Connecting to an Access Point

In order to establish communication between a client device and an AP, the client must know the Service Set IDentifier (SSID) of the network and then perform two special steps: authentication and association.Therefore, the communication relationship exists in three states:

• Unauthenticated and unassociated
• Authenticated and unassociated
• Authenticated and associated

SSID Discovery

There are two possible ways for the client to know the SSID of the network. Either the SSID is told to the client by the AP (often called Open Network mode), or the SSID has to be known by the client via some other method, such as being preprogrammed into the client device by the system administrator (often called Closed Network mode).
The AP broadcasts “management beacons,” typically every 100 milliseconds.These management beacons are a special kind of “Wi-Fi mating call,” containing all of the synchronization information that the client needs to know in order to associate with the network, including channel, supported speeds, timestamps, WEP status, and other capability information.With Open Network mode (see image), the SSID is included in the management beacon.

With Closed Network mode (see next image below), the client uses its preprogrammed knowledge of the SSID and broadcasts probe requests across all channels.The probe request includes the SSID of
the network that the client is attempting to communicate with. If the AP hears a probe request on its channel, along with a matching SSID, then the AP will answer back with a probe response.This probe response will contain synchronization details, similar to the information found in a management beacon.
Note that APs in Closed Network mode still broadcast a management beacon; however, the portion of the management beacon where the SSID would normally be found is blank.

Again, the key distinction between these two methods is the way in which the Service Set IDentifier (SSID) discovery is handled. In order to associate with an AP, a client must know the SSID of the network. Either the client can be told the SSID (Open Network), or it has to be preprogrammed in the client configuration (Closed Network).

Authentication

Authentication can occur using either “Open System” or “Shared Key” authentication (see image below). Null authentication, as its name implies, is a simple two-step process that does not require any
credentials to be supplied.The process begins when the client sends an Authentication Request frame to the AP.The AP responds with an Authentication Response frame, indicating either success or failure.

Shared Key authentication (see image below) is a four-step process that involves the client’s knowledge of the WEP key in order to be authenticated by the access point.The first step is that the client
sends an Authentication Request frame to the AP.The AP responds with 128 bytes of challenge text.
The client uses the WEP engine to encrypt the 128 bytes of random challenge text and then sends back a Challenge Response frame, containing 128 bytes of (encrypted) cipher text. In order to authenticate the client, the AP decrypts the cipher text and sees if it matches the original challenge text.This process is used to validate whether or not the client actually knows the shared secret of the
WEP key.The final step is for the AP to send an Authentication Result frame, indicating success or failure.

Association

Once the client has been authenticated, the next step is to associate with the access point (see image below).The client sends an Association Request frame (including the SSID) and the AP sends back an Association Response frame, indicating success or failure.