 6000 bps (= 2 x 3000 log2 2).
• Shannon’s Theorem for noisy Channel
• maximum data rate C = B log2 (1 + S/N) bits/sec B: bandwdith, S: signal
power, N: noise power
• S/N (Signal-to-noise ratio, SNR), usually measured as 10 log10S/N in db
= decibels, is called thermal noise ratio.
• Example: SNR = 20 db, 2 KHz bandwidth. The maximum data rate is
2000 x log2 (1 + 100) = 9230.241 bps
27
Modulation Techniques
 Analog modulation
• Used for transmitting analog data.
• shifts center frequency of baseband signal up to the radio carrier
• Analog modulation techniques
• Amplitude Modulation (AM): Not efficient. Example: Broadcast radio
• Frequency Modulation (FM): Example: Broadcast radio
• Phase Modulation (PM)
 Digital modulation
• digital data (0 and 1) is translated into an analog signal (baseband)
• Required if digital data has to be transmitted over a media that only allows
for analog transmission - old analog telephone system and wireless
networks
 Analog modulation techniques
• Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift
Keying (PSK)
• differences in spectral efficiency, power efficiency, robustness
28
Modulation Techniques
 An example of amplitude
modulation (AM):
• The top diagram shows the
modulating signal
superimposed on the carrier
wave.
• The bottom diagram shows
the resulting amplitudemodulated signal. Notice
how the peaks of the
modulated output follow
the contour of the original,
modulating signal.
29
Modulation Techniques
 An example of frequency
modulation (FM).
• The top diagram shows the
modulating signal
superimposed on the carrier
wave.
• The bottom diagram shows
the resulting frequencymodulated signal.
30
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio receiver
radio
carrier
31
Digital modulation
Modulation of digital signals known as Shift
Keying
 Amplitude Shift Keying (ASK):
• very simple
• low bandwidth requirements
• very susceptible to interference
 Frequency Shift Keying (FSK):
• needs larger bandwidth
• Binary FSK (BFSK): 1 is represented by
fc + k and 0 by fc – k.
 Phase Shift Keying (PSK):
• more complex
• robust against interference
1
0
1
t
1
0
1
t
1
0
1
t
32
Advanced Frequency Shift Keying
 bandwidth needed for FSK depends on the distance between the
carrier frequencies
 special pre-computation avoids sudden phase shifts
 MSK (Minimum Shift Keying)
 bit separated into even and odd bits, the duration of each bit is
doubled
 depending on the bit values (even, odd) the higher or lower
frequency, original or inverted is chosen
 the frequency of one carrier is twice the frequency of the other
 even higher bandwidth efficiency using a Gaussian low-pass
filter filtering out the unwanted signals  GMSK (Gaussian
MSK), used in GSM
33
Example of MSK
1
0
1
1
0
1
0
bit
data
even
0101
even bits
odd
0011
odd bits
signal
value
h l lh
- - ++
low
frequency
h: high frequency
n: low frequency
+: original signal
-: inverted signal
high
frequency
MSK
signal
t
No phase shifts!
34
Advanced Phase Shift Keying
 BPSK (Binary Phase Shift Keying):
• bit value 0: sine wave
• bit value 1: inverted sine wave
• very simple PSK
• low spectral efficiency
• robust, used e.g. in satellite systems
 QPSK (Quadrature Phase Shift Keying):
• 2 bits coded as one symbol
• symbol determines shift of sine wave
• needs less bandwidth compared to
BPSK
• more complex
 Often also transmission of relative, not
absolute phase shift: DQPSK - Differential
QPSK (IS-136, PHS)
Q
1
10
I
0
Q
11
I
00
01
A
t
11
10
00
01
35
Advanced Phase Shift Keying
 Quadrature Amplitude Modulation (QAM): combines
amplitude and phase modulation
• it is possible to code n bits using one symbol
• 2n discrete levels, n=2 identical to QPSK
• bit error rate increases with n, but less errors compared to comparable
PSK schemes
 Example:
16-QAM (4 bits = 1 symbol)
Q
Symbols 0011 and 0001 have the same phase φ,
but different amplitude a. 0000 and 1000 have
different phase, but same amplitude.
 used in standard 9600 bit/s modems
0010
0011
0001
0000
φ
I
a
1000
36
Multiple Access Techniques
 Multiplexing in 4 dimensions
•
•
•
•
frequency (f)
time (t)
code (c)
space (si)
channels ki
k1
k2
k3
k4
k5
k6
c
t
c
t
 Goal: multiple use of a shared
medium
s1
f
f
s2
c
 Important: guard spaces
needed!
