Coexistence issues for single-channel wireless mesh January 2008 Coexistence issues for single-channel wireless mesh Date: 2008-01-10 Authors: Name Address Company Phone Email Mathilde Benveniste 233 Mt Airy Road Basking Ridge, NJ 07920, US Avaya Labs-Research 973-761-6105 benveniste@ieee.org 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 <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, 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 <stuart.kerry@philips.com> 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 <patcom@ieee.org>. M. Benveniste, Avaya Labs

Coexistence issues for single-channel wireless mesh January 2008 Coexistence issues for single-channel wireless mesh Mathilde Benveniste benveniste@ieee.org M. Benveniste, Avaya Labs

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

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

Enterprise Wireless Mesh January 2008 Enterprise Wireless Mesh Non-mesh 802.11 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

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

‘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

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

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

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

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

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

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

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

January 2008 ‘Express Forwarding’ Reference: ‘Express’ Forwarding for Single-Channel Wireless Mesh, M. Benveniste, IEEE Doc 802.11-07-2452r2 “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

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

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

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

January 2008 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 M. Benveniste, Avaya Labs

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

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

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 11-20 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 11-20 delay 3 ms + Express Retransmission 5-hop VOIP delay 3 ms 2-hop VOIP delay <3 ms Flow 25-6 delay 3 ms Flow 11-20 delay <3 ms Substantially lower delays for all other flows M. Benveniste, Avaya Labs

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

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

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

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 802.11-07-2452r2 “Draft Text Changes for ‘Express Forwarding’ in a Mesh, M. Benveniste”, IEEE Doc 802.11-07-2453r4 “Performance Evaluation of ‘Express Forwarding’ for a Single-Channel Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc 802.11-07-2454r1 “Performance Implications of ‘Express Forwarding’ for a Single-Channel Mesh”, M. Benveniste, IEEE Doc 802.11-07-2814r1 “Effect of Contention Window Size on Express Forwarding Performance for Single-Channel Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc 802.11-07-2886r0 “More on Performance Evaluation of 'Express Forwarding' for Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc 802.11-08-0142r0 M. Benveniste, Avaya Labs