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802.11g Contention Period – Solution for Co-existence with Legacy

802.11g Contention Period – Solution for Co-existence with Legacy. Sunghyun Choi + , Olaf Hirsch * , Atul Garg * , Javier del Prado + + Philips Research and * Philips Semiconductors. Outline. Background Analysis of using RTS/CTS for .11g ERP

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802.11g Contention Period – Solution for Co-existence with Legacy

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  1. 802.11g Contention Period – Solution for Co-existence with Legacy Sunghyun Choi+, Olaf Hirsch*, Atul Garg*, Javier del Prado+ +Philips Research and *Philips Semiconductors S. Choi, et al., Philips

  2. Outline • Background • Analysis of using RTS/CTS for .11g ERP • 802.11g CP – a simple but efficient solution for co-existence • Revision made simple! – need to add only a single sentence into the draft S. Choi, et al., Philips

  3. Assumptions • .11g and .11b STAs co-exist in a BSS. • BSS Basic Rate set is equal to or a subset of .11b DSSS/CCK rates. • Legacy .11b STAs may not correctly see a pure OFDM ERP frame as a busy channel. S. Choi, et al., Philips

  4. Background • 802.11g/D2.1 - Using CCK-RTS/CTS to make .11b STAs set NAV during pure OFDM frame transmissions • Legacy Indication information element newly defined for the purpose • 02/051r0 - .11g and .11b collision avoidance via OFDM CP • Needed to add a new information element in beacons & some MAC operation changes S. Choi, et al., Philips

  5. Comments on RTS/CTS • It is a plausible solution apparently! • But, this will lead to high overhead and reduce the maximum system throughput compared to the pure OFDM network. • See the next! • It turns out that fragmentation should not be used for MSDU transmitted at a pure OFDM ERP rate and protected by RTS/CTS. • See the next! • Have to minimize the usage of RTS/CTS !!! S. Choi, et al., Philips

  6. Analytical Comparison • .11g two RTS/CTS options considered: • Long RTS: 2 Mbps rate & Long preamble (more realistic?) • Short RTS: 11 Mbps rate & Short preamble (as assumed in 02/065) • Theoretical throughput analysis • Assuming one transmitter and one receiver • See next … S. Choi, et al., Philips

  7. Transmission Time Comparison Preferred Choice S. Choi, et al., Philips

  8. Theoretical Throughput Comparison Preferred Choice S. Choi, et al., Philips

  9. .11g Fragmentation Problem • RTS/CTS protect only the first fragment and ACK. • The subsequent fragments are not protected! S. Choi, et al., Philips

  10. Complementary Solution • To reduce the usage of RTS/CTS … • 802.11g Contention Period (CP)! • Similar to OFDM CP of 02/051r0 … • .11g CP does not require any new information element!!! • Moreover, it can be achieved using a recommended practice as using CCK-RTS/CTS is according to 802.11g/D2.1. S. Choi, et al., Philips

  11. 802.11 MAC – CFP and CP • Superframe = CFP and CP • CFP starts with a beacon transmission • PCF during CFP and DCF during CP • (802.11e HCF during both CFP and CP) S. Choi, et al., Philips

  12. PCF Element and Frames • CF Parameter Set element • CF-END and CF-END + CF-Ack control frames • RA is broadcast group address S. Choi, et al., Philips

  13. PCF Operation during CFP • NAV is reset if CF-END (+ CF-ACK) is received • So, CFP ends with a CF-END (+ CF-ACK) S. Choi, et al., Philips

  14. .11g CP – Contention by .11g STAs Only! • .11g CP starts with a CF-END (+ CF-ACK) transmitted at an ERP rate S. Choi, et al., Philips

  15. During .11g CP … • .11g CP is part of CFP to .11b STAs!!! • So, .11g STAs do not need to use protection mechanisms (such as RTS/CTS and no fragmentation) during .11g CP • Is it true? Not really. See the next! S. Choi, et al., Philips

  16. Collision Example • The .11g ERP frame should have been protected with CCK-RTS/CTS! S. Choi, et al., Philips

  17. Solution • .11g STAs should start using protection mechanisms beginning T(.11b CP start) – T_extra, not beginning T(.11b CP start) S. Choi, et al., Philips

  18. Two Ways to Determine T_extra • T_extra = maximum transmission time of an MSDU at a .11g ERP rate • 4.8 msec for 2304 octect MSDU transmitted at 6 Mbps with 11 fragments • T_extra = duration of a pending frame exchange sequence, which cannot be finished by the upcoming T(.11b CP start) • Should be smaller than 4.8 msec • Can maximize .11g CP advantage at the cost of duration calculations! S. Choi, et al., Philips

  19. Why This Mechanism Works? • CF-Awareness is not optional !!! • According to 802.11-1999, 802.11 STAs shall • Understand CF Parameter Set elements • Preset NAV at Target Beacon Transmission Time (TBTT) when a CFP is scheduled to start • See 9.3.2.2 & Annex A.4.4.1 PICS PC3.1 • Reception of CF-END (+ CF-ACK) shall be supported by 802.11 STAs • See Annex A.4.4.2 PICS FR16 & RF17 S. Choi, et al., Philips

  20. What AP Needs to Do? • Include CF Parameter Set information element even if no need for CFP • When CFP is not actually needed, a CF-END follows a beacon with a SIFS time gap (or PIFS time gap in case of 802.11e). • Transmit CF-END or CF-END+CF-ACK at one of ERP mandatory rates S. Choi, et al., Philips

  21. What .11g STA Needs to Do? • When Bit 1 of Legacy Indication element is set to one, protection mechanisms are used only during .11b CP and the last part of .11g CP. • The length of the last part of .11g to use protection mechanisms can be determined according to one of two ways explained earlier. S. Choi, et al., Philips

  22. Single Change from 802.11g/D2.1 • 802.11b Clause 9.6 reads: “All frames with multicast and broadcast RA shall be transmitted at one of the rates included in the BSS basic rate set, regardless of their type or subtype.” • Add the following the above sentence: “For the Extended Rate PHY, control frames of subtypes CF-END and CF-END + CF-ACK may be transmitted at one of the Extended Rate PHY (ERP) mandatory rates irrespective of the BSS basic rate set.” S. Choi, et al., Philips

  23. Simulation Results S. Choi, et al., Philips

  24. OPNET Simulation Model • Revised our 11b model • Supports 2 OFDM–11g modulations (24 and 6 Mbps) + 4 11b modulations. • Slight change in the MAC model. S. Choi, et al., Philips

  25. Simulation Scenarios • 8 stations : 4 .11b and 4 .11g • Same load per STA • Network overloaded • 11g stations: • Data + Ack at 24 Mbps • RTS/CTS transmitted using 11b – 2Mbps with long preamble • 11b stations (long preamble): • Data at 11 Mbps • Control frames at 2 Mbps S. Choi, et al., Philips

  26. Simulation Scenarios • DCF: • 11g stations use RTS/CTS always • Beacon interval = 100 ms • 11g_CP_11b_CP • Beacon interval = 100 ms. • CFP_Max_Perido (in the beacon) = 50 ms • Beacon transmitted at 2 Mbps • CF-END transmitted right after the beacon at 24 Mbps so 11b stations don’t receive it • 11g CP: only 11g stations contend for the medium. RTS/CTS not used • 11b CP: both 11g and 11b stations contend for the medium. 11g stations use RTS/CTS • T_extra = 5 ms S. Choi, et al., Philips

  27. Simulation Scenarios • 11g_CP_11b_CP_T_extra_adjusted: • Same scenario as (2). • The T_extra is adjusted according to the duration of each frame Simulations results in next slides are for a frame size of 1500 bytes S. Choi, et al., Philips

  28. Aggregated Throughput S. Choi, et al., Philips

  29. 2nd simulation:11b traffic starts at 30 seconds Note: Although there is no 11b traffic, the 11g stations keep using RTS/CTS (always in the first scenario, during the 11b CP in the 2 last scenarios) S. Choi, et al., Philips

  30. Aggregate Throughput S. Choi, et al., Philips

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