t
s3
f
37
Frequency Multiplexing
 Separation of the whole spectrum into smaller frequency bands
 A channel gets a certain band of the spectrum for the whole time
 Advantages:
• no dynamic coordination
necessary
• works also for analog signals
k1
k2
k3
k4
k5
k6
c
f
 Disadvantages:
• waste of bandwidth
if the traffic is
distributed unevenly
• inflexible
• guard spaces
t
38
Time Multiplexing
 A channel gets the whole spectrum for a certain amount of time
 Advantages:
• only one carrier in the
medium at any time
• throughput high even
for many users
k1
k2
k3
k4
k5
k6
c
 Disadvantages:
f
• precise
synchronization
necessary
t
39
Time and Frequency Multiplexing
 Combination of both methods
 A channel gets a certain frequency band for a certain amount of
time
 Example: GSM
 Advantages:
k1
k2
k3
k4
k5
• better protection against
tapping
• protection against frequency
selective interference
• higher data rates compared
to code multiplex
 but: precise coordination t
required
k6
c
f
40
Code Multiplexing
 Each channel has a unique code
k1
k2
 All channels use the same spectrum
at the same time
 Advantages:
k3
k4
k5
k6
c
• bandwidth efficient
• no coordination and synchronization necessary
• good protection against interference and tapping
f
 Disadvantages:
• lower user data rates
• more complex signal regeneration
 Implemented using spread spectrum
technology
t
41
FHSS (Frequency Hopping Spread
Spectrum)
 Frequency Hopping Spread Spectrum (FHSS) is a transmission
technology used in wireless transmissions where the data signal is
modulated with a narrowband carrier signal that "hops" in a
random but predictable sequence from frequency to frequency as
a function of time over a wide band of frequencies.
 Discrete changes of carrier frequency
• The total bandwidth is split into many channels of smaller bandwidth.
Transmitter and receiver stay on one of these channels for a certain time and
hop to another channel.
• This system implements FDM and TDM.
• The pattern of channel usage is called the hopping sequence, the time spent
on a channel with a certain frequency is called the dwell time.
• sequence of frequency changes determined via pseudo random number
sequence
42
FHSS (Frequency Hopping Spread
Spectrum)
 Two versions
• Fast Hopping: several frequencies per user bit
• Slow Hopping: several user bits per frequency (not immune to narrowband
interference)
 Advantages
• frequency selective fading and interference limited to short period
• simple implementation
• uses only small portion of spectrum at any time
 Disadvantages
• not as robust as DSSS
• simpler to detect
43
FHSS (Frequency Hopping Spread
Spectrum)
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
td: dwell time
44
FHSS (Frequency Hopping Spread
Spectrum)
user data
modulator
modulator
frequency
synthesizer
transmitter
hopping
sequence
narrowband
signal
received
signal
data
demodulator
hopping
sequence
spread
transmit
signal
narrowband
signal
frequency
synthesizer
demodulator
receiver
45
DSSS (Direct Sequence Spread Spectrum)
 Direct Sequence Spread Spectrum (DSSS) is a transmission technology used
in wireless transmissions where a data signal at the sending station is
combined with a higher data rate bit sequence, or chipping code. The chipping
code which increases the signal's resistance to interference.
 XOR of the signal with pseudo-random number (chipping sequence)
• many chips per bit (e.g., 128) result in higher bandwidth of the signal
 Advantages
tb
• reduces frequency selective
user data
fading
0
1
XOR
• in cellular networks
tc
• base stations can use the
chipping
same frequency range
sequence
• several base stations can
01101010110101
=
detect and recover the signal
resulting
• soft handover
signal
 Disadvantages
01101011001010
• precise power control necessary
tb: bit period
46
(synchronization)
tc: chip period
DSSS (Direct Sequence Spread Spectrum)
spread
spectrum
signal
user data
X
transmit
signal
modulator
chipping
sequence
radio
carrier
transmitter
correlator
lowpass
filtered
signal
received
signal
demodulator
radio
carrier
products
sampled
sums
data
X
integrator
decision
chipping
sequence
receiver
47
Space Division Multiple Access
 Space division multiple access (SDMA) uses directional
transmitters/antennas to cover angular regions.
 Different areas/regions can be served using the same frequency
channel. This method is suited to
• Satellite system: a narrowly focused beam to prevent the signal from
spreading too widely.
• Cellular phone system: base station covers a certain transmission area (cell).
