The Controller includes all the lower level functionality necessary for a Bluetooth low energy device to communicate; it consists of the Physical Layer (PHY), the Link Layer (LL) and the controller side of the Host Controller Interface (HCI).
8.7.1. Physical Layer (PHY)
In the Physical Layer (PHY) the key block is the 1Mbps adaptive frequency-hopping Gaussian Frequency-Shift Keying (GFSK) radio. This operates in the unlicensed 2.4 GHz Industrial, Scientific, and Medical (ISM) band.
8.7.2. Link Layer (LL)
The Link Layer (LL) directly interfaces with the PHY; it is the hard real-time layer of the protocol stack as it must comply with all the timing requirements defined in the specification. Given that many of the calculations performed by the LL are computationally expensive, they are usually implemented with hardware accelerators. This helps prevent overloading of the Central Processing Unit (CPU) that runs all software layers in the stack, therefore the LL implementation is a combination of custom hardware and software. The functionality provided by the LL usually includes Preamble, Access Address, air protocol framing, CRC generation and verification, data whitening, random number generation and AES encryption. It is usually kept isolated from the higher layers of the protocol stack by an interface that hides this complexity and its real-time requirements.
The LL principally controls the Radio Frequency (RF) state of the device and manages the link state of the radio which is how the device connects to other devices. A Bluetooth low energy device can be a master, a slave, or both depending on the use case and the corresponding requirements. A master can connect to multiple slaves and a slave can be connected to multiple masters. Typically, devices such as smartphones or tablets tend to act as a master, while smaller, simpler, more memory-constrained devices such as standalone sensors generally adopt the slave role. A device can only be in one of the following five states: standby, advertising, scanning, initiating, or connected as shown in Figure 4.
Figure 4 Link Layer States
Advertisers transmit data without being connected, while scanners listen for advertisers. An initiator is a device that is responding to an advertiser with a connection request. If the advertiser accepts the connection request, both the advertiser and initiator will enter a connected state. When a device is in a connection state, it will be connected in one of two roles: master or slave. Typically, devices that initiate connections will be masters and devices that advertise their availability and accept connections will be slaves. Therefore, the Link Layer defines the following roles:
- Advertiser: a device sending advertising packets.
- Scanner: a device scanning for advertising packets.
- Master: a device that initiates a connection (initiator) and manages it later.
- Slave: a device that accepts a connection request and follows the master’s timing.
These roles can be logically grouped into two pairs: advertiser and scanner (when not in an active connection) and master and slave (when in a connection).
8.7.2.1. Bluetooth Device Address
The Bluetooth device address is the primary identifier of a Bluetooth device. This is just the same as an Ethernet Media Access Control (MAC) address which uniquely identifies a wired ethernet device. It is a 48-bit (6-byte) number that uniquely identifies a device among peers. There are two types of device addresses, and it is possible for a device to obtain one or both types:
- Public device address
This is the equivalent to a fixed, factory-programmed device address as used in BR/EDR devices as well. It must be registered with the Institute of Electrical and Electronics Engineers (IEEE) Registration Authority and should never change throughout the device’s lifetime.
- Random device address
This address can either be pre-programmed or dynamically generated at runtime on the device. There are numerous use cases in which such addresses are useful in Bluetooth low energy.
8.7.2.2. Advertising and Scanning
The Bluetooth low energy specification allows only one packet format and two types of packets, advertising and data.
Advertising packets are used for two purposes:
- To broadcast data for applications that do not need the overhead of a full connection establishment. This is used in Beacon applications.
- To discover slaves and connect with them so that data can be exchanged.
Data packets are used for user data transport between the master and the slave devices, in a bi-directional manner.
Finally, the Link Layer acts as a reliable data bearer since all received packets are checked against a 24-bit Cyclic Redundancy Check (CRC) and retransmissions are scheduled when the error checking mechanism detects a transmission failure. Since there is no pre-defined retransmission upper bound, the Link Layer will continuously resend the packet until it is finally acknowledged by the receiver.
8.7.3. Host Controller Interface – Controller side
The Host Controller Interface (HCI) interface at the Controller side, provides a mean of communication to the host via a standardized interface; the Bluetooth specification defines HCI as a set of commands and events for the host and the controller to interact with each other, along with a data packet format and a set of rules for flow control and other procedures. Additionally, the spec defines several transports, each of which augments the HCI protocol for a specific physical transport (UART, USB, SDIO, etc.).
