1

Wireless Networks

Ivan Marsic

Rutgers University

2

ISO OSI Protocol Stack

• Protocol at layer i doesn’t know about protocols at i1 and i1

1: Physical

2: LinkMAC

3: Network

4: Transport

• Radio spectrum• Infrared• Fiber• Copper

• IEEE 802.11 WiFi• IEEE 802.3 Ethernet• PPP (modems, T1)

• End-to-end (IP)• Routing• Address resolution

• Reliable (TCP)• Unreliable (UDP)

Application

3

Infrastructure vs. Ad Hoc (1)

(a) (b)

Internet/WebInternet/Web

802.11

802.3

Mobile Node

Access Point

Mobile Node

Internet/WebInternet/Web

802.11

802.3

Mobile Node

Access Point

Mobile Node

802.11

Mobile Node

Mobile Node

Mobile Node

Mobile Node

802.11

Mobile Node

Mobile Node

Mobile Node

Mobile Node

4

Infrastructure vs. Ad Hoc (2)

(c) (d)

5

Waves

6

EM Spectrum Allocation

Freq.

Freq.

Freq.902 MHz 928 MHz 2.4 GHz 2.4835 GHz 5.725 GHz 5.785 GHz

(AM radio)MF

(SW radio)HF

(FM radio - TV)VHF

(TV – Cell.)UHF SHFLF

30 KHz 300 KHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz

Vis

ible

Infr

are

d

X raysUV

Gammarays

1 KHz 1 MHz 1 GHz 1 THz 1 PHz 1 EHz

IEEE 802.11aHiperLAN II

IEEE 802.11b, gBluetooth

Microwave ovens

Cordless phonesBaby monitors

(old) Wireless LANs

ISM UNII

7

Communication Process

Information Source

Information Source TransmitterTransmitter ReceiverReceiver

NoiseSource

NoiseSource

DestinationDestinationCommunication

Channel

0 1 0 0 1 1 0 0 1 0

InputSignal:

SourceData:

Noise:

OutputSignal:

SamplingTimes:

0 1 0 0 1 1 0 0 1 0SourceData:

1 1 0 0 1 1 1 0 1 0DataReceived:

Bits in error

S

N

S+N

Decisionthreshold

8

Decibel definition

Link, channel,

repeater, or node

Power in Power out

9

Fourier Series Approximation

0

30

60 360180

90

120

150 210

240

270

300

330

30

6090

Angular phase of fundamental wave

Time

Resultant

Dis

pla

cem

en

t

Three dots runningat the same speedaround the circle indifferent lanes:

10

Phase Space

Time (t)

Am

plit

ud

e Period (T)

A

Phase ()

Frequency = 2 / T

A sin ( t )

Phase space:

A

02

/ 2

3 / 2

[rad / s]

11

Wireless Transmission and Receiving System

Modulator

Destination

Error Control Encoder

Source Decoder (Decompress)

Source Encoder (Compress)

Error Control Decoder

InformationSource

Demodulator

Tra

nsm

itte

r

Re

ceiv

er

CommunicationChannel

12

Modulation

Carrier signal

Analog message signal

Amplitude-modulated (AM) signal

Frequency-modulated (FM) signal

Phase-modulated (PM) signal

Carrier signal

Digital message signal

Frequency-shift-keying (FSK) signal

Amplitude-shift-keying (ASK) signal

Phase-shift-keying (PSK) signal

1 0 1 1 0

13

Modulation—PSK

135

225315

45

0001

10 11

90

180

2700 00

01

10

11

270

000

010

100

110

315225

180

135 90

45

111

001011

101

(a) (b) (c)

0000

0010

0100

0110

1000

1010

1100

1110

0001

0011

0101

0111

1001

1011

1101

1111

14

Example 2.1

3 bits

110

3 bits

100

3 bits

010Amplitude

Time

15

Gaussian r.v. and Q-function

x

p(x)

(2)

1

0

x

½

1

F(x)

0

(a) (b)

x

p(x)

