Wi-Fi Theory
Wi-Fi works over a set of standards defined by the Institute of Electrical and Electronics Engineers (IEEE 802.11 standards). With the use of standards every Wi-Fi devices can speak approximately the same language. The industry relies on this standards when building Wi-Fi devices and infrastructures. Over time the 802.11 standard evolved mainly in term of frequency, ranges. bandwidth and speed. The Wi-Fi Alliance is an organization promoting the interoperability of wireless devices. Wireless devices can be Wi-Fi alliance certified meaning that they can operate over the 802.11 standards and should not be object of any compatibility issues.
Wi-Fi transmitted data via radio-waves. These radio waves have different frequencies bands. For example, the two main frequency bands 2.4 GHz (802.11 b/g) and 5 GHz (802.11a). Others frequencies exist but are less common.
Using whether the 2.4 or 5 GHz frequency has its advantages and drawbacks. For example, while 2.4 GHz Wi-Fi have longer range than 5 GHz Wi-Fi, 5 GHz Wi-Fi have more frequency and is prone to less radio interference than 2.4 GHz Wi-Fi.
Wi-Fi 2.4 Ghz (802.11 b/g)
Two amendments works over 2.4Ghz 802.11/b and 802.11/g.
As we can observed in the image below, the 802.11 b/g amendments are split into 14 different channels. These channels are used to transmit data and avoid interference between devices. However, on the 2.4 GHz bandwidth most channels overlap, which can be the cause of interferences nevertheless. However, some channels such as channels 1, 6 and 11 are non overlapping. This is why it is recommended to configure the Wi-Fi on channels do not interfere with one and each others. When performing sniffing over the wireless network, we have to specify on which channel we want to sniff the data. At a given time, it is only possible to be on a single channel.
Depending on the regulatory of your country it is possible that some countries restrict the use of some channels. For example, in US and Europe most Wi-Fi devices can not transmit data over channel 14.
Wi-Fi 5 GHz (802.11 a)
The 802.11a amendment works over a 5 GHz bandwidth. The Wi-Fi 5 Ghz has a way more channels than the 2.4 GHz Wi-Fi. Moreover, unlike the 802.11 b/g amendments, there is no overlapping channels which is more unlikely to cause interferences and offers a better performance.
In the 5 GHz Wi-Fi, the speed is improved by the combination of two successive channels. Channels bonding means that two 20 MHz channels can bound with each others to form an approximately 40 MHz channel which can transmitted a way more data at a given time.
Channels Bonding
The image below illustrates channel bonding where two non-overlapping channels are combined to deliver a faster Internet experience to users. Chanel bonding is possible on 2.4 and 5 GHZ bandwidth, but a way more complicated to implement on 2.4 GHz due to the limited amount of space available.
Wi-Fi 2.4/5 GHz (802.11n)
The 802.11n amendment supports both 2.4 and 5 GHz bandwidth. It can supports channel bonding on both bandwidth. Thus, the combination of two 20 MHz channel are possible to form an approximately 40 MHz channel.
Wi-Fi 802.11ac
The 802.11ac amendment only supports 5 GHz bandwith. Unlike the 802.11n amendment, the 802.11ac amendment allows channel bonding up to 160 MHz.
Spatial Streams
Most Wi-Fi devices have more than one antenna to send and receive data. Multi antennas devices offers a better Wi-Fi signal. Spatial Streams can be described as the data being transmitted and received over multiple antennas. In Spatial Streams the data is divided into two or more chunk of data that are transmitted via multiple streams at the same time. The number of streams available relies mostly on the number of antennas of the Wi-Fi device. More antenna a device has more it is able to receive and send data simultaneously. In few words spatial streams allows for a reduction in the data transmission time and a better performance of the wireless communication.
The image below illustrates Spatial Streaming. On the left side we can observe three antennas for the transmitter (Tx). On the right side, three antennas are available for receiving data (Rx). In the image below, to simplify things, the data is divided into blue and yellow parts. On the transmitter side, two antennas are sending simultaneously parts of the same data streams and the receiver captures this data using the three antennas.
This notation is used by the industry for devices supporting spatial streams.
Transmitters X Receivers : Streams
It is important to note that the performance is always limited by the device supporting the least amount of streams. For example, if my Wi-Fi access point supports 4 spatial streams, but my phone only supports 2 spatial streams, only 2 streams will be used for data transmission.
Single vs Multi users MIMO
The acronym MIMO stands for Multiple In - Multiple Out. The concept is illustrated in the image just above where multiple antennas are used to transmit and receive data on a single device. Single vs Multi users MIMO refers to how many devices benefits from "MIMO" simultaneously.
In Single user MIMO, data can only be transmitted to one single device at the time. In other words, all streams are directed at a single device at a given time. The others devices have to wait for data transmission to be over with one device to be able to transmit data in its turn.
In Multi users MIMO, data can be transmitted over multiple devices at the same time. In the image below, the Wi-Fi access point supports 4 spatial streams. Because multi user MIMO is supported, it can dedicate two streams for one device and the two others to transmit data to others devices. It is not surprising that in environment with multiples devices, multi users MIMO offers a better wireless communication.
Beamforming
Beamforming is a way for Wireless devices to get the most of the signal. Instead for the signal to be sent uniformly in all directions, beamforming focuses the signal towards the devices which results in a better data transmission.
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