Mobile devices communicate only via the base station
48
Comparison
SDMA/TDMA/FDMA/CDMA
Approach
Idea
SDMA
segment space into
cells/sectors
Terminals
only one terminal can
be active in one
cell/one sector
Signal
separation
cell structure, directed
antennas
TDMA
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
all terminals are
active for short
periods of time on
the same frequency
synchronization in
the time domain
FDMA
segment the
frequency band into
disjoint sub-bands
CDMA
spread the spectrum
using orthogonal codes
every terminal has its all terminals can be active
own frequency,
at the same place at the
uninterrupted
same moment,
uninterrupted
filtering in the
code plus special
frequency domain
receivers
Advantages very simple, increases established, fully
simple, established,
robust
inflexible, antennas
Disadvantages typically fixed
inflexible,
frequencies are a
scarce resource
flexible, less frequency
planning needed, soft
handover
complex receivers, needs
more complicated power
control for senders
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA
capacity per km²
Comment
only in combination
with TDMA, FDMA or
CDMA useful
digital, flexible
guard space
needed (multipath
propagation),
synchronization
difficult
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
49
Voice Coding
 The voice coding process converts the analog signal into its
equivalent digital representation without any noticeable distortion.
 The devices that perform the analog to digital conversion (at the
sender) and the reverse digital to analog signal conversion (at the
receiver) are known as codecs (coder/decoder).
 The Pulse position modulation (PPM) is a technique used for
converting an analog signal into its digital representation.
• The position of a pulse relative to its unmodulated time of occurrence is
varied in accordance with the message signal.
• Disadvantage: Perfect synchronization is required.
 Pulse Code Modulation (PCM) is a technique of converting an
analog signal to a digital signal.
• The audio signal is converted in samples according to the frequency of the
signal.
• Every sample is then written in the stream without using any compression
techniques.
50
Voice Coding
 PCM consists of three stages: sampling of the analog signal,
quantization, and binary encoding.
• Sampling
• The codec converts the analog speech signal to its digital representation
by sampling the signal at regular intervals of time.
• The series of pulses produced after sampling the analogy signal is known
as pulse amplitude modulation (PAM) pluses whose amplitudes are
proportional to that of the original signal.
• Quantization: A fixed number of amplitude laves are used to represent the
amplitudes of the PAM pulses. The distortion could occur. It is called
quantization.
• Binary encoding: The sequence of quantized PAM pulses are represented by
bit streams.
 PCM are not suitable for wireless networks because of limited
bandwidth.
 Vocoders are devices that makes use of knowledge (distinct
features/characteristics) of the actual structure and operation of
human speech production organs. Only those characteristics are
encoded, transmitted, and decoded so that it can achieve voice
51
transfer at low bit rates.
Error Control
 Error-correcting codes/forward error correction
• include enough redundant information to enable the receiver to deduce the
correct transmitted data.
• Used in unreliable channel such as wireless links
 Error-detecting codes
• include only enough redundancy to allow the receiver to request a
retransmission.
• Used in reliable channel such as fiber
 N-bit codeword = m-bit data + r-bit check
 The number of bit positions in which two codewords differ is
called the Hamming distance.
 Example: Hamming distance is 3.
10001001
xor 10110001
00111000 3 bit difference
52
Error Control
 If two codewords are a Hamming distance d apart, it will require
d single-bit errors to convert one into the other.
 To detect d errors, we need a distance d+1 code (because there is
no way to convert a valid codeword into another valid codeword
with d changes.  The detail needs mathematical analysis).
 Example: A simple error - detection code: (check bit)
 A parity bit is chosen so that the number of 1 bits in the
codeword is even or odd.
000(0) - check bit
001 1
010 1
 That is Hamming distance of parity bit code is 2 = d + 1  can
detect d = 1 error
53
Error Control - Error Correction Codes
 To correct d errors, we need a distance 2d+1 code. (because d
changes is not enough to recover the original valid codeword but
only to convert to other valid codeword  The detail needs
mathematical analysis).
 Examples: Consider a code with four valid codewords:
•
•
•
•
•
0000000000, 0000011111, 1111100000, 1111111111
Hamming distance is 5. It can correct double errors.
If 0000000111 is received, the receiver knows the original
is 00000011111. But if a triple errors change 0000000000
to 0000000111, the error will not be corrected properly.
 Correct  round off to the nearest codeword.
54
Error Control
 m data bits  2m legal messages = codewords
 Examples: Consider a code with four valid codewords:
• 000000, 000111, 111000, 111111  differ by 3
• 011000, 101000, 110000, 111001, 111010, and 111100 are six invalid
code words a distance 1 from 111000.
•  each valid codeword has n invalid codewords within hamming distance
1. To correct these n invalid codewords with 1 bit error, n + 1 bit patterns
are required.
