The Impact of Channel Usage Information on the Throughput Achieved by 802.11-Style MACs in Urban Mesh Networks

1
The Impact of Channel Usage Information on the
Throughput Achieved by 802.11-Style MACs in Urban
Mesh Networks
Jonghyun Kim Stephan Bohacek Department of
Electrical and Computer Engineering University of
Delaware
2
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
3
Introduction

• Why does optimal spatial TDMA (STDMA) achieve
  higher throughput than CSMA/CA such as IEEE
  802.11?
• By eliminating collisions
• By ordering packets in an optimal way

4
Introduction
Destination A
A
C
Destination B (same side)
W
X
Y
Z
GW
T1
B
D
A
C
W
X
Y
Z
GW
Wait 1
T2
B
D
A
C
W
X
Y
Z
GW
T3
Wait 2
B
D
A
C
W
X
Y
Z
GW
T4
B
D
5
Introduction
Destination A
A
C
Destination C (opposite side)
W
X
Y
Z
GW
T1
B
D
A
C
W
X
Y
Z
GW
Wait 1
T2
B
D
A
C
W
X
Y
Z
GW
T3
B
D
GW can transmit 50 sooner
6
Introduction

• Goal

• Investigate how to increase the throughput by
  various 802.11-style MACs with different
  techniques and extending channel usage
  information so that the throughput may approach
  gradually and maximally to the throughput
  achieved by optimal STDMA.
• Investigate the main reasons (with percentage
  information) that optimal STDMA achieves higher
  throughput than 802.11

7
Introduction

• Topology

Gateway
Destination (mesh node)
3 Gateways with18 destinations
Downtown Chicago (2kmx2Km)
8
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
9
Throughput Metric
500
450
?2
400
Throughput
F bps
Peak
350
?4
flow1
?1
300
g?(F) (Kbps)
250
?3
flow1
?5
flow2
200
flow3
flow4
?6
150
flow5
flow6
100
200
250
300
350
400
450
500
500
F (Kbps)
Transmission rate to each destination ?
Average arrival rate at destination ?
Minimum average arrival rate
Peak value (Throughput)
10
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
11
Data-Rate Selection
40

Peak
35
SNR 18 dB
30
SNR1
Data-rate
25
20
Effective data rate (Mbps)
Link 1
15
10
5
0

0
10
20
30
40
50
60
R (Mbps)
Bit-rate provided by 802.11a (6,9,12,,54 Mbps)
Prob. of successfully decoding a packet transmitted over link i at bit-rate R
Maximum effective data rate over link i
Data-rate for link i
12
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
13
IEEE 802.11 Variants

• Standard 802.11

overhearing
S1
S2
ACK
DATA
DATA
R1
R2
overhearing
Node S1
Layer
State
NAV
DIFS
BO
BO remaining
DIFS
MAC
time
DATA frame from S2
ACK frame from R2
Receive
PHY
time
T1
DATA frame to R1
Transmit
time
DIFS
BO
Busy
BO
Transmit
DIFS
14
IEEE 802.11 Variants

• Stomp

S1
S2
ACK
DATA
R1
R2
Layer
State
DIFS
BO
MAC
time
DATA frame from S2
Receive
time
PHY
Decode PLCP header
Transmit
time
DIFS
BO
15
IEEE 802.11 Variants

• Stomp

SNRS1-S2
Red characters channel usage information S1
knows
S1
S2
SNRS1-R2
SNRS2-R2
SNRS1-R1
SNRR1-S2
Data-rate
Data-rate
R1
R2
SNRR1-R2
R1
Layer
State
DIFS
BO
BO remaining
MAC
time
DATA frame from S2
Receive
time
PHY
Decode PLCP header
DATA frame to R1
Transmit
time
DIFS
BO
Transmit
16
IEEE 802.11 Variants

• Packet Reordering

Buffer
Packet (destination A)
Packet (destination B)
Link 1
S1
S2
B
Conflict
overhearing
L3
Link 2
DATA
L1
L2
Link 3
A
R2
No conflict
Link 2
17
IEEE 802.11 Variants

• Capture of Stronger Signals

R
S1
S2
R
DATA frame from S2
Receive
time
PHY
T1
DATA frame from S1
Receive
time
T2
Received power
PS1 ? PS2
PS1
PS2
time
18
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
19
Idealized 802.11-Style MACs

• Transmitter-Transmitter Regional Channel Usage
  Information (TTCUI)

R5
T5
T2
T3
Transmitting nodes
R2
T2
T4
T5
T6
36Mbps region
6Mbps region
T1
R1
R3
T3
R4
T4
T4
R6
T6
Idealized version of 802.11 without RTS/CTS
20
Idealized 802.11-Style MACs

• Transmitter-Transmitter Regional Channel Usage
  Information (TTCUI)

36Mbps region
T1
R1
Cause a collision
R4
T4
21
Idealized 802.11-Style MACs

• Transmitter Regional Channel Usage Information
  (TCUI)

T2
T3
Transmitting nodes
R2
T2
Receiving nodes
R2
R4
36Mbps region
T1
R1
R3
T3
R4
T4
Half idealized version of 802.11 with RTS/CTS
22
Idealized 802.11-Style MACs

