1 / 14

Short Slot Time Option for TGg

Short Slot Time Option for TGg. Richard van Nee Woodside Networks. 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

cjoaquin
Télécharger la présentation

Short Slot Time Option for TGg

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Short Slot Time Option for TGg • Richard van Nee • Woodside Networks

  2. 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

  3. Preamble Data SIFS Preamble ACK DIFS Backoff 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·Tdata/(2*Tpreamble+Tdata+Tack+SIFS+DIFS+Tbackoff)

  4. Throughput .11g versus .11a 35 30 • red: 9µs slot time (.11a) • blue: 20µs slot time (current .11g) 25 Throughput [Mbps] 20 15 10 5 0 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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’

  10. Back Up Slides

  11. 18 16 14 12 Throughput [Mbps] 10 8 6 4 2 0 6 9 12 18 24 36 48 54 Raw Data Rate [Mbps] Throughput .11g versus .11a for 500 Byte Packets • 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

  12. 11.G Throughputs (In Mbps, 1500 Bytes Packets)

  13. 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

  14. Short Slots and IBSS • STAs indicate use of short slots in SSTS in their transmitted beacons

More Related