1. Coexistence issues for single-channel wireless mesh
January 2008
Coexistence issues for single-channel wireless mesh
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. Coexistence issues for single-channel wireless mesh
January 2008
Coexistence issues for single-channel wireless mesh
Mathilde Benveniste
M. Benveniste, Avaya Labs

3. January 2008
Abstract
Wireless meshes operating on a single channel have novel collision behavior that can impact nearby WLANs. The prevalence of hidden nodes and the interaction of contention-based access with multi-hop flows impose latency increases on both mesh and WLAN beyond what non-mesh experience suggests. Hidden nodes remain hidden after retrial, and their transmissions are dropped. The high correlation of sequentially forwarded frames on a multi-hop flow cause excessive delays to transmissions that have been involved in a collision. For backward compatibility, and for the contention-based access protocol to continue to be used, remedies are needed on the mesh side. We describe remedies we have in mind for this coexistence issue.
M. Benveniste, Avaya Labs

4. Introduction January 2008 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
Wireless mesh networks impact performance of WLANs
They operate on same RF spectrum as WLANs, often sharing the same channel
Mesh using a single channel exacerbates the impact on WLANs
Hidden node collisions are prevalent with such meshes
Multiple-hop flows of a mesh can cause substantial delays on both WLAN and mesh traffic
Possible remedies are described
M. Benveniste, Avaya Labs

5. Enterprise Wireless Mesh
January 2008
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

6. Wireless Mesh coexisting with WLANs
January 2008
Wireless Mesh coexisting with WLANs
Wired Network 11
Multi-hop transmission
Portal 1
WLAN AP 2 6
Station 5 9
Mesh Point 3
Mesh AP 7 10 4
Station 8
M. Benveniste, Avaya Labs

7. ‘Hidden node’ collisions with a single-channel mesh
January 2008
‘Hidden node’ collisions with a single-channel mesh
Characteristics of mesh networks using a single channel
A mesh point will typically not be able to hear most of the mesh points other than its neighbors
The neighbor of a neighbor is likely a hidden node
A wireless network without hidden nodes could be a single BSS
Such a mesh experiences more hidden node collisions than a WLAN system
A single WLAN experiences hidden node collisions on uplink transmissions only
WLANs with overlapping coverage areas are likely to use different channels
WLANs sharing same channel are separated by longer distances than mesh nodes
Such a mesh causes hidden node collisions for nearby WLAN devices
The mesh neighbors of a mesh node near a WLAN are hidden nodes for the WLAN as they all use the same channel
M. Benveniste, Avaya Labs

8. Hidden node collision between WLAN and mesh nodes
January 2008
Hidden node collision between WLAN and mesh nodes
A and C cannot hear each other; C completes its transmission first
B experiences a collision when D sends acknowledgement to C – regardless of whether D hears A
A must retry transmission
WLAN node
Mesh point B
((((
ACK X
Hidden node A Tx F E D C
Fig 1
M. Benveniste, Avaya Labs

9. Hidden node impact on dropped frames and throughput
January 2008
Hidden node impact on dropped frames and throughput
Hidden nodes increase collision rate
Collisions causing both hidden node transmissions to fail can lead to high rates of dropped frames, even in light traffic
Collisions repeat on retransmission as the retransmitting nodes cannot hear each other
When the ‘retry limit’ is reached frames are dropped
With adjustable data rates, high dropped frame rates lead to data rate reduction and low throughput
M. Benveniste, Avaya Labs

10. Hidden node collision ‘loop’
January 2008
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 X Tx C D
((((
(((( A B X Tx
Fig 2
M. Benveniste, Avaya Labs

11. Multi-hop flow impact on E-to-E delay
January 2008
Multi-hop flow impact on E-to-E delay
Multi-hop flows experience latencies at minimum the multiple of single-hop delays
A WLAN or mesh node involved in a collision can experience a longer delay than the delay of a nearby multi-hop flow
When a transmission is involved in a collision, it is at a disadvantage relative to transmissions attempted for the first time (because of the longer retry backoff)
If the failed transmission is near a multi-hop flow, it may have to wait for the entire multi-hop flow to complete before retransmission is possible
Applications with short frame inter-arrival times (e.g. HDTV) risk going unstable if near multi-hop flows
The faster the multi-hop flow completes, the sooner retransmission will succeed
M. Benveniste, Avaya Labs

12. Delay caused by multi-hop flow
January 2008
Delay caused by multi-hop flow
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
ACK Tx
(((( X
Hidden node B A F E D C Tx Tx Tx
Fig 3
M. Benveniste, Avaya Labs

13. Hidden node collision loop with multi-hop flow
January 2008
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 A B F
Fig 4
M. Benveniste, Avaya Labs

14. Possible remedies January 2008 Large retry window – all traffic ACs
Avoids dropped frames
Using a large backoff retry window reduces the probability of repeating a hidden node collision on retransmission
Express forwarding – top-priority ACs
Channel reservation for a forwarded frame reduces collision rate and benefits all
Expedited transmission reduces latency of all expedited flows
Non-expedited transmissions involved in collisions benefit as they will now wait a shorter time for nearby multi-hop flows to complete
M. Benveniste, Avaya Labs

15. January 2008
‘Express Forwarding’ Reference: ‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste, IEEE Doc r2
“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 and returned ACK is extended to keep all other neighbors silent long enough for forwarding to occur contention-free
Tx Hop 1
Channel reservation
Tx Hop 2
time
M. Benveniste, Avaya Labs

16. Hidden node collision ‘loop’
January 2008
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 2
M. Benveniste, Avaya Labs

17. Delay caused by multi-hop flow
January 2008
Delay caused by multi-hop flow
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 3
M. Benveniste, Avaya Labs

18. Hidden node collision loop & correlated transmissions
January 2008
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

19. January 2008
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

20. Fig 7. 802.11g Network January 2008 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

21. Fraction of Dropped Frames
January 2008
Fraction of Dropped Frames
There are practically no dropped frames with express forwarding
M. Benveniste, Avaya Labs

22. Mean Delays January 2008 Without Express Forwarding
5-hop VOIP delay 91 ms
2-hop VOIP delay 43 ms
Flow 25-6 is unstable
Flow delay 100 ms
With Express Forwarding
5-hop VOIP delay 4 ms
2-hop VOIP delay 3 ms
Flow 25-6 delay 4 ms
Flow delay 3 ms
+ Express Retransmission
5-hop VOIP delay 3 ms
2-hop VOIP delay <3 ms
Flow 25-6 delay 3 ms
Flow delay <3 ms
Substantially lower delays for all other flows
M. Benveniste, Avaya Labs

23. Observations Express forwarding reduced delays for all flows
January 2008
Observations
Express forwarding reduced delays for all flows
Express forwarding eliminated the negative effect of multi-hop flows
Question: How much do multi-hop flows hurt?
Answer: Remove multi-hop (‘long’) flows and see
M. Benveniste, Avaya Labs

24. January 2008
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

25. January 2008
Conclusions
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
Multi-radio/multi-channel meshes do not have the problems discussed above
M. Benveniste, Avaya Labs

26. January 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. Kaustubh, 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. Kaustubh, IEEE Doc r0
“More on Performance Evaluation of 'Express Forwarding' for Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc r0
M. Benveniste, Avaya Labs