## Site Planning

For optimal performance, do not locate access points near transformers, heavy-duty motors, fluorescent lights, microwave ovens, refrigerators, and other industrial equipment. See the site survey section for environment recommendations.

Signal loss can occur when metal, concrete, walls, or floors block transmission. Locate access points in open areas or add access points as needed to improve performance.

### Site Survey

A site survey analyzes the installation environment and provides recommendations for equipment and its placement. For detailed information about conducting a site survey with the Site Survey Utility, see the Site Survey section of his guide.

Intel recommends conducting a new site survey and developing a new coverage area floor plan when switching from frequency-hopping access points to direct-sequence access points.

Client Association Process

Access points recognize wireless clients as they associate with the access point. The access point keeps a list of the clients it services. Clients associate with an access point based on the following conditions:

- the signal strength between the access point and client
- clients currently associated with the access point
- the clients encryption and authentication capabilities and the type enabled
- the client supported data rates:
  - 802.11B: 1, 2, 5.5, and 11 Mbps
  - 802.11a: 6, 12, 24, 36, 48, and 54 Mbps

Clients perform preemptive roaming by intermittently scanning for access points and associating with the best available access point. Before roaming and associating with access points, clients perform scans to collect access point statistics and determine the direct-sequence channel used by the access point.

Scanning is an intermittent process where the wireless client sends out probe messages on all frequencies defined by the country code. The statistics enable a client to reassociate by synchronizing its frequency to the access point. The client continues communicating with that access point until it needs to switch cells or roam.

Clients perform full scans at start-up. In a full scan, a client uses a sequential set of channels as the scan range. For each channel in range, the client tests for Clear Channel Assessment (CCA). When a transmission-free channel becomes available, the client broadcasts a probe with the Network Name (SSID) and the broadcast BSSID. An access point-directed probe response generates a client Acknowledgment (ACK) and the addition of the access point to the access point table with a proximity classification. An unsuccessful access point packet transmission generates another client probe on the same channel. If the client fails to receive a response within the time limit, it repeats the probe on the next channel in the sequence. This process continues through all channels in the range.

Clients perform partial scans at programmed intervals, when missing expected beacons or after excessive transmission retries. In a partial scan, the client scans access points classified as proximate on the access point table. For each channel, the client tests for CCA. The client broadcasts a probe with the Network Name (SSID) and broadcast BSSID when the channel is transmission-free. It sends an ACK to a directed probe response from the access point, and updates the access point table. An unsuccessful access point packet transmission causes the client to broadcast another probe on the same channel. The client classifies an access point as out-of-range in the access point table if it fails to receive a probe response within the time limits. This process continues through all access points classified as proximate in the access point table.

A client can roam within a coverage area by switching access points. Roaming occurs when:

- An unassociated client attempts to associate or reassociate with an available access point.
- The supported rate changes or the client finds a better transmit rate with another access point.
- The received signal strength indicator (RSSI) of a potential access point exceeds the current access point.
- The ratio of good-transmitted packets to attempted-transmitted packets falls below a threshold.

A client selects the best available access point and adjusts itself to the access point direct-sequence channel to begin association. Once associated, the access point begins forwarding any frames that are addressed to the client. Each frame contains fields for the current direct-sequence channel. The client uses these fields to resynchronize to the access point again.

The scanning and association process continues for active clients. This process allows the client to choose the best network connection available by finding new access points and discarding out-of-range or deactivated access points.

Data Encryption

The Intel® PRO/Wireless LAN uses the Wired Equivalent Privacy (WEP) encryption and decryption algorithm specified in Section 8 of the IEEE 802.11 wireless LAN specification. WEP uses the same key for both encryption and decryption, and provides protection equivalent to that of a wired network, hence the "Wired Equivalent" portion of the name.

The IEEE 802.11 specification defines two types of authentication:

- Open system authentication is the default authentication service, in which all clients that request access to the network are accepted, with no actual verification. You should only use this system if it’s not necessary to positively validate the identity of the sender.
- Shared key authentication requires the exchange of an authentication key shared among all of the authentic access points and clients in the network. When a client requests access to the network, the access point sends a long random number encrypted with the shared key to the client. The client decrypts the number using the same key and sends it back to the access point, which only grants access to clients that return the correct number. The Intel® PRO/Wireless LAN Access Point supports 64-bit and 128-bit shared key authentication.

If you implement the shared key authentication mode, you must configure all access points and clients to use the same key.

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