Chapter 6 Wireless and Mobile Networks - SCS | UW6.2+6.3-2up.pdf · 6: Wireless and Mobile Networks 6-3 Chapter 6 outline 6.1 Introduction Wireless r6.2 Wireless links, characteristics

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6: Wireless and Mobile Networks 6-1

Chapter 6Wireless and Mobile Networks

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Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.


Chapter 6: Wireless and Mobile Networks

Background:r # wireless (mobile) phone subscribers now

exceeds # wired phone subscribers!r computer nets: laptops, palmtops, PDAs,

Internet-enabled phone promise anytime untethered Internet access

r two important (but different) challengesm wireless: communication over wireless linkm mobility: handling the mobile user who changes point

of attachment to network

2


Chapter 6 outline

6.1 Introduction

Wirelessr 6.2 Wireless links,

characteristicsm CDMA

r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)

r 6.4 Cellular Internet Accessm architecturem standards (e.g., GSM)

Mobilityr 6.5 Principles:

addressing and routing to mobile users

r 6.6 Mobile IPr 6.7 Handling mobility in

cellular networksr 6.8 Mobility and higher-

layer protocols

6.9 Summary


Elements of a wireless network

network infrastructure

wireless hostsr laptop, PDA, IP phoner run applicationsr may be stationary

(non-mobile) or mobilem wireless does not

always mean mobility

3




base stationr typically connected to

wired networkr relay - responsible

for sending packets between wired network and wireless host(s) in its “area”m e.g., cell towers,

802.11 access points




wireless linkr typically used to

connect mobile(s) to base station

r also used as backbone link

r multiple access protocol coordinates link access

r various data rates, transmission distance

4


Characteristics of selected wireless link standards

Indoor10-30m

Outdoor50-200m

Mid-rangeoutdoor

200m – 4 Km

Long-rangeoutdoor

5Km – 20 Km

.056

.384

1

4

5-11

54

IS-95, CDMA, GSM 2G

UMTS/WCDMA, CDMA2000 3G

802.15

802.11b

802.11a,g

UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellularenhanced

802.16 (WiMAX)

802.11a,g point-to-point

200 802.11n

Dat

a ra

te (M

bps)

data




infrastructure moder base station connects

mobiles into wired network

r handoff: mobile changes base station providing connection into wired network

5


Elements of a wireless networkad hoc moder no base stationsr nodes can only

transmit to other nodes within link coverage

r nodes organize themselves into a network: route among themselves


Wireless network taxonomy

single hop multiple hops

infrastructure(e.g., APs)

noinfrastructure

host connects to base station (WiFi,WiMAX, cellular) which connects to

larger Internet

no base station, noconnection to larger Internet (Bluetooth,

ad hoc nets)

host may have torelay through several

wireless nodes to connect to larger

Internet: mesh net

no base station, noconnection to larger

Internet. May have torelay to reach other a given wireless node

MANET, VANET

6


Chapter 6 outline

6.1 Introduction




r 6.4 Cellular Internet Accessm architecturem standards (e.g., GSM)





layer protocols

6.9 Summary


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

m decreased signal strength: radio signal attenuates as it propagates through matter (path loss)

m interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well

m multipath propagation: radio signal reflects off objects and the ground, arriving at destination at slightly different times

…. make communication across (even a point to point) wireless link much more “difficult”

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

m larger SNR – easier to extract signal from noise (a “good thing”)

r SNR versus BER tradeoffsm given physical layer:

increase power -> increase SNR->decrease BER

m given SNR: choose physical layer that meets BER requirement, giving highest thruput

• SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

10 20 30 40

QAM256 (8 Mbps)

QAM16 (4 Mbps)

BPSK (1 Mbps)

SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4


Wireless network characteristicsMultiple wireless senders and receivers create

additional problems (beyond multiple access):

AB

C

Hidden terminal problemr B, A hear each otherr B, C hear each otherr A, C can not hear each othermeans A, C unaware of their

interference at B

A B C

A’s signalstrength

space

C’s signalstrength

Signal attenuation:r B, A hear each otherr B, C hear each otherr A, C can not hear each other

interfering at B

8


Code Division Multiple Access (CDMA)

r used in several wireless broadcast channels (cellular, satellite, etc) standards

r unique “code” assigned to each user; i.e., code set partitioning

r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

r encoded signal = (original data) X (chipping sequence)

r decoding: inner-product of encoded signal and chipping sequence

r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)


Chapter 6 outline

6.1 Introduction




r 6.4 cellular Internet accessm architecturem standards (e.g., GSM)





layer protocols

6.9 Summary

9


IEEE 802.11 Wireless LANr 802.11b

m 2.4-5 GHz unlicensed spectrumm up to 11 Mbpsm direct sequence spread

spectrum (DSSS) in physical layer

• all hosts use same chipping code

r 802.11am 5-6 GHz rangem up to 54 Mbps

r 802.11gm 2.4-5 GHz rangem up to 54 Mbps

r 802.11n: multiple antennaem 2.4-5 GHz rangem up to 200 Mbps

r all use CSMA/CA for multiple accessr all have base-station and ad-hoc network versions


