1. Mobile Computing COE 446
Introduction
Tarek Sheltami
KFUPM
CCSE
COE
http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm
Principles of Wireless Networks
K. Pahlavan and P. Krishnamurth
February 15, 2014
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2. Introduction
Background:



# wireless (mobile) phone subscribers now
exceeds # wired phone subscribers!
computer nets: laptops, palmtops, PDAs,
Internet-enabled phone promise anytime
untethered Internet access
two important (but different) challenges


communication over wireless link
handling mobile user who changes point of
attachment to network
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3. Cellular Subscribers
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4. ..Cellular Subscribers
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5. Characteristics of selected wireless
link standards
54 Mbps
5-11 Mbps
1 Mbps
802.11{a,g}
802.11b
.11 p-to-p link
802.15
3G
UMTS/WCDMA, CDMA2000
384 Kbps
2G
IS-95 CDMA, GSM
56 Kbps
Indoor
Outdoor
Mid range
outdoor
Long range
outdoor
10 – 30m
50 – 200m
200m – 4Km
5Km – 20Km
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6. Introduction- Conventional Wireless
Communications
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7. Components of cellular network
architecture
MSC
cell
 connects cells to wide area net
 manages call setup (more later!)
 handles mobility (more later!)
 covers geographical
region
 base station (BS)
analogous to 802.11 AP
 mobile users attach
to network through BS
 air-interface:
physical and link layer
protocol between
mobile and BS
Mobile
Switching
Center
Public telephone
network, and
Internet
Mobile
Switching
Center
wired network
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8. Cellular networks: the first hop
Two techniques for sharing
mobile-to-BS radio spectrum
 combined FDMA/TDMA:
divide spectrum in frequency
channels, divide each
channel into time slots
frequency
 CDMA: code division
bands
multiple access
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time slots
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9. Cellular standards: brief survey
2G systems: voice channels


IS-136 TDMA: combined FDMA/TDMA (north
america)
GSM (global system for mobile
communications): combined FDMA/TDMA


most widely deployed
IS-95 CDMA: code division multiple access
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10. Cellular standards: brief survey
2.5 G systems: voice and data channels
 for those who can’t wait for 3G service: 2G extensions
 general packet radio service (GPRS)
 evolved from GSM
 data sent on multiple channels (if available)
 enhanced data rates for global evolution (EDGE)
 also evolved from GSM, using enhanced modulation
 Date rates up to 384K
 CDMA-2000 (phase 1)
 data rates up to 144K
 evolved from IS-95
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11. Cellular standards: brief survey
3G systems: voice/data


Universal Mobile Telecommunications Service
(UMTS)
 GSM next step, but using CDMA
CDMA-2000
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12. Cellular standards: brief survey
3G+ systems: voice/data

High Speed Downlink Packet Access

Hybrid Automatic Repeat Request

Fast cell site selection

Adaptive Modulation and Coding
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13. ?Why HSDPA

Comparison Between 3G & 3.5G.

Data Rate ( 2Mbps -----> 10 Mbps)

Modulation ( QPSK -----> QPSK&16QAM)

Transmission Time Interval (TTI) ( 10ms ----> 2ms )

14. HSDPA EVOLUTION

15. GSM: indirect routing to mobile
home
network
HLR
2
home MSC consults HLR,
gets roaming number of
mobile in visited network
correspondent
home
Mobile
Switching
Center
1
VLR
3
Mobile
Switching
Center
4
home MSC sets up 2nd leg of call
to MSC in visited network
mobile
user
visited
network
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Public
switched
telephone
network
call routed
to home network
MSC in visited network completes
call through base station to mobile
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16. GSM: handoff with common MSC

VLR Mobile
Switching
Center
old
routing

Handoff goal: route call via new
base station (without
interruption)
handoff initiated by old BSS
new
routing
old BSS
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new BSS
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17. GSM: handoff between MSCs

home network
correspondent
Home
MSC
anchor MSC
MSC
PSTN
MSC

MSC

(a) before handoff
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anchor MSC: first MSC
visited during call
 call remains routed
through anchor MSC
new MSCs add on to end
of MSC chain as mobile
moves to new MSC
IS-41 allows optional path
minimization step to
shorten multi-MSC chain
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18. GSM: handoff between MSCs

