1. 802.11g Contention Period – Solution for Co-existence with Legacy
Month 1998
doc.: IEEE /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

2. 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

3. 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

4. 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
02/051r g 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

5. 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

6. 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

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

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

9. .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

10. 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 g/D2.1.
S. Choi, et al., Philips

11. 802.11 MAC – CFP and CP Superframe = CFP and CP
March 2002
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

12. 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

13. 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

14. .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

15. 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

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

17. 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

18. 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

19. Why This Mechanism Works?
March 2002
Why This Mechanism Works?
CF-Awareness is not optional !!!
According to , STAs shall
Understand CF Parameter Set elements
Preset NAV at Target Beacon Transmission Time (TBTT) when a CFP is scheduled to start
See & Annex A PICS PC3.1
Reception of CF-END (+ CF-ACK) shall be supported by STAs
See Annex A PICS FR16 & RF17
S. Choi, et al., Philips

20. 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 e).
Transmit CF-END or CF-END+CF-ACK at one of ERP mandatory rates
S. Choi, et al., Philips

21. 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

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

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

24. OPNET Simulation Model
March 2002
Revised our 11b model
Supports 2 OFDM–11g modulations (24 and 6 Mbps) b modulations.
Slight change in the MAC model.
S. Choi, et al., Philips

25. 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

26. 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

27. 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

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

29. 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

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