• Since there are a total of 2n bit patterns
 (n + 1) x 2m ≤ 2n  (m + r + 1) x 2m ≤ 2m+r  m + r + 1 ≤ 2r
• Given m, this puts a lower limit on the number of check bits
needed to correct 1 error.
m = 7  7 + r + 1 ≤ 2r, 8 ≤ 2r - r  r = 4
55
Hamming Codes
 Bits are numbered from the left. Checkbits are bits
numbered powers of 2. {1,2,4,8, ...}. Each check bit
forces the parity of some collection of bits, including
itself, to be even or odd.
 To see which check bits the data in position k
contributes to, write k as a sum of powers of 2.
Data bits
Check bits
Check bits
Data bits
3
5
1+2
1+4
1
6
7
2+4 1+2+4
2
9
10
11
1+8 2+8 1+2+8
4
8
3 + 5 + 7 + 9 + 11 3 + 6 + 7 + 10 + 11 5 + 6 + 7 9 + 10 + 11
56
Constructing Hamming Codes
 Consider an ASCII code H (1001000). Use even
parity:
H
1001000
_ _ 1 _ 001 _ 000
Bit
Calculation
Result
1
(1 + 0 + 1 + 0 + 0) mod 2 = 0
0
2
(1 + 0 + 1 + 0 + 0) mod 2 = 0
0
4
(0 + 0 + 1) mod 2 = 1
1
8
(0 + 0 + 0 mod 2 = 0
0
The codeword is 00110010000.
57
Error Control - Hamming Codes
 When a codeword arrives, counter = 0.
If a check bit k does not have the correct parity, it adds k to the
counter.
 Supposed there is only one bit error.
If counter = 0  no error
If counter = 11  bit 11 in error.
ASCII
codeword
H 1001000
0 0 1 1 001 0 000
G 1100001
1 0 1 1 100 1 001
If G is received as (0) 0 1 1 100 1 001, 1st bit is incorrect.
If G is received as (1)(0){0} 1 100 1 001
1st and 2nd has errors.  3rd bit is incorrect.
58
Error Control - Cyclic Redundancy Check
 A major goal in designing error detection algorithms is to maximize the
probability of detecting errors using only a small number of redundant bits.
 In general, correcting is more expensive than detecting and re-transmitting.
 Add k bits of redundant data to an n-bit message
• want to use k << n to detect errors
• e.g., k = 32 and n = 12,000 (1500 bytes)
 Represent n-bit message as n-1 degree polynomial
• e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1
 Let k be the degree of some divisor/generator polynomial
• e.g., G(x) = x3 + x2 + 1
 Polynomial arithmetic is performed modulo 2.
10011011
11001010
01010001  EX-OR result.
 Sender & receiver agree upon a generator polynomial G(x).
59
Cyclic Redundancy Check
 Algorithm for computing the checksum
1. shift left r bits (append r zero bits to low order end of the
frame), i.e., M(x)xr
2. divide the bit string corresponding to G(x) into (xr)M(x).
3. subtract (or add) remainder of M(x)xr / G(x) from M(x)xr
using XOR, call the result T(x). Transmit T(x).
 Suppose that a transmission error E(x) has occured and
T(x)+E(x) arrives instead of T(x). Received polynomial T(x) +
E(x) = (T(x)+E(x))/G(x) = T(x)/G(x) + E(x)/G(x) = E(x)/G(x)
• E(x) = 0 implies no errors
 Divide (T(x) + E(x)) by G(x); remainder zero if:
• E(x) was zero (no error), or
• E(x) is exactly divisible by C(x)
60
CRC Example
M(x)=1101011011
C(x)=10011
k=4
1100001010
-------------10011 /11010110110000
10011
----10011
10011
----10110
10011
----10100
10011
----1110
P(x) = 1101011011 1110
61
Selecting G(x)
 Selecting G(x)
• All single-bit errors, as long as the xk and x0 terms have non-zero
coefficients.
• All double-bit errors, as long as G(x) contains a factor with at least three
terms
• Any odd number of errors, as long as G(x) contains the factor (x + 1)
• Any ‘burst’ error (i.e., sequence of consecutive error bits) for which the
length of the burst is less than k bits.
• Most burst errors of larger than k bits can also be detected
 International standards for G(x):
CRC-12 = x12+x11+x3+x2+x1+1
CRC-16 = x16+x15+x2+1  16 bit check sum.
 catches all single, double,odd errors.
 catches all burst errors of length < 16
 A simple shift register circuit can be constructed to compute and
verify the checksums in hardware.
