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 (Sensed - received = 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
SINR threhold
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
SINR threhold
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
C
SignalOfIn terest ( SoI )
SINR 
Interference( I )  Noise ( N )
A
SoI 
A
B
Ptra n smit
d AB
C
transmit
P
I 

d CB
C
B
A
Ptran
smit

d
AB
SINRBA 
C
Ptransmit
N 
d CB
13
Red < Blue = collision
Red signal >> Blue signal
X
A
C
B
D
Signal
power
SINR threhold
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
SINR threhold
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
SINR threhold
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
SINR threhold
Distance
19
A Project 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 don’t transmit
 Possible to guarantee no collisions?
20
A Project Idea!
X
A
C
B
D
Signal
power
SINR threhold
Sensitivity threshold
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
SINR threhold
T
Sensitivity threshold
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
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
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
 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