1. 802.11g NAV Propagation (based on 802.11g Draft 2.1 Jan-2002)
March 2002
doc.: IEEE /150
March 2002
802.11g NAV Propagation (based on g Draft 2.1 Jan-2002)
Terry Cole
AMD Fellow
Terry Cole, AMD
Terry Cole, AMD

2. March 2002
doc.: IEEE /150
March 2002
Introduction
We are routinely referring to RTS/CTS as a protection mechanism for OFDM in g draft.
The draft standard D2.1 contains a “legacy element” in all beacons and probe responses that is used by the AP to recommend protection mechanism such as the RTS/CTS.
Terry Cole, AMD
Terry Cole, AMD

3. March 2002
Clarification
To be more specific, RTS/CTS is being used to protect the OFDM traffic by propagating the network availability vector (NAV) to all the nodes, whether they be b or g.
The legacy element, normative material, and any informative material is better reworded to describe “NAV propagation by legacy modulation” rather than RTS/CTS protection of OFDM.
Terry Cole, AMD

4. March 2002
Additions
There are additional frames (besides RST/CTS) that are used to propagate and uniquely update the NAV.
RTS/CTS protects only the first of a fragmented data transmission. A data fragment frame and its ACK frame protect the subsequent data/ACK frames.
PS-Poll has an inferred duration of one ACK time one SIFS interval. In addition, any data or fragment frame following a PS-Poll propagates an updated NAV.
Beacons with non-zero CF time and CF-End (if used) propagate the NAV.
Terry Cole, AMD

5. Directed Data Implication
March 2002
Directed Data Implication
To be certain that OFDM traffic is not preempted by b stations:
RTS/CTS should be transmitted prior to an unfragmented sequence, not withstanding the value of RTSthreshold.
Data fragment frames and corresponding ACK frames should be transmitted using CCK modulation.
Implications are:
RTS/CTS is used only for unfragmented sequences.
Only the last data frame and ACK in a fragmented sequence and ACK is OFDM.
This is currently not covered by our gD2.1
We should address it.
Terry Cole, AMD

6. Power-Save Poll Implication
March 2002
Power-Save Poll Implication
To be certain that PS-Poll traffic is not preempted by b stations:
All PS-Poll frames and the immediate data response should be transmitted using CCK modulation.
Fragment sequences should also be protected using CCK modulation.
Implications are:
PS-Poll, unfragmented responses, and all but the last data and ACK frames of a fragmented response must be CCK.
The standard only covers a portion of this:
The frame is a control frame beacon and 9.6 requires it to be transmitted at one of the basic service set rates. Any mixed network contains only CCK rates in the basic service set, so the PS-Poll will always be sent using CCK if an b can join.
The data frame responses are not covered by our standard. We should address these.
Terry Cole, AMD

7. CF Signaling Implication
March 2002
CF Signaling Implication
To be certain that all attached b STAs respect the CF period:
The beacon should be transmitted using CCK modulation.
The CF-end frame, if used, should also be transmitted using CCK modulation.
The standard already covers these cases indirectly:
The beacon should have a broadcast address, and thus our gD2.1 requires it to be transmitted at one of the basic service set rates. Any mixed network contains only CCK rates in the basic service set, so the beacon will always be sent using CCK if an b can join.
The CF-end, if transmitted, is required to be broadcast, so the same reasoning applies.
Terry Cole, AMD

8. Motion Direct the editor to
March 2002
Motion
Direct the editor to
Re-label the legacy indication element in all normative (e.g ), explanative (e.g., 19.1 & 19.2), and any informative material (e.g. any informative annexes to be adopted) as follows
“Legacy indication element” changed to (->) “NAV propagation element using basic rate modulation”
“Protection mechanisms for OFDM frames” -> “propagation of NAV to all attached STAs by basic rate set modulated messages”
“CTS/RTS” -> “CTS/RTS frames, all data frames with the ‘more fragments’ field set to 1, all data frames sent in response to PS-Poll which are not proceeded in the frame sequence by a data frame with the ‘more fragments’ field set to 1, beacon frames with non-zero CF time, and CF-End frames”
Note in that transmission of beacon, PS-Poll, and CF-End frames shall also follow the constraints in 9.6.
Terry Cole, AMD