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Establishing Communications Between Wireless Devices

When you work with wireless networks, it is important to have a basic understanding of the communication that occurs between wireless devices. If you’re using an infrastructure wireless network design, the network has two key parts—the wireless client, also known as the station (STA), and the AP. The AP acts as a bridge between the STA and the wired network.

As with other forms of network communication, before transmissions between devices can occur, the wireless access point and the client must begin to talk to each other. In the wireless world, this is a two-step process involving association and authentication.

The association process occurs when a wireless adapter is turned on. The client adapter immediately begins scanning the wireless frequencies for wireless APs or, if using ad hoc mode, other wireless devices. When the wireless client is configured to operate in infrastructure mode, the user can choose a wireless AP with which to connect. This process may also be automatic, with the AP selection based on the SSID, signal strength, and frame error rate. Finally, the wireless adapter switches to the assigned channel of the selected wireless AP and negotiates the use of a port.

If at any point the signal between the devices drops below an acceptable level, or if the signal becomes unavailable for any reason, the wireless adapter initiates another scan, looking for an AP with stronger signals. When the new AP is located, the wireless adapter selects it and associates with it. This is known as reassociation.

With the association process complete, the authentication process begins. After the devices associate, keyed security measures are applied before communication can take place. On many APs, authentication can be set to either shared key authentication or open authentication. The default setting typically is open authentication. Open authentication enables access with only the SSID and/or the correct WEP key for the AP. The problem with open authentication is that if you don’t have other protection or authentication mechanisms in place, your wireless network is totally open to intruders. When set to shared key mode, the client must meet security requirements before communication with the AP can occur.

After security requirements are met, you have established IP-level communication. This means that wireless standard requirements have been met, and Ethernet networking takes over. There is basically a switch between 802.11 to 802.3 standards. The wireless standards create the physical link to the network, allowing regular networking standards and protocols to use the link. This is how the physical cable is replaced, but to the networking technologies there is no difference between regular cable media and wireless media.

Several components combine to enable wireless communications between devices. Each of these must be configured on both the client and the AP:

  • Service Set Identifier (SSID): Whether your wireless network is using infrastructure mode or ad hoc mode, an SSID is required. The SSID is a configurable client identification that allows clients to communicate with a particular base station. Only client systems configured with the same SSID as the AP can communicate with it. SSIDs provide a simple password arrangement between base stations and clients in a BSS network. ESSIDs are used for the ESS wireless network.
  • Wireless channel: RF channels are an important part of wireless communications. A channel is the frequency band used for the wireless communication. Each standard specifies the channels that can be used. The 802.11a standard specifies radio frequency ranges between 5.15 and 5.875GHz. In contrast, the 802.11b and 802.11g standards operate in the 2.4 to 2.497GHz ranges. Fourteen channels are defined in the IEEE 802.11b/g channel set, 11 of which are available in North America.
  • Security features: IEEE 802.11 provides security using two methods, authentication and encryption. Authentication verifies the client system. In infrastructure mode, authentication is established between an AP and each station. Wireless encryption services must be the same on the client and the AP for communication to occur.
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