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/BLUETOOTH MASTER/SLAVE COMMUNICATIONS AND SNIFF/SNIFF SUB-RATING
MODES
2 An Idle ACL Connection
2.1 PURPOSES AND ASSUMPTIONS
Before discussing sniff mode it is important to have a good understanding of Bluetooth communications at the
baseband level.
This section examines link maintenance for an established ACL link with no active traffic from higher protocol
layers (L2CAP and above). The process of establishing the link is outside the scope of this paper.
2.2 CLOCKS AND DATA COMMUNICATIONS
Data communications are inherently based on a master clock, but before digging into that, consider two non-
digital data analogies.
Case 1: A common clock is shared by all participants, a sporting event for example. The game clock is
displayed on the scoreboard for all to see. Everyone has access to the information and knows exactly when
the game will pause or end.
Moving back to the world of digital data, consider a wired serial connection with a flow control signal. The
sending side knows exactly when to send the data because the receiving side can use the flow control signal
to request the data. In Bluetooth wireless technology
1
each side of the connection has a clock that is used to
determine when to send, and listen for, data. The clock does not provide the time base for individual bits, but
rather acts in a manner similar to the flow control by defining when a packet of data should be sent and when
the receiver should listen for that data.
Case 2: Each participant has an independent clock. For example, consider this scenario.
Charles sends an email to Teddy that says, “I’ll call you tomorrow at nine AM, m y time”. The next day Teddy
is sitting at his desk 15 minutes before the appointed time contemplating Charles’s email and trying to decide
whether he should walk down to the cafeteria for a coffee, or stay at his desk and listen for the phone to ring.
His decision process must take into account these parameters:
1. What is the time difference between Teddy’s location and Charles’s location?
2. How likely is it that Charles will call exactly at nine and not a few seconds before or after?
3. How likely is it that Charles’s clock is running a bit slow and he will call 10 minutes late?
4. How likely is it that Charles’s clock is running a bit fast and he will call 10 minutes early?
5. How likely is it that Charles’s routine will be disrupted and he will not call at all?
We assume that Teddy actually wants to talk to Charles, so before he leaves his desk he must determine if a
trip to the cafeteria will cause him to miss the call. We also assume that Teddy wants a cup of coffee, so if
Charles does not call exactly by the stroke of nine, he must decide when to stop waiting and go for coffee.
Moving back to Bluetooth wireless technology, all of the parameters that Teddy must consider represent
important Bluetooth concepts.
In our example Teddy is listening for his phone to announce Charles’s call. In Bluetooth wireless technology,
the Slave device must listen for a transmission from the Master device.
1
This whitepaper only applies to Basic Rate/Extended Data Rate (BR/EDR) connections.