01. WiFi Basics

Terminology

Station (STA) = Client device connecting to AP.
Access Point (AP) = Networking hardware providing stations access to network.
Extended Service Set (ESS) = Infrastructure Network. ESS is comprised of one or more BSS, joined together by a common DS.
Distribution System (DS) = Link APs together in an ESS.
Independent Basic Service Set (IBSS) = "Ad-Hoc" Network. Do not require an AP, consists of at least one station.
Basic Service Set (BSS) = AP inside ESS.
BSSID = MAC address of BSS (AP).
ESSID = Name of ESS (identify a wireless network).
Card's Transmit (TX) Power =
- How far the card can transmit.
- Expressed in mW or dBm.
- dBm = 10*log_10(mW). 1 mW = 1 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm, 1W = 30 dBm...
- For example, Alfa AWUS306H has TX power = 1000mW (30dBm)
Card's Sensitivity =
- How well it can receive.
- Often overlooked in favor of TX Power.
- Usually measured in dBm.
- The more negative number, the better.
- Typical values for sensitivity in average cards = -80 to -90 dBm.
- Every -3dBm = double sensitivity.
Antenna's Sensitivity =
- Measured in dBi.
- Increase of 3 dBi = double antenna's effective range.

Spectrum	Year	Max Speed	Distance	Channel Bandwidth	Frequency	Congestion
802.11a	1999	54 Mbps	35m	20 MHz	5 GHz	Low
802.11b	1999	11 Mbps	35m	22 MHz	2.4 GHz	High
802.11g	2003	54 Mbps	38m	20 MHz	2.4 GHz	High
802.11n	2009	300/900 Mbps	70m/35m	20/40 MHz	2.4 / 5 GHz	High/Low
802.11ac	2013	1300 Mbps	35m	20/40/80/160 MHz	2.4 / 5 GHz	Low

Each channel is allotted 20MHz, separated by 5MHz
Total bandwidth = 100MHz
11 Channels
Channels overlap with each other => cause interferences
Some channels have better WiFi performance than others because they are non-overlapping => Channels 1, 6, 11 are non-overlapping
WiFi coverage better than 5GHz because lower frequencies can more easily penetrate solid objects
Non-WiFi interfence possible (e.g. Bluetooth)
Universal compatibility: 802.11 b/g/n

WiFi-2.4GHz-Spectrum

WiFi-5GHz-Spectrum

WiFi standard allows channel bandwidths of 10, 20, 22, 40, 80 and 160 MHz, but 10MHz is not used anymore.
80 MHz and 160 MHz can be used only with 5 GHz frequency.
Certain devices are not being able to connect to APs with channel widths > 40Mhz.
By default, the 2.4 GHz frequency uses a 20 MHz channel width.
20MHz channel width is wide enough to span one channel.
40 MHz channel width bonds two neighbouring 20 MHz channels together, forming a 40 MHz channel width => greater speed and faster transfer rates. In this case, there are:
- One "control" channel functions as the main channel: This main channel is used to send Beacon packets & data packets.
- One "auxiliary" channel (or extension channel): It is used to send other packets. The extension channel has to be contiguous with the edge of the control channel, without overlapping.
Notation for 40 MHz channel:
- HT40+ = Means that the frequency of the main ("control") channel is higher than the auxiliary channel. Warning: In this case, the main channel cannot be 1 because it would mean that "auxiliary" channel would be out of allowed frequency for the 2.4GHz band !
- HT40- = Otherwise.

HT20 High Throughput 20MHz, 802.11n
HT40 High Throughput 40MHz, 802.11n
HT40- High Throughput 40MHz, 802.11n, control channel is bellow extension channel.
HT40+ High Throughput 40MHz, 802.11n, control channel is above extension channel.
VHT20 Very High Throughput 20MHz, Supported by 802.11ac
VHT40 Very High Throughput 40MHz, Supported by 802.11ac
VHT80 Very High Throughput 80MHz, Supported by 802.11ac
VHT160 Very High Throughput 160MHz, Supported by 802.11ac