1 wireless medium access control romit roy choudhury wireless networking lectures duke university
TRANSCRIPT
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Wireless Medium Access Control
Romit Roy ChoudhuryWireless Networking LecturesDuke University
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Wired Vs Wireless Media Access
Both are on shared media.Then, what’s really the problem ?
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The Channel Access Problem
Multiple nodes share a channel
Pairwise communication desired Simultaneous communication not possible
MAC Protocols Suggests a scheme to schedule communication
• Maximize number of communications• Ensure fairness among all transmitters
AA CCBB
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The Trivial Solution
Transmit and pray Plenty of collisions --> poor throughput at high
load
AA CCBB
collision
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The Simple Fix
Transmit and pray Plenty of collisions --> poor throughput at high
load
Listen before you talk Carrier sense multiple access (CSMA) Defer transmission when signal on channel
AA CCBB
Don’ttransmit
Don’ttransmit
Can collisions still occur?Can collisions still occur?
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CSMA collisions
Collisions can still occur:Propagation delay non-zero between transmitters
When collision:Entire packet transmission time wasted
spatial layout of nodes
note:Role of distance & propagation delay in determining collision probability
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CSMA/CD (Collision Detection)
Keep listening to channel While transmitting
If (Transmitted_Signal != Sensed_Signal) Sender knows it’s a Collision ABORT
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2 Observations on CSMA/CD
Transmitter can send/listen concurrently If (Transmitted - Sensed = null)? Then success
The signal is identical at Tx and Rx Non-dispersive
The TRANSMITTER can detect if and when collision occurs
The TRANSMITTER can detect if and when collision occurs
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Unfortunately …
Both observations do not hold for wireless
Because …
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Wireless Medium Access Control
A B
C D
Distance
Signalpower
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Wireless Media Disperse Energy
A B
C D
Distance
Signalpower
A cannot send and listen in parallel
Signal not same at different locations
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Collision Detection Difficult
Signal reception based on SINR Transmitter can only hear itself Cannot determine signal quality at
receiver
A CD
B
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Calculating SINR
AB
C
α
α
CB
CtransmitC
B
AB
AAB
d
PI
d
PSoI
NNoiseIceInterferen
SoIterestSignalOfInSINR
transmit
=
=
+=
)()(
)(
α
α
CB
Ctransmit
AB
A
AB
dP
N
d
P
SINR
transmit
+
=
D
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A B
C D
Distance
Signalpower
Red signal >> Blue signal
X
Red < Blue = collision
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A B
C D
Distance
Signalpower
Important: C has not heard A, but can interfere at receiver B
X
C is the hidden terminal to A
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A B
C D
Distance
Signalpower
Important: X has heard A, but should not defer transmission to Y
X
X is the exposed terminal to AY
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Any Questions at this point?
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So, how do we cope withHidden/Exposed Terminals?
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A B
C D
Distance
Signalpower
X
How to prevent C from trasmitting?
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An Idea!
A B
C D
A node decides to intelligently choose a Carrier sensing threshold (T)
The node senses channel If signal > T, then node does not transmit If signal < T, then transmit
Possible to guarantee no collisions?
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A B
C D
Distance
Signalpower
X
An Idea!
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A B
C D
Distance
Signalpower
X
T
A Project Idea!Will this solve the wireless MAC problem? Do not transmit in this region
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Whatever the answer …
This is an example of a good class project
If you came up with the idea,Showed that it’s a new idea,
And evaluated it to demo how it performs
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The Emergence of MACA, MACAW, & 802.11
Wireless MAC proved to be non-trivial
1992 - research by Karn (MACA) 1994 - research by Bhargavan (MACAW)
Led to IEEE 802.11 committee The standard was ratified in 1999
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CTS = Clear To Send
RTS = Request To Send
IEEE 802.11
D
Y
S
M
K
RTS
CTS
X
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IEEE 802.11
D
Y
S
X
M
Ksilenced
silenced
silenced
silencedData
ACK
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802.11 Steps
All backlogged nodes choose a random number R = rand (0, CW_min)
Each node counts down R Continue carrier sensing while counting down Once carrier busy, freeze countdown
Whoever reaches ZERO transmits RTS Neighbors freeze countdown, decode RTS RTS contains (CTS + DATA + ACK) duration = T_comm Neighbors set NAV = T_comm
• Remains silent for NAV time
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802.11 Steps
Receiver replies with CTS Also contains (DATA + ACK) duration. Neighbors update NAV again
Tx sends DATA, Rx acknowledges with ACK After ACK, everyone initiates remaining countdown Tx chooses new R = rand (0, CW_min)
If RTS or DATA collides (i.e., no CTS/ACK returns) Indicates collision RTS chooses new random no. R1 = rand (0, 2*CW_min) Note Exponential Backoff Ri = rand (0, 2^i * CW_min) Once successful transmission, reset to rand(0, CW_min)
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But is that enough?
