Page 511 - From GMS to LTE

Bluetooth and Bluetooth Low Energy 497

Unlike ‘classic’ Bluetooth, Bluetooth Low Energy is not connection oriented and does
not natively offer a virtual serial interface over which applications can transfer their
data. Instead, the system is optimized to request or set remote values, referred to as
‘attributes’, that can range in length from a single byte to a long string. All values (attrib-
utes) are addressed via a 16‐bit ID, referred to as a ‘handle’, and communication over the
radio interface is kept as short and as power efficient as possible. In Bluetooth 4.0 and
4.1, packet size was limited to 27 bytes. This was extended in Bluetooth 4.2 for reasons
discussed later in this chapter. Figure 7.28 gives an overview of the BLE protocol stack
and the following sections will describe the different layers in more detail.
In practice, notebooks, smartphones and similar devices support classic Bluetooth
and Bluetooth Low Energy simultaneously. Many embedded devices such as sensors
and actuators, however, only support the low‐energy radio interface.

7.7.2 The Lower BLE Layers
The difference between ‘classic’ Bluetooth and BLE are already apparent on the physical
layer. Instead of using 1 MHz channels, BLE splits the 2.4 GHz ISM band into
40 channels of 2 MHz each. Like in the initial Bluetooth version, Gaussian Frequency
Shift Keying (GFSK) is used and data is sent at a fixed rate of 1 Mbit/s over a channel.
To keep the hardware as simple as possible, no other modulation methods are used to
increase the datarate in good signal conditions. In practice, BLE datarates are limited to
a few tens of kilobytes per second due to additional protocol overhead, as shown below.
Many connections, however, are configured with a significantly lower datarate to con-
serve power.
Out of the 40 channels, BLE reserves three channels for advertisement packets and
for connection establishment attempts. These channels are located before and after the
first 20 MHz channel used by Wi‐Fi and at the end of the 2.4 GHz ISM range. These
locations in the spectrum are typically less used by other networks, i.e. there is likely to
be less interference. All other 2 MHz channels are used for data transfer. Frequency
hopping is employed for coexistence with other radio technologies such as Wi‐Fi. The
hopping pattern of BLE is much slower compared to ‘classic’ Bluetooth. Channels are

Applications (e.g. hcitool, gatttool)

Generic Access Profile (GAP) Generic Attribute Profile (GATT)

Security Manager (SMP) Attribute Protocol (ATT)

L2CAP
Host Controller Interface

Link Layer

BLE Physical Layer

Figure 7.28 Bluetooth Low Energy protocol stack.