62
Error Control
 Convolution Coding
• Used for long bit streams in noisy channels.
• Two mechanisms: Sequential decoding and viterbi decoding
 Turbo codes are a class of recently-developed high-performance
error correction codes finding use in deep-space satellite
communications and other applications where designers seek to
achieve maximal information transfer over a limited-bandwidth
communication link in the presence of data-corrupting noise.
63
Computer Networks
 Internetworks
• Different networks are connected by means of machines called
gateways.
• A collection of interconnected networks is called an internetwork or
internet.
• A common form of internet is a collection of LANs connected by a
WAN.
 Network Software
•
•
•
•
•
Protocol Hierarchies
Design Issues for the Layers
Connection-Oriented and Connectionless Services
Service Primitives
The Relationship of Services to Protocols
64
Protocol Hierarchies
 Protocol Hierarchies
• The reduce design complexity, most networks are organized as a stack of
layers or levels.
• A protocol is an agreement between the communication parties.
• The entities comprising the corresponding layers on different machines are
called peers.
• The physical medium is the place through which actual communication
occurs.
• Between each pair of adjacent layers is an interface. It defines which
primitive operations and services the lower layer makes available to the
upper one.
 Network Architecture
• A network architecture is a set of layers and protocols used to reduce
network design complexity.
• A protocol stack is a list of protocols used by a certain system, one protocol
per layer.
65
Network Software
Protocol Hierarchies
Layers, protocols, and interfaces.
66
Protocol Hierarchies
The philosopher-translator-secretary architecture.
67
Protocol Hierarchies
Example information flow supporting virtual communication in layer 5.
68
Design Issues for the Layers
 Addressing: a specific destination needs to be
specified.
 Error Control: errors need to be detected and corrected.
 Flow Control: A fast sender is kept from swamping a
slow receiver with data.
 Multiplexing: the same connection is used for multiple,
unrelated conversations.
 Routing: a route must be chosen for a packet to
transmit.
69
Connection-Oriented and Connectionless
Services
 Connection-oriented: connection needs to be established
before communication: telephone
 Connectionless (datagram): connection needs not to be
established before communication: postal system
 Each service can be characterized by a Quality of Service
(QoS).
 Request-reply: the sender transmits a request; the reply
contains the answer.
 Reliable communication is communication where messages are
guaranteed to reach their destination complete and uncorrupted
and in the order they were sent.
 Why is unreliable communication used?
• Reliable communication is not available.
• The delay in a reliable service might not be acceptable such
as real-time applications.
70
Connection-Oriented and Connectionless
Services
Six different types of service.
71
Service Primitives
 A service is specified by a set of primitives
(operations) available to a user process to access the
service.
Five service primitives for implementing a simple connection72
oriented service.
Service Primitives
Packets sent in a simple client-server interaction on a
73
connection-oriented network.
Services to Protocols Relationship
• Services relate to the interfaces between layers.
Protocol relate to the packets sent between peer
entities.
The relationship between a service and a protocol.
74
Reference Models
 The OSI Reference Model
 The TCP/IP Reference Model
 A Comparison of OSI and TCP/IP
 A Critique of the OSI Model and Protocols
 A Critique of the TCP/IP Reference Model
 The OSI (Open Systems Interconnection) 7-Layer
Reference Model [ISO,1984] is a guide that
specifies what each layer should do, but not how each
layer is implemented.
 The TCP/IP Reference Model is not of much use but
the protocols associated with it are widely used.
75
Reference Model
• OSI Reference Model
1. Physical Layer - transmission of raw bits over a physical
channel.
2. Data Link Layer - provide an error-free point-to-point link to
transmit data and control frames (sequencing frames,
retransmission) between two directly connected nodes.
3. Network Layer - provide a point-to-point link between any
two switching nodes (routing, congestion control).
4. Transport Layer - provide a link between any two processes
in two hosts (connection-oriented or connectionless).
5. Session Layer - manage conversation between two peer
session entities.
6. Presentation Layer - present data in a meaningful format
(compress, encode, and convert data).
7. Application Layer - a variety of user applications (e-mail, ftp,
etc.).
76
ISO 7-Layer Reference Model
End host
End host
Application
Application
Various applications (FTP,HTTP,…)
Presentation
Presentation
Present data in a meaningful format
Session
Session
Provide session semantics (RPC)
Transport
Transport
Reliable, end-to-end byte stream (TCP)
Network
Network
Network
Network
Unreliable end-to-end tx of packets
Data link
Physical
Data link
Data link
Data link
Reliable
transmission (tx) of
frames
Physical
Physical
Physical
Unreliable
transmission
(tx) of raw bits
One or more
nodes
77
within the network
Reference Models
The OSI
reference
model.
