1. Performance comparison of RTS/CTS and ‘Express Forwarding’ for mesh
April 2008
Performance comparison of RTS/CTS and ‘Express Forwarding’ for mesh
Date:
Authors:
Name
Address
Company
Phone
Mathilde Benveniste
233 Mt Airy Road
Basking Ridge, NJ 07920, US
Avaya Labs-Research
Kaustubh Sinkar
1 Telcordia Drive, Piscataway, NJ 08854, US
Telcordia Technologies
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M. Benveniste (Avaya Labs)

2. April 2008
Performance comparison of RTS/CTS and ‘Express Forwarding’ for mesh Mathilde Benveniste Kaustubh Sinkar
M. Benveniste (Avaya Labs)

3. April 2008
Abstract
Latency on single-channel mesh can far exceed a hop-count nultiple of the latency seen in WLANs
Multi-hop flows cause the latency on other flows (either on mesh or co-channel WLANs) to increase as well
Among the causes are hidden node collisions
Q: Can RTS/CTS remedy the problem?
A: No, RTS/CTS may exacerbate the problem
M. Benveniste (Avaya Labs)

4. April 2008
Introduction
Express Forwarding and Express Retransmission have been proposed to alleviate delay increases caused by multi-hop transmissions in a single channel mesh [2]-[3]
They enable the co-existence of mesh with WLANs as they also improve the performance of all traffic flows that are not express forwarded
Increasing the contention window size following a collision helps avoid hidden node collision on retry
Performance studies of Express Forwarding covered meshes with both long and short multi-hop flows [4] [8]
We compare here Express Forwarding to the use of RTS/CTS in a mesh network with multi-hop flows
M. Benveniste (Avaya Labs)

5. Express Forwarding – Review Ref Doc 11-07/2452, 2453
April 2008
Express Forwarding – Review Ref Doc 11-07/2452, 2453
‘Express forwarding’ on a single-channel mesh reduces the end-to-end delay of selected frames by granting forwarding nodes immediate access to the channel
Criteria for express forwarding frames include:
Time sensitive QoS [TSQ] frames – e.g. VO/VI
Frames on paths traversing more than a specified number of hops
Single-hop frames are not express-forwarded
How it works
Forwarding delay is reduced by reserving the channel for a forwarded transmission longer, in order to enable the forwarding node to seize the channel
Reservation is done with no additional frames
Immediate access is thus given to nodes forwarding QoS traffic on a multi-hop path
Because neighbor nodes refrain from transmitting when a frame is forwarded, the collision rate is reduced
M. Benveniste (Avaya Labs)

6. Express Forwarding Illustration
April 2008
Express Forwarding Illustration
DT0
Channel busy at receiving node
NAV setting at all other neighbor nodes
Value in Duration field of TSQ frame
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 value longer than usual when a TSQ frame is transmitted to a forwarding node of a multi-hop path; the increment
DT0 >= 1 timeslot + max{0, (IP processing delay – AckTime – SIFS)}
The forwarding nodes, 2 and 3, adjust the Duration value on the received frame by subtracting the increment DT0 when setting their NAV
The non-forwarding neighbor nodes (e.g. 5) set NAV according to Duration field
M. Benveniste (Avaya Labs)

7. Express Retransmission
April 2008
Express Retransmission
Retransmission of a failed transmission typically involves backoff using a wider contention window
With express retransmission, a frame is retransmitted by dispensing with backoff and transmitting after SIFS following AckTimeout
An express forwarded frame is less likely to collide on retransmission because of its prioritization
Only the first retransmission attempt receives priority treatment
Prevents two express-forwarded frames from colliding repeatedly
Subsequent retransmission attempts less likely to lead to collision when using large CWmax
DT0
Frame + ACK time
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)

8. Performance Study _______ April 2008 802.11a Network
3 WLANs co-channel with a nearby single-channel mesh
Mix of wide and narrow contention windows
WLAN AC_VI parameters:
CWmin=15, CWmax=31
Mesh parameters:
CWmin=15, CWmax=1023
(reduce recurrence of hidden node collision)
Scenarios Studied*
EF Disabled: business as usual – EF disabled and RTS/CTS disabled
EF Disabled (RTS): EF disabled and RTS/CTS enabled for video flows
EF Enabled: EF and ERTX enabled and RTS/CTS disabled
_______
*All scenarios consider only high-priority traffic (VOIP and Video)
OPNET Modeler used for simulations
M. Benveniste (Avaya Labs)

9. Parameters April 2008 Parameter Value PHY 11a Slot Time 9 usec
Sifs Time
16 usec
Phy_CWmin 15
Phy_CWmax
31 or 1023
PLCP overhead control
20 usec
PLCP overhead data
Control Data Rate
24 Mbps
Difs Time
sifs + 2*slot_time = 34 usec
Eifs_time
difs + sifs + 24 Mbps
aifsn 2
Aifs [ac]
aifsn[ac] * slot_time + sifs_time = 34 usec
ACK tx rate
DATA tx rate
54 Mbps
M. Benveniste (Avaya Labs)

