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Sniff Mode Nsniff Timeout Use
Original Post: Sniff mode (SIG Forum) Date:
2000-04-04
The Bluetooth Specification , in section 10.8.2 states: "The slave has
to listen at Dsniff slot every sniff period, Tsniff
for a Nsniff attempt number of times. If the slave
receives a packet in one of the Nsniff attempt RX slots, it should
continue listening as long as it receives packets to its own AM_ADDR. Once it
stops receiving packets, it should continue listening for Nsniff
timeout RX slots or remaining of the Nsniff attempt number of RX
slots, whichever is greater."
This is all reasonably clear except the use of Nsniff
timeout, which can can be interpreted in several different incompatible
ways. These interpretations depend upon the meaning of "Once it stops
receiving packets". I am assuming that "receiving packets" means
"receiving packets with its AM_ADDR", since that phrase is included in
the preceding sentence. My problem is that I can see two different
interpretations for "Once it stops". Here are 2 possible
interpretations:
1)
"Once it stops receiving packets" means "As soon as it
fails to receive a packet or receives a packet with another AM_ADDR".
The slave will then set a slot counter to the Nsniff timeout
value. At each following receive slot it may or may not receive a packet that
has its AM_ADDR, but will always decrement the slot counter. When both the slot
counter equals zero and the Nsniff attempt period has
expired, it will stop receiving until the next Dsniff slot.
Possible Pseudo Code. Note: Denote the number of the RX slots,
in which the slave has to continue listening, AFTER receiving a packet with it's
own AM_ADDR, as Tlisten:
i = 1; For each RXslot(i)do
- {
- listen;
- Tlisten = max(Nsniff - i,
Nsniff_time_out - i);
- i = i + 1;
- }
2)
"Once it stops receiving packets" means "In the receive
slot following reception of the last packet with its AM_ADDR". The
slave will then set a slot counter to the Nsniff timeout value.
At each following receive slot it may or may not receive a packet with its
AM_ADDR. If it does not receive such a packet, then it will decrement the slot
counter. If both the slot counter reaches zero and the Nsniff
attempt period has expired, it will stop receiving until the next Dsniff
slot. If it does receive a packet with its AM_ADDR, then that packet becomes the
"last" packet and it restarts the slot counter to the full Nsniff
timeout value.
Possible Pseudo Code Note: Denote the number of the RX slots, in
which the slave has to continue listening, AFTER receiving a packet with it's
own AM_ADDR, as Tlisten. receive means 'receive a packet with it's own AM_ADDR'
i = j = 1; For each RXslot(i)do
- {
- listen;
- if (receive = true)
- then
- {
-
Tlisten = max(Nsniff - i, Nsniff_time_out);
-
j = 1;
- }
- else
- {
-
Tlisten = max(Nsniff - i, Nsniff_time_out - j);
-
j = j + 1;
- }
- i = i + 1;
- }
Conclusion
The second interpretation was seen to be the correct one. Some reasons are:
- In the first interpretation, one of the parameter (either Nsniff or
Nsniff_time_out) plays no role and i think, the comparison to find out
"whichever greater" is almost unnecessary.
- As cited from the Specs.: "... it should continue listening for
Nsniff_timeout RX slots or REMAINING of the Nsniff attempt number of RX slots,
whichever is greater." So, the word "REMAINING" is only for
Nsniff, and NOT for Nsniff_time_out. *
- Furthermore, the spec., pp 209 of 1082, says: "The sniff timeout
parameter determines for how many ADDITIONAL slots the slave must listen if it
..." That word "ADDITIONAL" here makes me again agree with the
second interpretation.
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