Wifi Signal 4 2 2011
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Introduction
2.4 GHz and 5 GHz are frequency bands each containing a number of channels. Ideally in radio communication (including WiFi) you do not want two close radio signals to be harmonically related. Download wifi signal 4.0 for free. System Tools downloads - WiFi Signal by Adrian Granados and many more programs are available for instant and free download. The bars on the Wi-Fi icon indicate signal strength. The higher the signal strength, the better the Wi-Fi network performance. Typically, if you have 3 or 4 bars, your Wi-Fi connection should be good. Once you’re connected, the different icons located on the lower right side of the taskbar show the state of your Wi-Fi connection. All the studies reviewed here were of Wi-Fi using the 2.4 GHz band, although there is also a 5 GHz band reserved for possible Wi-Fi use. Telecommunications industry-linked individuals and groups have claimed that there are no and cannot possibly be any health impacts of Wi-Fi ( Foster and Moulder, 2013; Berezow and Bloom, 2017).
This document covers the basics of how wireless technology works, and how it is used to create networks. Wireless technology is used in many types of communication. We use it for networking because it is cheaper and more flexible than running cables. While wireless networks can be just as fast and powerful as wired networks, they do have some drawbacks.
Reading and working through Learn Networking Basics before this document will help you with some of the concepts used in wireless networks.
In addition to some background information, this document covers six basic concepts:
- Wireless signals - what they are and how signals can differ.
- Wireless devices - the differences and uses for receivers and transmitters.
- Wi-Fi Modes - how networks are made up of clients, access points, or ad-hoc devices.
- Wi-Fi Signals - the unique characteristics of Wi-Fi, and how signals are organized.
- Power and Receiver sensitivity - how far each wireless device can go, and how well a router can listen and filter out interference and noise.
- Antennas - how the type of antenna changes the way the router broadcasts.
Reading through this material should take about an hour. Working through the activities, or diving deeper into the subject with a group may take longer.
What is a wireless signal?
Wireless signals are important because they can transfer information -- audio, video, our voices, data -- without the use of wires, and that makes them very useful.
Wireless signals are electromagnetic waves travelling through the air. These are formed when electric energy travels through a piece of metal -- for example a wire or antenna -- and waves are formed around that piece of metal. These waves can travel some distance depending on the strength of that energy.
For more on how electromagnetic signals work, check the #External Resources section at the end of this document.
Types of Wireless Signals
There are many, many types of wireless technologies. You may be familiar with AM and FM radio, Television, Cellular phones, Wi-Fi, Satellite signals such as GPS and television, two-way radio, and Bluetooth. These are some of the most common signals, but what makes them different?
Frequency
First of all, wireless signals occupy a spectrum, or wide range, of frequencies: the rate at which a signal vibrates. If the signal vibrates very slowly, it has a low frequency. If the signal vibrates very quickly, it has a high frequency. Frequency is measured in Hertz, which is the count of how quickly a signal changes every second. As an example, FM radio signals vibrate around 100 million times every second! Since communications signals are often very high in frequency, we abbreviate the measurements for the frequencies - millions of vibrations a second is Megahertz (MHz), and billions of vibrations a second is Gigahertz (GHz). One thousand Megahertz is one Gigahertz.
| Example Frequency Ranges Below we can see the span of frequencies that are commonly used in communications. Broadcast transmitters for AM, FM and Television use frequencies below 1000 MHz, Wi-Fi uses two bands at higher frequencies - 2.4 and 5GHz. Cellular phones use many different frequencies.
|
Modulation
In addition to having different frequencies, wireless signals can be different in the way they convey information. A wireless signal needs to be modulated--or changed--to send information. There are many types of modulation, and different technologies can use one or more types to send and receive information. In the two examples below -- AM and FM radio -- the M stands for modulation. The type of modulation is what makes them different.
Example one: AM radio. The A in AM comes from Amplitude - the energy or strength of the signal, operating at a single frequency. An un-modulated AM wave might look like:
And a modulated AM radio wave has higher and lower energy (amplitude) waves indicating higher and lower audio frequencies in the signal:
From left to right, we have the normal, un-modulated wave, then the lower amplitude wave (representing low points in audio waves), then the higher amplitude wave (representing crests or high points in audio waves).
A more detailed version of an AM signal is below:
The audio signal is the wave on the top, with the corresponding Amplitude Modulated wave below it.
Example two: FM radio. The F in FM comes from Frequency - defined by how quickly the wave vibrates every second. An un-modulated FM wave might look like:
And a modulated FM radio wave has higher and lower frequencies indicating higher and lower audio frequencies in the signal:
From left to right, we have the normal, un-modulated wave, then the lower frequency wave (representing lower audio amplitudes), then the higher frequency wave (representing higher audio amplitudes).
