Page 481 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 467

Master Slave
ID
ID packets on
‘wrong’ frequencies
Inquiry ID Inquiry
scan
FHS

ID
ID
Page FHS Page
scan
ID
Poll
Connection Connection
(Master) NULL (Slave)

Figure 7.8 Establishment of a connection between two Bluetooth devices.

connection. This is necessary, as it is not possible to be a master and a slave device at
the same time. However, as a device needs to be a slave in order to be contacted, this
feature allows a device to temporarily violate this rule to include another requesting
device in its piconet.
During an active connection, the power consumption of a device mainly depends on
its power class (see Section 7.2). Even while active, it is possible that for some time, no
data is to be transferred. Especially for devices such as smartphones, it is very important
to conserve power during these periods to maximize the operating time on a battery
charge. The Bluetooth standard thus specifies three additional power‐saving substates
of the connected state.
The first substate is the connection‐hold state. To change into this state, master and
slave have to agree on the duration of the hold state. Afterward, the transceiver can be
deactivated for the agreed time. At the end of the hold period, master and slave implic-
itly change back into the connection‐active state.
For applications that transmit data only very infrequently, the connection‐hold state
is too inflexible. Thus, the connection‐sniff state might be used instead, which offers the
following alternative power‐saving scheme. When activating the sniff state, master and
slave agree on an interval and the time during the interval in which the slave has to lis-
ten for incoming packets. In practice, it can be observed that the sniff state is activated
after a longer inactivity period (e.g. 15 seconds) and that an interval of several seconds
(e.g. 2 seconds) is used. This reduces the power consumption of the complete Bluetooth
chip to below 1 mW. If renewed activity is detected, some devices immediately leave the
sniff state even though this is not required by the standard.
With Bluetooth 2.1, an additional sniff‐subrating state was introduced to further
reduce power consumption, especially for human interface devices (HIDs). With the