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RTS/CTS
Does it solve hidden terminals ? Assuming carrier sensing zone =
communication zone
C
F
A B
E
D
CTS
RTS
E does not receive CTS successfully Can later initiate transmission to D.Hidden terminal problem remains.
E does not receive CTS successfully Can later initiate transmission to D.Hidden terminal problem remains.
CTS
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Hidden Terminal Problem
How about increasing carrier sense range ?? E will defer on sensing carrier no
collision !!!
CB DData
A
E
CTS
RTSF
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Hidden Terminal Problem
But what if barriers/obstructions ?? E doesn’t hear C Carrier sensing does not
help
CB DData
A
EF
CTS
RTS
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Exposed Terminal
B should be able to transmit to A RTS prevents this
CA B
E
D
CTSRTS
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Exposed Terminal
B should be able to transmit to A Carrier sensing makes the situation worse
CA B
E
D
CTSRTS
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Thoughts !
802.11 does not solve HT/ET completely Only alleviates the problem through RTS/CTS and
recommends larger CS zone
Large CS zone aggravates exposed terminals Spatial reuse reduces A tradeoff RTS/CTS packets also consume bandwidth Moreover, backing off mechanism is also wasteful
The search for the best MAC protocol is still on. However, 802.11 is being optimized too.Thus, wireless MAC research still alive
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Takes on 802.11
Role of RTS/CTS Useful? No? Is it a one-fit-all? Where does it not fit?
Is ACK necessary? MACA said no ACKs. Let TCP recover from losses
Should Carrier Sensing replace RTS/CTS?
New opportunities may not need RTS/CTS Infratructured wireless networks (EWLAN)
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MACA-BI [GerlaUCLA]
RTS/CTS/ACK are control overhead Needed to reduce it
Rx predicts trasmission from the Tx Traffic estimation (???)
If Rx thinks Tx has pending packets for Rx Rx transmits RTR to Tx Tx replies with Data
Improves MACA with no RTS/ACK improvement but not too much
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DBTMA [HaasCornell98]
A B X Y
RTS CTS Signal X
Rx Busy tone
A B X Y
CTS RTS Signal X
Tx Busy toneTx Busy tone
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Implicit MACKnowledgment
APs typically backlogged with traffic Persistent traffic possibility of optimzation
We propose an implicit ACK optimization Piggyback the CTS with ACK for previous
dialog802.11
Implicit ACK
Gain
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Hybrid Channel Access
The optimization timeline
T RRTS
CTS
Data
ACK
RTS
CTS
Data
ACK
T RRTS
CTS
Data
RTS
CTS +ACK
Data
T RRTS
CTS
Data
Poll +ACK
Data
RTS
CTS +ACK
Bac
kof
f
Bac
kof
fB
ack
off
Bac
kof
f
Poll +ACK
Data
Bac
kof
fB
ack
off
802.11802.11 Implicit ACKImplicit ACK Hybrid Channel AccessHybrid Channel Access
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Seedex [KumarUIUC03]
Forget channel reservation and backoff Instead, let nodes pick sequence of time slots
Decides to probably transmit in some, else listen Transmit slots chosen using a random seed Publishes the seed to 2-hop neighbors
When PT slots arrive, nodes transmit with Probability “p” “p” chosen as a function of overlapping neighbors
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Hot Research Topics
Power control increases spatial reuse Whisper in the room so that many people can talk
Rate control based on channel quality Expolit channel diversity
Utilize multiple channels to parallelize dialogs
Exploit spatial diversity Use directional antennas to interfere over smaller
region (next class)
… and many more topics
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Questions ?
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Announcements
Reviews: You are forgetting to appreciate the paper There is a reason why the paper was accepted
Please organize your papers/reviews Would be valuable later in career You never know what you will do after 5 years
Meet me with slides before you present Email for an appointment Don’t have to review if you are presenter
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Announcements
Review Template: Problem definition? Why is it important? Validity of models and assumptions Solution Evaluation
Email review to TA (CC to me) Bring print out to class Name-date-subject in email subject Will post example reviews on webpage
Some of you still doing summaries.
