The Case for Heterogeneous Wireless MACs
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The Case for Heterogeneous Wireless MACs
Chun-cheng ChenHaiyun Luo
Dept. of Computer Science, UIUC
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Problem#1: intra-BSS interference
Clear channel assessment (CCA): transmit iif RSS < CS threshold Downlink tx: all clients hear from AP
C
A B
Tx
RSS
CS Thrshld
timeCS
RSS
CS Thrshld
CS
RSS
CS Thrshld
CS
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Problem#1: intra-BSS interference
Clear channel assessment (CCA): transmit iif RSS < CS threshold Downlink tx: all clients hear from AP Uplink tx: clients may NOT hear from each other
C
A BCS Thrshld
RSS
CS
RSS
CS Thrshld
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Problem#1: intra-BSS interference
Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK)
C
A BRTSCTS
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Problem#1: intra-BSS interference
Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK)
C
A BDATAACK
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Problem#1: intra-BSS interference
Uplink tx: clients may NOT hear from each other Observation: they all hear from the AP Solution: four-way handshake (RTS/CTS/DATA/ACK)
Caveats RTS/CTS involves 29-37% overhead Not necessary for all clients
C
A B
D DATA
RTS
20bytes
PreambleHeader
72~144bits
48bits
@ basic_rate:2Mbps – 802.11b6Mbps – 802.11a/g
CCA is inconsistent.
When and with which client should RTS/CTS be enabled ?
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Problem#2: inter-BSS interference
Optimal channel assignment
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Optimal channel assignment
Clients on different BSS’s interfere with each other
Problem#2: inter-BSS interference
D ABC
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Hidden/exposed terminal problem Exposed receiver and hidden sender
CCA @ sender C and A is incomplete If RTS/CTS is enabled with C -> D, CCA @ A is complete
but CCA @ C is still incomplete
Problem#2: inter-BSS interference
D ABC
ExposedReceiver Hidden
Sender
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Hidden/exposed terminal problem Exposed receiver and hidden sender
When CCA is inconsistent, the sender with more complete CCA dominates starvation
Problem#2: inter-BSS interference
D ABC
Flow A->B always
dominates
Inconsistent CCA @ sender A, C
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Problem#2: inter-BSS interference
Hidden/exposed terminal problem Exposed receiver and hidden sender Hidden receiver
CCA at senders C and B is incomplete RTS/CTS helps only if D and A within communication
range
D ABC ACK
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Problem#2: inter-BSS interference
Can we have more orthogonal channels?
Dividing frequency band into more orthogonal channels does not serve bursty traffic well
Channel assignment may not be optimal
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Problem#2: inter-BSS interference
What about Receiver-initiated MAC ?
CCA @ receivers are still incomplete and inconsistent
D ABC
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Road Map
Motivations Goals Heterogeneous wireless MACs
Intra-BSS interference mitigation Inter-BSS interference mitigation
Evaluation Conclusion and future work
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Our Goals:
Collision avoidance
Starvation avoidance
Minimize MAC overhead
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Heterogeneous Wireless MACs
Idea: make the control tailored to “context” Turn on RTS/CTS only when necessary Control medium access from the node with better CCA
MAC protocols will be Context-Dependent Because contexts are heterogeneous, MAC protocols will
be heterogeneous
Approach Define a set of MAC protocols Learn from the context, apply the best MAC
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Intra-BSS interference mitigation
Example:
A node enables RTS/CTS only if another node does not hear its transmission
C
A B
D DATA
D doesn’t need to turn on RTS/CTS
A, B, C need to turn on RTS/CTS
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If signal propagation is symmetric: All stations initially turn off RTS/CTS
If signal propagation is asymmetric: Need explicit feedback relayed by AP
When to turn on/off RTS/CTS?
ACK
RSS
CSThrshld
One pkt trans. time
C
A B
D
DATAACK
ACK
RSS
CSThrshld
One pkt trans. time
ACK
RSS
CSThrshld A, C learned to turn on RTS/CTS!
One pkt trans. time
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Inter-BSS interference mitigation
Two hidden/exposed terminal problems not solved by 802.11:
Observation: node A and D have complete and consistent CCA!
Let A and D always initiate the transmission
D AC B D AC B
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When to use which MAC?
Example: All stations initially sender-initiated MAC:
Define a set of control messages for inter-operability RTS/CTS/ACK/RTR Diff. protocols may interpret them differently
D ABC
If success ratio too low switchMac().
DATA
Set one bit in DATA header to notify receiver of protocol switch!
RTR
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Success ratio
Topology: prev. example Using:
ns-2.28, TwoRayGround, 2Mbps basic rate, 11Mbps data rate CBR/UDP flows (flow 23 0~45sec; flow 01 10~45 sec), sender-initiated MAC initially
Metrics: throughput, success ratio
Evaluation
3 02 1
time (sec)
Normalized inst. thrput
Learning period
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Conclusion and future work
Incomplete CCA leads to high pkt loss Inconsistent CCA leads to starvation Context-dependent clear channel assessment can
be compensated with context-dependent, heterogeneous MAC protocols
Dynamic, context-aware, heterogeneous protocols provide us a new dimension for protocol design
Currently investigate global stability, learning algorithms
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Questions ?