The Case for Heterogeneous
Wireless MACs
Chun-cheng Chen
Haiyun Luo
Dept. of Computer Science, UIUC
1
Problem#1: intra-BSS interference
 Clear channel assessment (CCA): transmit iif
RSS < CS threshold
 Downlink tx: all clients hear from AP
RSS
CS Thrshld
RSS
CS
Tx
CS Thrshld
CS
C
time
RSS
A
CS Thrshld
B
CS
2
Problem#1: intra-BSS interference
 Clear channel assessment (CCA): transmit iif
RSS < CS threshold
 Downlink tx: all clients hear from AP
RSS
 Uplink tx: clients may NOT
hear from each other
CS Thrshld
C
RSS
A
CS Thrshld
B
CS
3
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
CTS
RTS
B
4
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
ACK
DATA
B
5
Problem#1: intra-BSS interference
 Uplink tx: clientsCCA
mayisNOT
hear from each other
inconsistent.
 Observation: they all hear from the AP
When
and with
which handshake
client should
RTS/CTS be enabled ?
 Solution:
four-way
(RTS/CTS/DATA/ACK)
 Caveats
 RTS/CTS involves 29-37% overhead
 Not necessary for all clients
C
Preamble Header
RTS
72~144
bits
20
bytes
48
bits
@ basic_rate:
2Mbps – 802.11b
6Mbps – 802.11a/g
A
B
D
DATA
6
Problem#2: inter-BSS interference
 Optimal channel assignment
6
1
11
1
11
6
6
1
11
1
11
6
1
7
Problem#2: inter-BSS interference
 Optimal channel assignment
C
D
A
B
 Clients on different BSS’s interfere with each other
8
Problem#2: inter-BSS interference
 Hidden/exposed terminal problem
 Exposed receiver and hidden sender
Exposed
Receiver
C
Hidden
Sender
D
A
B
 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
9
Problem#2: inter-BSS interference
 Hidden/exposed terminal problem
 Exposed receiver and hidden sender
Inconsistent CCA @ sender A, C
C
D
Flow A->B always
dominates
A
B
 When CCA is inconsistent, the sender with more complete
CCA dominates  starvation
10
Problem#2: inter-BSS interference
 Hidden/exposed terminal problem
 Exposed receiver and hidden sender
 Hidden receiver
C
D
AACK
B
 CCA at senders C and B is incomplete
 RTS/CTS helps only if D and A within communication range
11
Problem#2: inter-BSS interference
 Can we have more orthogonal channels?
6
1
11
1
11
6
6
1
11
1
11
6
1
 Dividing frequency band into more orthogonal
channels does not serve bursty traffic well
 Channel assignment may not be optimal
12
Problem#2: inter-BSS interference
 What about Receiver-initiated MAC ?
C
D
A
B
 CCA @ receivers are still incomplete and inconsistent
13
Road Map
 Motivations
 Goals
 Heterogeneous wireless MACs
 Intra-BSS interference mitigation
 Inter-BSS interference mitigation
 Evaluation
 Conclusion and future work
14
Our Goals:
 Collision avoidance
 Starvation avoidance
 Minimize MAC overhead
15
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
16
Intra-BSS interference mitigation
 Example:
A, B, C need to turn on RTS/CTS
C
A
B
D
DATA
D doesn’t need to turn on RTS/CTS
 A node enables RTS/CTS only if another node does not
hear its transmission
17
When to turn on/off RTS/CTS?
 If signal propagation is symmetric:
 All stations initially turn off RTS/CTS
RSS
RSS
CSThrshld
CSThrshld
ACK
C
ACK
One pkt trans. time
One pkt trans. time
ACK
DATA
A
D
B
RSS
CSThrshld
ACK
A, C learned to turn on RTS/CTS!
One pkt trans. time
 If signal propagation is asymmetric:
 Need explicit feedback relayed by AP
18
Inter-BSS interference mitigation
 Two hidden/exposed terminal problems not solved by
802.11:
C
D
A
B
C
D
A
B
 Observation: node A and D have complete and
consistent CCA!
 Let A and D always initiate the transmission
19
When to use which MAC?
 Example:
 All stations initially sender-initiated MAC:
Set one bit in DATA header to notify
receiver of protocol switch!
C DATA
RTR D
A
B
If success ratio too low  switchMac().
 Define a set of control messages for inter-operability
 RTS/CTS/ACK/RTR
 Diff. protocols may interpret them differently
20
Evaluation
 Topology: prev. example
 Using:
2
3
0
1
 ns-2.28, TwoRayGround, 2Mbps basic rate, 11Mbps data rate
CBR/UDP flows (flow 23 0~45sec; flow 01 10~45 sec),
sender-initiated MAC initially
 Metrics: throughput, success ratio
Normalized inst. thrput
Success ratio
time (sec) Learning period
time (sec)
21
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
22
Questions ?
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