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Backup slides onIEEE 802.11
Read for more details
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Today’s Discussions
IEEE 802.11 overview - some raw data Architecture PHY specifications – Spread Spectrum radios: FH & DS MAC specifications – DCF and PCF Synchronization, Power management, Roaming,
Scanning Security
Deliberations on 802.11 (DCF) MAC Hidden terminal & Exposed terminal issues Carrier sensing
Some other ideas & open challenges Could be interesting for the project
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IEEE 802.11 – An overview
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IEEE 802.11 in OSI Model
Wireless
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802.11 Scope & Modules
MAC Sublayer
MAC LayerManagement
PLCP Sublayer
PMD Sublayer
PHY LayerManagement
LLC
MAC
PHY
To develop a MAC and PHY spec for wirelessconnectivity for fixed, portable and moving stationsin a local area
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Applications
Single Hop Home networks Enterprise networks (e.g., offices, labs, etc.) Outdoor areas (e.g., cities, parks, etc.)
Multi-hops Adhoc network of small groups
(e.g.,aircrafts) Balloon networks (SpaceData Inc.) Mesh networks (e.g., routers on lamp-posts)
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802.11 Architecture – Two modes
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802.11 PHY Technologies
Two kinds of radios based on “Spread Spectrum” “Diffused Infrared”
Spread Spectrum radios based on Frequency hopping (FH) Direct sequence (DS)
Radio works in 2.4GHz ISM band --- license-free by FCC (USA), ETSI (Europe), and MKK (Japan) 1 Mbps and 2Mbps operation using FH 1, 2, 5.5, and 11Mbps operation using DSSS (FCC)
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Why Spread Spectrum ?
C = B*log2(1+S/N) . . . [Shannon] To achieve the same channel capacity C
Large S/N, small B Small S/N, large B Increase S/N is inefficient due to the logarithmic relationship
power
B
signalnoise, interferences
power
signal
Bfrequency
e.g. B = 30 KHz e.g. B = 1.25 MHz
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Spread Spectrum
Reduce effect of jamming Military scenarios
Reduce effect of other interferences More “secure”
Signal “merged” in noise and interference
Methods for spreading the bandwidth of the transmitted signal over a frequency band (spectrum)
which is wider than the minimum bandwidth required to transmit the signal.
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Frequency Hopping SS (FHSS)
2.4GHz band divided into 75 1MHz subchannels
Sender and receive agree on a hopping pattern (pseudo random series). 22 hopping patterns defined
Different hopping sequences enable co-existence of multiple BSSs
Robust against narrow-band interferences
f f f f f f f f f f f
One possible pattern
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FHSS due to [Lamarr1940]power
B
signalnoise, interferences
power
signal
Bfrequencyf f f f f f f f f f f
Invented by Hedy Lamarr (Hollywood film star) in 1940, at age of 27, with musician George Antheil
Simple radio design with FHSSData rates ~ 2 Mbps
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Direct Sequence SS
Direct sequence (DS): most prevalent Signal is spread by a wide bandwidth
pseudorandom sequence (code sequence) Signals appear as wideband noise to
unintended receivers
Not for intra-cell multiple access Nodes in the same cell use same code
sequence
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IEEE 802.11b DSSS
ISM unlicensed frequency band (2.4GHz)
Channel bandwidth: fhigh – flow = 22 MHz
1MHz guard band Direct sequence
spread spectrum in each channel
3 non-overlapping channels
Channel flow fhigh
1 2.401 2.423
2 2.404 2.428
3 2.411 2.433
4 2.416 2.438
5 2.421 2.443
6 2.426 2.448
7 2.431 2.453
8 2.436 2.458
9 2.441 2.463
10 2.446 2.468
11 2.451 2.473
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Diffused Infrared
Wavelength range from 850 – 950 nm For indoor use only Line-of-sight and reflected transmission 1 – 2 Mbps
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PHY Sublayers
Physical layer convergence protocol (PLCP) Provides common interface for MAC
• Offers carrier sense status & CCA (Clear channel assesment)
• Performs channel synchronization / training
Physical medium dependent sublayer (PMD) Functions based on underlying channel quality
and characteristics• E.g., Takes care of the wireless encoding
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PLCP (802.11b)
longpreamble
192us
shortpreamble
96us(VoIP, video)
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PLCP (802.11b)long
preamble192us
shortpreamble
96us(VoIP, video)
Note:
To send even one bit payloadreliably, you will have to form a packet with the PLCP preambleand the PLCP header.
This constraints protocol design
You cannot arbitrarily exchangecontrol messages.
What are the control messagesin IEEE 802.11 ?
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IEEE 802.11 MAC
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802.11 MAC (DCF)
CSMA/CA based protocol Listen before you talk CA = Collision avoidance (prevention is better than cure !!)
