IEEE 802.11 - Duke Electrical and Computer Engineering
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Transcript IEEE 802.11 - Duke Electrical and Computer Engineering
Wireless Medium Access Control
Romit Roy Choudhury
Wireless Networking Lectures
Duke University
1
Wired Vs Wireless Media Access
Both are on shared media.
Then, what’s really the problem ?
2
The Channel Access Problem
Multiple nodes share a channel
A
B
C
Pairwise communication desired
Simultaneous communication not possible
MAC Protocols
Suggests a scheme to schedule communication
• Maximize number of communications
• Ensure fairness among all transmitters
3
The Trivial Solution
A
B
C
collision
Transmit and pray
Plenty of collisions --> poor throughput at high load
4
Don’t
transmit
The Simple Fix
A
Transmit and pray
B
C
Can collisions still occur?
Plenty of collisions --> poor throughput at high load
Listen before you talk
Carrier sense multiple access (CSMA)
Defer transmission when signal on channel
5
CSMA collisions
spatial layout of nodes
Collisions can still occur:
Propagation delay non-zero
between transmitters
When collision:
Entire packet transmission
time wasted
note:
Role of distance & propagation
delay in determining collision
probability
6
CSMA/CD (Collision Detection)
Keep listening to channel
While transmitting
If (Transmitted_Signal != Sensed_Signal)
Sender knows it’s a Collision
ABORT
7
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
8
Unfortunately …
Both observations do not hold for wireless
Because …
9
Wireless Medium Access Control
A
C
B
D
Signal
power
Distance
10
Wireless Media Disperse Energy
A cannot send and listen in parallel
A
C
B
D
Signal
power
Signal not same at different locations
Distance
11
Collision Detection Difficult
D
A
B
C
Signal reception based on SINR
Transmitter can only hear itself
Cannot determine signal quality at receiver
12
Calculating SINR
B
A
D
SINR
C
SignalOfIn terest ( SoI )
Interferen ce ( I ) Noise ( N )
A
SoI
I
C
B
A
B
Ptra nsmit
A
Ptransmit
d AB
C
transmit
P
d CB
SINR
A
B
d AB
N
C
transmit
P
d CB
13
Red < Blue = collision
Red signal >> Blue signal
X
A
C
B
D
Signal
power
Distance
14
Important: C has not heard A, but can interfere at receiver B
C is the hidden terminal to A
X
A
C
B
D
Signal
power
Distance
15
Important: X has heard A, but should not defer transmission to Y
Y
X is the exposed terminal to A
X
A
C
B
D
Signal
power
Distance
16
Any Questions
at this point?
17
So, how do we cope with
Hidden/Exposed Terminals?
18
How to prevent C from trasmitting?
X
A
C
B
D
Signal
power
Distance
19
An Idea!
A
C
B
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?
20
An Idea!
X
A
C
B
D
Signal
power
Distance
21
Do not
transmit in
this region
Will this solve the wireless MAC problem?
A Project Idea!
X
A
C
D
B
Signal
power
T
Distance
22
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
23
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
24
IEEE 802.11
RTS = Request
To Send
CTS = Clear
To Send
M
S
Y
RTS
D
CTS
X
K
25
IEEE 802.11
silenced
M
S
Data
Y
D
silenced
ACK
X
silenced
silenced
K
26
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
27
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)
28
But is that enough?
29
RTS/CTS
Does it solve hidden terminals ?
Assuming carrier sensing zone = communication zone
E
RTS
F
CTS
A
B
C
CTS
D
E does not receive CTS successfully Can later initiate transmission to D.
Hidden terminal problem remains.
30
Hidden Terminal Problem
How about increasing carrier sense range ??
E will defer on sensing carrier no collision !!!
E
RTS
F
CTS
A
B
C
Data
D
31
Hidden Terminal Problem
But what if barriers/obstructions ??
E doesn’t hear C Carrier sensing does not help
E
RTS
F
CTS
A
B
C
Data
D
32
Exposed Terminal
B should be able to transmit to A
RTS prevents this
E
RTS
CTS
A
B
C
D
33
Exposed Terminal
B should be able to transmit to A
Carrier sensing makes the situation worse
E
RTS
CTS
A
B
C
D
34
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
35
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)
36
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
37
DBTMA [HaasCornell98]
RTS
A
Tx Busy tone
Rx Busy tone
CTS
B
X
Y
Tx Busy tone
CTS
A
Signal X
RTS
B
Signal X
X
Y
38
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 dialog
802.11
Gain
Implicit
ACK
39
Hybrid Channel Access
The optimization timeline
R
T
R
RTS
CTS
RTS
CTS +ACK
RTS
RTS
CTS
Data
Backoff
Data
Data
Poll +ACK
Data
Data
CTS
Data
ACK
Backoff
Backoff
CTS
ACK
R
Backoff
RTS
T
Backoff
T
Hybrid Channel Access
Implicit ACK
Poll +ACK
RTS
CTS +ACK
Data
Backoff
802.11
40
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
41
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
42
Questions ?
43
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
44
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.
45
Backup slides on
IEEE 802.11
Read for more details
46
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
47
IEEE 802.11 – An overview
48
IEEE 802.11 in OSI Model
Wireless
49
802.11 Scope & Modules
To develop a MAC and PHY spec for wireless
connectivity for fixed, portable and moving stations
in a local area
LLC
MAC
PHY
MAC
Sublayer
PLCP Sublayer
PMD Sublayer
MAC Layer
Management
PHY Layer
Management
50
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)
51
802.11 Architecture – Two modes
52
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)
53
Why Spread Spectrum ?
C = B*log2(1+S/N)
To achieve the same channel capacity C
. . . [Shannon]
Large S/N, small B
Small S/N, large B
Increase S/N is inefficient due to the logarithmic relationship
power
power
signal
noise, interferences
frequency
signal
B
B
e.g. B = 30 KHz
e.g. B = 1.25 MHz
54
Spread Spectrum
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.
Reduce effect of jamming
Military scenarios
Reduce effect of other interferences
More “secure”
Signal “merged” in noise and interference
55
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
One possible pattern
f
f
f f f f f
f f f f
Different hopping sequences enable co
existence of multiple BSSs
Robust against narrow-band interferences
56
FHSS due to [Lamarr1940]
power
power
signal
noise, interferences
frequency
B
f
f
signal
f f f f f
f f f f
B
Simple radio design with FHSS
Data rates ~ 2 Mbps
Invented by Hedy Lamarr (Hollywood film
star) in 1940, at age of 27, with musician
George Antheil
57
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
58
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
59
Diffused Infrared
Wavelength range from 850 – 950 nm
For indoor use only
Line-of-sight and reflected transmission
1 – 2 Mbps
60
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
61
PLCP (802.11b)
long
preamble
192us
short
preamble
96us
(VoIP, video)
62
PLCP (802.11b)
long
preamble
192us
Note:
To send even one bit payload
reliably, you will have to form
a packet with the PLCP preamble
and the PLCP header.
This constraints protocol design
You cannot arbitrarily exchange
control messages.
What are the control messages
in IEEE 802.11 ?
short
preamble
96us
(VoIP, video)
63
IEEE 802.11 MAC
64
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 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
65
802.11 MAC (DCF)
66
Physical Carrier Sense & Backoff
67
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
68
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
69
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
70
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
71
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
72
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
73
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
74
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
75
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
76
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
77
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.
78
Active scanning example
79
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
80