• Transmitter Regional Channel Usage Information
  (TCUI)

Cause a collision
36Mbps region
T1
T5
R5
R1
23
Idealized 802.11-Style MACs

• Transmitter and Receiver Regional Channel Usage
  Information (TRCUI)

T6
T2
R2
R6
R5
T1
R1
T5
R7
R3
T3
R4
T4
T7
T2
T3
Transmitting nodes
T4
T5
Receiving nodes
R2
R4
Idealized version of 802.11 with RTS/CTS
24
Idealized 802.11-Style MACs

• Global Channel Usage Information

T6
T2
R2
R6
R5
T1
R1
T5
R7
R3
T3
R4
T4
T7
T2
T3
Transmitting nodes
T4
T5
T6
T7
Receiving nodes
R2
R3
R4
R5
R6
R7
25
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
26
Simulation Environment

• Simulation Set-up

of gateways 1, 3, 5
of destinations 18, 36, 54, 72, 90
of topology samples 10
of topologies 150
City map Downtown Chicago (2Km x 2Km)
Size of topology 6 x 6 city block randomly chosen from city map
Application traffic CBR with 1344 bytes and F bps
MAC protocol 802.11a with transmission power of 18 dBm
Mobility UDel Mobility Model
Channel gain UDel Channel Model
Packet simulator QualNet
Methodology used to determine the throughput Golden section method and bootstrap percentile confidence interval
27
Simulation Environment

• MAC types

Type 802.11-style MAC algorithm
A Without RTS/CTS (baseline)
B With RTS/CTS transmitted at 6Mbps
C With CTS-to-self transmitted at 6Mbps
D Aloha-like
E Without RTS/CTS and capture of stronger signals
F Without RTS/CTS and with stomp, packet reordering, capture of stronger signals
G TTCUI with 6Mbps region
H TCUI with 6Mbps region
I TRCUI with 6Mbps region
J Global knowledge
K Optimal STDMA
Stomp, packet reordering, capture of stronger
signals
Idealized
28
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
29
Simulation Results
of gateways 1
of gateways 3
of gateways 5
3.5
3.5
3.5
3
3
3
2.5
2.5
2.5
Ratio of throughput
2
2
2
1.5
1.5
1.5
1
1
1
0.5
0.5
0.5
20
40
60
80
100
20
40
60
80
100
20
40
60
80
100
of destinations
A (Without RTS/CTS) baseline
B (With RTS/CTS)
C (With CTS-to-self )
D (Aloha)
E (Without RTS/CTS and capture of stronger signal)
F (Without RTS/CTS and with stomp, packet
reordering, capture of stronger signal )
G (TTCUI)
H (TCUI)
IJ (TRCUI GK)
K (Optimal STDMA)
30
Outline
Introduction Throughput Metric Date-Rate
Selection IEEE 802.11 Variants Idealized
802.11-style MACs Simulation Environment Simulatio
n Results Conclusions
31
Conclusion

• Significant improvement does not occur with
  perfect knowledge of only nearby transmitters.
• Significant improvement starts to occur with
  perfect knowledge of nearby transmitters and
  receivers.
• Half collisions will occur with knowing perfect
  channel activity around only transmitter.
• Half performance improvement by optimal STDMA is
  because of elimination of collisions. The other
  is because of ordering packets in an optimal way.

32
Thanks
Any questions, comments, suggestions ?
E-mail jonghyun_at_udel.edu
bohacek_at_udel.edu
33
UDel Models Website
http//udelmodels.eecis.udel.edu
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Extra Slides
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Extra Slides
Frame in 802.11a PHY
Question How much collision occur in this
scheme?
CBR data size 1344 bytes UDP header size 8 IP
header size 20 DATA MAC header size 28 MSDU
size 1372 PSDU size 1400 bytes
SIFS 16 us DIFS 34 us EIFS 94 us SLOT_TIME
9 us CW_MIN 15 (contention window) CW_MAX
1023
For DATA
Service 16 bits
Signal 24 bits
PLCP preamble 96 bits
MAC Hdr 28 bytes
MSDU 1372 bytes
Tail 6 bits
Pad
Duration 4.44 us
Duration 203.37 us
Duration 20 us
Bit rate 54 Mbps (varies)
Bit rate 6 Mbps (fixed)
Our objective is to use the channel during this
time if conditions are satisfied based on sender,
receiver, and duration from DATA MAC header.

• Conditions
• If receiver is not the node that is now receiving
  the frame above.
• If the link between sender and receiver is not a
  carrier sensing neighbor of the link between this
  node and the nodes receiver which is determined
  later if MAC does not have a frame received from
  network layer now or it already have a frame, but
  it stops counting back-off due to channel busy.
  Need to adjust back-off time.

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This is the current last point
of trials are increased in four times
Tracking line (B point)
Tracking line (A points)
Confidence Interval
Golden Section Method and bootstrap to find
confidence interval are used to find optimal
capacity (i.e., bit rate) in urban mesh network.
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