1. NAV Protection Mathilde Benveniste Avaya Labs, Research July 2003
doc.: IEEE /457
NAV Protection
Mathilde Benveniste
Avaya Labs, Research
July 2003

2. doc.: IEEE /457
July 2003
NAV protection
Current NAV protection is not adequate to provide reliable virtual carrier sensing
The NAV of a station should be set or reset by considering all stations or functions involved
The NAV rules do not protect QBSS properly when OBSS exists
M. Benveniste -- Avaya Labs, Research

3. HCF Polling NAV Reset HC overrules the NAV values
doc.: IEEE /457
July 2003
HCF Polling NAV Reset
HC overrules the NAV values
HC can reset NAV of QSTAs by sending CF-END
HC can reset NAV of QSTAs by sending QoS (+)CF-poll to itself with Dur/ID = 0
Source: 11-01/272
M. Benveniste -- Avaya Labs, Research

4. EDCF NAV Reset This is rooted from 802.11-1999
doc.: IEEE /457
July 2003
EDCF NAV Reset
This is rooted from
If no PHY-RXSTART.indication is detected from the PHY during a period with a duration of (2 x aSIFSTime) + (CTS_Time) + (2 x aSlotTime) starting at the PHY-RXEND.indication corresponding to the detection of the RTS frame, the STA may reset the NAV.
Source: 11-01/272
M. Benveniste -- Avaya Labs, Research

5. July 2003
NAV Problem
Several sources of NAV setting; the NAV should be set always to the longest value.
The problem arises when NAV must be reset because a source requests so. How can we know which source set the NAV in order to cancel it?
M. Benveniste -- Avaya Labs, Research

6. July 2003
NAV Problem Solution
Define NA: address TXOP holder causing NAV to be set
Keep a set of the n longest NAV values, ANAV. Discard the shorter NAV values. For each NAV retain the NA of the node setting it.
When an HC sets the NAV, the NA is the address of the HC
When the NAV is set through an RTS, the NA is the address of the station sending the RTS
In general, a station will refrain from transmitting if a ANAV component >0
When a component of ANAV expires it becomes 0
A new NAV is retained among the n ANAV components if it exceeds the length of the shortest NAV retained, which it replaces.
Reset the ANAV component of the source requesting reset. Set a ANAV[NA]=0 when NA coincides with the address of a node responsible for NAV cancellation, unless at least one NAV value has been discarded and none of the non-discarded NAV values has since expired
M. Benveniste -- Avaya Labs, Research

7. Example – 2 NAV values retained in ANAV
July 2003
Example – 2 NAV values retained in ANAV
NAV value=3 NA=XC
Reset NAV NA=XA
10, XA
9, XA
6, XA
0, XA
ANAV …
2, XB
3, XC
0, XC
Discarded=F
Discarded=T
Discarded=F
Discarded=F
Time +1 +3 +1
NAV setting requests received from XA, XB, and XC
Discarded is set to True when ANAV[XB] is discarded
Discarded is reset to False when ANAV[XC] expires
XA requests NAV reset 5 time-units later, and the ANAV[XA] is cleared
M. Benveniste -- Avaya Labs, Research

8. Proposed Normative Text Changes*
July 2003
Proposed Normative Text Changes*
Insert at the beginning of
A QSTA may track multiple NAV values. The NAV setting of the QSTA is the maximum of these NAV values for the QSTA. When a QSTA retains a new NAV value that QSTA shall also save the TXOP holder address setting the NAV value. A QSTA may retain a subset consisting of the longest NAV values; the shorter are discarded. A NAV value is removed when it expires. A NAV value is reset to 0 when such a request comes from the address stored for the NAV value, at least one NAV value has been discarded but none of the non-discarded NAV values has since expired.
When a QSTA updates it’s sets a NAV value setting using the duration value from a QoS (+)CF-Poll containing the BSSID of this a QBSS, that QSTA shall also save the TXOP holder address, which is the MAC address from the Address1 field of the frame containing the QoS (+)CF-Poll. If an RTS frame is received and the MAC address in the TA field in the RTS frame matches the saved TXOP holder address, the QSTA shall send the CTS frame after SIFS, without regard for, and without resetting, its NAV. The saved TXOP holder address shall be cleared whenever the NAV is reset or when the NAV counts down to 0. A non-AP QSTA that receives a CF-End frame containing the BSSID of this a QBSS shall reset its NAV the corresponding NAV value to 0. A non-AP QSTA that receives a QoS CF-Poll with a MAC address in the Address1 which matches the HC's MAC address and a Duration/ID value equal to zero shall reset the NAV corresponding to the address in Address 1 of the received QoS CF-Poll clear its NAV.
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*Additions appear in red; deletions are underlined in blue.
M. Benveniste -- Avaya Labs, Research