April 10, 2023 1

Mobile Computing COE 446

IntroductionTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

April 10, 2023 2

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

Introduction

April 10, 2023 3

Cellular Subscribers

April 10, 2023 4

Cellular Subscribers..

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Characteristics of selected wireless link standards

384 Kbps384 Kbps

56 Kbps56 Kbps

54 Mbps54 Mbps

5-11 Mbps5-11 Mbps

1 Mbps1 Mbps

802.15

802.11b

802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid rangeoutdoor

200m – 4Km

Long rangeoutdoor

5Km – 20Km

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Introduction- Conventional Wireless Communications

April 10, 2023 7

Mobile Switching

Center

Public telephonenetwork, andInternet

Mobile Switching

Center

Components of cellular network architecture

connects cells to wide area net manages call setup (more later!) handles mobility (more later!)

MSC

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

cell

wired network

April 10, 2023 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

CDMA: code division multiple access

frequencybands

time slots

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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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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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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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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

Why HSDPA?

Comparison Between 3G & 3.5G. Data Rate ( 2Mbps -----> 10 Mbps)

Modulation ( QPSK -----> QPSK&16QAM)

Transmission Time Interval (TTI) ( 10ms ---->

2ms )

HSDPA EVOLUTION

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Public switched telephonenetwork

mobileuser

homeMobile

Switching Center

HLR home network

visitednetwork

correspondent

Mobile Switching

Center

VLR

GSM: indirect routing to mobile

1 call routed to home network

2

home MSC consults HLR,gets roaming number ofmobile in visited network

3

home MSC sets up 2nd leg of callto MSC in visited network

4

MSC in visited network completescall through base station to mobile

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Mobile Switching

Center

VLR

old BSSnew BSS

old routing

newrouting

GSM: handoff with common MSC Handoff goal: route call via

new base station (without interruption)

handoff initiated by old BSS

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home network

Home MSC

PSTN

correspondent

MSC

anchor MSC

MSCMSC

(a) before handoff

GSM: handoff between MSCs 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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home network

Home MSC

PSTN

correspondent

MSC

anchor MSC

MSCMSC

(b) after handoff

GSM: handoff between MSCs 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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Segmenting the Telecom Market

Cordless

WiMAX

WiFi

DSL / Cable

3G

Mobile

Local

Fixed

Narrowband Broadband

Dialup

Cellular

POTS

The Evolution from Audio to Video

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WiMAX: A new paradigmWIMAX

Any Operator

VoIP, Data, Video

100 Mbps

Consumer Products

Internet IEEE

Intel & Others

$20 - $40 / month

3G+

Incumbent Operator

Voice and Data

30 Mbps

$200 Handsets

Telecom ITU

Qualcomm

$50 - $70 / month

April 10, 2023 21

Potential of networking: move bits everywhere, cheaply, and with desired performance characteristics

Break the space barrier for information

Network provides “connectivity”

Networks

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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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Connectivity.

Building Blocks links: coax cable, optical fiber... nodes: general-purpose workstations...

Direct connectivity: point-to-point

multiple access

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Connectivity..

Indirect Connectivity switched networks

=> switches

inter-networks => routers

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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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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

April 10, 2023 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

Data Oriented

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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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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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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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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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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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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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ALOHA-Based Wireless Random Access Techniques (Pure ALOHA) ..

April 10, 2023 35

ALOHA-Based Wireless Random Access Techniques (Slotted ALOHA) ..

April 10, 2023 36

ALOHA-Based Wireless Random Access Techniques (R-ALOHA) ..