Look at the table below to see the various frequencies and types of modulation each uses:. Nearly every device or technology uses a different wireless frequency and modulation. This means most devices can only understand a very specific kind of wireless signal. When a device sends out a wireless signal, it is called a transmitter. When another device picks up that wireless signal and understands the information, it is called a receiver.
In the case of FM radio, there is one transmitter--owned and operated by the radio station--and many receivers that people listen to the station with. When a device has both a transmitter and a receiver, it is sometimes called a transceiver. Devices such as routers can both transmit and receive, which is what makes them useful for building networks--you probably want to be able to send messages to your neighbors and out to the world, as well as receive messages!
Quick Activity: What devices do you own or use frequently that are transmitters, receivers or transceivers? Fill in some examples below each type:. Do you use more transmitters, receivers, or transceivers throughout the day? What is different about the way you use each of these? When building a network, you will be using Wi-Fi technology, which has some unique characteristics you will need to know.
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Each frequency band used in Wi-Fi is divided up into multiple "channels". Each channel is similar to rooms at a party - if one room is crowded it is hard to carry on a conversation. You can move to the next room, but that might get crowded as well. As soon as the building is full, it becomes difficult to carry on a conversation at the party.
If you are setting up a mesh network -- all of the mesh links will need to be on the same channel. The available channels vary depending on where you are in the world. For example, in the United States channels 12, 13 and 14 are not allowed for Wi-Fi, as those frequencies are used by TV and satellite services.
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If you are building networks in the United States, you can only use channels 1 through In the rest of the world, channels 1 through 13 are generally usable, and in a few places channel 14 is available. Despite that, the best channels in the United States and most of the world to use for 2. This will minimize interference caused by partially overlapping Wi-Fi signals:.
You could use other sets of Wi-Fi channels, as long as they are 5 channels apart - for instance 3, 8 and This may not be optimal though, as channels 1 and 2 would be unused, and in many places in the world channel 13 is not available.
Introduction to Wi-Fi Wireless Antennas
Wherever you are, try and check what channels are most in use, and plan your network to use a channel that doesn't overlap. This may not always be true -- more and more wireless equipment is starting to use the 5GHz. In the United States, only channels available for building mesh networks are 36, 40, 44, 48, , , , , and The best place to check what is allowed in your area is online. Links are provided in External Resources at the end of this document. When setting up your wireless network, you will need to think about what frequency band to use, and what channel to use. 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. In some cases, usually with more business or professional oriented equipment you can find the information for transmit power and receive sensitivity. For instance, a light bulb might be 40 watts.
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A router will have an output power of mW, which is times less! A dBm is a relative measurement using logarithms. One milliwatt is 0 dBm. This is the scale that many network designers use to calculate if longer wireless links will work. About 25 to 50 meters. About 50 to meters. About 5 to 10 kilometers. About 10 to 20 kilometers or more. Wireless transmitter power is only one half of the connection.
What is a wireless signal?
This is also known as the receive sensitivity. The receive sensitivity values are generally rated in dBm, and are usually in the range of dBm to dBm. 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. 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. The optimal signal range for outdoor wireless equipment is between dBm and dBm. This will ensure the connection can maintain the highest bandwidth possible.
Wireless routers have different types of antennas. Some routers will have antennas built in, and sometimes the routers will have a choice of antenna you can attach to the router. There are many specific types of antennas, but three basic types are used most of the time, and will be useful in building a wireless network. The first type of antenna is also the most common--omnidirectional. Using omnidirectional antennas has the benefit of creating connections in any direction. If there is enough signal between nodes, they should connect.
The all-direction strength of these antennas comes with the drawback of transmitting a weaker signal. Since the signal is going in all directions, it spreads out and gets weaker with distance very fast.
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If nodes or clients are far away, they may not connect well. Also, if there are only nodes or clients in one direction of the router, then the signals going in the opposite direction are wasted:. The next type of antenna is known as directional--it sends out a signal in a more focused way. There are two main types of directional antennas:. Sector Antenna Focused Antenna Sector antennas send out a pie-shaped wedge of signal - it can be anywhere between 30 degrees and degrees wide. These are often long, rectangular antennas that are separate or integrated in to a router.
A focused antenna sends out a narrow beam of signal - it is normally around 5 to 10 degrees wide, but it can be a little wider as well. These are often dishes or have a mesh bowl reflecting signal behind them. Using directional antennas has the benefit of increasing the distance a signal will travel in one direction, while reducing it in all other directions. Since the signal is all going one way, the power that would be sent out in all directions with omnidirectional nodes is now focused, increasing the power in that direction. It can also decrease the interference received at the node.
There are fewer signals coming in to the antenna, since the node is only listening to signals from the direction it is pointing. This reduces the signals it needs to sort out, and allows it to focus on other signals more, increasing the quality of those connections. However, directional antennas also have the drawback of requiring more planning to create links in your neighborhood. Since you are defining and limiting the areas where wireless signals go, you need to think about how those signals cover your neighborhood.
If there are areas that are then left out, how will those areas be included in the network? Also, the node has a very powerful signal in a single direction.
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If omnidirectional units, or lower power units such as laptops, are connecting to the node, they may not connect properly. The laptop will hear the node very well, but the directional node may not hear the laptop. This will create the situation where it looks like there is a strong signal, but you cannot connect. Networking concepts are important when dealing with wireless.