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

2. July 2002
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

3. 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)

4. 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

5. July 2002
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

6. 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

7. 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

8. 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

9. 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 , and adding the following text to clause : ‘As an optional mode, a slot time of 9 µs shall be used if the Short Slot Time Subfield as defined in is equal to one’

10. July 2002
Back Up Slides

11. 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 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)
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

13. 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

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