Do WiFi devices transmit packets when they are just turned on?

Question

I read a lot about WiFi sensors being used to track smart phones in Retail environment. The location triangulation is done on basis that a smart phone has its WiFi turned ON, be it in connected or unconnected state.

Case 1 : WiFi turned ON but unconnected

Why should a smart phone which has its WiFi turned ON need to transmit the packets, unless the user 'scans' for nearby WiFi networks?

Case 2 : WiFi turned ON and connected

Why should a smart phone transmit any packets, unless the user is browsing the net?

In both the above cases, there is a high chance that most of the time the WiFi device does not send any packet, which means none of the WiFi sensors detect it. If that is true, then the whole idea behind WiFi sensor based triangulation in Retail goes for toss, clearly with so many companies working on this, I must be wrong. Please answer with more than a yes or no, as to which packets are generally sent in both the above scenarios.

Answer 1

If wifi is turned on it will periodically search for new networks. This happens even if you are already connected to one, as it allows the device to connect to a 'better' network, if available.

Scanning/network discovery can be done in two ways. First is passive when a device listens to surrounding access point's (AP) beacon frames. These are basically advertisements for their network. The second method is called active. This is the most likely explanation of how the technology you mentioned works. Active scanning is when the device sends out a probe frame asking for available APs. These are generally ones that you have associated with previously, e.g. Your home network. These probes can be listened to from nearby 802.11 (wifi) devices, therefore tracking you.

Active and passive scanning

801.11 frames

Answer 2

As mentioned in @AndrewLeeming answer, one of the causes for data transmission data is scanning. It's not necessary but normally it will be performed to find a network to connect to (or a better network in case of already connected). Active scanning can be turned off for power saving reasons. Passive scanning doesn't involve transmissions, so it's irrelevant to this question.

However, the most important reason for WiFi devices to transmit packets while connected is to let the AP know that the client is still available. Otherwise the AP will drop the link after a certain period of time without activity. Additionally, the clients might be in power save mode and instruct the AP not to transmit data to them. From time to time the client will inquire the AP to see if there are any pending packets for it.