1. Performance implications of wireless mesh coexistence with WLANs
December 2007
Performance implications of wireless mesh coexistence with WLANs
Date:
Authors:
Name
Address
Company
Phone
Mathilde Benveniste
233 Mt Airy Road
Basking Ridge, NJ 07920, US
Avaya Labs-Research
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M. Benveniste, Avaya Labs

2. Performance implications of wireless mesh coexistence with WLANs
December 2007
Performance implications of wireless mesh coexistence with WLANs
Mathilde Benveniste
M. Benveniste, Avaya Labs

3. December 2007
Introduction Reference: “Wireless mesh networks: Performance implications for WLANs,” M. Benveniste, Interop New York, Oct 2007
WLANs (wireless LANs)* operate in the 2.4 GHz and 5 GHz unlicensed RF bands
Wireless mesh networks operate on same RF spectrum
QoS challenges for VoIP and multimedia
if mesh multi-hop transmissions use a single channel, which is also used by nearby WLANs
_______
*The WiFi Alliance certifies WLAN devices for compliance with IEEE standard and interoperability
M. Benveniste, Avaya Labs

4. Enterprise Wireless Mesh
December 2007
Enterprise Wireless Mesh
Non-mesh
End-User Devices
Converged “Wired” Enterprise Networking Infrastructure
MP-PDA
Laptop c
MP-Portal
Switch/Router
WiFi telephone
WiFi telephone
MP-Desktop
IP Network
MP-HDTV
MP- AP
Wireless/ Wireline Demarcation Point
(Managed) Service Provider Network
MP-Printer
MP-Camera
Wireless Mesh
M. Benveniste, Avaya Labs

5. About meshes December 2007 Purpose of a wireless mesh
to extend the area of wireless broadband and reduce need for wiring
for temporary extensions to LAN/WLANs (such as trade shows or multi-venue sporting events) without re-working wiring/cabling
Mesh characteristics
Mesh points will typically not be able to hear most of the mesh points other than their neighbors
The neighbor of a neighbor will likely be a hidden node
M. Benveniste, Avaya Labs

6. Coexisting Wireless Mesh and WLAN
December 2007
Coexisting Wireless Mesh and WLAN
Wired Network 11
WLAN AP
Station
Multi-hop transmission
Portal 1 2 6 5
Mesh Point 9 3
Mesh AP 7 10 4
Station 8
M. Benveniste, Avaya Labs

7. QoS Challenges with meshes
December 2007
QoS Challenges with meshes
Traffic Concentration Points
“Hidden Node” Collisions
Correlated Traffic: Multi-hop Flows
M. Benveniste, Avaya Labs

8. Traffic Concentration
December 2007
Traffic Concentration
Excessive latency for VoIP/multimedia frames caused by mesh bottlenecks that arise from improper traffic engineering
Single radio mesh points are inadequate for a wireless mesh used to connect multiple fully loaded APs
Multi-channel (-radio) mesh is needed as backbone for WLAN networks
Sufficient radios must be available at traffic concentration points, such as the point of interface with wired network for traffic to be distributed to multiple channels
Single-channel mesh simply extends coverage range of a WLAN through multiple-hop transmissions
M. Benveniste, Avaya Labs

9. “Hidden node” collisions in single-channel mesh
December 2007
“Hidden node” collisions in single-channel mesh
A wireless mesh using CSMA/CA (or EDCA) on a single channel experiences more hidden node collisions than a WLAN system
WLANs with overlapping coverage areas are likely to use different channels
Hidden node collisions may repeat on retransmission and lead to dropped frames, even in light traffic
The retransmitting nodes cannot hear each other
A multi-hop flow with a transmission that causes but survives a hidden node collision can cause substantial delays to other traffic
The longer retry backoff of the failed transmission place it at a disadvantage in accessing the channel relative to the forwarded transmissions that follow
WLAN traffic is as vulvnerable as mesh traffic
M. Benveniste, Avaya Labs

10. December 2007
Examples
M. Benveniste, Avaya Labs

11. Hidden node collision caused by the Ack
December 2007
Hidden node collision caused by the Ack
A and C cannot hear each other; C completes its transmission first
B experiences a collision when D sends acknowledgement to C
A must retry transmission
WLAN node
Mesh point
((((
ACK X
Hidden node B A Tx F E D C
Fig 1
M. Benveniste, Avaya Labs

12. Single transmission failures
December 2007
A cannot hear either C or D
B experiences a collision when C transmits; A must retransmit
If D’s ACK collides with the retransmission, A must retransmit again
If D’s ACK collides with the ACK to A’s transmission, C must retransmit
Remedy: Use large backoff window on retransmission
WLAN node
Mesh point X
ACK
((((
RTX
Hidden node Tx
(((( X
Hidden node D A B C D
Fig 2
M. Benveniste, Avaya Labs