The type of modulation various technologies use to communicate can be very different, and are often not compatible. Satellite equipment cannot speak directly to your laptop or smartphone, which uses Wi-Fi to send and receive information. This is because the radios in different devices can listen only to certain types of modulations and frequencies.
As an example, some broadcast radio receivers have a switch to select between AM and FM signals, for two reasons: they use different frequencies to transmit, and they use different modulation types. If you try and listen to an AM signal with a radio in FM mode, it won’t work. The opposite is also true - in AM mode, an FM signal doesn’t make sense to the receiver. It is important that transmitters and receivers use the same frequencies and modulation types to communicate.
Devices in your daily life use many types of wireless signals. Look at the table below to see the various frequencies and types of modulation each uses:
| Technology or device | Type of wireless signal |
| |
| |
| |
| |
|
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.
Receivers and Transmitters
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:
| Transmitter | Receiver | Transceiver |
| Examples: | Examples: | Examples: |
Do you use more transmitters, receivers, or transceivers throughout the day? What is different about the way you use each of these?
Wi-Fi Signals
When building a network, you will be using Wi-Fi technology, which has some unique characteristics you will need to know.
There are two types of Wi-Fi signal, based on the frequencies they use:
- 2.4GHz - A lower frequency, this is the more common Wi-Fi technology in use today. Many devices use it, so the signals can become more crowded and interfere with each other. It can pass through walls and windows fairly well.
- 5GHz - This higher frequency technology is used by fewer devices, and can sometimes achieve higher speeds because the frequencies are less crowded. It cannot pass through walls and windows as well as the 2.4GHz band signals, so the range of 5GHz technology is often shorter.
These two types of Wi-Fi are called the Frequency Bands, or just Bands for short.
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.
2.4GHz Band
For the 2.4GHz band, there are 14 channels total. Unfortunately, these channels overlap, so they aren’t all usable at the same time. 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. If you are building networks in the United States, you can only use channels 1 through 11. 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 2.4GHz band equipment are channels 1, 6, and 11. 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 13. 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. Wherever you are, try and check what channels are most in use, and plan your network to use a channel that doesn't overlap.
5GHz Band
The 5GHz frequency band is much wider and has more channels, so the diagram is a bit more extensive. Fortunately, these channels do not overlap, so you don’t have to worry about picking non-standard channels like in the 2.4GHz band.
There are many more channels available in the 5GHz band, so it should be easier to select a channel in this band that doesn’t cause interference. 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, 149, 153, 157, 161, and 165. There are other channels available for Access Points or other types of community networks, but those channels won’t work with mesh wireless. 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.
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.
Antennas
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.
Omnidirectional Antennas
An omnidirectional antenna sends a signal out equally in all directions around it.
Using omnidirectional antennas has the benefit of creating connections in any direction. You don’t have to do as much planning to connect with multiple neighbors or buildings. 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. 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:
Directional Antennas
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 120 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. It won’t hear signals behind it or to the sides as well or at all. 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. 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.
Quick Activity: What are the best uses for the different kinds of antennas?
| Antenna Type | Best Uses |
| Omnidirectional Sector Focused | ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ |
What would the best antennas to use for building a neighborhood network?
Definitions
- Omnidirectional
- When a node has an omnidirectional antenna attached, it can send and receive wireless signals in all directions around it equally. The signal is actually strongest out to the “sides” of the antenna. Very little or no signal comes out of the “ends” of the antenna.
- Directional antenna
- When a node has a directional antenna attached, the wireless signal is very strong in one direction, and has a very weak or no signal in every other direction. This generally forms a cone or wedge shaped area from the front of the antenna.
- Receive sensitivity
- The minimum level of a received signal required for a device to understand the signal.
- Access point
- A device that allows wireless devices to connect to a wired network using Wi-Fi.
- Watt
- A unit of power, usually written “W”. The most common power levels for Wi-Fi devices are in the range of milliwatts - or thousandths of a watt.
- dBm
- An abbreviation for the power ratio in decibels (dB) of the power referenced to one milliwatt (mW). 0 dBm is equal to 1 milliwatt.
Related Information
We recommend you work through Learn Networking Basics if you haven’t already. Networking concepts are important when dealing with wireless.
External Resources
If you are interested in learning more about Wi-Fi and wireless technology, there is a lot of information out there. Good books to read for background and more information include How Radio Signals Work by Sinclair (ISBN 0070580588), and 802.11 Wireless Networks: The Definitive Guide by Gast (ISBN 0596100523).
