1 characteristics of wireless environment. 2 radio propagation mechanism
TRANSCRIPT
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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)