0 z

Pr{x z}

(c) (d) 1 2010 6

10 5

10 4

10 3

10 2

10 1

10 0

3 4

z

Q(z)

16

Effect of Noise on Signal

Decisionthreshold

0

VA

VB

Symbol A

Symbol B

ModulatorModulator DemodulatorDemodulatorbits (symbols) in bits (symbols) out

Noise ~ N(0,)Noise ~ N(0,)

Bit-to-waveform mapper

Decision device

++

(a)

(b)

Noi

se p

df

P(AR|BT)

P(BR|AT)

Channel

17

Probability of Error for2,4-PSK

2 4010 7

10 6

10 5

10 4

10 3

10 2

10 1

6 8

SNR per bit, Eb/N0 (dB)

Pro

ba

bil

ity

of

bit

err

or,

Pe

10

BPSK

QPSK

12 14

18

Discrete vs. Continuous Channel

(b) (c)

0 1 0 0 1 1 0 0 1 0

InputSignal:

SourceData:

Noise:

OutputSignal:

(a)

(1,0,1) (1,1,1)

(0,0,1)

(1,0,0)

19

Signals as Vectors

s(t)

t5 V

T

Example 3-bit message: 1 0 1

A three-bit signal waveform

0 T 2T 3T

p1(t)

p2(t)

p3(t)

Orthogonal function set (Basis vectors)

s1(t)

s2(t)

s3(t)

(0,0,0)

(1,1,1) (1,1,1)

(1,1,1)

(1,1,1) (1,1,1)

(1,1,1)

(1,1,1)

(b)

(a)

(c)

(d)

20

11S

N 1

1S

N rS = 2

1,1 1,1

S

rN= 1/2 1, 1 1, 1

N

rS = 2

1,1 1,1

S

rN= 1/2 1, 1 1, 1

N

1, 1,1

1, 1,1

1, 1, 1

1, 1, 1

1,1, 1

1,1, 1

111

S

N

1,1,1

1, 1,1

1, 1,1

1, 1, 1

1, 1, 1

1,1, 1

1,1, 1

111

S

N

1,1,1

Geometric Representation

(c)(a) (b)

21

Signal Space

ST

N

SR)(2 NSBT

(b)(a)

22

Locus of Error-Causing Signals

(b)(a)

h

O'N

ST

Noise sphere,radius ,centered on ST

O

Signal sphere,radius

SR = ST + N

BTS2

BTN22h

O'

2h

O'

23

Error Detection and Correction

(a) (b)

1,1 1,1

1,1 1,1Validmessage

Validmessage

Errormessage

Errormessage

r2 = 22r1 = 2

r3 = 23

1, 1, 1

1,1,11, 1,1

1, 1,1

1,1, 1

111

r2 = 22r1 = 2

r3 = 23

1, 1, 1

1,1,11, 1,1

1, 1,1

1,1, 1

111

24

Wave Interactions

(a)

(b)

Rough ceiling

Transmitter

“Knife-edge”obstacle

Receiver 1

s

s1

s0 + s1' + s2'

Receiver 2

s1'' + s2''

s0

s2

Rough ceiling

Transmitter

“Knife-edge”obstacle

Receiver 1

s

s1

s0 + s1' + s2'

Receiver 2

s1'' + s2''

s0

s2

25

Interference & Doppler Effect

(a) (b)

s0

s1

s2 s0+s1+s2

s0

s1

s2 s0+s1+s2vv

26

Multipath Fading (1)

Transmittedsignal

Transmittedsignal

Rec

eiv

edLO

S s

igna

l

Rec

eiv

edLO

S s

igna

l

ReceivedNLOS signals

ReceivedNLOS signals

Time

Time

27

(b)(a)

Plane-Earth Model

Receiver

Source

Receiver

Source

Receiver

Source

Receiver

Source

28

Delay Spread

(a) (b)

Tx Rx

2

1

Tx Rx

2

1

Delay (s)

Pat

h po

wer

(dB

)

0 1 2 3 4 5 6

mxd

Delay (s)

Pat

h po

wer

(dB

)

