Figure 813 Paging Sequences
In Standby, no connection has been established and the station can use all of its capacity to support Page Scan. If desired, the station can place ACL connections in the Hold mode or even use the Park mode before entering Page Scan. SCO connections are preferably not interrupted by Page Scan. In this case, Page Scan may be interrupted by the reserved SCO slots which have higher priority than Page Scan.
8.4.2.1.2 Connection State
At the connection state, a Bluetooth station becomes the member of a piconet. The station can be in one of the following four connection modes: Active, Sniff, Hold, and Park. These connection modes are used to save power and to allow stations to communicate with different piconets.
Active Mode: In this mode the station can actively communicate on the wireless channel. A piconet is allowed to have up to eight active mode stations at the same time. The master schedules the transmission based on traffic requirements to and from the slaves. In addition, the master supports regular transmissions to keep slaves synchronized to the channel. Active slaves listen during the master-to-slave slots. If an active slave is not addressed, it may sleep until the next new master transmission. A periodic master transmission is required to keep the slaves synchronized to the channel. Since the slaves only need the channel access code for synchronization, any packet type can be used for this purpose.
Sniff Mode: Slave stations can change to a power-saving mode in which the station's activity is decreased. In this Sniff mode the slave station listens to the piconet less often, thus reducing power consumption. If a slave participates on an ACL link, it has to listen to the master traffic every ACL slot. The time interval between consecutive listening events is programmable by the application.
Hold Mode: Slave stations can change into another power-saving mode. During the Connection state, the ACL link to a slave can be in Hold mode. This means that the slave temporarily does not support ACL packets while continuing to support possible SCO links. While in Hold mode, the station can execute procedures such as scan, page, inquire, or communicate with another piconet. Before the slave enters Hold mode, the master and slave have to agree on the time duration that the slave can remain in this mode. Once this time expires, the station will return to Active mode, synchronize to the traffic, and wait for instructions from the master.
Park Mode: In this mode the slave station is still synchronized to the piconet but does not communicate. Parked stations give up their active member address and only occasionally listen to the traffic from the master to synchronize and check for broadcast messages. To support parked slaves, the master establishes a beacon channel where one or more slaves are parked. The master transmits on this channel at constant time intervals a series of beacon slots.
8.4.2.2 Piconet Establishment and Operation
The piconet is governed entirely by its master. The master's address determines the frequency hopping sequence and the channel access code (the code that uniquely identifies the piconet). Since Bluetooth is based on TDD, every Bluetooth station has an internal system clock which determines the timing and hopping of the radio transceiver. This clock has no relation to the time of day and can therefore be initialized at any value, does not require adjustments, and is never turned off. For synchronization with other units, only offsets are used and added to the native internal clock. To ensure synchronization, the master sets the timing of all slaves based on its clock by transmitting to the slaves its clock reading. The slaves add an offset value to their native clocks so they can be synchronized to the master clock. Since the clocks are free-running, the offsets have to be updated regularly. Establishing piconet is controlled by link control state. Figure 8.14 shows the piconet establishment sequence.
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