1 mac protocols for wireless networks: interaction between physical layer and mac nitin h. vaidya...
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MAC Protocols for Wireless Networks:Interaction between Physical Layer and
MAC
Nitin H. VaidyaUniversity of Illinois at Urbana-Champaign
www.crhc.uiuc.edu/wireless
© 2004 Vaidya
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Joint work with
Jason FuemmelerXue Yang
Romit RoyChoudhuryJungmin So
Pradeep KyasanurVenugopal Veeravalli
Funded in part by
National Science FoundationMotorola Center for Communications
NSF graduate fellowshipVodafone graduate fellowshipMotorola graduate fellowship
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Our Research
MAC/Routing/Transport protocols for wireless
Distributed algorithms (leader election, clock sync, ...)
Misbehavior in wireless networks
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Wireless Ad Hoc Networks
Formed by wireless hosts that may be mobile
Without necessarily using infrastructure
Routes between nodes may potentially contain multiple hops
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Ad Hoc Networks
EA
B CD
X
Z
Ad hoc connectivity
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Hybrid Environments
Infrastructure + Ad hoc connectivity
EA
B CD
BS1 BS2
X
Z
infrastructure
Ad hoc connectivity
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Wireless Capacity
Wireless capacity limited
In dense environments, performance suffers
How to improve performance ?
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Improving Wireless Capacity
Exploit physical resources
Exploit diversity
Examples ...
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Add Spectrum
More bandwidth
Example: Multiple channels in IEEE 802.11
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Improve Spatial ReusePower/Rate Control
A B C D
A B C D
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Exploit Infrastructure
Infrastructure provides a tunnel to forward packets
EA
B CD
BS1 BS2
X
Z
infrastructure
Ad hoc connectivity
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Exploit Antennas
Diversity antenna
Steered beam directional antenna
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Path Diversity
Multiple paths to a destination
Multiple next-hops to a destination
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Exploiting Diversity
Exploiting physical layerrequires suitable protocols
Routing
Medium access control (MAC)
Link
Network
Transport
PhysicalLayer
Upper layers
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Medium Access Control (MAC)
MAC protocols coordinate wireless channel access
May be centralized or distributed
Distributed protocols suit ad hoc & hybrid networks
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MAC Protocols
Need to design MAC protocols to exploitphysical layer capabilities
Proof by example …
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Outline
CSMA Protocols:
Warning: Work-in-progress
Directional antennas & Multiple channels:
Brief discussion, time permitting
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Carrier Sense Multiple Access (CSMA)
Listen-before-you-talk
A host may transmit only if the channel is idle
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Carrier Sense Multiple Access (CSMA)
Implementation using Carrier Sense (CS) threshold
If received power < CS threshold Channel idle
Else channel busy
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Carrier Sense Multiple Access (CSMA)
D perceives idle channel although A is transmitting
AB C
D
distance
po
we
r
D’s CS Threshold
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Carrier Sense Multiple Access (CSMA)
D perceives busy channel when A transmits
AB C
D
distance
po
we
r D’s CS Threshold
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Transmission Reliability
Reliability depends on SIR = S / I (ignoring noise)
SIR requirement depends on modulation scheme (rate)
AB C
D
distance
po
we
r
SI
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Warning
SIR and SINR used in this talk interchangeably
SINR = S / (I + N) S = signal I =
interference N = noise
SIR = S / I (when Noise ignored)
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Packet Transmission
The transmitter must choose Transmit rate Transmit power CS Threshold
Prior work typically focuses on choosing 1 parameter,
keeping others fixed
Jointly adapting 2-of-3 potentially better strategy
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Joint Adaptation2-of-3
Which 2-of-3 ?
Practical considerations might eventually prove one combination superior
Our on-going work
Transmit Rate & CS threshold Transmit Power & CS threshold
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Joint Control:
Transmit Power and CS Threshold
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Assumed Protocol Requirements
Distributed decision-making
Choose transmit power to meet desired SINR (despite future transmitters)
Not destroy on-going transmissions
E to F
A to B
C to D
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Observation 1
AB
distance
po
we
r
Higher transmit power increasesreceived signal power S at the intended receiver
Low transmit power
S
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Observation 1
AB
distance
po
we
r
High transmit power
Higher transmit power increasesreceived signal power S at the intended receiver
S
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Observation 2
AB J
distance
po
we
r
For a fixed CS threshold,high transmit power closest interferer farther
Decreases interference I
Low transmit power
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Observation 2
AB J
distance
po
we
r
For a fixed CS threshold,high transmit power closest interferer farther
Decreases interference I
High transmit power
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Observations 1 & 2
Higher transmit powerincreases signal S,decreases interference Iat the intended receiver
SIR = S/I improves
Double dipping!
