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it is possible to form a piconet where an MP3 player streams audio to a headset while it
receives commands (e.g. volume control commands) from a third device such as a
remote control.

7.6.5 The Human Interface Device (HID) Profile
An application that has become more popular in recent years allows connection of
input devices such as keyboards and mouses to devices such as notebooks and tablets.
Although most wireless mouses use a proprietary radio protocol and USB receiver,
wireless connectivity of keyboards used in combination with tablets is based on
Bluetooth technology as no proprietary receiver can be connected to such a device. The
profile used for this application is the HID profile.
The HID profile establishes two L2CAP connections. The first connection is used for
a control channel on which data is transferred synchronously, that is, in a request and
response manner. The second L2CAP connection is required for the HID interrupt
channel that is used for carrying asynchronous information, for example, notifications
when the user has pressed or released a key. As the HID is a generic profile, information
stored in the SDP database informs the host device what kind of input or output mes-
sages the device supports. Input messages can, for example, be keyboard notifications
or mouse movements. Output messages can be sent by the host device, for example, to
a force feedback joystick.
As HID devices are usually battery driven, power consumption has to be as low as
possible. On the Bluetooth side, host and HID device therefore enter the Bluetooth sniff
mode after the establishment of the L2CAP control and interrupt channels. A typical
sniff rate observed in practice is 40 milliseconds. Sniff subrating can be used to further
reduce power consumption between keyboard activity input messages or between
mouse movement notifications.
Figure 7.27 shows an abbreviated HID input message that was sent from a keyboard
to a notebook. As can be seen in the figure, the message size is only 19 bytes and thus
very small despite the ACL, L2CAP and HID protocols stacked on each other.
Furthermore, the message shows that PSM 13 was used to establish the HID interrupt
channel. The payload of the message is only a single byte (0x04h), which represents the
lowercase ‘a’ character that the user has pressed on the keyboard.

7.7 Bluetooth Low Energy

7.7.1 Introduction
The idea behind ‘classic’ Bluetooth as described in this chapter so far is to establish a
communication channel over which data flows continuously, such as, for example, an
audio stream. While power consumption has been an important focus since the begin-
ning, there are applications for devices that have very limited battery capacity and
report or require information over the air very infrequently. For such devices, the rela-
tively small power consumption of Bluetooth is still too high. Examples of such applica-
tions and devices that are now emerging as part of the Internet of Things (IoT) [23] are
small wireless sensors, e.g. for temperature, humidity and magnetic fields; wearable
devices such as sensors for heart rate and blood pressure; and actuators such as