Transcript Part 4

Chapter 4
Communications,
Theory and Media
Postacademic Interuniversity Course in Information Technology – Module C1
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A three layers model.
Networks Layer
Postacademic Interuniversity Course in Information Technology – Module C1
Connectivity
Internet & Transport Layer
Interoperability
Applications Layer
p2
Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Parallel Transmission
In computers, data is structured in bytes
Clock
Disadvantages :
Differences in propagation delay
Cost of multiple channels
Consequence :
Restricted to very short distances
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Serial Transmission
Parallel in
Serial out
b7
Serial in
Parallel out
b0
Serial Data
b0b1b2b3b4b5b6b7
Clock
Transmission rate expressed in bits/second
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Serial Transmission
with clock/data multiplexing
Serial Data + Clock
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Digital Data Communications
011001
TX
Modem
Analog communication channel
011001
RX
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Encoding and Decoding
• Transmitter (Tx)
– Input : stream of binary numbers
– Output : stream of analog signals suitable for
transmission over long distances
• Receiver (Rx)
– Input : stream of analog signals
• generated by transmitter
• distorted by transmission channel
– Compares each input signal with all signals which
could have been transmitted and decides from
which one the input is a distorted image.
– Output : stream of binary numbers, preferably
identical to the input of the transmitter
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Analog Transmission Channel
Characterized by :
• Bandwidth
– Difference between highest and lowest frequency of
sine waves which can be transmitted
Received power
B
Frequency
– Number of possible state changes per second
• Signal to Noise ratio
– S/N = (signal power) / (noise power)
– S/N determines number of distinct states which can
be distinguished within a given observation interval
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Binary vs. Multi-bit encoding
t
t
0
1
0
0
01 10 00 11
Modulation rate = 1/t (in Baud)
Data rate = (1/t) Log 2 n (in b/s)
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Shannon’s Theorem
DataRate <= B.Log2(1+S/N)
B : Channel Bandwidth (in Hertz)
S/N : Signal to Noise ratio
Example:
Telephone channel,
B = 3000 Hz, S/N = 1000
DataRate <= 30 000 b/s
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Error Detection Example
Belgian Bank Account Numbers
• Bank account number structure
– Bank identification : 3 digits
– Account number : 7 digits
– Error detection : 2 digits
• The ten first digits modulo 97 are appended for error
detection purposes.
• This algorithm allows detection of all single digit errors
• Example :
– 140-0571659-08. 1400571659 MOD 97 = 08
– 140-0671659-08. 1400671659 MOD 97 = 01
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Error detection and correction
Length of messages :
k + r <= LMax
Informative message:
k bits
Redundancy:
r bits, f(inf.mess.)
# Messages send:
2k
# Messages received:
2 k+r
i=1
Hamming Distance (X-Y): |Xi-Yi|
k+r
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Error detecting codes
k = 1; r = 1; red.bit = inf.bit.
01
11
00
00
Hd = 2
11
10
Single bit errors are detected if
hamming distance between legitimate messages > 1.
No guessing is possible as erroneous messages are
at equal distances from several correct ones.
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Error correcting codes
k = 1; r = 2; red.bits = inf.bit.
011
010
111
110
001
000
000
Hd = 3
111
101
100
Hamming distance between legitimate messages > 2.
This implies that each erroneous message is closer
to one correct message than to any other.
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Error correcting codes
Required Overhead
for single bit error correction
k+r < 2r
information
redundancy
Overhead
1
<= 4
<= 11
<= 26
<= 57
<= 120
<= 247
2
3
4
5
6
7
8
200 %
75 %
36 %
19 %
11 %
6%
3%
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Error Correction
• Error detecting codes
– Correction by retransmission of erroneous blocks
– If few errors, very low overhead
– Most common approach to error correction in data
communications
• Error correcting codes
– Very high overhead with short data blocks
– Longer data blocks can have multiple errors
– Used when retransmission impossible or impractical
– Also used when error rate rather high.
– Error correcting codes for long blocks, with multiple
errors exist and are used (trellis encoding)
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Snell’s Law
sin 1
sin 2
2
n2
n1
n2

n1
1
=
n2
n1
n2
n1
c
>c
n2 < n1
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Optical Fibers
(step index)
n2 < n1
n2
n1
Protective coating
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Multimode Fiber
Diameter : > 50 
Step index fiber
Graded index fiber
Low cost but limited bandwidth * distance
due to multimode dispersion
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Multimode Dispersion
t
t
Step index fiber :
< 50 MHz.Km
Graded Index Fiber : < 1000 MHz.Km (1990)
< 5000 MHz.Km (2000)
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Monomode Fiber
Diameter : < 5 
Only one propagation mode possible
Higher cost due to end equipment
but enormous bandwidth*distance product
10 Gb/s over 500 Km optical sections (1995)
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Wave Domain Multiplexing
Each color can carry an independent data flow.