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Towards a Protocol
Node A transmits a packet to B
Interference sources:1.Transmitters already active before A begins
talking2.Transmitters that transmit after A begins talking
A B C
D
distance y
x
z
E
F
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Interference from “Already Active” Sources
E is transmitting before A starts transmitting Such interference by design less than CS
threshold
A B C
D
E
F
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Interference from Future Sources
C begins transmitting after A starts its transmission
C must have sensed idle channel despite A’s transmission
A B C
D
E
F
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Transmit Power & CS Threshold Selection[Fuemmeler04]
How to ensure that a transmission will be completed reliably ?
Analysis suggest that transmit power and CS threshold should be adapted together
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Notation
g(x) = gain at distance x from the transmitter
Received power = transmit power * gain
Distance x
G
ain
g(x
)
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Notation
Pt = transmit power Pcs = Carrier-sense threshold
SINR threshold for reliability = noise
k = estimated number of interferers
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Power Control & CS ThresholdTwo Constraints
= design constant
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Sensitivity Constraint
Loud transmitters should be sensitive Whisperers may be insensitive
Limits interference to ongoing transmissions
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Interference Constraint
CS threshold = interference margin per interferer
Initial interference < CS Threshold it can be modeled as an equivalent future interferer (alternatively it can be added to η)
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Interference Constraint
Chosen transmit power also serves as an indication of tolerable interference to the corresponding transmission
Chosen transmit power serves dual roles of limiting interference to others, and limiting interference to self
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Performance Evaluation
Static Protocol
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Static Protocol
Pick a fixed value of k
Solve the two constraints to obtain transmit power and CS threshold
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Ring Topology
N nodes spaced equally around a ring
Each node transmits to its immediate counterclockwise neighbor
Symmetric topology
46k = number of interfererers
Ag
gre
gat
e t
hro
ug
hp
ut
(kb
ps)
N = 128
N = 8
Ring Topology(Static Protocol: k fixed)
Single flow throughput = 767 kbps SINR > 10 dB
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Random Topology
x = Txo = Rx
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Random Topology(Fixed Receive Power)
CS Threshold (Watts)
Ag
gre
gat
e t
hro
ug
hp
ut
(kb
ps)
x = Topology 1o = Topology 2
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Random Topology(Fixed Transmit Power)
CS Threshold (Watts)
Ag
gre
gat
e t
hro
ug
hp
ut
(kb
ps)
x = Topology 1o = Topology 2
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Static Protocol(fixed k)
Ag
gre
gat
e t
hro
ug
hp
ut
(kb
ps)
x = Topology 1o = Topology 2
k
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Failure Rate(Static Protocol: fixed k)
Pa
cke
t fa
ilure
ra
tex = Topology 1o = Topology 2
k
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Performance Evaluation
Dynamic Protocol
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Dynamic Protocol
Estimate k = number of interferers
Solve the two constraints to determinetransmit power and CS threshold
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Parameter k
k : the tunable knob
Co-location approximation introduces error: Error handled by increasing dynamic range of
k
Intuitively, k is a measure of total interference as a multiple of that from a “worst-case interferer”
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Estimation of Optimal k
Preliminary protocol : Details omitted here
Obtain an initial estimate on “reasonable” non-zero k starting at k = 0:
Increase k on packet failure Decrease k on success
Gradient-descent search: Motivated by the throughput and failure rate
versus k curves
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Throughput Comparison (kbps)Random Topologies
Protocol Topology 1 Topology 2
Fixed receive power
3071 3710
Fixed transmit power
3627 3944
Static k 4780 5218
Dynamic k 4955 5365
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Dynamic Protocol
Allows each node to use different k,transmit power and CS threshold
Can improve performance compared toglobally constant parameters
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Ongoing Work
Current dynamic protocol slow to adapt,may not work well in presence of rapid mobility
Small scale effects (fading) not accounted for May potentially be accounted for by adding
interference margin
Need to co-exist with other dynamic mechanisms such as backoff in 802.11
Fairness issues
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Related Work
Power-Controlled Multiple Access (PCMA):Monks et al. Receiver-based mechanism using busy-tone (BT) Data power & BT power chosen independently We use data for both purpose: only 1 power to choose
Zhu et al. (Intel): Globally constant CS threshold
Muqattash et al.: Need separate control channel, or a priori coordination
Congnitive radios: Sahai et al. (Allerton’04)
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Summary
Joint adaptation of CS threshold & transmit power beneficial
Need further work to make protocol more robust
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Impact of Protocols Overheads onCS Threshold
Skip?
