1. Long SlotTime Option for RTS/CTS Procedure
May 2007
Long SlotTime Option for RTS/CTS Procedure
Date:
Authors:
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Y. Inoue & K.Kawamura (NTT)

2. May 2007
Abstract
This submission contains a proposal called Long SlotTime option for RTS/CTS procedure which is simple modification of legacy RTS/CTS handshake to be effective in the outdoor deployments.
Y. Inoue & K.Kawamura (NTT)

3. May 2007
Rationale
802.11y systems will be widely deployed in outdoor environments
There will be more opportunity to see the “hidden station problem”
Although RTS/CTS procedure is defined in the base standard to minimize the effect of hidden station problem, some problems are still remained.
This proposal provides a solution for the issue in current RTS/CTS procedure
Y. Inoue & K.Kawamura (NTT)

4. Issue in current RTS/CTS Procedure
May 2007
Issue in current RTS/CTS Procedure
DIFS/
AIFS
RTS
STA#1
Collision!
hidden AP
RTS
SlotTime
STA#2
Hidden STA could start its transmission even when another station is sending the RTS to the AP
This is because SlotTime is shorter than the RTS frame duration
Y. Inoue & K.Kawamura (NTT)

5. Solution: Long SlotTime Option
May 2007
Solution: Long SlotTime Option
DIFS/
AIFS
RTS
STA#1
hidden
CTS AP
SIFS
NAV
LongSlotTime
STA#2
Use LongSlotTime for the RTS transmission can minimize the collision probability of RTS frames
Y. Inoue & K.Kawamura (NTT)

6. LongSlotTime: How long should it be?
May 2007
LongSlotTime: How long should it be?
A station may not realize that a hidden node started a transmission of RTS frame until it receives CTS frame
 LongSlotTime should be
LongSlotTime >= TRTS_Duration + SIFS + a
where a is the time to detect a frame.
Y. Inoue & K.Kawamura (NTT)

7. May 2007
Pros and Cons
Pros.
LongSlotTime option effectively minimizes the collision probability of RTS frames from different stations.
Cons.
Increased overhead.
Y. Inoue & K.Kawamura (NTT)

8. Some Results Prob. Successful Transmission [%] May 2007 100 80 60 40
CW = 127
CW = 1023 60
LongSlotTime Option
Prob. Successful Transmission [%]
Legacy RTS Transmission 40 20
CW = 15 5 10 15 20 25 30
Number of STAs
Y. Inoue & K.Kawamura (NTT)

9. May 2007
Maximum Throughput (1)
All frames are assumed to be sent using 6M bit/s mode.
Long SlotTime: 72 [us]
Since the SlotTime for RTS transmission is increased, maximum throughput will be decreased.
Y. Inoue & K.Kawamura (NTT)

10. Maximum Throughput (2) Similar calculation with the previous page.
May 2007
Maximum Throughput (2)
Similar calculation with the previous page.
Data & ACK: 24M bit/s
RTS & CTS: 6M bit/s
Long SlotTime: 72 [us]
Y. Inoue & K.Kawamura (NTT)

11. Simulation Result Maximum throughput is also decreased by collision.
May 2007
Simulation Result
Maximum throughput is also decreased by collision. 2 4 6 8 10 12 14 16 18 20
Offered Traffic [M bit/s]
Throughput [M bit/s]
w/ Long SlotTime Option
w/o Long SlotTime Option
Data Rate = 24M bit/s
Coverage Class = 0
Better throughput performance will be expected when the Long SlotTime option is used.
Y. Inoue & K.Kawamura (NTT)

12. May 2007
Summary
Use of LongSlotTime in transmitting RTS frames minimizes the collision probability of RTS frames in the hidden station scenarios.
802.11y should provide this option for the purpose of outdoor deployment.
Y. Inoue & K.Kawamura (NTT)

13. May 2007
Questions?
Y. Inoue & K.Kawamura (NTT)

14. May 2007
Straw Poll
How many people would like to see the LongSlotTime Option in the next version of IEEE y draft?
Yes:
No:
Abstain:
Y. Inoue & K.Kawamura (NTT)

15. May 2007
Motion
Y. Inoue & K.Kawamura (NTT)