Page 483 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 469

HCI commands + HCI commands +
user data user data
Bluetooth module 1 Bluetooth module 2

HCI commands HCI commands
User data Link manager User data Link manager

LMP packets LMP packets

ACL connection
Time multiplex of LMP packets + user data
Figure 7.9 Communication between two link managers via the LMP.

7.4.4 The HCI Interface
The next layer of the Bluetooth protocol stack is the Host Controller Interface (HCI). In
most Bluetooth implementations, this interface is used as a physical interface between
the Bluetooth chip and the host device. Exceptions include, for example, headsets,
which implement all Bluetooth protocol layers in a single chip because of their physical
size and the limitation of using Bluetooth only for a single application, that is, voice
transmission.
By using the HCI interface, the device (host) and the Bluetooth chip (controller)
can exchange data and commands for the link manager with each other by using
standardized message packets. Two physical interface types are specified for
the HCI.
For devices like notebooks, USB is used to connect to a Bluetooth chip. The Bluetooth
standard references the USB specifications and defines how HCI commands and data
packets are to be transmitted over this interface.
The second interface for the HCI is a serial connection, the universal asynchronous
receiver and transmitter (UART). Apart from power levels, this interface is identical to
the RS‐232 interface used in the PC architecture. While an RS‐232 interface is limited
to a maximum speed of 115 kbit/s, some Bluetooth designs use the UART interface to
transfer data with a speed of up to 1.5 Mbit/s. This is necessary, as the maximum
Bluetooth datarate far exceeds the ordinary speed of an RS‐232 interface used with
other peripheral devices. The bandwidth that is used on the UART interface is left to
the developers of the host device.