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Doc.: IEEE 802.11-01/372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 1 A New Approach to the NAV Author: Matthew.

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Presentation on theme: "Doc.: IEEE 802.11-01/372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 1 A New Approach to the NAV Author: Matthew."— Presentation transcript:

1 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 1 A New Approach to the NAV Author: Matthew Sherman AT&T Labs - Research 180 Park Avenue Florham Park, NJ Date: June 12, 2001

2 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 2 What is the NAV Mechanism for implementing Virtual Carrier Sense –Allows STA to know channel is busy when CCA may not be triggered Presented conceptually as a count down timer –Counts down time till channel is idle again –Wireless Medium considered busy if NAV non-zero –See Clause Description of NAV and rules effecting it distributed throughout –Cant find all the rules in one section –Takes detailed reading to understand all effects and exceptions

3 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide NAV Rules Set NAV at start of CFP if > current setting ( ) –CFP parameter values from Beacon or Probe Response –Must update NAV prior to first Tx in BSS –Originating PC does not set NAV –Recurring value (Automatic set every CFP) –Set regardless of which BSS Beacon CFP parameters are from Do they over write each other? -> YES! Set NAV for Valid duration if > current settings ( ) –Dont set NAV if frame addressed to STA –Implied dont set NAV if source STA –Nonrecurring value Set NAV for Ack+SIFS if PS-Poll ( ) Auto reset when NAV expires (9.2.1) Reset NAV if CF-End ( ) –Ignores address in CF-End Reset NAV even if CF-End for another BSS! –Reset even if NAV not set for CFP (set for frame sequence) May be resetting NAV even it was set for a frame sequence! May Reset NAV if RTS with no CTS and no frame start after CTS time ( ) Ignore NAV to respond ACK (9.2.8) Ignore NAV for Poll response ( ) Do not respond or TX for any other frame if NAV set (3.36)

4 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 4 Some Observations Affecting Mechanism –CFP parameters –Duration field –Arrival / non arrival of some frame types Independent of duration field Both recurring, and non-recurring sets occur Resets are implied –Reset if counter counts to zero Resets can also be caused by events –Received frame –Did not receive frame NAV is ignored for some frame types

5 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 5 Deficiencies in NAV Not well suited for dealing with multiple PCF parameter sets –Parameter set can interact, and be unstable May appear to be constantly changing if hear 2 PCFs with different parameter sets Never really sure which CFP parameter set will be obeyed –CF_End from one BSS can reset CFP for another BSS Duration and CFP parameters mechanisms interact –CF_End may reset NAV during frame sequence Frame sequences may inappropriately ignore NAV –Not sensitive to context of NAV

6 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 6 Additional HCF NAV Rules CFB protected via duration values rather than CFP parameters (9.1.3) –Protecting mini-CFP rather than specific frame sequence –Also uses CFP parameters (9.10.1) Within TXOP Respond to frames from TXsrc with NAV set ( ) –TXsrc own TxOp Set as RA from first (+)CF-Poll of CFB if own BSSID Clear if NAV=0, update NAV from different source, Container from TXsrc with Non-final=0, Non-QoS data or Non Container Management received Clear if CF-End from BSSID –Same as existing rules? –Ignore NAV for RTS, Probe, Data (any type) Respond CTS if NAV set ( ) –CFP only or CFB as well? –Respond even if NAV set for ongoing frame sequence Respond RR to CC if NAV not set (except for CC) ( ) –What if set for CFP? Assumed NAV ignored for RR under CFP? If NAV set, respond only QoS CF-Ack(no data) to QoS data types, even if (+)CF-Poll ( ) –This does not make sense since means cant be polled by HC

7 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 7 Undocumented HCF NAV Rules (From 01/308) If NAV set for TxOP, reset if –QoS data frame with NF=0 –Any management type frame. –The TXOP expires –WM idle for DIFS TXOP extended by time borrowed by HC –Means NAV setting must be updated as well

8 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 8 NAV Usage During TXOP This diagram shows the periods covered by the NAV, based on Duration field values in the various frames of a TXOP during which the polled ESTA initiates several QoS Data frame exchanges. Taken from 01/308r0 by Michael Fischer

9 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 9 So What Should the NAV Be? Functionally it is a set of counters - not one counter Each counter has a context –Set of parameters describing what counter is counting If counter is set, transmission is suppressed in some circumstances Resets due to frames received / not received only affect specific counters, not all counters, depending on context If =0, counter does not suppress transmission Many different realizations possible, and may only require one counter with additional registers –Virtual (Software) NAVs This is a generalization of the existing NAV