78
TCP/IP Reference Model
 TCP/IP Reference Model
• The internet layer defines an official packet format and
protocol called IP (Internet Protocol) and specifies how IP
packets are routed from the source to the destination.
• The transport layer is designed to allow peer entities to talk.
• TCP (Transmission Control Protocol) is a reliable
connection-oriented protocol that allows a byte stream to
be delivered.
• UDP (User Datagram Protocol) is an unreliable,
connectionless protocol for applications.
• The application layer contains all the higher-level protocols.
• The host-to-network layer points out that the host has to
connect to the network.
79
Reference Models
The TCP/IP reference model.
80
Reference Models
Protocols and networks in the TCP/IP model initially.
81
Connection-Oriented Networks
 The X.25 protocol, adopted as a standard by the
Consultative Committee for International Telegraph
and Telephone (CCITT), is a connection-oriented
network protocol.
 Frame relay is connection-oriented network with no
error control and no flow control.
 ATM (asynchronous transfer mode) is a dedicatedconnection switching technology that organizes digital
data into 53-byte cell units and transmits them over a
physical medium using digital signal technology.
82
ATM Virtual Circuits
A virtual circuit.
An ATM cell.
83
ATM Reference Model
 The physical layer deals with the physical medium.
• The PMD (Physical Medium Dependent) sublayer interfaces
to the actual cable.
• The TC (Transmission Convergence) sublayer converts back
forth a bit stream to a cell stream.
 The ATM layer deals with cells and cell transport.
 The ATM adaptation layer deals with segmentation and
re-assembly.
• The SAR (Segmentation And Reassembly) sublayer breaks
up packets into cells and put them back.
• The CS (Convergence Sublayer) is used to offer different
kind of services to the upper layers.
84
The ATM Reference Model
The ATM reference model.
85
The ATM Reference Model
The ATM layers and sublayers and their functions.
86
Shortcomings of the ATM Reference Model
 Each 53-byte cell has a 5-byte header. This constitutes a
significant control overhead.
 Complex mechanisms are required for ensuring fairness among
connections and provisioning quality of service.
 Complex packets scheduling is required due to the varying
Ea
delays.
 The high cost and complexity of dvices.
 Lack of scalability
87
IEEE 802 Standards
 IEEE 802 standards defines the physical and data link layer for LANs.
The important ones are marked with *. The ones marked with 
88
are hibernating. The one marked with † gave up.
IEEE 802 Standard
 The physical layer in a LAN deals with the actual physical
transmission medium used for communication.
• Some commonly used physical media: twisted pair, coaxial cable, optical
fiber, and radio waves.
 In IEEE 802 Logical Link Control (LLC) forms the upper half of
the data link layer. Medium access control (MAC) forms the
lower sublayer.
• error-controlled, flow-controlled
• Adds an LCC header, containing sequence and acknowledgement numbers.
 LLC provides three service options:
• Unreliable datagram service
• Acknowledged datagram service
• Reliable connection-oriented service
89
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
90
IEEE 802 Standard
 The medium access control sublayer (MAC)
• It directly interfaces with the physical layer.
• It provides services such as addressing, framing, and medium access
control.
 The Pure Aloha Protocol (by Abramson in 1970s) is one of oldest
MAC protocol in which a station transmits the data whenever it
is available. Then, the station listens to the channel to see if a
collision occurred. If the frame was destroyed, the station waits
for a random length of time and tries again.
 In slotted Aloha (by Roberts in 1972) a computer is not permitted
to send whenever a carriage return is typed but wait for a time
slot.
91
Carrier Sense Multiple Access (CSMA)
 Protocols in which stations listen for a carrier and act accordingly
are called carrier sense protocols.
 1-persistent CSMA
Channel Busy  Continue sensing until free and then grab.
Channel Idle  Transmit with probability 1.
Collision  Wait for a random length of time and try again.
 Nonpersisten CSMA:
Channel Busy  Wait for a random length of time and try again.
Channel Idle  Transmit.
Collision  Wait for a random length of time and try again.
 p-persistent CSMA:
Channel Busy  Continue sensing until free (same as idle).
Channel Idle  Transmit with probability p, and defer transmitting until the
next slot with probability q = 1-p.
Collision  Wait for a random length of time and try again.
92
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various
93
random access protocols.
CSMA/CD
 Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is a
protocol for carrier transmission access in Ethernet networks.
• In CSMA/CD, any device can try to send a frame at any time.
