March 2013
doc.: IEEE 802.11-13/0293r01
Clarification on Beacon Transmission
Rules
Date: 2013-03-19
Authors:
Name
Daniel COHN
Company
Lantiq
Yaron ALPERT
Lantiq
Adrian Stephens
Intel
Corporation
Submission
Address
Phone
email
Zarhin 13,
Ra’anana,
Israel
Zarhin 13,
Ra’anana,
Israel
+972 9 960
5555
[email protected]
+972 9 960
5555
[email protected]
[email protected]
Slide 1
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
January 2013
doc.: IEEE 802.11-13/0293r01
Problem Statement
• 802.11-2012 text on medium access rules for beacon
transmission can lead to confusion
• Clause 10.1.3.2 states:
“At each TBTT, the AP shall schedule a Beacon frame as the
next frame for transmission according to the medium access
rules specified in Clause 9”
• But, when EDCA is the medium access procedure, “rules
specified in clause 9” don’t appear to always allow this
• This is because a different access function may choose a
backoff value that will grant it access to the medium before
the access function transmitting the beacon (AC_VO)
Submission
Slide 2
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
January 2013
doc.: IEEE 802.11-13/0293r01
Problem Statement
• Clearly the intention of the standard is for beacons to be
transmitted at the TBTT before frames mapped to other ACs
• In addition, to decrease beacon transmission jitter, it can be
beneficial to allow the use a different IFS and contention
window for beacon transmission than for other frames in
AC_VO
– There is a precedent for this, e.g. Channel Switch Announcement is
transmitted after PIFS and no backoff,
9.3.2.3.4 PIFS
The PIFS may be used as described in the following list and shall not be used
otherwise:
<snip>
A STA transmitting a Channel Switch Announcement frame as described in
10.9
Submission
Slide 3
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
January 2013
doc.: IEEE 802.11-13/0293r01
Proposed Solution(1)
• Modify the text in 10.1.3.2 as follows:
At each TBTT, the AP shall schedule a Beacon frame as the next
transmission, according to the medium access rules specified in Clause 9
and shall ensure that no other frame exchange is initiated by the AP prior
to the transmission of the Beacon. When the Beacon frame is scheduled,
the AP shall transmit it immediately if the medium has been idle for at
least PIFS or, if the medium is busy, after the medium has been idle for an
AP-selected random number of slots following PIFS.
Submission
Slide 4
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
January 2013
doc.: IEEE 802.11-13/0293r01
Proposed Solution(2)
• Modify the text in “10.1.3.3 Beacon generation in an IBSS” as
follows:
c) Wait for the period of the random delay, decrementing the random
delay timer using the same algorithm as for backoff, except that PIFS is
used as the medium idle period before the backoff procedure is invoked
Submission
Slide 5
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
January 2013
doc.: IEEE 802.11-13/0293r01
Backward Compatibility
• AP behavior per existing text (wait AIFS if idle free or AIFS
+ backoff is medium busy) is compliant with proposed text so
the proposed solution is backward compatible
Submission
Slide 6
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq
March 2013
doc.: IEEE 802.11-13/0293r01
• Motion-1: To authorize the Editor to incorporate the
text changes proposed in contribution IEE 11-13-029301-000m to the draft TGmc Specification Document.
•
• Yes: ____________; No:
_________________; Abstain:________________
• [Result of Motion]
Submission
Slide 7
Matt Fischer, Broadcom ; Daniel Cohn, Lantiq