10 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 10 What is appropriate context? Collection of parameters required to determine applicability of counter Context may be explicit, or implicit –Implicit if dedicated counter Prior to HCF, sufficient context would include –Event causing set (CFP or Duration) –Frame Type Causing set (if Duration) Need to know if RTS –BSSID (if CFP) –CFP Parameters (if CFP) Additional context required for HFC

11 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 11 Rules for Improved legacy NAV Set dedicated CFP NAV counter at start of CFP for each BSSID –Multiple CFP NAVs (Recommended 7 minimum) –CFP parameter values from Beacon or Probe Response If no counter assigned for BSSID set up new counter If no counters available, overwrite BSSID (other than own) for least recently updated May also filter on Beacon signal strength keeping strongest beacons –Recurring value (Automatic set every CFP) –Must update NAV for own BSSID prior to first Tx in BSS –Originating PC does not set NAV Set Duration NAV for Valid duration if > current setting –One dedicated Duration NAV –no CFP sets or resets –Dont set NAV if source or destination STA –Nonrecurring value Set NAV for Ack+SIFS if PS-Poll Auto reset when NAV expires If CF-End, Reset CFP NAV for corresponding BSSID Reset Duration NAV if RTS with no CTS and no frame start after CTS time Ignore All NAVs to respond with ACK or to Poll –Respond only Ack or Null if NAV set for Duration or BSS CFP other than own –Set More Data bit if have data to respond with, but suppressed by NAV –Respond normally if only own BSS CFP NAV set Do not respond or transmit for any other frame if any NAV set RED indicates changes from existing Rules!

12 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 12 Block Diagram of NAV Function … Other NAV For implementation shown, CFP Counters are used to count time to next CFP as well, and use Set bit to differentiate between 2 counts Own BSS CFP NAV Set 0 Counter 0 Context 0 CFP NAV 2 Set 1 Counter 1 Context 1 Duration NAV Counter N Duration Context Set N-1 CFP NAV N Counter N-1 Context N-1 Full NAV Counter Sets and Resets not Shown

13 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 13 Additional HCF NAV Rules Ignore own BSS CFP NAV for responding to –RTS (Respond CTS) –Probe (Respond Probe Response) –CC (Respond RR as appropriate) Create CFB NAV –Set by Duration value –Non-recurring set Set CFB NAV on QoS Poll from HC –Set from Duration field in Poll Reset CFB NAV if New QoS Poll from HC –Set to new Duration field from Poll (Even if smaller) Reset CFB NAV if CF-End from Own BSS Ignore CFB NAV for responding to –RTS (Respond CTS) –Probe (Respond Probe Response) –CC (Respond RR as appropriate)

14 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 14 New TxSup NAV Capability Add new TX suppression (TxSup) NAVs Customized to individual STA environment Use Management Action Frame to Load context –Allow Multicast & Broadcast Addressing Same parameters as CFP NAV Same response rules as CFP NAV Takes precedence over CFP NAVs –(except Own BSS CFP) –If out of NAVs replace oldest CFP NAV with new TxSup NAV If multiple TxSup NAVs always keep most recent

15 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 15 Software NAV Realization Single NAV Register –Each bit in Register represents one NAV state Full NAV Mask defining Valid NAVs Other NAV Mask –excludes own BSS CFP & CFB NAVs Each NAV has following Context –NAV Type (Duration, CFB, CFP, TxSup) –Frame Type of last set (Null if not set or CFP) –Time of last set –CF Period (Null if Duration or CFB) –CF Length (Null if Duration or CFB) –BSSID (Null if Duration or TxSup)

16 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 16 Software NAV Realization Cont. Maintain Time ordered Pending Event list –Recommended minimum of 16 Events supported Event context –Time of Event –NAV Number (Bit of NAV effected) –Type of event (Set or Reset) –Qualifiers RTS Auto Reset –Cancel if CTS or start of response frame detected

17 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 17 Soft NAV Realization Cont. 2 Timer set for Current Event If timer expires, implement event / set next event If frame received –Check if potential NAV impact –Process based on rules presented earlier in this document

18 doc.: IEEE /372r0 A New Approach to the NAV June, 2001 Matthew Sherman, AT&T Labs - ResearchSlide 18 Other Capabilities Make NAV state accessible via MIB –Include full NAV context


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