January 2002
doc.: IEEE 802.11-01/630r2a
HC Recovery and Backoff Rules
Sunghyun Choi and Javier del Prado
Philips Research USA
Briarcliff Manor, New York
[email protected]
Submission
Slide 1
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
References
• IEEE 802.11e/D1.3
• IEEE 802.11-01/412r0: “Aligning 802.11e HCF
and 802.11h TPC operations ” by A. Soomro,
S.Choi and J. del Prado
• IEEE 802.11-01/128r1: “HCF Channel Access
And Inter-BSS Channel Sharing” by J.-M. Ho,
et al.
Submission
Slide 2
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Introduction
Status as of 802.11e/D1.3
Submission
Slide 3
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Problem Statement
• 802.11e/D2.0 has both recovery and backoff
rule of HC, which look inconsistent
• HC backoff is not needed if the HC has the
full control over the medium
• However, backoff is desirable if Overlapping
BSS (OBSS) exists
• HC Backoff may result in collision with its own
ESTAs in the QBSS
Submission
Slide 4
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
802.11e/D1.3 Says …
• Clause 9.10 of D2.0 reads:
“A HC may perform a backoff following an interruption
of a frame exchange sequence due to lack of an
expected response from an (E)STA, or due to
detection of CCA busy within a CFP or CFB. This
backoff shall occur under the rules of the EDCF,
using dot11CWmin[7] and dot11AIFS[7].”
Submission
Slide 5
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
802.11e/D2.0 Says …
• And Clause 9.10.1.2 of D2.0 reads:
“ESTAs, including the HC, are required to respond within
any frame exchange sequence after a SIFS period. If the
beginning of reception of an expected response, as
detected by the occurrence of PHY-CCA.indication(busy)
at the ESTA which is expecting the response, does not
occur during the first slot time following SIFS, that ESTA
may initiate recovery by transmitting after PIFS from the
end of the last transmission. This recovery after PIFS is
only permitted by the ESTA expecting the response. This
ESTA is the HC in case of a QoS (+)CF-Poll frame, and is
the TXOP holder in case of a QoS data type frame
transmitted during a CFB”
Submission
Slide 6
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Status as of 802.11e/D2.0
• …therefore, after sending a QoS (+)CF-Poll
frame, the HC senses the medium and:
– If PHY-CCA.indication busy is not detected
• The HC may recover transmission by sending a
second frame (ref. 9.10.1.2)
• The HC may perform backoff (ref. 9.10)
– If PHY-CCA.indication busy is detected
• The HC remains silent during the rest of the CFB
(ref. 9.10.1.2)
• The HC may perform backoff (ref. 9.10); but, will
not due to 9.10.1.2
Submission
Slide 7
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
But…
• After sending QoS (+)CF-Poll frame, detecting
PHY-CCA.indication(busy) is enough, and it is not
needed to receive the frame correctly
• Was the QoS (+)CF-Poll received correctly?
• Is the frame received by the HC from the Polled (E)STA?
• If these two things are not true, it results in the
loss of the TXOP. But, no way to find it out!
Submission
Slide 8
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Proposed Solution
Submission
Slide 9
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Proposed Solution (1)
• If PHY-CCA.indication (busy) is not detected
within PIFS after the HC sending any frame
which requires response, e.g., QoS (+)CFPoll, the HC may recover transmission or
perform backoff both after PIFS
– Which one to do is up to the HC algorithm, and
out of the scope of 802.11e spec.
– For example, the HC may want to backoff if it
knows the OBSS existence.
Submission
Slide 10
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Example
• HC backoff (if OBSS detected)
QoS CFPoll
HC Backoff
PIFS
• HC recovery
QoS CFPoll
QoS CFPoll
PIFS
Submission
Slide 11
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Proposed Solution (2)
• If PHY-CCA.indication (busy) is detected within
PIFS after the HC sending a QoS (+)CF-Poll
frame:
– The HC needs to have PHY-RXSTART.indication
before PHY-CCA.indication (idle).
– If PHY-RXSTART.indication happens, the HC assumes
that the TXOP was granted successfully.
– If PHY-RXSTART.indication does not happen, it could
mean a collision of the QoS (+)CF-Poll with another
frame, e.g., from neighboring HC, so HC may
perform backoff after PHY-CCA.indication (idle).
Submission
Slide 12
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Example
• Successful TXOP grant
CCA(busy)
QoS CF-Poll
CCA(idle)
Wait till end of
Channel Busy
polled TXOP
RXSTART
PIFS
• Potential collision with OBSS
CCA(idle)
CCA(busy)
QoS CFPoll
Channel Busy
PIFS
PIFS
Submission
Backoff
Slide 13
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Proposed Solution (3)
•
If PHY-CCA.indication(busy) is detected within
PIFS after the HC sending an RTS or non-CFPoll QoS Data(+) frame:
– If there were PHY-RXSTART.indication, PHYCCA.indication(idle), and PHY-RXEND.indication, and
the response is not received correctly, e.g., FCS error,
the HC may retransmit the frame or transmit another
frame after SIFS from PHY-CCA.indication(idle)
– If there was not PHY-RXSTART.indication before PHYCCA.indication (idle), it may perform backoff as it
may mean a collision
Submission
Slide 14
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Example
• Successful frame transmission w/
unsuccessful response reception
CCA(busy)
QoS Data
CCA(idle)
Channel Busy
PIFS
SIFS
QoS Data
RXSTART RXEND & FCS Error
• Potential collision with OBSS
CCA(busy)
QoS Data
Channel Busy
Backoff
PIFS
PIFS
Submission
CCA(idle)
Slide 15
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
HC Backoff Rules
• For the random backoff procedure HC shall use:
–
–
–
–
Fixed MIB value: dot11AIFSHC = 1 (i.e., AIFSHC = PIFS)
Fixed MIB value: CWHC = dot11CWmin = dot11CWmax
CWHC could be 3 (ref. 01/128r1)
The random backoff will reduce collisions with other
HCs
• The HC may set AIFS [i] for every i to > PIFS+
CWHC
– Increasing the AIFS [i] will avoid collisions with EDCF
access
Submission
Slide 16
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Summary
• HC recovery or backoff depending on PHYRXSTART.indication(.) as well as PHYCCA.indication(.)
• Random backoff with fixed contention
window size for HCs to avoid collisions with
other HCs
• Increasing AIFS[i] for EDCF access to avoid
collision with HC
• A best-effort & simple approach to handle
OBSS problem
Submission
Slide 17
S. Choi, Philips Research
January 2002
doc.: IEEE 802.11-01/630r2a
Motion
• Empower the editor to incorporate the
normative text found in the word document
01/630r1, and to change wording to make
the revised draft to be consistent with the
above changes.
Submission
Slide 18
S. Choi, Philips Research