99.99.0/24CORPORATE USERMAC

10.99.99.0/24Corporate UserMAC: 0000.0001.0001

Figure 9-1 A Simple Wireless Network

148 CCNA Wireless Official Exam Certification Guide

Clients have more than one way to find an AP and associate with it. A client can passively

scan the network and listen on each frequency for beacons being sent by an AP, or it can

use an active scan process and send a probe request in search of a specific AP. Users of the

UserNet would likely actively scan the network, whereas a guest would passively scan.

The detailed method of client interaction is covered in Chapter 16, “Wireless Clients.”

Getting back to the association process, a client scans the channels hoping to hear a bea-

con from an AP or actively sends a probe request. If a probe response is received or a bea-

con is heard, the client can attempt to associate with the SSID received in that probe

response or beacon.

The next step is to authenticate and associate with the AP. When the client chooses an

SSID, it sends an authentication request. The AP should reply with an authentication re-

sponse. After this occurs and a “Success” message is received, an association request is

sent, including the data rates and capabilities of the client, followed by an association re-

sponse from the AP. The association response from the AP includes the data rates that the

AP is capable of, other capabilities, and an identification number for the association.

Next, the client must determine the speed. It does this by determining the Received Signal

Strength Indicator (RSSI) and signal-to-noise ratio (SNR), and it chooses the best speed to

send at based on these determinations. All management frames are sent at the lowest rate,

whereas the data headers can be sent faster than management frames, and the actual data

frames at the fastest possible rate. Just as the client determines its rates to send, the AP, in

turn, does the same. Now that the client is associated, it can attempt to send data to other

devices on the network.

Sending to a Host on Another Subnet

When a client is associated with an AP, the general idea is to send data to other devices. To

illustrate this, first try to send data between Client A in Figure 9-2, which is on the User-

Net network, and Client B, which is on the Guest network. Although a typical network

would not allow guest users to send traffic to internal WLAN users for security purposes,

this will provide an example of how the connection works.

The two clients are clearly on two different subnets, so the rules of how IP works are still

in play. The clients cannot send traffic directly to each other. Based on normal IP rules,

they would first determine that the other is not on the same subnet and then decide to use

a default gateway to relay the information. If a client has never communicated with the de-

fault gateway, it uses Address Resolution Protocol (ARP) to resolve its MAC address. The

process would appear as follows:

Step 1. Client A wants to send traffic to Client B.

Step 2. Client A determines that the IP address of Client B is not on the same subnet.

Step 3. Client A decides to send the traffic to the default gateway of 10.99.99.5.

Step 4. Client A looks in its ARP table for a mapping to the gateway, but it is not there.

Step 5. Client A creates an ARP request and sends to the AP, as seen in Figure 9-3.

Client BGuest UserMAC: AAAA.BBBB.CCCC