256 ITU‐T's Technical Reports and Specifications 2.3.2.3 Radio network infrastructure The radio subsystem is the most important system on a wireless sensor node, since it is the primary energy consumer among the three highlighted application scenarios. Modern low power, short range transceivers consume between 15 and 300 milliwatts of power when sending and receiving. A key hardware observation is that low power radios consume approximately the same amount of energy when in receive or transmit mode. This energy is consumed if the radio is on, whether or not it is receiving actual data. The actual power emitted out of the antenna only accounts for a small fraction of the transceiver's energy consumption. A significant fraction goes to internal operation. As a result, the overall cost of radio communication can be dominated in some cases by the receiver power consumption – a metric that is often ignored in wireless studies. Transmission Range The transmission range of a wireless system is controlled by several key factors. The most intuitive factor is that of transmission power. The more energy put into a signal, the farther it should travel. The relationship between power output and distance travelled is a polynomial with an exponent of between 3 and 4 (non‐line of sight propagation). So to transmit twice as far through an indoor environment, 8 to 16 times as much energy must be emitted. Other factors in determining range include the sensitivity of the receiver, the gain and efficiency of the antenna and the channel encoding mechanism. In general, wireless sensor network nodes cannot exploit high gain, directional antennas because they require special alignment and prevent ad‐hoc network topologies. Omni‐directional antennas are preferred in ad‐hoc networks because they allow nodes to effectively communicate in all directions. Both transmission strength and receiver sensitivity are measured in dBm. Typical receiver sensitivities are between −85 and −110 dBm. Transmission range increases can be achieved by either increasing sensitivity or by increasing transmission power. When transmitting at 0 dBm, a receiver sensitivity of ‐85 dBm will result in an outdoor free space range of 25‐50 meters, while a sensitivity of −110 dBm will result in a range of 100 to 200 meters. The use of a radio with a sensitivity of −100 dBm instead of a radio with −85 dBm will allow you to decrease the transmission power by a factor of 30 and achieve the same range15. Bit Rate Unlike many high performance data networks, wireless sensor networks do not require high bit rates. 10‐100 Kbps of raw network bandwidth is sufficient for many applications. Radio bandwidth has a more significant impact on node power consumption and its lifetime in case of a battery powered node. As bit rates increase, transmission times decrease. As the highest instantaneous energy consumer, it is essential that the radio remain off as much as possible. By increasing the bit rate without increasing the amount of data being transmitted, the radio duty cycle is decreased. ____________________ 15 The dB scale is a logarithmic scale where a 10 dB increase represents a 10x increase in power. The baseline of 0 dBm represents 1 milliwatt, so 1 watt is 30 dBm.