802.11 LAN architecture

r wireless host communicates with base stationm base station = access

point (AP)r Basic Service Set (BSS)

(aka “cell”) in infrastructure mode contains:m wireless hostsm access point (AP): base

stationm ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP

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802.11: Channels, association

r 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesm AP admin chooses frequency for APm interference possible: channel can be same as

that chosen by neighboring AP!r host: must associate with an APm scans channels, listening for beacon frames

containing AP’s name (SSID) and MAC addressm selects AP to associate withm may perform authentication [Chapter 8]m will typically run DHCP to get IP address in AP’s

subnet


802.11: passive/active scanning

AP 2AP 1

H1

BBS 2BBS 1

1223 4

Active Scanning: (1) Probe Request frame broadcast

from H1(2) Probes response frame sent from

APs(3) Association Request frame sent:

H1 to selected AP (4) Association Response frame

sent: H1 to selected AP

AP 2AP 1

H1

BBS 2BBS 1

12 3

1

Passive Scanning:(1) beacon frames sent from APs(2) association Request frame sent:

H1 to selected AP (3) association Response frame sent:

H1 to selected AP

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IEEE 802.11: multiple accessr avoid collisions: 2+ nodes transmitting at same timer 802.11: CSMA - sense before transmitting

m don’t collide with ongoing transmission by other noder 802.11: no collision detection!

m difficult to receive (sense collisions) when transmitting due to weak received signals (fading)

m can’t sense all collisions in any case: hidden terminal, fadingm goal: avoid collisions: CSMA/C(ollision)A(voidance)

AB

CA B C

A’s signalstrength

space

C’s signalstrength


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender1 if sense channel idle for DIFS then

transmit entire frame (no CD)2 if sense channel busy then

start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff

interval, repeat 2802.11 receiver- if frame received OK

return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

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Avoiding collisions (more)

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

r sender first transmits small request-to-send (RTS) packets to BS using CSMAm RTSs may still collide with each other (but they’re short)

r BS broadcasts clear-to-send CTS in response to RTSr CTS heard by all nodes

m sender transmits data framem other stations defer transmissions

avoid data frame collisions completely using small reservation packets!


Collision Avoidance: RTS-CTS exchange

APA B

time

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

RTS(A)

RTS(B)

reservation collision

RTS(A)

defer

13


framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: addressing

Address 2: MAC addressof wireless host or AP transmitting this frame

Address 1: MAC addressof wireless host or AP to receive this frame

Address 3: MAC addressof router interface to which AP is attached

Address 4: used only in ad hoc mode


Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addraddress 1 address 2 address 3

802.11 frame

R1 MAC addr H1 MAC addr dest. address source address

802.3 frame

802.11 frame: addressing

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framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

TypeFromAP

SubtypeToAP

More frag

WEPMoredata

Powermgt

Retry RsvdProtocolversion

2 2 4 1 1 1 1 1 11 1

802.11 frame: moreduration of reserved transmission time (RTS/CTS)

frame seq #(for reliable ARQ)

frame type(RTS, CTS, ACK, data)


hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

routerr H1 remains in same IP subnet: IP address can remain same

r switch: which AP is associated with H1?m self-learning (Ch. 5):

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

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802.11: advanced capabilities

Rate Adaptationr base station, mobile

dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies

QAM256 (8 Mbps)QAM16 (4 Mbps)BPSK (1 Mbps)

10 20 30 40SNR(dB)

BE

R

10-1

10-2

10-3

10-5

10-6

10-7

10-4

operating point

1. SNR decreases, BER increase as node moves away from base station2. When BER becomes too high, switch to lower transmission rate but with lower BER


802.11: advanced capabilitiesPower Managementr node-to-AP: “I am going to sleep until next

beacon frame”mAP knows not to transmit frames to this

nodemnode wakes up before next beacon frame

r beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sentmnode will stay awake if AP-to-mobile frames

to be sent; otherwise sleep again until next beacon frame

16


Mradius ofcoverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: personal area network

r less than 10 m diameterr replacement for cables

(mouse, keyboard, headphones)

r ad hoc: no infrastructurer master/slaves:

m slaves request permission to send (to master)

m master grants requestsr 802.15: evolved from

Bluetooth specificationm 2.4-2.5 GHz radio bandm up to 721 kbps


802.16: WiMAXr like 802.11 & cellular:

base station modelm transmissions to/from

base station by hosts with omnidirectionalantenna

m base station-to-base station backhaul with point-to-point antenna

r unlike 802.11:m range ~ 6 miles (“city

rather than coffee shop”)

m ~14 Mbps

point-to-multipoint

point-to-point

Documents

Chapter 6 Wireless and Mobile Networks - SCS | UW6.2+6.3-2up.pdf · 6: Wireless and Mobile Networks 6-3 Chapter 6 outline 6.1 Introduction Wireless r6.2 Wireless links, characteristics