Robust for interference Explicit acknowledgment requested from receiverExplicit acknowledgment requested from receiver
• for unicast frames Only CSMA/CA for Broadcast frames
Optional RTS/CTS offers Virtual Carrier Sensing RTS/CTS includes duration of immediate dialog Addresses hidden terminal problems
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802.11 MAC (DCF)
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Physical Carrier Sense & Backoff
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MAC Management Layer
Synchronization Finding and staying with a WLAN
• Uses TSF timers and beacons
Power Management Sleeping without missing any messages
• Periodic sleep, frame buffering, traffic indication map
Association and Reassociation Joining a network Roaming, moving from one AP to another Scanning
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Synchronization
Timing Synchronization Function (TSF) Enables synchronous waking/sleeping Enables switching from DCF to PCF Enables frequency hopping in FHSS PHY
• Transmitter and receiver has identical dwell interval at each center frequency
Achieving TSF All stations maintain a local timer. AP periodically broadcasts beacons containing
timestamps, management info, roaming info, etc.• Not necessary to hear every beacon
Beacon synchronizes entire BSS• Applicable in infrastructure mode ONLY
Distributed TSF (for Independent BSS) more difficult
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Power management Battery powered devices require power efficiency
LAN protocols assume idle nodes are always ON and thus ready to receive.
Idle-receive state key source of power wastage
Devices need to power off during idle periods Yet maintain an active session – tradeoff power Vs throughput
Achieving power conservation Allow idle stations to go to sleep periodically APs buffer packets for sleeping stations AP announces which stations have frames buffered
when all stations are awake – called Traffic Indication Map (TIM)
• TSF assures AP and Power Save stations are synchronized • TSF timer keeps running when stations are sleeping
Independent BSS also have Power Management Similar in concept, distributed approach
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Roaming & Scanning
Stations switch (roam) to different AP When channel quality with current AP is poor
Scanning function used to find better AP Passive Scanning Listen for beacon from different Aps Active Scanning Exchange explicit beacons to determine
best AP
Station sends Reassociation Request to new AP If Reassociation Response successful Roaming
If AP accepts Reassociation Request AP indicates Reassociation to the Distribution System Distribution System information is updated Normally old AP is notified through Distribution System
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MAC management frame
Beacon Timestamp, Beacon Interval, Capabilities, ESSID,
Supported Rates, parameters Traffic Indication Map
Probe ESSID, Capabilities, Supported Rates
Probe Response Timestamp, Beacon Interval, Capabilities, ESSID,
Supported Rates, parameters same for Beacon except for TIM
Association Request Capability, Listen Interval, ESSID, Supported Rates
Association Response Capability, Status Code, Station ID, Supported Rates
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MAC Management Frame
Reassociation Request Capability, Listen Interval, ESSID, Supported Rates,
Current AP Address
Reassociation Response Capability, Status Code, Station ID, Supported Rates
Disassociation Reason code
Authentication Algorithm, Sequence, Status, Challenge Text
Deauthentication Reason
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Security Range of attacks huge in wireless
Easy entry into the network Jamming, selfish behavior, spatial overhearing
Securing the network harder than wired networks Especially in distributed environments
WEP symmetric 40 or 128-bit encryption WPA: Wi-Fi protected access
Temporal key integrity protocol (TKIP) – better User authentication
IEEE 802.11i – Efforts toward higher security
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PLCP
PLCP has two structures. All 802.11b systems have to support Long preamble. Short preamble option is provided to improve efficiency
when trasnmitting voice, VoIP, streaming video.
PLCP Frame format PLCP preamble
• SFD: start frame delimiter PLCP header
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PLCP Header
8-bit signal or data rate (DR) indicates how fast data will be transmitted
8-bit service field reserved for future 16-bit length field indicating the length of
the ensuing MAC PDU (MAC sublayer’s Protocol Data Unit)
16-bit Cyclic Redundancy Code
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Power management approach
Allow idle stations to go to sleep station’s power save mode stored in AP
APs buffer packets for sleeping stations. AP announces which stations have frames buffered Traffic Indication Map (TIM) sent with every Beacon
Power Saving stations wake up periodically listen for Beacons
TSF assures AP and Power Save stations are synchronized stations will wake up to hear a Beacon TSF timer keeps running when stations are sleeping synchronization allows extreme low power operation
Independent BSS also have Power Management similar in concept, distributed approach
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Scanning
Scanning required for many functions. finding and joining a network finding a new AP while roaming initializing an Independent BSS (ad hoc) network
802.11 MAC uses a common mechanism for all PHY. single or multi channel passive or active scanning
Passive Scanning Find networks simply by listening for Beacons
Active Scanning On each channel Send a Probe, Wait for a Probe
Response Beacon or Probe Response contains information necessary
to join new network.
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Active scanning example
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Collision Detection
What is the aim of collision detection ?
It’s a transmitter’s job:
To determine if the packet was successfully received without explicitly asking the receiver
It’s a transmitter’s job:
To determine if the packet was successfully received without explicitly asking the receiver