0 1 2 3 4 5 6

mxd

29

Discrete-time Delay Model

g(t, 0) g(t, ) g(t, 2) g(t, 3) g(t, N)

r(t)

š(t)

g(t, 0) g(t, ) g(t, 2) g(t, 3) g(t, N)

r(t)

š(t)

30

Multipath Fading (2)

Direct path (1 component)

Delay spread (2 components)Flat Fading

Delay spread (2 components)Frequency Selective Fading

Doppler spread (2 components)Fast Fading

0

0.05

0.05

0.1

0.1

0.5 1.0 1.5 2.0 2.5 3.00

0.15

0

0.05

0.05

0.1

0.1

0.5 1.0 1.5 2.0 2.5 3.00

0.15

0

0.05

0.05

0.1

0.1

0.15

3.00 0.5 1.0 1.5 2.0 2.5

0.2

0

0.05

0.05

0.1

0.1

0.15

3.00 0.5 1.0 1.5 2.0 2.5

0.2

0.5 1.0 1.5 2.0 2.5 3.00

0

0.05

0.05

0.1

0.5 1.0 1.5 2.0 2.5 3.00

0

0.05

0.05

0.1

0.05

0.05

0.2

0.5 1.0 1.5 2.0 2.5

0.15

3.00

0

0.1

0.05

0.05

0.2

0.5 1.0 1.5 2.0 2.5

0.15

3.00

0

0.1

31

Error Probabilities

2 40 6 8

SNR per bit (dB)

Pro

ba

bil

ity

of

bit

err

or,

Pe

10

BPSKAWGN

12 1410 5

10 4

10 3

10 2

10 1

10 0

BPSKRayleigh

16 18 20

32

Medium Access Control (MAC)

• Controls who gets to transmit when• Avoids “collisions” of packet

transmissions

Station 1

Station m

1

2

Station 2

m

Packetarrival rate Receiver

33

Coordination Problem

34

Collisions

Receiver electronics detects collision Receiver broadcasts info about collision (jam)

= total time to detect collision = RTT of the most distant station

ReceiverReceiver

Station 1

Station m

Station 2 Station 2

ReceiverReceiver

Station 1

Station m

Station 2 Station 2

35

Channel State

Assumption:There is always at least one station in need of transmission

Objective:Maximize the fraction of time for the “Successful transmission” state( or: minimize the duration of “Idle” and “Collision” )

Time

SuccessfultransmissionIdle

Error/Collision

36

Multiaccess vs. Multiplexing

MUX

Ordering of packets on higher capacity link

Receiver

Ordering of packets on shared medium

37

Static multiaccess schemes: TDMA and FDMA

Deterministic Schemes

m1 3

time

2 m1 32 m1 32

TDMA

freq

uenc

y

m1 3

time

2 m1 32 m1 32

TDMA

freq

uenc

y

m

321

time

m

321

m

321

FDMA

freq

uenc

y

m

321

time

m

321

m

321

FDMA

freq

uenc

y

38

Poisson Arrivals Model

0

5

10

15

20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

arrivals per time unit (n)

per

cen

t o

f o

ccu

rren

ces

(%)

39

Parameter

t

txmit

Transmitter

Receiver

< 1

t

txmit

» 1

L

C

txmit

Propagation constant :

• Ratio of propagation delay vs. packet transmission time

40

Vulnerable Period

• Packet will not suffer collision if no other packets are sent within one packet time of its start

