Characteristics of Wireless Environment

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1 Characteristics of Characteristics of Wireless Environment Wireless Environment

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Characteristics of Wireless Environment. Radio Propagation Mechanism. Characteristics of Wireless Channel. Path loss P r /P t = O(d - γ ), where d: distance γ : 2 (free space), 5 (strong attenuation) Fading: fluctuation of signal strength Fast fading: due to multipath propagation - PowerPoint PPT Presentation

Transcript of Characteristics of Wireless Environment

Page 1: Characteristics of  Wireless Environment

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Characteristics of Characteristics of Wireless EnvironmentWireless Environment

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Radio Propagation MechanismRadio Propagation Mechanism

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Characteristics of Wireless Characteristics of Wireless ChannelChannel Path loss

Pr/Pt = O(d-γ), where d: distance• γ: 2 (free space), 5 (strong attenuation)

Fading: fluctuation of signal strengthFast fading: due to multipath propagationSlow fading: occurs when objects absorb the

transmissionMay reduced by diversity or adaptive modulation

InterferenceAdjacent channel interference guard bandCo-channel interference cellular, directional

antenna, dynamic channel allocation Inter-symbol interference adaptive

equalization Doppler shift

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Multiple Access TechniquesMultiple Access Techniques

FDMA OFDM

TDMA Hard to compute good schedules in a distributed fashion. Schedule needs to be traffic dependent. Need synchronized clocks in hardware to implement

slots CDMA

FHSS DSSS

SDMA

Duplexing FDD TDD

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CDMA DSSSCDMA DSSS

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

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

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

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

decoding: inner-product of encoded signal and chipping sequence

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

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CDMA Encode/DecodeCDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

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CDMA: two-sender interferenceCDMA: two-sender interference

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Wireless MACWireless MAC

MAC (Medium Access Control) Sharing a Single Broadcast Medium among Multiple Users Contention : Most Widely Used, Suffer from Collision Non-Contention : Reservation/Round-Robin, Collision Free

Wireless MAC vs. Ad Hoc MAC ? Ad Hoc Network: Multi-Hop Wireless Network

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ALOHA and CSMAALOHA and CSMA ALOHA - University of Hawaii (1970)

Transmit whenever it has data to send. Listen to the acknowledgement feedback from the receiver. If a collision occurs (no ACK), retransmits after a random

delay. Utilization: pure Aloha = 18.5%, slotted Aloha = 37%

CSMA - Kleinrock (1975) Listen (Carrier Sense) before transmission 1. If channel is idle, transmit 2. Otherwise, do one of the followings:

• Wait until channel become idle and transmit 1 Persistent-CSMA• Wait until idle and transmit with probability p p Persistent-CSMA• Defer transmission and try again after a random delay NP-CSMA

Carrier sense not foolproof• Propagation delay (also a problem in wireline).• Can sense only at transmitter; but collision happens at receiver (a

wireless problem).

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CSMA/CA (Collision Avoidance)CSMA/CA (Collision Avoidance) RTS/CTS Dialog before Data Transmission

RTS (Request To Send : Sender) / CTS (Clear To Send : Receiver) / DATA

Contention Window

How about CSMA/CD (Collision Detection) ? Need the ability to Listen while transmitting to detect collision The strength of its own transmission would mask all other

signals on the air

Frame

Frame

Frame

Frame

DIFS DIFS DIFS

ContentionWindow

DataArrival

A

Backoff = Uniform[0, CW]

Remaining BackoffB

C

D

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Hidden and Exposed Terminal ProblemHidden and Exposed Terminal Problem

PacketTransmissi

onFrom A to

B PacketTransmissi

onFrom C to

B

Time

Collision

C and A are Hidden Terminals relative to each other – one can’t sense the other’s transmission

A transmits to B

A CB

C wants to transmit to B. It does not hear A’s transmission, accesses the channel and collides

PacketTransmissi

onFrom B to

A

Time

C is an Exposed Terminal relative to B. B’s transmission inhibits C, although there would be no collision at the receiver (D). If C were to transmit.

A CB D

Packet can be transmitted from C to D,But don’t it.

B transmits to A

C wants to transmit to D. It hears B’s transmission, and unnecessarily defers, although it could transmit in parallel as A can’t hear C’s transmission

PacketTransmissi

onFrom B to

A

WasteResource

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Sense Carrier at ReceiverSense Carrier at Receiver

Busy Tone Multiple Access (BTMA)Receiver sounds a tone when busy receiving.Carrier sense on busy tone before

transmission.Perfect solution. But need a busy tone channel

and extra interface. Channel gains on data and busy tone channels may be different.

