1. Long Slot Directive Matthew Fischer Broadcom mfischer@broadcom.com
November 2002
doc.: IEEE /xxx
November 2002
Long Slot Directive
Matthew Fischer
Broadcom
Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

2. D4.0 - Long Slot Use Switch to long slot When long-slot STA associates
November 2002
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
November 2002
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
November 2002
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
November 2002
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
Matthew Fischer, Broadcom

6. RX “foreign” beacon Beacon contains TSF and LSTT
November 2002
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
November 2002
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
November 2002
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