10. 802.11a Mesh (3 hops max) April 2008 Physical layer rates17 18 21 1 2 3 26 25 4 5 6 11 12 33 34 32 31 14 22 29 30 20 27 28
VIDEO (L)
VOIP
VIDEO (H) 19
TOTAL LOAD: 23 Mbps
Network configuration
TX RANGE: 25 m
next-hop neighbors don’t hear each other
Physical layer rates
54 Mbps
24 Mbps
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, Mbps
payload size: 200 bytes, inter-arrival 20 ms
WLAN devices
Mesh points
M. Benveniste (Avaya Labs)

11. Average ETE Delay (sec)
April 2008
Average ETE Delay (sec)
M. Benveniste (Avaya Labs)

12. Express Fwd Disabled (RTS) Express Fwd with Exp Rtx
April 2008
Average ETE Delay (ms)
Express Fwd Disabled
Express Fwd Disabled (RTS)
Express Fwd with Exp Rtx
node_0 --> node_3 22 44 2
node_0 --> node_6
2,698
8,610 3
node_0 --> node_12
3,583
13,728 6
node_3 --> node_0 19 31
node_6 --> node_0
2,562
7,756
node_12 --> node_0
3,448
10,506 7
node_17 --> node_18 12 11
node_20 --> node_19 92 4
node_21 --> node_22
node_22 --> node_14
node_25 --> node_26
node_27 --> node_19 8
2,200 5
node_28 --> node_26
node_29 --> node_30 9
node_30 --> node_29
node_31 --> node_32
node_33 --> node_34 28
349
Express-forwarded
flows
WLAN flows
M. Benveniste (Avaya Labs)

13. ETE Delays CDF: Portal to Node 3
April 2008
ETE Delays CDF: Portal to Node 3
Portal -> Node3 Delay
Node3 -> Portal Delay
M. Benveniste (Avaya Labs)

14. ETE Delays CDF: Portal to Node 6
April 2008
ETE Delays CDF: Portal to Node 6
Portal -> Node6 Delay
Node6 -> Portal Delay
M. Benveniste (Avaya Labs)

15. ETE Delays CDF: Portal to Node 12
April 2008
ETE Delays CDF: Portal to Node 12
Portal -> Node12 Delay
Node12 -> Portal Delay
M. Benveniste (Avaya Labs)

16. April 2008
Dropped Frames
M. Benveniste (Avaya Labs)

17. April 2008
Retransmissions
M. Benveniste (Avaya Labs)

18. Summary of Results April 2008 RTS/CTS impact
At faster data rates (11g/a/n), differences in the size of data part become less significant
The increased number of transmissions becomes most relevant
The number of collisions increases as a consequence
Delay in successfully completing transmission increases
RTS/CTS offers no advantage with wireless meshes on faster channels
Express forwarding (with express retransmission) benefits
ETE delay for all 3-hop VOIP flows is reduced by at least 90%
All other flows (both on the mesh or in neighboring BSS) also enjoy substantial delay reduction
Express Forwarding causes fewer frames to be dropped
Express Retransmission reduces retransmissions and end-to-end delay further
Traffic concentration at Portal
The flows going thru the portal experience very long delays if no express forwarding is allowed
Express forwarding (with express retransmission) on the multi-hop flows reduces delays substantially on all flows thru the portal, both multi-hop and single-hop flows
M. Benveniste (Avaya Labs)

19. April 2008
Conclusions
Use of RTS/CTS cannot alleviate the negative impact of multi-hop flows of a single-channel mesh
In fact, it can exacerbate this impact
It causes flows with short frame interarrival times to become unstable on fast channels – e.g. 11a/g/n
Express Forwarding
reduces end-to-end delay and jitter of the multi-hop flows substantially
causes fewer frames to be dropped
Other (not express forwarded) traffic also benefits substantially
Express Retransmission further enhances these benefits
Express Forwarding and Express Retransmission enable multi-hop QoS traffic to get through a single-channel mesh fast with minimum impact on other traffic (WLAN and mesh)
M. Benveniste (Avaya Labs)

20. April 2008
References
“Wireless mesh networks: Performance implications for WLANs,” M. Benveniste, Interop New York, Oct 2007
“‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste”, IEEE Doc r2
“Draft Text Changes for ‘Express Forwarding’ in a Mesh, M. Benveniste”, IEEE Doc r4
“Performance Evaluation of ‘Express Forwarding’ for a Single-Channel Mesh”, M. Benveniste and K. Sinkar, IEEE Doc r1
“Performance Implications of ‘Express Forwarding’ for a Single-Channel Mesh”, M. Benveniste, IEEE Doc r1
“Effect of Contention Window Size on Express Forwarding Performance for Single-Channel Mesh”, M. Benveniste and K. Sinkar, IEEE Doc r0
“Coexistence issues for single-channel wireless mesh”, M. Benveniste, IEEE Doc r0
“More on Performance Evaluation of ‘Express Forwarding’ for Mesh”, M. Benveniste and K. Sinkar, IEEE Doc r0
M. Benveniste (Avaya Labs)