More Wireless Basics
This article explains the types of wireless signal. What makes these signals different? What is the difference between a transmitter and a receiver? What do you call a device that both transmits and receives? Make sure you can name two types of antennas and the reason for choosing each type.
Power and Receiver Sensitivity
Many people want to know how far wireless signals will go. Knowing this is important for planning a network, as the power of the routers will affect the design of the network, and how much equipment is needed.
Different Wi-Fi routers can have very different power levels. Some are much stronger: they have more speaking or transmitting power than others. Some are very good listeners: they have what is called a better receive sensitivity. These two elements define how well wireless devices will connect, and how far away a receiving Wi-Fi router can be.
Manufacturers do not usually publish information about their router's transmit power or receive sensitivity. Instead, the manufacturer will give a generic "range" rating to their routers, usually relative to each other. In some cases, usually with more business or professional oriented equipment you can find the information for transmit power and receive sensitivity.
A router's transmit power can be measured with two scales - milliwatts (mW) or dBm:
- A milliwatt is one thousandth (that's 1/1000) of a single watt - which is a generic measurement of power. For instance, a light bulb might be 40 watts. A router will have an output power of 100mW, which is 400 times less!
- A dBm is a relative measurement using logarithms. One milliwatt is 0 dBm. 10 milliwatts is 10 dBm; 100 milliwatts is 20 dBm, and so on. This is the scale that many network designers use to calculate if longer wireless links will work.
A few examples of the transmit power levels in common Wi-Fi hardware is below:
10mW (10dBm): Laptop or smartphone, or very low cost Wi-Fi router.
About 25 to 50 meters
100mW (20dBm): Indoor home or office router.
About 50 to 100 meters
100mW (20dBm): Outdoor sector router.
About 5 to 10 kilometers
500mW (1/2 Watt or 27dBm): Outdoor, long-distance focused routers.
About 10 to 20 kilometers or more
Wireless transmitter power is only one half of the connection. The Wi-Fi receiver has a range of power levels it can hear - the "listen power" in the diagram above. This is also known as the receive sensitivity. The receive sensitivity values are generally rated in dBm, and are usually in the range of -40dBm to -80dBm. The negative number indicates a very small signal - tiny fractions of a milliwatt.
Below we have an example of two routers in relatively close range. They have a good connection because the signal strength between them is strong.
As a receiver moves away from a wireless router, the signal it hears will get "quieter" - in other words, the power it receives will go down. Below, we can see the same routers, but with more distance between them. In this case, the routers have a weaker connection because the signal is near the limit of what the routers can hear. The speed between the routers will be less.
If the router moves too far away from the transmitter, it won't be able to receive any signal, either due to the signal being too weak or other signals interfering, and the routers will disconnect. Below we can see the two routers have disconnected, as there isn't enough signal.
The optimal signal range for outdoor wireless equipment is between -40dBm and -60dBm. This will ensure the connection can maintain the highest bandwidth possible.