WiFi, technically specified in the IEEE 802.11 set of standards, is one of the most widely deployed wireless standards in the world. Chance are the device you are using to read this article has is WiFi enabled. WiFi is a straightforward extension of Ethernet, with some slight adaptations for using radio instead of copper wire as the communication channel.
Like Ethernet, WiFi has no central process that controls which device is allowed to transmit data at any point in time. Instead, each device decides on its own, and all devices must work together to guarantee good shared channel performance. With Ethernet, the protocol senses when the communication channel is busy and waits until it is free before sending data (carrier sensing). Ethernet also adds a collision detection protocol as an optimization. If a collision is detected, nodes stop sending data and use a randomized backoff algorithm to decide when to begin sending again. Together, these algorithms form the carrier-sense multiple access with collision detection algorithm (CSMA/CD). WiFi uses a similar strategy to Ethernet, but is restricted by the communication channel — there is no reliable way to detect collisions using radio waves. Therefore, instead of collision detection, WiFi uses a collision avoidance strategy defined by the carrier-sense multiple access with collision avoidance algorithm (CSMA/CA).
Using WiFi’s collision avoidance algorithm, each sender transmits data only when the channel is idle, like Ethernet. If the channel is idle, WiFi sends an entire data frame, and then waits for explicit acknowledgement from the receiver before proceeding with the next frame. This strategy works very well if there is light load on the network, but as the WiFi network gets more and more busy, the number of collisions will rise and network performance will suffer. This is one of the reasons why heavily used public WiFi networks usually offer such dismal performance.
The Many Versions of WiFi
While the WiFi protocol is fairly straightforward, there is still innovation and improvements to be had, as evidenced by the number of different versions of WiFi around. The versions differ based on the radio band they operate on, the wavelengths they use, and the data rates that they support. In general, as with all wireless networks, lower frequencies generally have better range but less bandwidth than high frequencies.
The 802.11 standard uses several different radio frequencies as the communication channel, depending on the particular WiFi version in use. For example, the \(b\) and \(g\) standards both use the 2.4 GHz frequency and the \(ac\) standard uses the 5 GHz frequency, includes advanced modulation techniques, and increases the total bandwidth of the channel. Newer WiFi systems also allow use of multiple radios to transmit multiple data streams in parallel, further increasing throughput.
The 802.11 standard uses either the 2.4 GHz or 5 GHz radio band. This band is often subject to interference from anything from microwave ovens, cordless phones, Bluetooth devices, and even USB hubs. This shared spectrum means that ideal WiFi conditions rarely, if ever, exist. The wide deployment of WiFi networks also creates performance challenges as several overlapping WiFi networks compete for access to the radio. For example, though your Internet connection and wireless router may be technically capable of reaching 50 Mbps download speeds, as soon your neighbour starts streaming their HD video using the same radio channel, your bandwidth can be cut in half. Generally, WiFi can provide no bandwidth or latency guarantees due to using a shared radio channel and dealing with the interference and collisions that this implies.
This article provides an introduction to WiFi. If you want to learn more, there are several great resources to choose from: