802.11g Contention Period – Solution for Co-existence with Legacy Month 1998 doc.: IEEE 802.11-98/xxx March 2002 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

Outline Background Analysis of using RTS/CTS for .11g ERP March 2002 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

Assumptions .11g and .11b STAs co-exist in a BSS. March 2002 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

March 2002 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

Comments on RTS/CTS It is a plausible solution apparently! March 2002 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. Have to minimize the usage of RTS/CTS !!! S. Choi, et al., Philips

Analytical Comparison March 2002 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

Transmission Time Comparison March 2002 Transmission Time Comparison Preferred Choice S. Choi, et al., Philips

Theoretical Throughput Comparison March 2002 Theoretical Throughput Comparison Preferred Choice S. Choi, et al., Philips

.11g Fragmentation Problem March 2002 .11g Fragmentation Problem RTS/CTS protect only the first fragment and ACK. The subsequent fragments are not protected! S. Choi, et al., Philips

Complementary Solution March 2002 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

802.11 MAC – CFP and CP Superframe = CFP and CP March 2002 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

PCF Element and Frames CF Parameter Set element March 2002 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

PCF Operation during CFP March 2002 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

.11g CP – Contention by .11g STAs Only! March 2002 .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

During .11g CP … .11g CP is part of CFP to .11b STAs!!! March 2002 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

March 2002 Collision Example The .11g ERP frame should have been protected with CCK-RTS/CTS! S. Choi, et al., Philips

March 2002 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

Two Ways to Determine T_extra March 2002 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

Why This Mechanism Works? March 2002 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

March 2002 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

March 2002 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

Single Change from 802.11g/D2.1 802.11b Clause 9.6 reads: March 2002 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

March 2002 Simulation Results S. Choi, et al., Philips

OPNET Simulation Model March 2002 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

Simulation Scenarios 8 stations : 4 .11b and 4 .11g Same load per STA March 2002 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

Simulation Scenarios DCF: 11g_CP_11b_CP March 2002 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

Simulations results in next slides are for a frame size of 1500 bytes Simulation Scenarios March 2002 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

Aggregated Throughput March 2002 Aggregated Throughput S. Choi, et al., Philips

2nd simulation: 11b traffic starts at 30 seconds March 2002 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

March 2002 Aggregate Throughput S. Choi, et al., Philips