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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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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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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C

A BDATAACK

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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


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


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


D ABC

Flow A->B always

dominates

Inconsistent CCA @ sender A, C

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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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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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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

21time (sec)

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 ?

the case for heterogeneous wireless macs

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