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Chapter 4 — Building a Roomba Bluetooth Interface 67

How Bluetooth Works

Bluetooth is a radio protocol for transmitting digital information. It sits in the 2.4 GHz spec-
trum along with many other protocols and devices like 802.11b/g (Wi-Fi), cordless phones,
X10 video cameras, wireless mice and keyboards, and microwave ovens. The 2.4 GHz spectrum
is one of the few places on the airwaves where one can broadcast without needing an FCC
license. The 2.4 GHz band is part of the ISM (industrial, scientific, medical) set of bands that
are free to use by anybody.
And that makes it a crowded place to live for a wireless protocol. Having so many technologies
existing in close physical proximity using the same band can cause interference. Bluetooth
attempts to avoid interference through frequency hopping spread spectrum. This is a technique
whereby instead of transmitting on a single channel, it transmits a little, hops to another fre-
quency, transmits a little more, hops again, and so on. By hopping very fast (many times a
second), the chances of hitting interference go down since not all frequencies are equally
noisy.

One End of the Spectrum

This spread spectrum technique was first invented during World War II by movie star Hedy
Lamarr. Her version used a piano roll to switch between 88 frequencies and was meant to
foil enemy eavesdropping. She probably never imagined that in the following millennium
people would use her invention to enhance listening in millions of households.

Bluetooth Power Classes

Bluetooth can transmit information at a variety of speeds, from 1.1 kbps to 2.1 Mbps (depend-
ing on the application) and at one of three power levels, or classes:

Class 1: 100 mW power, about 100 meter range
Class 2: 2.5 mW power, about 10 meter range
Class 3: 1 mW power, about 1 meter range

Many early Bluetooth devices were Class 3 or badly designed and Bluetooth got a bad repu-
tation for being too low power and not very useful. In actuality, Bluetooth devices can have
a range that equals or exceeds Wi-Fi. And thanks to advances in low-power circuitry and
improvements to the Bluetooth specification, Bluetooth has started to deliver on its promise
of PANs.