In the following discussion AP is the acronym for a fixed access point and STA (short for "station") refers to the wireless device, such as a laptop computer, that wants to connect to the network. The AP and STA talk to each other using wireless messages. We will assume that the AP is connected to a wired network that the STA wants to access.
To help understand the process by which the STA connects to the AP and starts to send data, we'll run through a simplified overview first. This describes the sequence of events that occur in systems that are not using security. Let's assume that the AP is already turned on and operating. The AP advertises its presence by transmitting short wireless messages at a regular interval, usually about 10 times a second. These short messages are called beacons and allow wireless devices to discover the identity of the AP.
Now suppose that someone powers up a laptop with a Wi-Fi network adapter installed (the STA). After the initialization phase, the STA will start to search for an AP. It may have been configured to look for a particular AP, or it may be prepared to connect to any AP, regardless of identity. There are a number of different radio frequencies (called channels) that could be used so the STA must tune into each channel in turn and listen for beacon messages. This process is called scanning. The process can be accelerated by probing, as explained later in this chapter.
The STA may discover several APs in a large network and must decide to which it intends to connect; often this decision is made based on signal strength. When the STA is ready to connect to the AP, it first sends an authenticate request message to the AP. The original IEEE 802.11 standard defined the authenticate messages as part of the security solution, but they are not used for this purpose in Wi-Fi (for reasons why, see Chapter 6). Because, in our scenario, we are not using security, the AP immediately responds to the authenticate request by sending an authenticate response indicating acceptance.
Now that the STA has permission to connect to the AP, it must take one more step before the connection is complete. In IEEE 802.11 the concept of "connection" is called association. When an STA is associated with an AP, it is eligible to send data to and receive data from the network. The STA sends an association request message and the AP replies with an association response indicating successful connection. After this point, data sent from the STA to the AP is forwarded onto the wired LAN to which the AP is connected. Similarly, data from the wired LAN intended for delivery to the STA is forwarded by the AP.
This overview scenario describes the sequence of events by which an STA joins a network. Many details have been left out in the interests of simplicity. Some of the details are brought out in the rest of this chapter.
In IEEE 802.11 there are three types of messages:
We won't discuss control messages in detail here, but management messages are important for you to understand the process of connecting to a Wi-Fi LAN. The rest of this section describes the management messages and the processes they support.
Beaconing is the method by which the access point tells the world it is ready for action and maintains timing in the network. Beacons are management frames that are regularly sent out by the AP, typically about ten times a second. The beacon contains useful information such as the network name and the capabilities of the AP. For example, the beacon can tell the STA whether the AP supports the new security provisions of the IEEE 802.11 standard.
When a station turns on, it can listen for beacons, hoping to find an access point with which to connect. You might think that ten beacons a second would be plenty for the STA to find the right access point quickly. However, remember that there are multiple frequency channels and that if the STA has to go to each frequency and wait for 0.1 seconds, it could take a while to complete the scan (in other words, the search all the channels). Furthermore, if you are already connected and want to find a new access point because your signal strength is getting weak, you must find the new access point very rapidly to avoid disruption. For this reason, the STA has the option to send a probe request message. This is basically the equivalent of shouting "hello, anyone there?" when entering a dark cave. If any access points receive the probe request, they immediately reply with a probe response that looks essentially like a beacon message. In this way, an STA can rapidly learn about the access points in its area.
Connecting to an AP
Remember that the process of connecting to an AP is called association. When you want to connect, you send an association request; the access point may reply with an association response. If that response is positive, you are now associated with the access point.
If there are multiple access points on the same network, your STA might choose to move its association from the current AP to a new one. First it should disconnect from the old AP using a disassociation message. Then it connects to the new AP using a reassociation message. The reassociation message has some information about the old AP that can be useful to make the handover smoother. The information allows the new AP to talk to the old AP to confirm that the roam has taken place.
Once you are associated and after authentication has been performed, you can start sending data. In most cases data is exchanged between the STA and the AP. In fact, this is the normal method even if you are sending data to another STA. First, you send to the AP and then you allow the AP to forward to the STA. Often data will go to the AP and then be forwarded on to an Ethernet LAN or to an Internet gateway. To facilitate this, each IEEE 802.11 data frame going to or from the AP has three addresses. Two may be considered the "final" source and destination, and the third is the "intermediate" address that of the access point through which the message passes.
When you are sending from the STA to the AP, there is one source address that of the STA that sent the message and two destination addresses. One destination address specifies the AP and the other specifies the eventual destination for the message. Similarly data from the access point to the STA has one destination address (the STA) and two source addresses the AP and also the originator of the message.