Introduction to Wi-Fi

Wireless LANs and Wi-Fi

In a wired network, a device reaches the local infrastructure through an Ethernet cable. This can provide a predictable link, but it limits the device to places where a cable is available. A wireless local area network removes that condition on the segment between the device and the access point. The radio segment ends at the AP, which bridges frames to a local distribution network. It does not automatically provide Internet access. Internet access also depends on the router or gateway, the service provider link, and the networks beyond the local network.

Wi-Fi lets computers, smartphones, tablets, and home automation devices connect to a local network and exchange data over radio. Instead of a cable, it uses radio waves. The term WLAN (Wireless Local Area Network)describes a category of local networks, while IEEE 802.11 defines its radio mechanisms. "Wi-Fi" is the Wi-Fi Alliance trademark and certification name, also used informally for compliant devices. Wi-Fi is not a synonym for the Internet because it describes local wireless access rather than the external network.

How Wi-Fi works on a local network

Wi-Fi carries data frames between a station and an access point over a radio link. A transmitter encodes digital data in a signal, while the receiver performs the reverse operation and checks the received frame. Frame exchanges include addressing, medium access, acknowledgements, and retransmissions when a transmission fails. A typical infrastructure network uses two basic types of devices.

1. The Access Point (AP) and wireless router

   The Access Point provides the radio link and forwards frames to the local network. A router additionally forwards IP packets between networks. A home gateway often combines an AP, router, NAT, DHCP, and firewall in one enclosure. On its external side it may connect to a modem, optical network terminal, or another provider device, but the Wi-Fi standard itself does not provide Internet access.

2. The wireless client or station (STA)

   This is a device using a Wi-Fi interface, such as a laptop, smartphone, game console, or television. It needs a network adapter, also called a Wireless Network Interface Card (WNIC). The station sends frames to the AP and receives frames from the local network. Its software may also select an access point, band, and transmission parameters.

Wi-Fi primarily uses the $2.4$ GHz and $5$ GHz bands. The $6$ GHz band is also available in some countries. The lower band usually propagates farther and contains fewer non-overlapping wide channels, while the higher bands can offer wider channels with greater attenuation through walls. Unlicensed spectrum does not mean that there are no rules. The regulatory domain sets available channels, maximum power, protection requirements for other spectrum users, and permitted operating modes. In the United States and elsewhere, equipment must follow the applicable regulations, and 6 GHz operation also depends on device certification and configuration.

The IEEE 802.11 family of standards

Devices must agree on frame formats, medium access procedures, and radio parameters before they can exchange data. These elements are described by standards from the Institute of Electrical and Electronics Engineers (IEEE). IEEE 802.11 separates functions between the medium access control (MAC) and physical (PHY) layers. The family of requirements for wireless LANs is called IEEE 802.11.

IEEE 802.11 has been extended with new PHY layers, wider channels, more spatial streams, and management mechanisms. Lettered designations such as 802.11b, 802.11g, 802.11n, and 802.11ac identify particular amendments or capability generations. The designation alone does not specify the channel width, number of antennas, or achieved throughput.

To simplify device labels, the Wi-Fi Alliance introduced generation names. Its official guide maps Wi-Fi 4 to 802.11n, Wi-Fi 5 to 802.11ac, and Wi-Fi 6 to 802.11ax. Wi-Fi 1, Wi-Fi 2, and Wi-Fi 3 are retrospective shorthand found in some material, but they are not generation names assigned by that guide.

Note. These values are maximum PHY rates for particular channel widths, modulation settings, and numbers of spatial streams. User data throughput is lower and depends on distance, interference, shared airtime, and protocol overhead.

A device advertises only the modes it supports, and a link uses the highest mode that both endpoints can decode under current radio conditions. The 600 Mbps value for 802.11n assumes a particular channel width, modulation, coding, and four spatial streams. The 9.6 Gbps value for 802.11ax likewise describes a configured PHY aggregate, not a speed delivered to one application. Retransmissions and shared airtime reduce the useful rate further. The application sees the rate after those protocol costs.

Fundamental elements of a Wi-Fi network

Wi-Fi networks are described with concepts for identifying a network, accessing a channel, and grouping access points.

• SSID (Service Set Identifier)

  An SSID is the identifier of a service set. In a typical configuration it is the name shown in a network list, such as "CoffeeShop_WiFi" or "HomeNetwork". It distinguishes networks in the same area, although an administrator can disable broadcasting of the identifier.

• Channels

  A band is divided into channels with a defined width. An AP and its client must use compatible channels, while devices in range share transmission time. For 20 MHz channels, 1, 6, and 11 are a common North American planning choice. Other regulatory domains and channel widths offer different options. Channel numbers and bandwidth therefore have to be interpreted together with the local rules. Overlapping transmissions and congestion increase delay and retransmissions.

• BSS (Basic Service Set)

  A BSS is the most basic building block of a Wi-Fi network. It consists of a single Access Point and all the wireless clients (stations) that are associated with it. The AP coordinates the radio exchange and forwards traffic between associated stations when the network permits that communication. All devices within a BSS can communicate with each other through the AP.

• ESS (Extended Service Set)

  In a larger building, two or more BSSs can be connected through a Distribution System. The system can use Ethernet or another transport mechanism. Access points often share an SSID so that a client can select another AP, but a shared SSID does not define an ESS and does not guarantee a handoff without interruption. Roaming requires support on the relevant devices and can cause a short interruption. The client evaluates signal and policy information, then performs reassociation or another transition procedure.

Limits of radio transmission

Radio is a shared medium that is sensitive to interference and propagation conditions. These properties affect delay, throughput, and coverage.

• Shared medium and collisions

  Unlike a dedicated Ethernet cable, a radio channel is shared. Devices use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). A station defers transmission when it detects activity. Two stations may still fail to hear each other or start transmitting at a similar time. The mechanism reduces collisions, but it does not eliminate them.

• Interference:

  Wi-Fi signals can be disrupted by other systems using the same band and by neighboring networks. In the 2.4 GHz band, relevant sources include Bluetooth devices, microwave ovens, and transmissions on overlapping channels. Interference raises the background level, reduces available airtime, and can cause retransmissions.

• Security Concerns:

  Because a radio signal can be received outside the room, a network needs authentication and frame protection. WEP is no longer considered adequate. WPA2 (Wi-Fi Protected Access 2) commonly uses CCMP with AES, while WPA3-Personal uses SAE and still requires a compatible cipher suite. A mode name alone does not protect a weak password or an incorrect configuration. The AP and station must also agree on the security profile before protected data frames can be exchanged.

• Range and Signal Degradation:

  Radio signals weaken with distance, while walls, floors, furniture, and people can absorb, reflect, or scatter them. Transmit power, receiver sensitivity, channel width, and interference also matter. Network performance depends in part on the Signal-to-Noise Ratio (SNR), the ratio between desired Wi-Fi signal power and unwanted background power. The selected modulation and coding scheme changes as this ratio changes, so the nominal rate can fall before a link disconnects.