Each device senses whether the line is idle and therefore
available to be used.
• If it is available, the device begins to transmit its first frame. If
another device has tried to send at the same time, a collision is
said to occur and the frames are discarded. Each device then
waits a random amount of time and retries until successful in
getting its transmission sent.
• When there is collision, the station wait some time between 0
to 2n - 1 slotted time at the n's trial. This is called back-off
algorithm. Usually, after 16 trials the station gives up.
94
IEEE 802.3 Standard
 IEEE 802.3 is standard using Carrier Sense Multiple
Access/Collision Detection (CSMA/CD). It is commonly
referred to as the Ethernet standard.
 IEEE 802.3 supports data transfer rate up to 10 Mbps.
 Fast Ethernet (IEEE 802.3u) specifies data transfer rate up to 100
Mbps.
 The 802.3 committee decided to keep 802.3 for the fast Ethernet
(802.3u).
• Backward compatible
• A new protocol might have problems.
• Get job done before the technology changed.
 Gigabit Ethernet (IEEE 802.3z) specifies data transfer rate up to
1 Gbps.
95
IEEE 802.3 Physical Layer
 10Base2 means that is operates at 10 Mbps, uses
baseband signaling, and support segments up to 200
meters.
 10Base-T became dominant due to its use of existing
wiring and the ease of maintenance .
The most common kinds of Ethernet cabling.
96
Fast/Gigabit Ethernet
 100Base-T4 – 4 twisted pairs achieve 100 Mbps.
The original fast Ethernet cabling.
Gigabit Ethernet cabling.
97
Ethernet MAC Sublayer Protocol
 Preamble – used for sender and receiver to synchronize their
clock.
 Addresses
•
•
•
•
unique, 48-bit unicast address assigned to each adapter
example: 8:0:e4:b1:2
broadcast: all 1s, the set of all recipient nodes
Multicast: first bit is 1,a group of recipient nodes
Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.
98
Wireless LAN: 802.11
 A wireless LAN is one in which a mobile user can connect to a
local area network (LAN) through a wireless (radio) connection.
 A standard, IEEE 802.11, specifies the technologies for wireless
LANs.
 It is designed to work in two modes:
• In the presence of a base station: access point
• In the absence of a base station: ad hoc networking
 Physical Layer
• It supports three different physical layers:
• Frequency hopping spread spectrum (FHSS)
• Direct sequence spread spectrum (DSSS)
• Infrared
• Clear channel assessment (CCA): It provides mechanisms for sensing the
wireless channel and determine whether or not it is idle.
 MAC Sublayer follows carrier sense multiple access with
collision avoidance (CSMA/CA).
99
Wireless LANs
(a) Wireless networking with a base station.
(b) Ad hoc networking.
100
IEEE 802.11 Standard
 The 802.11 task group has the object to develop MAC layer and
physical layer specifications for wireless connectivity.
 The 802.11a task group created a standard for wireless LAN
operations in the 5 GHz frequency baud, where data rates of up to
54 Mbps are possible.
 The 802.11b task group created a standard for wireless LAN
operations in the 2.4 GHz Industrial, Scientific, and Medical (ISM)
band, which is freely available for use throughout the world.
 The 802.11c task group devised standards for bridging operations.
 The 802.11d task group published the definitions and requirements
for enabling the operation of the 802.11 standard in countries
where the 802.11 standard is not adopted yet.
 The 802.11e task group defined an extension of the
802.11 standard for quality of service (QoS).
101
IEEE 802.11 Standard
 The 802.11f developed specifications for implementing access
points and distribution systems.
 The 802.11g task groups extended the 802.11b standard to support
high-speed transmissions of up to 54 Mbps in the 2.4 GHz
frequency.
 The 802.11h task groups developed the MAC layer standard that
comply with European regulations for 5 GHz wireless LAN.
 The 802.11i group is working on mechanisms for enhancing
security in the 802.11 standard.
 The 802.11j task group is working on mechanisms for enhancing
security in the 802.11 MAC physical layer protocols to
additionally operate in the newly available Japanese 4.9 GHz and 5
GHz bands.
 The 802.11n defines standardized modifications to the 802.11
102
MAC and physical layers to allows at least 100 Mbps.
Wireless Networks
 Wireless networks are computer networks that use radio frequency
channels as their physical medium for communication.
 The first wireless radio communication system was invented by
Guglielmo Marconi in 1897.
 Radio and television broadcasting are common applications of
wireless communications techniques.
 The wireless communications industry includes cellular telephony,
wireless LANs, and satellite-based communication networks.
 In cellular networks a fixed based station serving all mobile
phones in its coverage area is called a cell.
