Chapter 7 outline · 7.5 Principles: addressing and routing to mobile users ... UMTS/WCDMA,...

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Chapter 7 outline7.1 Introduction

Mobility 7 5 Principles:Wireless

7.2 Wireless links, characteristics

7.5 Principles: addressing and routing to mobile users7 6 M bil IPcharacteristics

CDMA 7.3 IEEE 802.11

wireless LANs (“wi fi”)

7.6 Mobile IP 7.7 Handling mobility in

cellular networkswireless LANs ( wi-fi ) 7.4 Cellular Internet

Access 7.8 Mobility and higher-

layer protocols architecture standards (e.g., GSM) 7.9 Summary

Wireless and Mobile Networks (SSL) 7-112/5/2017

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Characteristics of selected i l ss li k st d dswireless link standards450 802.11n

802.11ac

5-11

54

802.11b

802.11a, g4G: LTE

802.11a, g point-to-point

bps)

1

4

802.15

Enhanced 3G: HSPA

a ra

te (M

b

056

.384

2G: GSM CDMA

3G: UMTS/WCDMA, CDMA2000

Dat

a

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

Long-rangeoutdoor

.056 2G: GSM, CDMA

12/5/2017Wireless and Mobile Networks (SSL) 7-2

200m – 4 Km 5Km – 20 Km

3

Elements of a wireless network

wireless hosts laptops, phones, sensors run applications may be stationary or

b lnetwork

infrastructure

mobile wireless does not always

mean mobility


4

Elements of a wireless networkf d

base station typically connected to

Infrastructure mode

yp ywired network

forwards packets between wired

t k d i lnetwork

infrastructure

network and wireless hosts in its “area” e.g., 802.11 access

point (AP)point (AP)


5

802.11: mobility within same subnet it h hi h AP i

switch

router switch: which AP is associated with H1? self-learning (Ch. 6) - switchg ( )

switch will see frame from H1 and “remember” which switch port can be

d h H1

AP 2

AP 1

used to reach H1 H1 moves but remains

in same IP subnet - IP AP 2

H1

in same IP subnet IP address can remain the same (See subsection 7.3.4, page 546 of textbook)page 546 of textbook)

Suppose the two APs are connected directly to a router instead of to a switch. Mobility ?

Not covered in this class:


Not covered in this class: mobile IP (section 7.6) and mobility in cellular networks (section 7.7)

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Elements of a wireless network

ad hoc mode no base stations no base stations nodes can only transmit

to other nodes within link coveragelink coverage

nodes organize themselves into a network: need protocol pto route packets among themselves


7

Wireless network taxonomy

single hop multiple hops

lots of research on routing protocols

single hop mult ple hops

infrastructuremode (e g APs)

hosts connect to base station (WiFi,

ll l WiMAX)

wireless nodes serve as relays towards

Internet e gmode (e.g., APs) cellular, WiMAX) which connects to

Internet

Internet, e.g.,community mesh net

no base station, no

noinfrastructure

no base station, noconnection to Internet

(Bluetooth 802 11

,connection to Internet;

wireless nodes serve as relays to

(Bluetooth, 802.11ad hoc mode)

reach some nodes:MANET, VANET


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Wireless Link Characteristics (1)W L ( )Differences from wired link ….

decreased signal strength: radio signal attenuates more as it propagates (path loss)

int f n f m th s u s: i l ss n t k interference from other sources: wireless network frequencies (e.g., 2.4 GHz) shared by cordless phones, sensors, etc.; devices (motors) interfere as well

multipath propagation: radio signal reflects off objects multipath propagation: radio signal reflects off objects, reaching destination at slightly different times

…. make communication across a wireless link more “difficult” – may need link layer protocol to provide reliable delivery

Wireless and Mobile Networks (SSL) 7-8

y

12/5/2017

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Wireless Link Characteristics (2) SNR (signal-to-noise ratio) tradeoffs

for a given modulation technique:increase power or reduce noise ->

10-1

10-2

increase power or reduce noise increase SNR

-> decrease BER for a fixed bit error rate (BER): R

10-3

10-4

for a fixed bit error rate (BER): larger SNR –> higher bit rate for the same bandwidth

for a given SNR: choose

BER 10-5

10-6

for a given SNR: choose modulation technique that meets BER requirement to get highest bit rate QAM256 (8 Mbps)

10 20 30 40SNR(dB)

10-7

SNR may change with mobility dynamically adapt modulation

technique (rate)

QAM16 (4 Mbps)

BPSK (1 Mbps)


technique (rate) 12/5/2017

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Wireless network characteristics Multiple wireless senders and receivers create

additional problems (carrier sensing is unreliable):

CA B C

A’ i l C’s signal

AB

dd l bl

A’s signalstrength

C s signalstrength

Hidden terminal problem B, A hear each other B, C hear each other

space

Signal attenuation (fading): B, A hear each other

A, C cannot hear each other-> A, C unaware of interference at receiver B

B, C hear each other A, C cannot hear each other,

unaware of interference at B


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IEEE 802.11: multiple accessb f CSMA - sense before transmitting

can’t sense all transmissions: hidden terminal, fading llisi d t ti is diffi lt l collision detection is difficult – strong transmit signal,

weak receive signal

New goal: avoid collisions hence CSMA/CANew goal: avoid collisions, hence CSMA/CA

CA B C

B

C

A’s signalstrength

C’s signalstrength

A

space


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IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if channel sensed idle for DIFS then

ftransmit entire frame2 if channel sensed busy then

choose random backoff time

sender receiver

DIFS whenever channel sensed idle, timer

counts down transmit when timer reaches zero

data

if no ACK, increase random backoff interval, repeat 2

802.11 receiverSIFS

ACK- if frame received OKreturn ACK after SIFS (no collision detection due to hidden terminal and

f di bl )

ACK


fading problems)

12/5/2017

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Collision Avoidance - idea senders transmit small request-to-send (RTS)

packets to AP using CSMA RTS packets may collide with each other (but

they’re short) AP b d t l t d (CTS) i t AP broadcasts clear-to-send (CTS) in response to

received RTS CTS heard by all nodesS heard by all nodes

the sender selected then transmits data frame other stations defer transmissions

Try to avoid long data frame collision by using small reservation packets


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RTS-CTS exchange (use is optional)

APA B

reservation collision

DATA (A)defer

time

defer


15

CS 356 Fall 2017 - The end


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Chapter 7 outline · 7.5 Principles: addressing and routing to mobile users ... UMTS/WCDMA,...

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