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Long Slot Directive

Long Slot Directive. Matthew Fischer Broadcom mfischer@broadcom.com. D4.0 - Long Slot Use. Switch to long slot When long-slot STA associates When knowledge of neighboring co-channel long-slot BSS exists Causes lock-up problem if a legacy device wanders in, then leaves

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Long Slot Directive

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  1. Long Slot Directive Matthew Fischer Broadcom mfischer@broadcom.com Matthew Fischer, Broadcom

  2. D4.0 - Long Slot Use • Switch to long slot • When long-slot STA associates • When knowledge of neighboring co-channel long-slot BSS exists • Causes lock-up problem if a legacy device wanders in, then leaves • 2 or more G-BSS overlapping • Legacy device causes cascade of switching to long slot • G-BSS’s reinforce each other’s long slot state after legacy device leaves Matthew Fischer, Broadcom

  3. Solution to Lock-up • Establish Short Slot as default operation • AP reverts to Long Slot operation when: • A long-slot STA is associated • A co-channel legacy BSS is detected • No longer need neighboring BSS using long slot rule which included Long Slot G-BSS • Long Slot G-BSS will be covered by LSTT updates • But place time limit on long slot operation • LSTT = Long Slot Termination Time (1 second?) • Send LSTT in outgoing beacons, use latest LSTT Matthew Fischer, Broadcom

  4. AP (+ IBSS STA) behavior • Rules for switching to Long Slot • If Long-slot associated, or legacy BSS sighted • At TBTT, before beaconing • If Long Slot required according to local conditions, compute local required time duration for long slot • TBTT+MLSE • MLSE = Minimum Long Slot Epoch = ~1 second? • If TBTT+MLSE > LSTT • Update LSTT <= TBTT+MLSE • Else, no change to LSTT • Send LSTT in outgoing beacons, PR frames Matthew Fischer, Broadcom

  5. STA+AP behavior • Keep track of latest received LSTT • update to “latest” of received LSTT values • Not “last received” but “latest long slot epoch end-time” • I.e. similar to NAV update – use “later” of local and received values • At all times: If LSTT>TSF • Use Long Slot, otherwise, revert to Short Slot • Unless your associated AP has ShortSlot=0 Matthew Fischer, Broadcom

  6. RX “foreign” beacon • Beacon contains TSF and LSTT • Easy to calculate delta from time of beacon reception to foreign LSTT • Use delta to adjust foreign LSTT to local TSF time-base • Compare foreign LSTT to local LSTT • Update as appropriate Matthew Fischer, Broadcom

  7. LSTT Minimum Resolution • Resolution must be less than: • Minimum separation between • Receipt of beacon • Transmission of beacon • I.e., must guarantee that all STA TSF will have advanced by at least one LSTT tick between TX of any LSTT and next possible RX of LSTT • To insure that LSTT does not get constantly pushed forward • Otherwise, two AP may trade same LSTT indefinitely • (would require constant beaconing to occur…) Matthew Fischer, Broadcom

  8. The bits • ShortSlot bit in capability element • No longer a directive – really only a capability • Add LSTT field to nonERP indication element: • 1 bit – LSTTValid • Need this or some reserved value, (0x000000?), to indicate valid LSTT field • 23 bits - LSTT • Allows MLSE up to 4.19 seconds • MLSE cannot exceed LSTTbits minus one • To allow wrap detection • To deal with TSF deltas among STA • May consider longer MLSE – to allow more hops Matthew Fischer, Broadcom

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