tstart

tstart txmit

Vulnerable periodtstart txmit

Current packetCollides with

the head of thecurrent packet

Collides withthe tail of the

current packet

Time

41

ALOHA Protocols

TimePacketArrivals

Departures

1 2 3 4 5 6 7

1 2 3 4 5 6 7

Time

Departures

1 2 3 4 5 6 7

1 2 3 4 5

ALOHA

SlottedALOHA

PacketArrivals

42

Transmission Success Rate

Time

Departures Slot 1

Arrivals atStation 1

Slot 2 Slot 3 Slot 4

Departures Slot 1

Arrivals atStation k

Slot 2 Slot 3 Slot 4

Time

Time

(a) – Slotted ALOHA (b) – Pure ALOHA

PacketTime

Time

Receiver

1

k

43

ALOHA and Slotted ALOHA State Diagram

SendWait for

start of slot

ALOHA

Abort

ACK arrived /

Collision /

End

Backoff CWmax /Backoff CWmax /

S-ALOHA

Wait for backoffS-ALOHA

Wait for ACK

Increase backoff

ALOHA

SendWait for

start of slot

ALOHA

Abort

ACK arrived /

Collision /

End

Backoff CWmax /Backoff CWmax /

S-ALOHA

Wait for backoffS-ALOHA

Wait for ACK

Increase backoff

ALOHA

44

Analysis of Slotted ALOHA (1)

ASSUMPTIONS FOR ANALYSIS:• All packets require 1 slot for x-mit• Poisson arrivals, arrival rate • Collision or perfect reception (no errors)• Immediate feedback (0, 1, e)• Retransmission of collisions

(backlogged stations)• No buffering or infinite set of stations

(m = )i 1 i i 1 i 2

Time Slots

45

Backlogged Stations

• “Fresh” stations transmit new packets

• “Backlogged” stations re-transmit collided packets

ReceiverReceiver

Fresh Station

Backlogged Station

/m

/m

“Fresh” station

“Backlogged” station

“Fresh” station

“Backlogged” station

46

ALOHA System Model (1)

• In equilibrium state, system input equals system output

= S = GeG

SystemReceiverReceiver

/m

/m/m

/m

System Input =

System Output = S

Transmission Attempts = G

“Fresh” station

“Backlogged” station

47

ALOHA System Model (2)

Channel

Stations Receiver

G S

ChannelG

SGP0

G(1 P0)

48

Analysis of Slotted ALOHA (2)

• 0 < < 1, since at most 1 packet / slot

• Equilibrium: departure rate = arrival rate

• Backlogged stations transmit randomly

• Retransmissions + new transmissions:Poisson process with parameter G >

• The probability of successful x-mit: S=GP0,where P0=prob. packet avoids collision

• No collision => no other packets in the same slot: GGetAtAPGGPS 0)()1(0

49

Efficiency of ALOHA’s

S-ALOHA: In equilibrium, arrival rate = departure rate: = GeG

Max departure rate (throughput) = 1/e 0.368 @ G = 1

0 0.5 1.0 1.5 2.0 3.0

0.1

0.2

0.3

0.4Slotted ALOHA: S = Ge–G

Pure ALOHA: S = Ge–2G

G (transmission attempts per packet time)

Arrival rate

Equilibrium

S(t

hrou

ghpu

t p

er p

acke

t tim

e)

50

Unslotted (Pure) ALOHA

• Assume: all packets same size, but no fixed slots

• The packet suffers no collision if no other packet is sent within 2 packets long: S=GP0=Ge2G

• Max throughput 1/2e 0.184 @ G = 0.5• Less efficient than S-ALOHA, but

simpler, no global time synchronization

i

51

Markov chain for S-ALOHA

P10

0 1 2 3 4

P21

P12

P00 P11 P22

P02

P03P04

P33 P44

P32

P23

52

Instability of Slotted ALOHA

Desiredstablepoint

Departure rateGe G

Unstableequilibrium

Undesiredstablepoint

mqa

G = (m n)qa nqr

G = mqrG = mqa

Arrival rate(m n)qa

n = mn = 0

G = 0

S(t

hrou

ghpu

t per

slo

t)

G (transmission attempts per slot)

Negative drift Positive drift

53

Carrier Sensing (CSMA)

• Listen before talk (unlike ALOHA, where talk when you need to)

54

CSMA/CD

1. Wait until the channel is idle.

2. When the channel is idle, transmit immediately and listen while transmitting.

3. In case of a collision, stop the packet transmission, and then wait for a random delay and go to step 1.

IEEE 802.3 (Ethernet)

55

Basic CSMA Protocols

CSMA Protocol

Transmission Rules

Nonpersistent If medium is idle, transmit.If medium is busy, wait random amount of time and sense channel again.