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Goal
To identify how protocol overheads affect optimal carrier-sense threshold
Assume fixed transmit power
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Observation
AB
po
we
r
Higher CS Threshold brings closest interferer closer
Higher interferences at B Lower SIR Lower transmit rate
I J
CS threshold
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Small Carrier Sense Threshold(Sensitive Nodes)
Sensitive transmitters must be far away
With fixed transmit power, interference reduces
Transmit rate can be increased
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Small Carrier Sense Threshold(Sensitive Nodes)
AB X
Y
PQ
Smaller CS threshold Fewer simultaneous transmissions, but at higher rate
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Large Carrier Sense Threshold(Insensitive Nodes)
AB
Higher CS threshold More simultaneous transmissions, but at lower rate
M
N
R
S
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Optimal Carrier Sense Threshold
CS Threshold = 0 Only one transmission at a time
CS Threshold = ∞ All hosts transmit together
Optimal somewhere in between
Protocol overhead affect the optimal CS threshold …
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Impact of Protocol Overheads
Two components
Rate-independent overhead: Propagation delay, backoff slot in 802.11
Rate-dependent overhead: Collision cost
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Impact of Protocol Overhead
Rate-independent
Rate-dependent
Single rate Rtransmissionat a time
Two rate R/2transmissionsat a time
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Impact of Protocol Overhead
Can be beneficial to havemore simultaneous transmission atsmaller transmit rate
One approach: Divide bandwidth into multiple channels Previously investigated by others
Our approach: Increase “density” of transmitters
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Impact of Protocol Overheads [Yang05]
Increase density of transmitters,while decreasing transmission rate
Reduces rate-independent overhead
Conclusion:
Optimal CS threshold larger whenrate-independent overhead is considered
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Outline
CSMA Protocols
Directional Antennas & Multiple Channels
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Antenna Capabilities
Fixed beam antennas Omnidirectional
antennas, directional antennas with a fixed beam pattern
Movable beam antennas Switched, steered,
reconfigurable, adaptive, smart …
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Antenna Capabilities
Protocols designed for fixed beam antennas inadequate with movable beam antennas
Movable directional beams introduce additional benefits and challenges Deafness Neighbor discovery Utilizing long links
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Deafness [RoyChoudhury04]
S transmitting to D A cannot carrier-sense S A sends packet to S, but gets no response
D
A
S
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Deafness
Two difficult-to-distinguish possibilities Collision at S S is looking in a different direction
D
A
S
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Deafness
Two difficult-to-distinguish possibilities Collision at S S is looking in a different direction
Need different responses to the two events
Similarities to the problem of distinguishing between packet losses due to congestion and corruption in context of TCP-over-wireless.
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Deafness
Deafness arises due to directionality& the ability to change beam directions
Need to adapt MAC protocol to handle deafness
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Multi-Channel Environments
Skip
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Multi-Channel Environments
Multiple Channels available in IEEE 802.11 3 channels in 802.11b 12 channels in 802.11a
Utilizing multiple channels may improve throughput
1
defer
1
2
Single channel Multiple Channels
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Issues
Using k channels does not necessarily translate into proportional throughput improvement
Hosts limited by number of transceivers
Nodes on different channels cannot talk to each other
1 2
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Multi-Channel Hidden Terminals
Consider the following naïve protocol
Static channel assignment
Communication takes place on receiver’s channel
Sender switches to receiver’s channel to transmit
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Multi-Channel Hidden Terminals
A B C
Channel 1
Channel 2
A transmit on channel 1
C is on channel 2
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Multi-Channel Hidden Terminals
A B C
C switches to channel 1 and transmits
Channel 1
Channel 2
Collision occurs at B
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Multi-Channel Environments[So04,So03,Kyasanur04]
Protocols need to be designed to maximize channel utilization despite
Transceiver limitations
Multi-channel hidden terminals
Switching delays
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Conclusion
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Conclusion
Newer chipsets & radios are allowing more flexibility
Timescale over which parameters can be changed shrinking
•Ideal scenario: Per-packet switching
Makes it possible to exert more control on the physical layer
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Conclusion
Necessary to exploit physical layer capabilities to maximize performance
Need suitable MAC and Routing protocols
Many physical layer characteristics interesting: Transmit power, transmit rate, carrier-sense
threshold Antenna Channel diversity Switching delays Multi-user diversity
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Conclusion
State-of-the-Art:
Protocol mechanisms designed for particular physical layer capability
Holy grail:
Adaptive protocols that learn physical layer characteristics and adapt to them
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Additional Slides
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Physical Layer ParametersAffecting MAC
Transmit rate Transmit power Antenna Single or multi-channel Single or multi-band Switching delays …
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Analysis (details omitted)[Fuemmeler04]
Approximations to facilitate analysis:
Noise known
Sender & receiver effectively co-located
Sender senses same power level as the receiver
Approximation error handled by adaptation mechanism in the protocol
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Interference
From already on-going transmissions From future interferers
Interference from on-going transmissions can be modeled explicitly or as an equivalent future interferer
We choose the latter option, which fortuitously works out (but former can be used as well)
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Illinois Wireless Wind Tunnel
New project : To develop an environment for repeatable experiments on wireless networks
To be built in an anechoic chamber
“Scaling” of wireless network and environment to account for differences in mobility and obstacle size in the real network and experimental network
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Other Research
Energy efficient protocols for wireless networks
TCP over wireless networks
Handling misbehavior in wireless networks
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Ring Topology(Transmit Power fixed)
CS Threshold (Watts)
Ag
gre
gat
e t
hro
ug
hp
ut
(kb
ps)
N = 128
N = 8