There are also excellent documents on Wikipedia about Wi-Fi and wireless signals. Similarly, an Internet search will most likely answer any questions you can think of, as wireless is a very popular technology.
For more information on what frequencies are available in your country or regulatory area, please see this article on Wikipedia on wireless channels.
Documentation
- Commotion Construction Kit
- Wireless + Networking
Carrier Sense (CS) is one of the most integral parts of modern Wi-Fi networks. Fundamentally, Wi-Fi is a multiple access link, which means that it is shared and requires vastly different protocol design and architecture than a point to point circuit (as any Masters graduate student studying computer science could tell you). In addition, random access to the medium is distributed across all stations on the network. Wi-Fi does not pass tokens, reserve the medium with bit-mappings, or use any other control mechanisms dictating which stations have access to transmit.
This distributed nature makes carrier sense (and subsequent medium contention) core components of network operation and efficiency. The practical implications are seen in Wi-Fi engineer's daily work. We perform site surveys, develop non-overlapping 1, 6, 11 channel plans, optimize channel re-use, and perform capacity planning all in the name of performance optimization for a network that runs over an unguided medium with random access distributed between stations. Understanding these principles of design and the protocol operation behind them are important for engineers to adequately and successfully build wireless networks.
The precursors to the IEEE 802.11 and Wi-Fi protocol design were developed in the pure ALOHAnet protocol, slotted ALOHA, and finally Carrier Sense Multiple Access (CSMA) developed for the 802.3 Ethernet specification.
As I've previously written, 802.11 Wi-Fi is based on CSMA/CA, whereas 802.3 Ethernet is based on CSMA/CD. What's the deal, one little letter difference? Well that one little letter represents a significant underlying difference between the two network protocols. What they both have in common is the need to perform Carrier Sense (CS) for medium idle/busy detection on a Multiple Access (MA) network segment (hence the CSMA portion). What differs are how stations determine if the medium is idle (Collision Detection versus Collision Avoidance), and how they are granted access to the medium once it is found to be idle (1-persistent for Ethernet versus p-persistent for Wi-Fi, where 'p' indicates the probability of transmission when the medium is found to be idle). This is largely due to the inherent differences of electromagnetic signalling over guided versus unguided media (copper or fiber cabling versus the air). For an overview of collision avoidance and medium contention, see my previous series on Wireless QoS.
Let's review how carrier sense is performed.
Wi-Fi Carrier Sense Overview
Wi-Fi carrier sense is composed of two separate and distinct functions, Clear Channel Assessment (CCA) and the Network Allocation Vector (NAV). From a high level perspective, CCA is physical carrier sense which listens to received energy on the radio interface. NAV is virtual carrier sense which is used by stations to reserve the medium for mandatory frames which must follow the current frame.
It is important to note that CCA is not the same as the NAV. CCA indicates a busy medium for the current frame, whereas NAV reserves the medium as busy for future frames that are required to be transmitted immediately following the current frame.
Clear Channel Assessment
CCA is defined in the IEEE 802.11-2007 standards as part of the Physical Medium Dependant (PMD) and Physical Layer Convergence Protocol (PLCP) layer. For reference, the IEEE layer specifications include PMD, PLCP, MAC/LLC as shown below:
Clear Channel Assessment is composed of two related functions, carrier sense (CS) and energy detection (ED).
Carrier sense refers the ability of the receiver to detect and decode an incoming Wi-Fi signal preamble. In addition, CCA must be reported as BUSY when another Wi-Fi signal preamble is detected, and must be held as BUSY for the length of the received frame as indicated in the frame's PLCP Length field. Typically, any incoming Wi-Fi frame whose PLCP header can be decoded will cause CCA to report the medium as busy for the time required for the frame transmission to complete.
The PLCP header Length field indicates either the number of microseconds required for transmission of the full frame MPDU payload (DSSS), or the number of octects carried in the frame MPDU payload (OFDM) which is then used in combination with the Rate field (which identifies the modulation used for the payload) to determine the time required for MPDU transmission. Either way, the Length or Rate + Length fields give the receiver the information required to de-modulate the frame and determine how long the medium will be busy.
For reference, here is the PLCP frame header format for the DSSS PHY:
Energy detection (ED) refers to the ability of the receiver to detect the non-Wi-Fi energy level present on the current channel (frequency range) based on the noise floor, ambient energy, interference sources, and unidentifiable Wi-Fi transmissions that may have been corrupted but can no longer be decoded. Unlike carrier sense which can determine the exact length of time the medium will be busy with the current frame, energy detection must sample the medium every slot time to determine if the energy still exists. In addition, energy detection requires a pre-defined threshold which determines if the reported energy level is adequate to report the medium as busy or idle. This is typically referred to as the ED threshold level or CCA sensitivity level. The ED threshold is usually much lower for valid Wi-Fi signals that can be decoded using carrier sense than it is for non-Wi-Fi signals. For example, the noise ED threshold must be 20 dB greater than the corresponding Wi-Fi signal ED threshold for most PHYs.