1-persistent If medium is idle, transmit.If medium is busy, continue sensing until channel is idle;then transmit immediately.

p-persistent If medium is idle, transmit with probability p.If medium is busy, continue sensing until channel is idle;then transmit with probability p.

56

CSMA Protocols State Diagram

Sense Send End Increase backoff

Wait random

time

Wait for IFS

Wait 1 slot

Idle (p-persistent)Pr(1-p) / Idle (p-persistent)

Pr(p) /

No Collision(CSMA/CD) /

ACK received(CSMA/CA) /

Abort

Wait for backoff

Backofftoo large /

Jam

Idle(CSMA/CA) /

Busy(nonpersistent) /

Busy (1-persistent& p-persistent) /

Idle (1-persistent& nonpersistent) /

Timeout(CSMA/CA) /

Collision(CSMA/CD) /

Backoff OK /

Sense Send End Increase backoff

Wait random

time

Wait for IFS

Wait 1 slot

Idle (p-persistent)Pr(1-p) / Idle (p-persistent)

Pr(p) /

No Collision(CSMA/CD) /

ACK received(CSMA/CA) /

Abort

Wait for backoff

Backofftoo large /

Jam

Idle(CSMA/CA) /

Busy(nonpersistent) /

Busy (1-persistent& p-persistent) /

Idle (1-persistent& nonpersistent) /

Timeout(CSMA/CA) /

Collision(CSMA/CD) /

Backoff OK /

57

Nonpersistent CSMA

Station 1

Time

Station 2

Station 3

Station m

Vulnerableperiod

Packet A Packet B

Packet C

Idleperiod

Vulnerableperiod

1 1

1

0 1 1

Successful transmission(period = 1)

Collision(period = 1+Y)

Y

58

Efficiency of CSMA protocols

(a)

(b)SlottedALOHA

Pure ALOHA

1-persistent CSMA

NonpersistentCSMA

NonpersistentCSMA/CD

S(t

hrou

ghpu

t per

pac

ket t

ime)

G (transmission attempts per packet time)0.1

0.2

0.4

0.6

0.8

1.0

1 10 100 1000

= 0.01

SlottedALOHA

Pure ALOHA

1-persistent CSMA

NonpersistentCSMA

NonpersistentCSMA/CD

S(t

hrou

ghpu

t per

pac

ket t

ime)

G (transmission attempts per packet time)0.1

0.2

0.4

0.6

0.8

1.0

1 10 100 1000

= 0.01

g = (2)

g (transmission attempts per slot time)

1 2

Arrival rate

Equilibrium

Departure rate

1 ge g

1 e g

Thr

ough

put p

er s

lot

time

g = (2)

g (transmission attempts per slot time)

1 2

Arrival rate

Equilibrium

Departure rate

1 ge g

1 e g

Thr

ough

put p

er s

lot

time

59

Delay vs. Arrival Rate

Arrival rate

Ave

rage

pac

ket

dela

y

ALOHA

CSMA

TDMA

Max

imum

cha

nnel

tran

smis

sion

rat

e

60

Hidden and Exposed Terminals

Range of B’s transmissionsRange of A’stransmissions

Hidden Terminal Exposed Terminal

CAB

CAB

AB

CD

AB

CD

61

CSMA/Basic Atomic Exchange

Data Packet

Receiver

Sender

Covered Station

Busy

Busy

Busy

Hidden Station

Busy

Access to medium deferred

Time

IFS(2)

Idle contentionperiod

Vulnerableperiod =

Vulnerable period = Packet time

62

CSMA/MACA Atomic Exchange

RTS Data Packet

CTS

Access to medium deferred

Receiver

Sender

Covered Station

Busy

Busy

Busy

Hidden Station

Busy

Access to medium deferred

Time

IFS(2)

Idle contentionperiod

IFS IFS

Vulnerableperiod =

Vulnerable period= RTS + IFS +

63

RTS/CTS Exchange (1)

CAB

RTS(N-bytes)

64

RTS/CTS Exchange (2)

CAB

CTS(N-bytes)

