1: ZigBee Essential Facts and Features > Radio Communication

Radio Communication

This page details the radio frequency bands available to ZigBee networks. In a ZigBee network, radio transmission/reception is handled by IEEE 802.15.4 (the lowest level in the software stack).

Frequency Bands

IEEE 802.15.4 (and therefore ZigBee) was designed to operate in unlicensed radio frequency (RF) bands. The unlicensed RF bands are not the same in all territories of the world, but IEEE 802.15.4 employs three possible bands, at least one of which should be available in a given territory. The three bands are centred on the following frequencies: 868, 915 and 2400 MHz.

The characteristics and geographical applicability of these RF bands are shown in the diagram on the right, and explained below:

• Each RF band is divided into a number of channels
• There are 27 channels across the three RF bands, numbered 0 to 26
• The three RF bands have different data rates: 20, 40 and 250 kbps (with increasing frequency)

The 868- and 915-MHz frequency bands offer certain advantages such as fewer users, less interference, and less absorption and reflection. However, the 2400-MHz band is far more widely adopted for a number of reasons, including:

• Worldwide availability for unlicensed use
• Higher data rate (250 kpbs) and more channels
• Lower power (transmit/receive are on for a shorter time due to higher data rate)

Channel Selection

Energy detection functionality is included that can be used by higher software layers to avoid interference between radio communications. The best frequency channel can be selected at initialisation.

Range of Transmission

The range of a radio transmission is the distance the radio communication can travel before it becomes undetectable. This is dependent on the operating environment - for example, inside or outside a building. The best results are obtained in an open area. Inside a building, the range is reduced due to absorption, reflection, diffraction and standing wave effects caused by walls and other solid objects.

With a standard device (around 0 dBm output power):

• In an open area, a range of over 200 metres can typically be achieved (Jennic has measured in excess of 450 metres)
• In a building, a range of 30 metres can typically be achieved

However, high-power modules (greater than 15 dBm output power) can achieve a range of 5 times greater than a standard module.

	Note that the range between devices can be extended by employing a topology that uses intermediate nodes (Routers) as stepping stones when passing data to the destination. Such topologies will be described later in this course.

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