 The first-generation (1G) cellular networks used analogy signal
technology.
• They used frequency modulation.
• Voice communication
• Example: advanced mobile phone system (AMPS)
103
Cellular Systems
 The second-generation (2G) cellular systems used digital
transmission mechanisms such as TDMA and CDMA.
• Voice communication
• Example: global system for mobile communication (GSM) in Europe, IS136 in States, Personal Digital System (PDS) in Japan.
 The present system is called 2.5 G. General packet Radio Services
(GPRS) has been deployed for data communication.
 The third-generation (3G) systems provides services such as
enhanced multimedia, bandwidth up to 2 Mbps.
• Standards: wideband code division multiple access (W-CDMA), universal
mobile telecommunications system (UMTS)
 The fourth-generation (4G) systems provides further
improvements such as higher bandwidth, enhanced multimedia,
universal access, and portability across all types of devices.
104
Wireless Local Area Network (WLAN)
 The wireless Local Area Network (WLAN) is a type of local-area
network that uses radio waves to communicate between nodes.
 A stationary node called an access point (AP) coordinates the
communication between nodes.
 The two main standards for WLANs are the IEEE 802.11 standard
and European Telecommunications Standards Instititue (ETSI)
HIPERLAN standard.
 Wireless personal area networks (WPANs) are short-distance
wireless networks.
 Bluetooth is a popular WPAN specification.
• Work within 10 m.
• Bluetooth Special Interest Group (SIG) including Ericsson, Intel, IBM,
Nokia, and Toshiba is the driving force for Bluetooth.
 The IEEE 802.15 is a standard for WPAN.
105
Ad Hoc/Hybrid Wireless Network
 An ad hoc wireless network is an autonomous system of mobile
nodes connected through wireless links. It doesn’t have any fixed
infrastructure.
 Hybrid networking combines the advantages of infrastructurebased and less networks.
• Example: multi-hop cellular network (MCN), integrated cellular and ad hoc
relaying system (iCAR), multi-power architecture for cellular networks
(MuPAC).
106
Network Standardization
 Who’s Who in the Telecommunications World: ITU
 Who’s Who in the International Standards World: ISO, ANSI,
NIST, IEEE
 Who’s Who in the Internet Standards World
•
•
•
•
IAB (Internet Architecture Board)
A Request for Comments (RFC) is a formal document from the Internet.
IRTF (Internet Research Task Force)
IETF (Internet Engineering Task Force)
 Main sectors: Radiocommunications (ITU-R),
Telecommunications Standardization (ITU-T), Development
(ITU-D)
 Classes of Members: National governments, Sector members,
Associate members, Regulatory agencies
107
Wireless systems: overview of the
development
cellular phones
satellites
1983:
AMPS
1982:
Inmarsat-A
1984:
CT1
1986:
NMT 900
1987:
CT1+
1988:
Inmarsat-C
1991:
CDMA
1991:
D-AMPS
1989:
CT 2
1992:
Inmarsat-B
Inmarsat-M
1993:
PDC
1994:
DCS 1800
analogue
wireless LAN
1980:
CT0
1981:
NMT 450
1992:
GSM
cordless
phones
1991:
DECT
1998:
Iridium
2000:
GPRS
1997:
IEEE 802.11
1999:
802.11b, Bluetooth
2000:
IEEE 802.11a
2001:
IMT-2000
2003:
IEEE 802.11g
digital
4G – fourth generation: when and how?
199x:
proprietary
200?:
Fourth Generation
(Internet based)
Areas of research in mobile communication
 Wireless Communication
•
•
•
•
transmission quality (bandwidth, error rate, delay)
modulation, coding, interference
media access, regulations
...
 Mobility
•
•
•
•
location dependent services
location transparency
quality of service support (delay, jitter, security)
...
 Portability
• power consumption
• limited computing power, sizes of display, ...
• usability
109
Simple reference model used here
Application
Application
Transport
Transport
Network
Network
Data Link
Physical
Radio
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Medium
110
Influence of mobile communication to the
layer model
Application layer
Transport layer
Network layer
Data link layer
Physical layer
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
service location
new applications, multimedia
adaptive applications
congestion and flow control
quality of service
addressing, routing,
device location
hand-over
authentication
media access
multiplexing
media access control
encryption
modulation
interference
attenuation
frequency
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Metric Units
 The metric prefixes are typically abbreviated by their
first letters, with the units greater than 1 capitalized.
 m is for milli and µ is for micro.
 For storage, Kilo means 210. For communication, 1Kbps means 1000 bits per second.
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