13. Hidden node collision ‘loop’
December 2007
Hidden node collision ‘loop’
A and D cannot hear each other
C cannot receive when A transmits
B cannot receive when D transmits
A and D retransmit unsuccessfully until retry limit is reached
Without a remedy, both frames will be dropped!
WLAN node
Mesh point
Remedy: Use large backoff window on retransmission X Tx C D
((((
(((( A B X Tx
Fig 3
M. Benveniste, Avaya Labs

14. Correlated Traffic: Multi-hop Flows exacerbate contention
December 2007
Correlated Traffic: Multi-hop Flows exacerbate contention
Substantial delay can be caused if the frame surviving a hidden node collision belongs to a multi-hop flow
Successful retransmission may be delayed for any of the following reasons:
The same hidden node collisions repeat on retransmission
Another collision may be experienced with the forwarded frame
The frame experiencing collision will yield the channel to forwarded frames because of its longer retry backoff window
M. Benveniste, Avaya Labs

15. Hidden node collision loop & correlated transmissions
December 2007
A and C cannot hear each other
D cannot receive when A transmits
B cannot receive when C transmits
A and C retransmit repeatedly without success
Without a remedy, both frames will be dropped
WLAN node
Mesh point
If transmission to D succeeds, D’s forwarded frame will preempt A’s retransmission due to A’s longer backoff
A’s transmission is delayed
The sooner D & E complete their transmissions the sooner A will be able to transmit Tx
(((( Tx X E Tx D C
Remedy: Use large backoff window on retransmission &
Express Forwarding for multi-hop flow A B F
Fig 4
M. Benveniste, Avaya Labs

16. Acknowledgment induced collision & correlated transmissions
December 2007
Acknowledgment induced collision & correlated transmissions
A cannot hear C or D, and vice versa
B experiences a collision when D sends acknowledgement to C
A must retry transmission
WLAN node
Mesh point
D and E will preempt A’s retransmission due to A’s the longer backoff
A’s transmission is delayed
The sooner D and E complete their transmissions the sooner A will be able to transmit
Remedy: Use Express Forwarding
ACK Tx
(((( X
Hidden node B A F E D C Tx Tx Tx
Fig 5
M. Benveniste, Avaya Labs

17. December 2007
“Express Forwarding” Reference: ‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste, IEEE Doc
“Express forwarding” is a prioritization method for multi-hop transmissions
Can be used selectively for VoIP/multi-media
Prioritization thru next hop reservation
The Duration field on the frame is extended to keep all other neighbors silent longer
Simulation results show substantial latency reduction (50-95%)
Tx Hop 1
Channel reservation
Tx Hop 2
time
M. Benveniste, Avaya Labs

18. December 2007
Example Reference: “Performance Evaluation of ‘Express Forwarding’ for a Single-Channel Mesh,” M. Benveniste and K. Kaustubh, IEEE Doc
802.11g Mesh
Mix of multi-hop and single-hop VoIP calls and video users transmitting on the same channel as an independent WLAN
Single mesh portal
Data rate 54 Mbps; Ack rate 24 Mbps
M. Benveniste, Avaya Labs

19. Fig 7. 802.11g Network December 2007 Data Range Ack Range
Nodes 2, 3, 4 & 5 likely to draw shorter backoff than Node25
A collision with the ACK leads to longer backoff for Node25 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
ANIMATED
If the ACK from 2 (or 3) causes collision at Node 6, retransmission of frame from Node 25 will wait till multi-hop TX P->5 completes
The sooner the latter completes, the sooner the transmission 25->6 will complete
Express Forwarding reduces time of transmission P->5, thus shortens delay for flow 25->6
With Express Forwarding, the ACK from Node 2 prevents collision by Node 25 with subsequent transmission from Node 2
M. Benveniste, Avaya Labs

20. December 2007
Mean Delays
Delays are substantially shorter without the multi-hop flows
Express Forwarding reduces the negative effect of multi-hop flows on other traffic
M. Benveniste, Avaya Labs

21. December 2007
Conclusions
Multi-radio/channel meshes must be used to alleviate heavy traffic concentration
In a single-channel mesh, hidden node collisions and multi-hop flows can cause severe delays and/or dropped frames to both mesh and WLAN traffic
Remedies include large backoff windows on retransmission and Express Forwarding for multi-hop flows
M. Benveniste, Avaya Labs

22. December 2007
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 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