Hardware Reference
In-Depth Information
Figure 2-2. Frequency channels
The standard uses a technique called
frequency hopping spread spectrum
, in which the
radio
hops
between channels on each connection event using the following formula:
channel = (curr_channel + hop) mod 37
The value of the
hop
is communicated when the connection is established and is there‐
fore different for every new established connection. This technique minimizes the effect
of any radio interference potentially present in the 2.4 GHz band across any single
channel, especially since WiFi and classic Bluetooth are prevalent in this band and
devices might experience heavy interference near devices with a strong transmission
power.
The modulation chosen to encode the bitstream over the air is Gaussian Frequency Shift
Keying (GFSK), the same modulation used by classic Bluetooth and several other pro‐
prietary low-power wireless protocols. The modulation rate for Bluetooth Low Energy
is fixed at 1 Mbit/s, which is therefore the upper physical throughput limit for the tech‐
nology.
In practice, however, as with any other protocol stack, this upper limit
is never actually reached when it comes to application throughput,
due mainly to protocol overheads in each of the different layers.
Link Layer
The Link Layer is the part that directly interfaces with the PHY, and it is usually imple‐
mented as a combination of custom hardware and software. It is also the only hard real-
time constrained layer of the whole protocol stack, since it is responsible for complying
with all of the timing requirements defined by the specification. It is therefore usually
kept isolated from the higher layers of the protocol stack by means of a standard interface
