HOST CONTROLLER INTERFACE
Some link controller hardware may include an HCI layer above the link manager. This firmware layer is used to isolate the Bluetooth baseband and link manager from a transport protocol such as USB or RS-232. This allows a standard host processor interface to Bluetooth hardware. An HCI driver on the host is used to interface a Bluetooth application with the transport protocol. Currently three transport mechanisms are supported: USB, RS-232, and UART. The HCI layer is illustrated in Fig. 3. Using HCI, a Bluetooth application can access Bluetooth hardware without knowledge of the transport layer or other hardware implementation details.