1. doc.: IEEE 802.11-07/2814r0 Submission Slide 1 Performance implications of wireless mesh coexistence with WLANs Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at.http:// ieee802.org/guides/bylaws/sb-bylaws.pdfstuart.kerry@philips.compatcom@ieee.org Date: 2007-11-12 Authors: NameAddressCompanyPhoneEmail Mathilde Benveniste 233 Mt Airy Road Basking Ridge, NJ 07920, US Avaya Labs-Research973-761-6105benveniste@ieee.org

2. doc.: IEEE 802.11-07/2814r0 Submission Slide 2 Performance implications of wireless mesh coexistence with WLANs Mathilde Benveniste benveniste@ieee.org

3. doc.: IEEE 802.11-07/2814r0 Submission Slide 3 Introduction Reference: “Wireless mesh networks: Performance implications for WLANs,” M. Benveniste, Interop New York, Oct 2007 802.11 WLANs (wireless LANs)* operate in the 2.4 GHz and 5 GHz unlicensed RF bands Wireless mesh networks will operate on the same RF spectrum QoS challenges arise for VoIP and multimedia –if mesh multi-hop transmissions use a single channel, which is also used by WLANs nearby _______ *The WiFi Alliance certifies WLAN devices for compliance with IEEE 802.11 standard

4. doc.: IEEE 802.11-07/2814r0 Submission Slide 4 Enterprise Wireless Mesh MP- AP MP-Printer MP-PDA MP-Camera MP-Portal Converged “Wired” Enterprise Networking Infrastructure c Wireless/ Wireline Demarcation Point Switch/Router Wireless Mesh Laptop WiFi telephone Non-mesh 802.11 End-User Devices MP-HDTV MP-Desktop (Managed) Service Provider Network IP Network

5. doc.: IEEE 802.11-07/2814r0 Submission Slide 5 About meshes Purpose of a wireless mesh –to extend the area of wireless broadband and eliminate need for much 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

6. doc.: IEEE 802.11-07/2814r0 Submission Slide 6 Wireless Mesh and WLAN 5 9 7 10 6 4 3 2 1 8 Wired Network Portal Mesh Point Mesh AP Station Multi-hop transmission 11 WLAN AP Station

7. doc.: IEEE 802.11-07/2814r0 Submission Slide 7 QoS Challenges in meshes 1.Traffic Concentration Points 2.“Hidden Node” Collisions 3.Correlated Traffic: Multi-hop Flows

8. doc.: IEEE 802.11-07/2814r0 Submission Slide 8 Traffic Concentration Points  Without proper traffic engineering, bottlenecks arise, which increase the latency experienced by VoIP/ multimedia frames Sufficient radios must be available at traffic concentration points, such as the point of interface with wired network Single radio mesh points are inadequate for a wireless mesh used to connect multiple fully loaded APs  Multi-channel (-radio) mesh needed as backbone for WLAN networks Single-channel mesh simply extends coverage range of a WLAN through multiple-hop transmissions

9. doc.: IEEE 802.11-07/2814r0 Submission Slide 9 “Hidden node” collisions in single-channel mesh  Because of hidden nodes, a mesh using a single channel with CSMA/CA (or EDCA) for access experiences more collisions than a BSS (where most stations hear each other)  Hidden node collisions may repeat on retransmission and lead to dropped frames, even in light traffic  The problem is exacerbated in the presence of multi-hop transmissions, leading to substantial delays

10. doc.: IEEE 802.11-07/2814r0 Submission Slide 10 Examples

11. doc.: IEEE 802.11-07/2814r0 Submission Slide 11 Acknowledgment induced collision A and C cannot hear each other B experiences a collision when D sends acknowledgement to C A must retry transmission F EDC (((((((( ACK X Hidden node Tx Fig 1 A B

12. doc.: IEEE 802.11-07/2814r0 Submission Slide 12 Single transmission failures A cannot hear either C or D B experiences a collision when C transmits A BCD Tx (((((((( X Hidden node Fig 2 D Remedy: Use large backoff window on retransmission X ACK (((((((( RTX Hidden node If D’s ACK collides with the retransmission, A must retransmit again If D’s ACK collides with the ACK to A’s retransmission, C must retransmit

13. doc.: IEEE 802.11-07/2814r0 Submission Slide 13 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! A B Tx X C D (((((((( (((((((( X Fig 3 Remedy: Use large backoff window on retransmission

14. doc.: IEEE 802.11-07/2814r0 Submission Slide 14 Correlated Traffic: Multi-hop Flows exacerbate contention  When a multi-hop flow frame causes a hidden node collision to another frame, the latter could experience substantial delay  Successful retransmission may be delayed for any of the following: 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 window

15. doc.: IEEE 802.11-07/2814r0 Submission Slide 15 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 Hidden node collision loop & correlated transmissions A B D C (((((((( (((((((( Tx X X Fig 4 E Remedy: Use large backoff window on retransmission & Express Forwarding for multi-hop flow Tx 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 completes its transmission the sooner A will be able to transmit

16. doc.: IEEE 802.11-07/2814r0 Submission Slide 16 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 F EDC ACK Tx (((((((( X Hidden node Tx Fig 5 A B Tx 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

17. doc.: IEEE 802.11-07/2814r0 Submission Slide 17 “Express Forwarding” Reference: ‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste, IEEE Doc 802.11-07-2452  “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 Channel reservation time

18. doc.: IEEE 802.11-07/2814r0 Submission Slide 18 Example Reference: “Performance Evaluation of ‘Express Forwarding’ for a Single-Channel Mesh,” M. Benveniste and K. Kaustubh, IEEE Doc 802.11-07-2454 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

19. doc.: IEEE 802.11-07/2814r0 Submission Slide 19 P 1 2 3 45 VIDEO (L) VIDEO (H) VIDEO (L) VOIP 17 18 211325 24 1120 6 78 2216 10 12 1914 9  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 ANIMATED A collision with the ACK leads to longer backoff for Node25 Nodes 2, 3, 4 & 5 likely to draw shorter backoff than Node25 Data Range Ack Range 802. Fig 7. 802.11g Network

20. doc.: IEEE 802.11-07/2814r0 Submission Slide 20 Mean Delays Delays are substantially shorter without the multi-hop flows Express Forwarding reduces the negative effect of multi-hop flows on other traffic

21. doc.: IEEE 802.11-07/2814r0 Submission Slide 21 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 packets Remedies include large backoff windows on retransmission and Express Forwarding for multi-hop flows

22. doc.: IEEE 802.11-07/2814r0 Submission Slide 22 References 1.“Wireless mesh networks: Performance implications for WLANs,” M. Benveniste, Interop New York, Oct 2007 2.“‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste”, IEEE Doc 802.11-07-2452r1 3.“Draft Text Changes for ‘Express Forwarding’ in a Mesh, M. Benveniste”, IEEE Doc 802.11-07-2453r1 4.“Performance Evaluation of ‘Express Forwarding’ for a Single-Channel Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc 802.11-07-2454r1