Everything about different WiFi modes of operation

Everything about different WiFi modes of operation

Wireless adapters, or network interface controllers (NICs for short), are network cards with the 802.11 standards that let a machine connect to a wireless network. A station is any device that has such a card. Access points, also known as AP or hotspots, can let nearby WiFi-equipped stations access a wired network to which the access point is directly connected. The 802.11 standard defines two operating modes: infrastructure mode and ad hoc mode.

Wireless network in infrastructure mode

In infrastructure mode, each station computer (STA for short) connects to an access point via a wireless link. The set-up formed by the access point and the stations located within its coverage area is called the basic service set (BSS). They form one cell. Each BSS is identified by a BSSID, a 6-byte (48-bite) identifier. In infrastructure mode, the BSSID corresponds to the access point's MAC address:

WiFi infrastructure mode (802.11b)

It is possible to link several access points together (or, more precisely, several BSS's) using a connection called a distribution system (DS) in order to form an extended service set or ESS. The distribution system can also be a wired network, a cable between two access points, or even a wireless network:

Creating an ESS by linking two APs using a DS

An ESS is identified with an ESSID, Extended Service Set Identifier. But what is ESSID? It is a 32-character identifier (in ASCII format) that acts as its name on the network. The ESSID, often shortened to SSID, shows the network's name and, in a way, acts like a first-level security measure because it is necessary for a station to know the SSID to connect to the extended network.

When a roaming user goes from one BSS to another while moving within the ESS, their machine's wireless network adapter can switch access points depending on the quality of the signal it receives from different access points. Access points communicate using a distribution system to trade information about the stations and, if necessary, transmit data from mobile stations. This feature that lets stations move "transparently" from one access point to another is called roaming.

Communicating with the access point

When a station joins a cell, the cell sends a probe request on each channel. This request contains the ESSID that the cell is configured to use and the traffic volume that its wireless adapter can support. If no ESSID is set, the station listens to the network for an SSID.

Each access point broadcasts at regular intervals (about ten times a second) a signal called a beacon, that gives information on its BSSID, its characteristics, and, if applicable, its ESSID. The ESSID is automatically broadcast by default, but it is possible (and recommended) to disable this option.

Whenever a probe request is received, the access point checks the ESSID and the traffic volume request found in the beacon. If the given ESSID matches the access point, the access point sends a response containing synchronization data and information on its traffic load. This way, the station that receives the response can check the quality of the signal being sent by the access point in order to determine how far away it is. Generally, the closer an access point is, the higher its data transfer capacity is.

So, a station within range of multiple access points (which have the same SSID) may choose the access point offering the best balance of capacity and current traffic load.

When a station is within range of several access points, the station chooses which one to connect to.

Ad Hoc mode

What is Ad Hoc mode? In this mode, wireless client machines connect to one another to form a peer-to-peer network, meaning a network in which every machine acts as both a client and an access point at the same time:

WiFi ad hoc mode

The set-up formed by the stations is called the independent basic service set, or IBSS for short. An IBSS is a wireless network that has at least two stations and uses no access point. Therefore, the IBSS, forms a temporary network that lets people in the same room exchange data. It is identified by an SSID, just like an ESS in infrastructure mode.

In an ad hoc network, the range of the independent BSS is determined by each station's range. That means that if two stations on the network are outside each other's range, they will not be able to communicate, even if they can "see" other stations. Unlike infrastructure, ad hoc mode has no distribution system that can send data frames from one station to another. An IBSS is, by definition, a restricted wireless network.

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