65

RTS/CTS Exchange (3)

CAB

N-bytes Packet

Defer(N-bytes)

66

Components of 802.11 LANs

Ad hoc network does not have distribution system nor access point

Distribution system

Station

Wireless medium

Access point

67

IBSS and Infrastructure BSS

Independent BSS (IBSS) Infrastructure BSS

Access point

68

Extended Service Set (ESS)

DS

t = 1 t = 2

BSS1 BSS2 BSS3

AP3AP2AP1

69

802.11 Network Services

Service Provider Description

Distribution Distribution Service used by stations to exchange MAC frames when the frame must traverse the DS to get from a station in one BSS to a station in another BSS.

Integration Distribution Frame delivery to an IEEE 802 LAN outside the wireless network.

Association Distribution Used to establish a logical connection between a mobile station and an AP. This connection is necessary in order for the DS to know where and how to deliver data to the mobile station.

Reassociation Distribution Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another.

Disassociation Distribution Removes the wireless station from the network.

Authentication Station Establishes identity prior to establishing association.

Deauthentication

Station Used to terminate authentication, and by extension, association.

Privacy Station Provides protection against eavesdropping.

MSDU delivery Station Delivers data to the recipient.

70

States and Services

State 1:Unauthenticated,

Unassociated

Class 1Frames

SuccessfulAuthentication

State 2:Authenticated,Unassociated

State 3:Authenticated

and Associated

De-authenticationNotification Disassociation

Notification

SuccessfulAuthentication orRe-association

De-authenticationNotification

Class 1 & 2Frames

Class 1, 2 & 3Frames

71

802.11 Interframe Spacings

Busy Frame transmission.....

Contention period

Backoff slots

DIFS

PIFS

SIFS

Time

Defer access Select slot using binary exponential backoff

72

Basic 802.11 Transmission Mode

TimeDIFS

Receiver

Sender

Data

SIF

S

ACK

4 3 2 1 0

BackoffBusyBusy

BusyBusy

73

802.11 Protocol State Diagram – Sender

Sense Send End

Increase CW & Retry count

Wait for end of transmission

Wait for DIFS

ACK error-free /

Abort

Wait for IFS

Sense

SenseCountdown for backoff

while medium idle

Busy /

Idle /

Busy /

Idle /

Busy /

Backoff == 0 /

Timeout /

Backoff 0 /

Retry Retrymax /

New

pac

ket

/

Retry Retrymax /

Wait for EIFS

1

SetBackoff

Idle /

1

1

ACK in error /

74

802.11 Protocol State Diagram – Receiver

ReceiveSend ACK

EndWait for

SIFS

Wait for EIFS

Packet in error /

Packet error-free /

75

Example: Infra BSS

Assume Station A has a single packet to transmit to B

A B

Accesspoint

DIFSStation A

AP

Station B

DIFS Data SIF

S

ACK

Example: backoff = 4

4,3,2,1,0

Data

SIF

S

ACK

Time

76

Timing Diagrams

Timing of successful frame transmissions under the DCF.

Frame retransmission due to ACK failure.

Frame retransmission due to an erroneous data frame reception.

Frame

DIFS

SIFSACK

FrameDIFS

Backoff

SIFSACK

Busy FrameDIFS

Backoff

SIFSACK

FrameEIFS

Backoff

SIFSACK

Busy FrameDIFS

Backoff FrameACK Timeout

Backoff

SIFSACK

(a) Time

(b)

(c)

Packet arrival,channel idle

No backoff

77

Backoff Mechanism

The backoff mechanism of 802.11 MAC.The Frame* transmission time includes the RTS/CTS exchange and the MAC layer ACK.CP: Contention period.