For more information on the CCA sensitivity level of each PHY, see the 'Further Reading' notes at the end of this post.
As an interesting side note, when evaluating the RF environment with a spectrum analyzer, the ED threshold level value may be implemented differently than Wi-Fi stations. This may cause the spectrum analyzer's duty cycle metric to be out of alignment with how a Wi-Fi station would interpret the same environment. For example the Cisco Spectrum Expert SaGE v2 chipset reports duty cycle as any energy above the noise floor which is typically -92 to -95 dBm, whereas a client adhering to DSSS specifications transmitting at 50mW would only mark the medium as busy based on ED of -76 dBm or greater (per IEEE 802.11-2007 section 15.4.8.4). This difference is worth noting, but in my experience is not cause for significant concern when evaluating an environment for noise or interference.
Network Allocation Vector
In addition to CCA determining the medium idle/busy state for the current frame and noise, the NAV allows stations to indicate the amount of time required for transmission of required frames immediately following the current frame. This is important to reserve the medium as busy for these mandatory frames.
The importance of NAV virtual carrier sense is to ensure medium reservation for frames critical to operation of the 802.11 protocol. Typically these are control frames, but not always. They include 802.11 acknowledgements, subsequent data and acknowledgement frames as part of a fragment burst, and data and acknowledgement frames following an RTS/CTS exchange.
Wifi Signal 4 2 2011 Bmw
The NAV is carried in the 802.11 MAC header Duration field which is part of the MPDU payload, and is encoded at a variable data rate as discussed previously. Therefore, not all stations within the area may be able to decode the MPDU due to insufficient SNR. However, all stations within the area should still be able to decode the PLCP header used for CCA. Since the critical frames that the NAV protects use shorter inter-frame spacing values (SIFS for example), those frames should still gain access to the medium before any other station attempts to transmit and stations will detect a busy medium through CCA carrier sense. However, the added protection through NAV reservation is lost.
Stations that are able to decode the 802.11 header extract the Duration field value and use it mark the medium as busy for the amount of time specified (in microseconds). Therefore, the transmitter should follow the strict rules defined in the 802.11 specification for calculation of the NAV value to be sent within frames. Stations (including APs) not adhering to the specification could reserve too much time in order to hog the medium, as has been accused of at least one infrastructure vendor in the past. (I won't name names, but they know who they are :)
Note - The NAV timer also accounts for time required for appropriate inter-frame spacing, such as SIFS intervals between data and acknowledgement for example.
Summary and Recap
Every Wi-Fi station must first determine the state of the medium as idle or busy through the process of carrier sense, prior to being allowed to perform pro-active collision avoidance and ultimately transmit a frame. Carrier sense is composed of clear channel assessment and the network allocation vector, which together allow for adequate sensing of the physical environment as well as reserve the medium for frames critical to the operation of the protocol.
If you're like me, you find the physical layer properties of Wi-Fi networks fascinating. These fundamental concepts of carrier sense, CCA, NAV, modulation, encoding and the underlying physical properties of radio frequency propagation form the building blocks for most of the advanced mobile technologies that form our modern civilization. It's exciting to understand these concepts and see real-world examples in front of us everyday that are shaping our culture and our lives.
My intent is to convey my joy of these concepts to you, my readers, and inspire the same passion for Wi-Fi as I have. We are a new generation of RF engineers, not all that different from generations past who invented the telegraph, radio communications, and amateur ham radio operators.
Cheers,
Andrew
A Note on Nomenclature
PHY stands for Physical Layer Specifications.
DSSS stands for Direct Sequence Spread Spectrum, and is used in the original 802.11 PHY at 1 and 2 Mbps.
HR-DSSS stands for High Rate DSSS, and is used in the 802.11b PHY at 5.5 and 11 Mbps.
ERP stands for Extended Rate Physical, and is used with OFDM encoding with 802.11g PHY at 6 - 54 Mbps.
OFDM stands for Orthogonal Frequency Division Multiplexing, and is used in the 802.11a PHY at 6 - 54 Mbps.
HT stands for High Throughput, and is used with OFDM encoding with 802.11n PHY at 6.5 - 600 Mbps.
Further Reading
The following references are worthwhile reading for those looking for more information:
CSMA Persistence - 'Computer Networks: Fifth Edition' by Andrew S. Tanenbaum and David J. Wetherall
The following sections in the IEEE 802.11-2007 standard: