Radio Layer

This link operates in the unlicensed ISM band around 2.4GHz and

uses spread spectrum communication. The band extends from 2400

to 2483.5 MHz in a vast majority of countries and this whole range is

utilized for optimizing the spectrum spreading.

However, for some countries with a smaller ISM band a down-scaled version is also provided.

For spread spectrum, the frequency hopping (FH) technique is used. As multiple uncoordinated networks may exist in this band and cause interference, fast FH and short data packets are used. This is because the error rate may be high, especially due to strong interference from microwave ovens that operate at this frequency.

CVSD coding has been adopted for voice, which can withstand high

bit error rates. Also the packet headers are protected by a highly

redundant error correction scheme to make them robust to errors.

The frequency hops are fixed at 2402+k MHz, where k= 0,1,...,78.

The nominal hop rate is 1600 hops per second.

This gives a single hop slot of 625 microseconds. The modulation used is Gaussian pre-filtered Binary FSK. The Gaussian filter has BT=0.5. The

transmitter power is fixed at 0dBm for a 10m range while it can be

increased to 20dBm for a 100m range.

Various restrictions are specified on clock accuracies and drift, spurious emissions and radio frequency tolerances.