DIF

S

Frame*

Frame*CP

CP CPCP

STA 1

STA 2

STA 3

5 4 3 2 1 0

2 1 0

9 8 7 6 5 4 3 2 1 0

Frame*

3 2 1 0

7 6 5 4 3 2 1 0

4 3 2 1 0

4 3 2 1 0

3 2 1 0

DIF

S

DIF

S

DIF

S

DIF

S

7 6 5 4 3 2 1 0

Frame*

Remainder Backoff

1 0

Frame*

4 3 2 1 0

4 3 2 1 0

78

RTS/CTS Transmission Mode

RTS

SIF

S

TimeData

CTS

SIF

S

SIF

S

ACK

DIFS

Access to medium deferred

Receiver

Sender 4 3 2 1 0

Backoff

DIFS

Covered Station

BusyBusy

BusyBusy

BusyBusyNAV (RTS)

NAV (CTS)

NAV (Data)

8 7 6

Backoff

5 4

DIFS

Hidden Station

BusyBusyBackoff

Access to medium deferred

79

802.11 MAC Frame Format

FC D/I Address Address Address SC Address Frame body FCS FC D/I Address Address Address SC Address Frame body FCS

Protocol Type Subtype To From MF RT PM MD W Oversion DS DS

Protocol Type Subtype To From MF RT PM MD W Oversion DS DS

bits 2 2 4 1 1 1 1 1 1 1 1

bytes 2 2 6 6 6 2 6 0 to 2312 4

FC = Frame controlD/I = Duration/Connection IDSC = Sequence controlFCS = Frame check sequence

DS = Distribution systemMF = More fragmentsRT = RetryPM = Power management

MD = More dataW = Wired equivalent privacy (WEP) bitO = Order

80

802.11 Performance Analysis

Appl

STA 1

Appl

2

Appl

m

Channel

cr r r

81

3 2 1 03 2 1 07 6 5 4 37 6 5 4 35 4 3 2 1 05 4 3 2 1 0

Idle

DIFSBckof

Inter-eventeligible period 2b

Event 4: Packet 2transmission

RTSBackoff

Idle

Inter-eventeligible period 3

Event 5: Packet 2re-transmission

Bsy

DIFS

Cha

nnel

stat

e

Event 1:Packet 1 arrival

IdleBusyBusy Busy

DIFSBackoff

Inter-eventeligible period 1

RTS

SIF

S

CTS

Data

SIF

S

SIF

S

ACK

DIFS

Idle

Inter-eventeligible period 2a

Event 2: Packet 1transmission

Another stationtransmits

Event 3:Packet 2 arrival

Busy

Time

RTS

CTSSIF

S

SIF

S

BusyBusy

DataDataBackoff

4 3 2 1 04 3 2 1 0

82

Transmission Example

Busy

6

Busy/success

Idle

DIFS

Actual channel stateobserved by AP

12

transmitattempt

Idle BusyBusy

789101112

Channel stateobserved by A

RTSSIFS SIFS

DataSIFS

ACK

DIFS

CTS Idle

Busy

RTS RTS

DIFS

23456

Idle

Idle

Busy/collision

83

0CW(0)

K = 1C = 0

K = 0C = 0

K = 5C = 0

K = C = 0

K = 1C = 0

01

11

51

1

pbusy

pidle

pbusy

pidle

pbusy

pidle

pbusy

pidle

pidle pidle

pbusy

1CW(1)

pidle pidle

pbusy

5CW(5)

pidle pidle

pbusy

CW(5)

pidle pidle

pbusy

CW(1)

pcoll~pcoll~

CW(1)

pcoll~pcoll~

CW(1)

pcoll~pcoll~

CW(2)

pcoll~pcoll~

CW(2)

pcoll~pcoll~

CW(5)

pcoll~pcoll~

CW(5)

pcoll~pcoll~

CW(5)

pcoll~pcoll~

CW(5)

pcoll~pcoll~

CW(5)

pcoll~pcoll~

psuccpidle~psuccpidle~ psuccpidle

~psuccpidle~

psuccpbusy/CW(0)~psuccpbusy/CW(0)~

(psuccpbusy pcoll)/CW(0)~ ~(psuccpbusy pcoll)/CW(0)~ ~

psuccpbusy/CW(0)~psuccpbusy/CW(0)~

CW(5)

pcoll~pcoll~

pbusy/CW(0)

pidle

psucc~psucc~

1

84

802.11 Protocol Architecture

802.11 MAC

802.11bDSSS

802.11aOFDM

802.11DSSS

802.11FHSS

85

802.11 PHY Frame Using DSSS

PLCP headerPLCP preamble

Payload(variable)