In 2000
40 colors carrying each 10 Gb/s or
80 colors carrying each 2.5 Gb/s
were commercially available
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Optical amplifiers
Pump
laser
Erbium
doped fiber
Erbium atoms are pumped into a higher
energy state by the light of the pump laser,
they fall back in synchronism with the
incoming light, amplifying it.
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Optical Switching
From IEEE Com.Mag.V39,N1, Jan 2001.
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Coaxial Cables
Insulator
Conductor
Conductor
Protective coating
Monomode propagation for all data applications
Transmission rates up to some Gb/s
Distance limited by electrical attenuation
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Twisted Pairs
Performance highly
dependant on cable
quality
Transmission speed up
to several 100 Mb/s
for distances of up to
100 m.
with better cables
(class 5 or 6)
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Contents
• Communications Theory
– Parallel vs. serial transmission
– Transmission Capacity (Shannon)
– Error detection and correction
• Communications Media
– Optical fibers
– Coaxial cables
– Twisted pairs
– Wireless
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Wireless Communications
Why ?
Mobile terminals
Cost of wiring
Why Not ?
Lower data rates
Lower reliability
Potential Lack of Security
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Wireless Communications
Main restriction:
Limited availability of bandwidth
in the electromagnetic spectrum
Some solutions:
• Displace some heavy users
• Reuse of frequencies at different locations
(cellular radio, infrared LAN’s)
• Sharing of a set of frequencies
(spread spectrum radio)
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Reuse of Frequencies
P(r) = Pt/r2
r
transmitter
rmax =
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Pt
Pmin
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Cellular Radio
Automatic handover allows
continuous operation of mobiles
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Cellular Radio
k = number available frequencies per cell
S = Area of a cell
n = Number of simultaneous calls per km2
pt = Power of transmitter
pm= Minimal power at receiver input
n =k/S
p t = pm * S
With smaller cells,
- more antenna sites are needed ...
- more simultaneous calls are possible
- transmitted power can be reduced
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Cellular Radio
in practice
Flanders
Postacademic Interuniversity Course in Information Technology – Module C1
Ardennes
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Digital Cellular Telephony
Name
DECT
GSM
DCS
1800
Freq.(MHz)
1880-1890
890-915
1710-1785
935-960
1805-1880
# rad.ch.
12
124
248
P.max.(W)
0.25
2
1
r.max.(Km)
0.2
35
8
Voicerate (Kb/s)
32
13
13
Capacity (E/Km2)
10 000
1000
2000
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Infrared LAN’s
Limited to line of sight, within a few meters.
IR technology also widely used for data transfers
between laptops, PDA’s and mobile phones.
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Bluetooth
Micro-lan’s designed to eliminate desktop wires
and connectors on hand-held devices
Frequency : 2.4 GHz unlicensed general purpose band
Power : Class 3 = 1 mW (Cl. 2 = 10 mW, Cl.1 = 100 mW)
Distances : Class 3 = 5m, up to 100m with class1.
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Wireless Local Loop
Radio is sometimes cheaper
than digging the streets !
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Wireless Local Loop
Using planes or balloons
could be simpler than
getting building permits
for antennas !!!
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Microwave Links
• Cost effective for line of sight communications (30 Km)
• High transmission capacity (several Mb/s)
• transmission impaired by heavy rain
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Satellite Communications
Geostationary
36000 Km
Round trip Delay = 240 ms
High power ground stations
Used for TV and paging broadcasting and for point to
point links where terrestrial lines would be unpractical
or too expensive
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Satellite Communications
Low power ground stations
Low Orbit
Short round trip delays
Upcoming applications:
Narrowband mobile
communications
(TFTS airline phones,
IRIDIUM mobile phones,
INMARSAT communications, ...)
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Introduced concepts
•
•
•
•
•
•
•
•
•
Serial transmission
Baud rate vs. throughput
An upper limit for transmission capacity
Error detection and correction
Optical transmission and switching
Coaxial cables and Twisted pairs
Cellular radio and handover
Point to point radio
Geostationary and low orbit satellites
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