home network
correspondent
Home
MSC
anchor MSC
MSC
PSTN
MSC

MSC

(b) after handoff
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anchor MSC: first MSC
visited during cal
 call remains routed
through anchor MSC
new MSCs add on to end
of MSC chain as mobile
moves to new MSC
IS-41 allows optional path
minimization step to
shorten multi-MSC chain
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19. Segmenting the Telecom Market
Narrowband
Broadband
Mobile
Cellular
WiMAX
3G
Local
WiFi
Cordless
Fixed
POTS
Dialup
DSL / Cable
The Evolution from Audio to Video
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20. WiMAX: A new paradigm
+3G
WIMAX
Incumbent Operator
Any Operator
Voice and Data
VoIP, Data, Video
Mbps 30
Mbps 100
Handsets $200
Consumer Products
Telecom ITU
Internet IEEE
Qualcomm
Intel & Others
month / $70 - $50
month / $40 - $20
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21. Networks
Potential of networking:
 move bits everywhere ,
cheaply , and with desired
performance characteristics
 Break the space barrier for
information
 Network provides “connectivity”

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22. What is “Connectivity ” ?


Direct or indirect access to every other
node in the network
Connectivity is the media needed to
communicate if you do not have a direct
pt-pt physical link.
 Tradeoff: Performance characteristics
worse than true physical link!
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23. .Connectivity


Building Blocks
 links: coax cable, optical fiber...
 nodes: general-purpose workstations...
Direct connectivity:
 point-to-point

multiple access
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24. Connectivity..

Indirect Connectivity
 switched networks
=> switches

inter-networks
=> routers
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25. Connectivity …


Internet:
 Best-effort
(no performance
guarantees)
 Packet-by-packet
A pt-pt physical link:
 Always-connected
 Fixed bandwidth
 Fixed delay
 Zero-jitter
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26. Wired and Wireless Multiple
Access





Most multiple access were originally developed for
wired networks
Requirements for wired & wireless networks are
different
The main difference between wired and wireless
channels are availability of BW and reliability of
transmission
The wired medium is moving toward optical media
with enormous BW and very reliable transmission
BW of wireless systems always limited because of
the air medium
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27. Wired and Wireless Multiple
..Access


Wireless medium always suffers from multi-path
and fading, which causes serious threat to reliable
data transmission over the communication link
Wireless have evolved around voice and data
application
Wireless Networks
Voice
Oriented
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Data Oriented
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28. Wired and Wireless Multiple
Access..



Voice oriented networks are designed for
relatively long telephone conversation as
the main application, therefore exchange of
several Mbytes of information in both
directions
Data oriented networks are designed for
bursts of data (packet switching)
Wireless networks assigns a time slot, a
portion of frequency, or a code to user
preferably for the entire length of the
conversation.
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29. ALOHA-Based Wireless Random Access
( Techniques (Pure ALOHA





MT transmits an information packets when the
packet arrives from the upper layers of the protocol
stack
MTs say “hello” to the air interface as the packet
arrives
Each packet is encoded with an error-detection
code
The BS checks the parity of the received packet, if
it is OK, it sends a short ACK packet
If no ACK received the packet is assumed lost in a
collision and it is transmitted again with randomly
selected delay to avoid repeated collisions
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30. ALOHA-Based Wireless Random Access
(.. Techniques (Pure ALOHA


Advantages

Simple

No synchronization between MTs
Disadvantage

Low throughput under heavy load conditions

Max throughput for pure ALOHA 18%
What is the max throughput of pure ALOHA network
with large number of users and transmission range of 1
Mbps?
Max Throughput = 1 Mbps X 18% = 180 Kbps
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31. ALOHA-Based Wireless Random Access
(.. Techniques (Slotted ALOHA





Transmission time is divided into time slots
BS transmits beacon signal for time and all MTs is
divided into time slots to this beacon signal
When MT generates a packet, it is buffered and
transmitted at the start of the next time slot
Assuming equal length packet, either we have a
complete collision or no collision
Throughput of slotted ALOHA = 36%, which is still
low
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32. ALOHA-Based Wireless Random Access
(.. Techniques (R-ALOHA


Time slots are divided into contention
periods and contention free periods
During contention interval, an MT uses very
short packets to contend for the upcoming
contention free intervals that will be used for
transmission of the long information packets
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33. ALOHA-Based Wireless Random Access
..Techniques

Disadvantages of ALOHA-based
Random Access:



The main drawback of ALOHA based contention is
the lack of efficiency caused by collision and
retransmission
Users don’t take into account what other users are
doing when they attempt to transmit data packets
There is no mechanisms to avoid collision
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34. ALOHA-Based Wireless Random Access
(.. Techniques (Pure ALOHA
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35. ALOHA-Based Wireless Random Access
(.. Techniques (Slotted ALOHA
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36. ALOHA-Based Wireless Random Access
(.. Techniques (R-ALOHA
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