1. November 2007
Effect of Contention Window Size on ‘Express Forwarding’ Performance for Single-Channel Mesh
Date:
Authors:
Name
Address
Company
Phone
Mathilde Benveniste
233 Mt Airy Road
Basking Ridge, NJ 07920, US
Avaya Labs-Research
Kaustubh Sinkar
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M. Benveniste (Avaya Labs)

2. November 2007
Effect of Contention Window Size on ‘Express Forwarding’ Performance for Single-Channel Mesh Mathilde Benveniste Kaustubh Sinkar Avaya Labs - Research
M. Benveniste (Avaya Labs)

3. Introduction November 2007
Express Forwarding and Express Retransmission have been proposed to alleviate delay increases caused by multi-hop transmissions in a single channel mesh [2]-[3]
Increasing the contention window size following a hidden node collision helps avoid collision when retransmitting
This presentation shows the performance of ‘Express Forwarding’ with different contention window sizes
M. Benveniste (Avaya Labs)

4. Express Forwarding – Review Ref Doc 11-07/2452, 2453
November 2007
Express Forwarding – Review Ref Doc 11-07/2452, 2453
‘Express forwarding’ reduces the end-to-end delay of selected frames by granting forwarding nodes immediate access to the channel
Criteria for express forwarding frames are:
Time sensitive QoS [TSQ] frames – e.g. VO/VI
Frames on paths traversing more than a specified number of hops
Other
Single-hop frames are not express-forwarded
‘Time critical’ frames are single-hop frames that do not yield priority to express forwarded frames; such frames are
Top-priority management frames
Top-priority frames experiencing longer delay than a specified limit
M. Benveniste (Avaya Labs)

5. Express Forwarding Illustration Ref Doc 11-07/2452, 2453
November 2007
Express Forwarding Illustration Ref Doc 11-07/2452, 2453
DT0
NAV setting at receiving node
DTI
NAV setting at all other neighbor nodes
Value in Duration field
Channel
1-2
Frame
ACK
2-3
3-4
time 5
3-hop path 1-4 1 2 3 4
ANIMATED
The Duration field is set at a value longer than usual when a TSQ frame is transmitted to a forwarding node of a multi-hop path; DT0 added
The forwarding nodes, 2 and 3, adjust the Duration value on the received frame by subtracting an increment DTI when setting their NAV
Nodes with a ‘time critical’ frame subtract the increment DT0 from Duration field
The non-forwarding neighbor nodes (e.g. 5) sets NAV by Duration field
M. Benveniste (Avaya Labs)

6. Express Retransmission Ref Doc 11-07/2452, 2453
November 2007
Express Retransmission Ref Doc 11-07/2452, 2453
Conventional retransmission typically involves backoff and use of a wider contention window
With ‘express retransmission’, backoff is dispensed and frame is retransmitted within DT0 following ACKtimeout
Because of its prioritization, an express retransmitted frame is less likely to collide with one that is not
Only the first retransmission attempt receives priority treatment
Prevents two express retransmitted frames from colliding repeatedly
DT0
NAV setting at receiving node
DTI
NAV setting at all other neighbor nodes
Value in Duration field of TSQ frame
Channel
TSQ
TSQ
time
Frame
Retransmission
ACKtimeout
M. Benveniste (Avaya Labs)

7. Performance Evaluation
November 2007
Performance Evaluation
M. Benveniste (Avaya Labs)

8. November 2007
Scenarios
802.11g Network
Mix of wide and narrow contention windows
(WLAN flows set according to default values)
Wide contention windows
(To reduce hidden node collision recurrence)
All scenarios consider only high-priority traffic (VOIP and Video) – worst-case scenarios
OPNET Modeler used for simulations
M. Benveniste (Avaya Labs)

9. Parameters November 2007 PHY 11g Slot Time 9 usec Sifs Time 10 usec
Phy_CWmin 15
Phy_CWmax
1023
PLCP overhead control
20 usec
PLCP overhead data
Control Data Rate
24 Mbps
Difs Time
sifs + 2*slot_time = 28 usec
Eifs_time
difs + sifs + 24 Mbps
aifsn 2
Aifs [ac]
aifsn[ac] * slot_time + sifs_time = 28 usec
ACK tx rate
DATA tx rate
54 Mbps
M. Benveniste (Avaya Labs)