“In band” solutionUse virtual carrier sensing. Used in 802.11

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IEEE 802.11 WLANIEEE 802.11 WLAN

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IEEE802.11 ETSI BRAN

802.11f : Inter Access Point ProtocolUMTS Integration

IEEE 802.11

802.11e : QoS Enhancements

802.11i : Security Enhancements

802.11h

DFS & TPC802.11a

5GHz

54Mbps

802.11g

2.4GHz

20Mbps

802.11b

2.4GHz

11Mbps

802.11

2.4GHz

2Mbps

MAC

PHY

HiperLAN

DFS &TPC

5GHz

54Mbps

Wireless LAN StandardsWireless LAN Standards

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802.11 802.11b 802.11g 802.11a Hperlan2Hperlan2

Frequency2.4~2.4835 GHz

(83.5MHz)2.4~2.4835 GHz

(83.5MHz)2.4~2.4835 GHz

(83.5MHz)

5.150~5.350 GHz5.725~5.825 GHz

(455MHz)

5.150~5.350 GHz5.470~5.725 GHz

(300MHz)

5.150~5.350 GHz5.470~5.725 GHz

(300MHz)

Modulation DBPSK, DQPSKDBPSK/CCK,DQPSK/CCK

CCK,OFDM

OFDMBPSK, QPSK

16QAM, 64QAM

OFDMBPSK, QPSK

16QAM, 64QAM

OFDMBPSK, QPSK

16QAM, 64QAM

Max. PHY rate 1,2 Mbps 1,2,5.5,11Mbps

54 Mbps(1,2,5.5,6,9,11,12,18,24,

36,48,54Mbps)

54 Mbps(6,9,12,18,24,36,48,54Mbps)

54 Mbps(6,9,12,18,24,36,48,54Mbps)

54 Mbps(6,9,12,18,24,36,48,54Mbps)

Max. DataRate(layer 3)

1.2 Mbps 5 Mbps 22~32 Mbps 32 Mbps 32 Mbps32 Mbps

MAC CSMA/CA CSMA/CA CSMA/CA CSMA/CA TDMA/TDDTDMA/TDD

Connectivity Connection-lessConnection-lessConnection-lessConnection-less Connection-oriented

Connection-oriented

Fixed Networksupport

Fixed Networksupport EthernetEthernet EthernetEthernet EthernetEthernet EthernetEthernet

Ethernet,IP, ATM, UMTS,FireWire, PPP

Ethernet,IP, ATM, UMTS,FireWire, PPP

WLAN StandardsWLAN Standards

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IEEE 802.11 Protocol StackIEEE 802.11 Protocol Stack

For centralized contention-free channel access

For distributed contention-based channel access

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Possible Network TopologiesPossible Network Topologies

BSS mode ESS mode

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802.11: Channels, association802.11: Channels, association

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

that chosen by neighboring AP! host: must associate with an AP

scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address

selects AP to associate withmay perform authenticationwill typically run DHCP to get IP address in AP’s

subnet

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IEEE 802.11: multiple accessIEEE 802.11: multiple access

avoid collisions: 2+ nodes transmitting at same time

802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node

802.11: no collision detection! difficult to receive (sense collisions) when transmitting

due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal,

fading goal: avoid collisions: CSMA/C(ollision)A(voidance)

AB

CA B C

A’s signalstrength

space

C’s signalstrength

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IEEE 802.11 MAC Protocol: DCFIEEE 802.11 MAC Protocol: DCF

802.11 sender1 if sense carrier idle for DIFS then

transmit entire frame (no CD)2 if sense (physical or virtual) carrier

busy then Choose random backoff interval in [0,

cw]counts down while medium is idleCount-down is supended if medium

becomes busytransmit when backoff interval 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)

DCF is a CSMA/CA protocol 802.11 DCF is suitable for

multi-hop ad hoc networking

sender receiver

DIFS

data

SIFS

ACK

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Distributed Coordination Function Distributed Coordination Function (DCF)(DCF)

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Binary Exponential BackoffBinary Exponential Backoff

Backoff Counter is randomly selected from [0,CW],where CW is contention window

For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax) CW 2 (CW+1)-1

If successful transmission, CW CWmin

Reduces the collision probability

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Avoiding collisions (more): RTS/CTSAvoiding collisions (more): RTS/CTS

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

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

Sender first transmits small request-to-send (RTS) packets to AP using CSMA RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes

sender transmits data frame other stations defer transmissions

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RTS/CTS Mechanism (Optional)RTS/CTS Mechanism (Optional) RTC/CTS solves HTP But, non-negligible

overhead If frame size > RTSthreshhold,

• RTS-CTS-DATA-ACK Otherwise,

• DATA-ACK

802.11b

tslot 20usec

SIFS 10usec

PIFS SIFS + tslot

DIFS SIFS + 2*tslot

EIFS > DIFS

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Priorities in 802.11Priorities in 802.11

