doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 1 Short Slot Time Option for TGg Updated Version Richard van Nee, Woodside Networks Sean Coffey, Texas Instruments

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 2 TGg Slot Time Proposal Context Strong support for enhancing.11g throughput by introducing the possibility for short slot-times Key issues were raised regarding the proposal This presentation addresses these issues and will be accompanied by a text document

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 3 TGg Slot Time Current draft 11g use pure IEEE a, except for slot time which is the longer b slot time Longer slot time significantly reduces throughput compared to a at 5 GHz TGg currently has no migration path towards an 11g-only network that is as fast as a at 5 GHz

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 4 Throughput and Slot Time Each packet involves a significant wait time of SIFS+DIFS+Backoff, that is proportional to the slot time MAC Throughput = Rate·T data /(2*T preamble +T data +T ack +SIFS+DIFS+T backoff ) DataPreamble ACKPreamble SIFS DIFSBackoff

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide Raw Data Rate [Mbps] Throughput [Mbps] Throughput.11g versus.11a 31 Mbps throughput for.11a versus less than 25 Mbps for.11g at 54 Mbps mode Note: Throughput estimates without RTS/CTS using 1500 bytes packet length red: 9µs slot time (.11a) blue: 20µs slot time (current.11g)

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 6 Short Slot Time One of the intents of the initial 11g draft was to create a migration path to pure a in 2.4 GHz Current 11g draft does not fully meet this goal Solution: have an optional mode to use the 9 µs a short slot time

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 7 How does it work? Introduce Short Slot Time Subfield in Capability Information Field (Extended Capability Information bit, or Beacon Information Element) Access point signals the optional short slot time mode in beacon by setting Short Slot Time Subfield to 1 If a non-short slot time device* associates, access point sets Short Slot Time Subfield to 0 All.11g devices change slot time when seeing a change in the beacon Short Slot Time Subfield * non-short slot time device: Legacy 11b device or 11g device with Short Slot Time Subfield = 0 in its association request

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 8 Impact on Legacy Devices Impact at association when b0=0 (short slot time) :  Legacy devices associate using a slot time that is too long  No impact on receiving (assuming beacons are transmitted on an 11b rate), but some minor impact on transmitting association request due to:  Legacy backoff delay is on average four times as long as that of an.11g device using a short slot time  Impact is a somewhat larger delay in association because the legacy association effectively gets a lower priority than.11g short slot time packets After association of first legacy device, no further impact on legacy devices as access point switches back to.11b slot time

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 9 Summary of Benefits Seamless migration path towards an.11g network that is as fast as.11a More efficient use of the wireless medium in the 2.4 GHz band when feasible Possibility to significantly improve throughput in both the mandatory.11g mode and the two optional modes (CCK/OFDM and PBCC) More future-proof standard, anticipates a 2.4 GHz network setting where there will be no legacy.11b devices

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 10 Issues to be Addressed 1.20 us cannot be denied association 2.9us overlapping BSS should not disadvantage adjacent 20us BSS 3.Choice of slot time shouldn’t be constrained to 9us and 20us only 4.IBSS mode not defined

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 11 Issue 1. 20us STA cannot be denied association If a 20 us device associates, the AP has to revert back to the 20 us mode immediately after the first Beacon subsequent to the association

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 12 Issue 2. 9us overlapping BSS should not disadvantage adjacent 20us BSS In the case of a network composed of only short slot-time devices, but with knowledge of a neighboring co-channel BSS having longer slot-time traffic, the AP shall switch to the longest slot-time of the overlapping BSS’s

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 13 Issue 3. Choice of slot time shouldn’t be constrained to 9us and 20us only b implementations usually need longer than 9us for CCA It may be advantageous to enable.11g slot- times longer then 9us, but shorter than 20us to allow for the throughput enhancements AND the longer.11b CCA requirements

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 14 Issue 3. Choice of slot time shouldn’t be constrained to 9us and 20us only (cont) Three Possibilities A.Include a variable slot time with the following format: 20us minus the short slot-time subfield (4bit number between 0 – 15 or ) B.Include a 9us, a 15us and a 20us slot-time option C.Only include the existing 9us.11a slot-time and 20us.11b slot time (per original proposal)

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 15 Issue 4. IBSS Mode not defined Recommended practice: Only enable 20us long slot-time for IBSS mode

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 16 Back Up Slides

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide 17 Motion Instruct the editor to add the text contained in document……………… titled…………..an optional short slot time mode to the draft TGg standard…………

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide Raw Data Rate [Mbps] Throughput [Mbps] Throughput.11g versus.11a for 500 Byte Packets 16.8 Mbps throughput for.11a versus 11.7 Mbps for.11g at 54 Mbps mode red: 9µs slot time (.11a) blue: 20µs slot time (.11g) Note: Throughput estimates without RTS/CTS, using 500 bytes packet length

doc.: IEEE /433r1 Submission Richard van Nee, Sean Coffey July 2002 Slide G Throughputs (In Mbps, 1500 Bytes Packets) Phy Rate.11g with RTS/CTS 11g without RTS/CTS, long slot time 11g. with short slot time