Short Slot Time Option for TGg July 2002 Short Slot Time Option for TGg Richard van Nee Woodside Networks

July 2002 TGg Slot Time Current draft 11g use pure IEEE 802.11a, except for slot time which is the longer 802.11b slot time Longer slot time significantly reduces throughput compared to 802.11a at 5 GHz TGg currently has no migration path towards an 11g-only network that is as fast as 802.11a at 5 GHz

Throughput and Slot Time July 2002 Throughput and Slot Time Data Preamble ACK SIFS DIFS Backoff Each packet involves a significant wait time of SIFS+DIFS+Backoff, that is proportional to the slot time MAC Throughput = Rate·Tdata/(2*Tpreamble+Tdata+Tack+SIFS+DIFS+Tbackoff)

July 2002 Throughput .11g versus .11a 35 30 red: 9µs slot time (.11a) blue: 20µs slot time (current .11g) 25   Throughput [Mbps] 20 15 10 5 6 9 12 18 24 36 48 54 Raw Data Rate [Mbps] 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

July 2002 Short Slot Time One of the intents of the initial 11g draft was to create a migration path to pure 802.11a in 2.4 GHz Current 11g draft does not fully meet this goal Solution: have an optional mode to use the 9 µs 802.11a short slot time

July 2002 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

Impact on Legacy Devices July 2002 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

July 2002 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

July 2002 Motion Add an optional short slot time mode to the draft TGg standard to support a 9 µs slot time. This is done by adding a Short Slot Time Subfield to clause 7.3.1.4, and adding the following text to clause 19.4.3.8.4: ‘As an optional mode, a slot time of 9 µs shall be used if the Short Slot Time Subfield as defined in 7.3.1.4 is equal to one’

July 2002 Back Up Slides

Throughput .11g versus .11a for 500 Byte Packets July 2002 Throughput .11g versus .11a for 500 Byte Packets 2 4 6 8 10 12 14 16 18 9 24 36 48 54 Raw Data Rate [Mbps] Throughput [Mbps] red: 9µs slot time (.11a) blue: 20µs slot time (.11g)   16.8 Mbps throughput for .11a versus 11.7 Mbps for .11g at 54 Mbps mode Note: Throughput estimates without RTS/CTS, using 500 bytes packet length

11.G Throughputs (In Mbps, 1500 Bytes Packets) July 2002 11.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 6 4.8 5.3 5.5 9 6.5 7.4 8.0 12 9.4 10.3 18 10.2 12.8 14.4 24 12.0 15.6 18.0 36 19.9 24.1 48 16.0 23.2 29.0 54 16.6 24.5 31.1

July 2002 Overlapping BSSs If a .11g BSS overlaps with a legacy BSS using long slot times, the .11g AP can: Set SSTS=0 to switch to long slot times Keep using short slots as long as no legacy device associates. This does have some throughput impact on the legacy devices in the overlap region, as they use a longer backoff delay

July 2002 Short Slots and IBSS STAs indicate use of short slots in SSTS in their transmitted beacons