CTS and ACK have priority over RTS

After channel becomes idle If a node wants to send CTS/ACK, it transmits

SIFS duration after channel goes idle If a node wants to send RTS, it waits for DIFS

> SIFS

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Ranges and ZonesRanges and Zones

Transmission rangeFrame can be

successfully received Carrier-sensing

zone (C-Zone)Signal can be detected,

but not decoded. Interfering range

Receiving node can be interfered by another transmission collision

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Collisions are not completely avoidedCollisions are not completely avoidedin IEEE 802.11 !!in IEEE 802.11 !!

H does not sense any signal during D’s DATA tx H may transmit Collision in E’s reception

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Energy Conservation: PowerEnergy Conservation: Powercontrolcontrol Power control has two potential benefit

Reduced interference & increased spatial reuse Energy saving

If C reduces transmit power, it can still communicate with D Reduces energy consumption at node C Allows B to receive A’s transmission (spatial reuse)

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Point Coordination Function (PCF)Point Coordination Function (PCF)

To provide real-time service Poll-and-response MAC for nearly Isochronous service In infrastructure BSS only – Point Coordinator (PC)

resides in AP Alternating Contention-Free Period (CFP)

and Contention Period (CP)

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Contention Free OperationContention Free Operation

Two consecutive frames are separated by SIFS

CFP lengths depend on traffic amountMaximum length announced by AP; used for

NAV set

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framecontrol

durationaddress

1address

2address

4address

3payload CRC

2 2 6 6 6 2 6 0 - 2312 4

seqcontrol

802.11 frame: addressing802.11 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

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Internetrouter

AP

H1 R1

AP MAC addr H1 MAC addr R1 MAC addr

address 1 address 2 address 3

802.11 frame

R1 MAC addr AP MAC addr

dest. address source address

802.3 frame

802.11 frame: addressing802.11 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: more802.11 frame: more

duration of reserved transmission time (RTS/CTS)

frame seq #(for reliable ARQ)

frame type(RTS, CTS, ACK, data)

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hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same 802.11: mobility within same subnetsubnet

router

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

switch: which AP is associated with H1?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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WEB: Wired Equivalent PrivacyWEB: Wired Equivalent Privacy

RSA RC4 algorithm with 40-bit secret keyData encryption and integrity

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Other MAC Layer FunctionalitiesOther MAC Layer Functionalities

Synchronization Quasi periodic beacon frame are transmitted by AP (may

be deferred if medium is busy) Beacon contains time stamp

Power management Sleep and awake states

• Sleeping stations wake up periodically Sender has to buffer the data if receiver is on sleep

state Roaming

Active scanning: send a probe on each channel and waiting for response

Passive scanning: listen into medium to find other network

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The Other IEEE 802.11 EffortsThe Other IEEE 802.11 Efforts 802.11e

Provides QoS support by differentiating traffic streams Applicable to 802.11 PHY a, b, and g

802.11h Supplementary to MAC layer so as to comply with European

regulations for 5 GHz WLAN 802.11i

Security enhancement 802.11n

Enhancement for higher throughput (> 100 Mbps ) Decrease overhead within 802.11 protocol

• Packet preamble, CW, ACK, IFS parameters 802.11r

Speed up handoff between APs (Fast BSS-Transition) Important for VoWLAN

802.11s Support mesh networks

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HIPERLANHIPERLAN

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HIPERLAN StandardsHIPERLAN Standards ETRI BRAN Project HIPERLAN/1

RLAN without a wired infrastructure

Suited to both ad hoc and infra-based net

5.15 GHz, 17.1 GHz : ~23.5 Mbps

HIPERLAN/2 Short range (~200m)

wireless access to IP, ATM, other infra-based net

To integrate WLANS into cellular systems

5 GHz: 6 ~ 54 Mbps HIPERACCESS

(HIPERLAN/3) HIPERLINK

(HIPERLAN/4)

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HIPERLAN/1 – EY-NPMAHIPERLAN/1 – EY-NPMA

Elimination Yield Non-Preemptive MAEfficiency: 8 ~ 83% for packet size 50B ~ 2KB

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HIPERLAN/2HIPERLAN/2

IEEE 802.a (54Mbps) + QoS + handoff + data integrity

To integrate WLANs into cellular system (3G+)

ATM-compatible WLAN CO Fixed size packets support QoS

MAC: based on TDMA/TDD 2msec MAC frame consists of

• BCH: broadcast control• FCH: frame control• ACH: access feedback control• DL: downlink data• UL: uplink data• DiL: direct link (for Ad Hoc)