Synchronization (128bits)

SFD (16 bits)

Signal (8 bits)

Service (8 bits)

Length (16 bits)

HEC (16 bits)

86

IEEE 802.11b BER vs. SNR

SNR (dB) 5 0 5 10 10

10 6

10 5

10 4

10 3

10 2

10 1

BE

R

DBPSK

DQPSK

CCK 5.5 CCK 11

87

IEEE 802.11b Throughput vs. SNR

1

2

3

4

5

6

7

0

SNR (dB)

Thr

ough

put

(Mb

ps)

DBPSK

DQPSK

CCK 5.5

CCK 11

0 5 10 15 5 20

88

11Mbps

5.5Mbps

2Mbps

1MbpsDBPSK

DQPSK

DQPSK/CCK

DB

PS

K/C

CK

160 m270 m400 m

550 m

W-LAN Transmission Rates

11 Mbps 8 % of coverage area 1 Mbps 47 % of coverage area Low probability of having good link!!Lucent ORiNICO 802.11b

outdoors, no obstruction—ideal conditions!

Access Point

Mobile Node

Obstacle

89

Asymmetry

B

A

Access Point Hearing Range

Access Point Transmission Range

AP1

3

90

Receiver-Based Autorate MAC Protocol

NAV updated using ratespecified in the data packet

AB

CD

1: RTS2: CTS

3: Data

CTS at 1 Mbps

RTS at 2 Mbps

Data at 1 Mbps

91

IEEE 802.11b Channels

NOTE: The 12 channels in 802.11a do NOT overlap

1:2.

412

2:2.

417

3:2.

422

4:2.

427

5:2.

432

6:2.

437

7:2.

442

8:2.

447

9:2.

452

10:

2.45

7

11:

2.46

2

22 MHz

5 MHz

2.4 GHz 2.483 GHz

92

Power Conservation

93

Comparison of 802.11’s

Standard 802.11a 802.11b 802.11g

Number of channels

Interference

Bandwidth

Power consumption

Range/penetration

Upgrade/compatibility

Price

http://www.nwfusion.com/techinsider/2002/0520wlan/0520feat1.html

94

Route Discovery in DSR (1)

FB

D

E G

H

KJ

I

Y

Z

LA

C

95

Route Discovery in DSR (2)

Broadcast Tx

Represents a node that has received RREQ for H from C

F

C

B

D

E G

H

KJ

I

Y

Z

LA

RREQ[C]

RREQ[C]

96

Route Discovery in DSR (3)

F

C

B

D

E G

H

KJ

I

Y

Z

LA

RREQ[C, E]

RREQ[C, B]

97

Route Discovery in DSR (4)

F

C

B

D

E G

H

KJ

I

Y

Z

LA

RREQ[C, E, G]

RREQ[C, B, A]

RREQ[C, B, A]

98

Route Discovery in DSR (5)

F

C

B

D

E G

H

KJ

I

Y

Z

LA

RREP[C, E, G, H]

RREQ[C, B, A, K]

99

Route Discovery in AODV (1)

F

C

B

D

E G

H

KJ

I

LA

100

Multihop Throughput

Challenge: more hops, less throughputLinks in route share radio spectrumExtra hops reduce throughput

Throughput = 1

Throughput = 1/2

Throughput = 1/3

101

Cellular Hierarchy

High-tier

Low-tier

Satellite

High Mobility Low MobilityWide Area

Regional Area

Local Area

High-tier

Low-tier

Satellite

High Mobility Low MobilityWide Area

Regional Area

Local Area

102

Hybrid Wireless Networks

Infrastructure + MANET

(a) (b) (c)

Access Point

Mobile NodeObstacle

103

Community Mesh Network

WiMAXTower

Mesh NetworkDSLCable NetworkWiMAXFiber Backbone

CableOperator

CentralOffice

Internet