November 2007 Effect of Contention Window Size on ‘Express Forwarding’ Performance for Single-Channel Mesh Date: 2007-11-09 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 Kaustubh Sinkar 908-696-5284 ksinkar@avaya.com 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)

November 2007 Effect of Contention Window Size on ‘Express Forwarding’ Performance for Single-Channel Mesh Mathilde Benveniste Kaustubh Sinkar Avaya Labs - Research M. Benveniste (Avaya Labs)

Introduction November 2007 Express Forwarding and Express Retransmission have been proposed to alleviate delay increases caused by multi-hop transmissions in a single channel mesh [2]-[3] Increasing the contention window size following a hidden node collision helps avoid collision when retransmitting This presentation shows the performance of ‘Express Forwarding’ with different contention window sizes M. Benveniste (Avaya Labs)

Express Forwarding – Review Ref Doc 11-07/2452, 2453 November 2007 Express Forwarding – Review Ref Doc 11-07/2452, 2453 ‘Express forwarding’ reduces the end-to-end delay of selected frames by granting forwarding nodes immediate access to the channel Criteria for express forwarding frames are: Time sensitive QoS [TSQ] frames – e.g. VO/VI Frames on paths traversing more than a specified number of hops Other Single-hop frames are not express-forwarded ‘Time critical’ frames are single-hop frames that do not yield priority to express forwarded frames; such frames are Top-priority management frames Top-priority frames experiencing longer delay than a specified limit M. Benveniste (Avaya Labs)

Express Forwarding Illustration Ref Doc 11-07/2452, 2453 November 2007 Express Forwarding Illustration Ref Doc 11-07/2452, 2453 DT0 NAV setting at receiving node DTI NAV setting at all other neighbor nodes Value in Duration field 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 a value longer than usual when a TSQ frame is transmitted to a forwarding node of a multi-hop path; DT0 added The forwarding nodes, 2 and 3, adjust the Duration value on the received frame by subtracting an increment DTI when setting their NAV Nodes with a ‘time critical’ frame subtract the increment DT0 from Duration field The non-forwarding neighbor nodes (e.g. 5) sets NAV by Duration field M. Benveniste (Avaya Labs)

Express Retransmission Ref Doc 11-07/2452, 2453 November 2007 Express Retransmission Ref Doc 11-07/2452, 2453 Conventional retransmission typically involves backoff and use of a wider contention window With ‘express retransmission’, backoff is dispensed and frame is retransmitted within DT0 following ACKtimeout Because of its prioritization, an express retransmitted frame is less likely to collide with one that is not Only the first retransmission attempt receives priority treatment Prevents two express retransmitted frames from colliding repeatedly DT0 NAV setting at receiving node DTI 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)

Performance Evaluation November 2007 Performance Evaluation M. Benveniste (Avaya Labs)

November 2007 Scenarios 802.11g Network Mix of wide and narrow contention windows (WLAN flows set according to default values) Wide contention windows (To reduce hidden node collision recurrence) All scenarios consider only high-priority traffic (VOIP and Video) – worst-case scenarios OPNET Modeler used for simulations M. Benveniste (Avaya Labs)

Parameters November 2007 PHY 11g Slot Time 9 usec Sifs Time 10 usec Phy_CWmin 15 Phy_CWmax 1023 PLCP overhead control 20 usec PLCP overhead data Control Data Rate 24 Mbps Difs Time sifs + 2*slot_time = 28 usec Eifs_time difs + sifs + ACK @ 24 Mbps aifsn 2 Aifs [ac] aifsn[ac] * slot_time + sifs_time = 28 usec ACK tx rate DATA tx rate 54 Mbps M. Benveniste (Avaya Labs)

802.11g Network November 2007 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, 0.16 Mbps payload size: 200 bytes, inter-arrival 20 ms Network configuration TX RANGE: 391 m 5-hop path, next-hop neighbors don’t hear each other Physical layer rates Data @ 54 Mbps ACK @ 24 Mbps 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 Traffic Load: 17.2 Mbps 5 Video(L)= 5*1.464=7.32 Mbps 2 Video(H)= 2*4.14=8.28 Mbps 10 VOIP= 10*0.16=1.6 Mbps Single VOIP flow 1600bpf *1000/20 fps=0.08 Mbps VIDEO (L) flow 11712bpf * 1000/8 fps=1.464 Mbps VIDEO (H) flow 11712bpf* 1000/2.83fps=4.14 Mbps TOTAL LOAD: 17 Mbps M. Benveniste (Avaya Labs)

Contention Windows A B November 2007 Nodes CWmin CWmax 15 1023 1 2 3 4 15 1023 1 2 3 4 5 7 8 11 31 12 13 16 17 18 19 24 25 Nodes CWmin CWmax 15 1023 1 2 3 4 5 7 8 11 12 13 16 17 18 19 24 25 M. Benveniste (Avaya Labs)

Mean Delays (ms) A B November 2007 M. Benveniste (Avaya Labs) Flows Express Forwarding Disabled Express Forwarding Enabled Express Forwarding With Express Re-TX Portal->Node_5 496.07 6.31 5.24 Portal->Node_16 322.78 5.22 4.55 Portal->Node_17 323.80 5.50 4.36 Portal->Node_18 403.43 6.17 4.92 Portal->Node_22 472.21 6.76 6.61 Node_5->Portal 266.27 4.98 3.73 Node_7->Node_8 1.85 0.95 1.01 Node_8->Node_7 2.24 1.04 1.22 Node_11->Node_20 2287.42 4.15 5.25 Node_12->Node_10 18.33 2.68 3.07 Node_13->Node_21 51.94 2.82 3.49 Node_16->Portal 26.25 2.83 3.19 Node_17->Portal 60.76 2.20 2.33 Node_18->Portal 80.86 3.37 3.17 Node_19->Node_14 32.40 2.03 2.37 Node_24->Node_9 82.19 3.00 2.89 Node_25->Node_6 2850.28 6.18 Express Forwarding Disabled Express Forwarding Enabled Express Forwarding With Express Re-TX 90.76 4.20 3.39 36.68 2.90 2.64 40.97 2.77 43.18 3.32 36.16 5.21 4.50 68.67 4.04 2.18 4.89 1.49 1.08 5.53 1.47 1.36 137.82 3.37 2.72 9.05 1.90 14.03 1.83 1.67 8.87 2.69 2.56 11.80 1.89 1.55 23.38 2.84 2.11 11.71 1.79 1.53 10.79 1.96 1.77 1119.60 3.70 2.94 M. Benveniste (Avaya Labs)

November 2007 Conclusion Increasing contention windows helps reduce hidden node collision recurrence Express Forwarding reduces end-to-end delay of multi-hop flows substantially Other (not express forwarded) traffic also benefits substantially from Express Forwarding Express Forwarding eliminates the negative effect of multi-hop transmission correlation that exacerbates hidden node collisions [1] M. Benveniste (Avaya Labs)

November 2007 References “Performance implications of wireless mesh coexistence with WLANs”, M. Benveniste”, IEEE Doc 802.11-07-2814r0 “‘Express’ Forwarding for Single-Channel Wireless Mesh”, M. Benveniste”, IEEE Doc 802.11-07-2452r1 “Draft Text Changes for ‘Express Forwarding’ in a Mesh, M. Benveniste”, IEEE Doc 802.11-07-2453r1 “Performance Evaluation of ‘Express Forwarding’ for a Single- Channel Mesh”, M. Benveniste and K. Kaustubh, IEEE Doc 802.11-07-2454r1 M. Benveniste (Avaya Labs)