10. 802.11g Network November 2007 Traffic description
VIDEO (L): Low Resolution, 1.4 Mbps
payload size: 1464 bytes, inter-arrival 8 ms
VIDEO (H): High Resolution, 4.2 Mbps
payload size: 1464 bytes, inter-arrival 2.83 ms
VOIP : G711, 0.16 Mbps
payload size: 200 bytes, inter-arrival 20 ms
Network configuration
TX RANGE: 391 m
5-hop path, next-hop neighbors don’t hear each other
Physical layer rates
54 Mbps
24 Mbps P 1 2 3 4 5
VIDEO (L)
VIDEO (H)
VOIP 17 18 21 13 25 24 11 20 6 7 8 22 16 10 12 19 14 9
Traffic Load: Mbps
5 Video(L)= 5*1.464=7.32 Mbps
2 Video(H)= 2*4.14=8.28 Mbps
10 VOIP= 10*0.16=1.6 Mbps
Single VOIP flow 1600bpf *1000/20 fps=0.08 Mbps
VIDEO (L) flow 11712bpf * 1000/8 fps=1.464 Mbps
VIDEO (H) flow 11712bpf* 1000/2.83fps=4.14 Mbps
TOTAL LOAD: 17 Mbps
M. Benveniste (Avaya Labs)

11. Contention Windows A B November 2007 Nodes CWmin CWmax 15 1023 1 2 3 415
1023 1 2 3 4 5 7 8 11 31 12 13 16 17 18 19 24 25
Nodes
CWmin
CWmax 15
1023 1 2 3 4 5 7 8 11 12 13 16 17 18 19 24 25
M. Benveniste (Avaya Labs)

12. Mean Delays (ms) A B November 2007 M. Benveniste (Avaya Labs) Flows
Express Forwarding Disabled
Express Forwarding Enabled
Express Forwarding With Express Re-TX
Portal->Node_5
496.07
6.31
5.24
Portal->Node_16
322.78
5.22
4.55
Portal->Node_17
323.80
5.50
4.36
Portal->Node_18
403.43
6.17
4.92
Portal->Node_22
472.21
6.76
6.61
Node_5->Portal
266.27
4.98
3.73
Node_7->Node_8
1.85
0.95
1.01
Node_8->Node_7
2.24
1.04
1.22
Node_11->Node_20
4.15
5.25
Node_12->Node_10
18.33
2.68
3.07
Node_13->Node_21
51.94
2.82
3.49
Node_16->Portal
26.25
2.83
3.19
Node_17->Portal
60.76
2.20
2.33
Node_18->Portal
80.86
3.37
3.17
Node_19->Node_14
32.40
2.03
2.37
Node_24->Node_9
82.19
3.00
2.89
Node_25->Node_6
6.18
Express Forwarding Disabled
Express Forwarding Enabled
Express Forwarding With Express Re-TX
90.76
4.20
3.39
36.68
2.90
2.64
40.97
2.77
43.18
3.32
36.16
5.21
4.50
68.67
4.04
2.18
4.89
1.49
1.08
5.53
1.47
1.36
137.82
3.37
2.72
9.05
1.90
14.03
1.83
1.67
8.87
2.69
2.56
11.80
1.89
1.55
23.38
2.84
2.11
11.71
1.79
1.53
10.79
1.96
1.77
3.70
2.94
M. Benveniste (Avaya Labs)

13. November 2007
Conclusion
Increasing contention windows helps reduce hidden node collision recurrence
Express Forwarding reduces end-to-end delay of multi-hop flows substantially
Other (not express forwarded) traffic also benefits substantially from Express Forwarding
Express Forwarding eliminates the negative effect of multi-hop transmission correlation that exacerbates hidden node collisions [1]
M. Benveniste (Avaya Labs)

14. November 2007
References
“Performance implications of wireless mesh coexistence with WLANs”, M. Benveniste”, IEEE Doc r0
“‘Express’ Forwarding for Single-Channel Wireless Mesh”, M. Benveniste”, IEEE Doc r1
“Draft Text Changes for ‘Express Forwarding’ in a Mesh, M. Benveniste”, IEEE Doc r1
“Performance Evaluation of ‘Express Forwarding’ for a Single- Channel Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc r1
M. Benveniste (Avaya Labs)