4.6 WiFi stations
A WiFi station determines whether it is in range of an AP by transmitting an enquiry, known as a probe request frame, and waiting for a response. If more than one AP responds, the station will choose to communicate with the one that has the strongest signal. A probe request frame initiates the WiFi connection and is an example of a management frame – a type of frame that does not carry any message data.
Just like the nodes on an Ethernet network, each station must have a means of being uniquely identified by a MAC address. Every message data frame sent must contain the MAC address of the source, destination and access point, as well as other management data that enables the frames to be correctly sequenced and errors to be detected.
Because all the stations in a WiFi network share the same communication channel, only one station at a time can be allowed to send data. So a station waits until it detects a period of inactivity and then uses a special protocol which prevents two or more stations sending data at exactly the same time. The exchanges involved in these protocols are another example of management data.
The WiFi standards do not define any upper limit on the number of stations that can join a network, though some particular equipment manufacturers may specify a limit. (We've seen one which stipulates a maximum of 128 stations connected to any one AP.) However, as the number of communicating stations increases, the channel capacity available for each station decreases. A point will eventually be reached when the network becomes too congested to provide an adequate service.