3Com 09-1765-001 manual Interface User Guide, Security

Theory of Operation 27

The AirConnect Mobile IP (roaming across routers) feature enables a wireless client on the Internet to move from one subnet to another while keeping its IP address unchanged. The scanning and associating process continues for active wireless clients. This allows the wireless clients to find new APs and discard out-of-range or deactivated APs. By testing the airwaves, the wireless clients can choose the best network connection available.

Set the wireless client for Mobile IP as specified in the AirConnect Network

Interface User Guide.

Security

Security involves two distinct areas: authentication and privacy. Authentication ensures that only authorized users access the wireless network. Privacy ensures that communication between authenticated users and the network cannot be intercepted or overheard. The Access Control List provides authentication using the wireless LAN service area and a system administrator-supplied list of all the wireless client MAC addresses authorized to access the Home Service Area. Privacy is ensured by enabling the 40-bit WEP encryption option.

Supporting CAM and Continuously Aware Mode (CAM) stations leave their radios on continuously to PSP Stations hear every beacon and message transmitted. These systems operate without any

adjustments by the AP.

A beacon is a uniframe system packet broadcast by the AP to keep the network synchronized. A beacon includes the wireless LAN service area, the AP address, the Broadcast destination addresses, a time stamp, Delivery Traffic Indicator Maps, and the Traffic Indicator Message (TIM).

Power Save Polling (PSP) stations power off their radios for long periods. When a wireless client in PSP mode associates with an AP, it notifies the AP of its activity status. The AP responds by buffering packets received for the wireless client. The PSP-mode wireless client wakes up to listen to the AP beacon every nth Beacon Interval where n is a PSP-mode value from the 1 to 5-range; the Beacon Interval is set on the wireless client. When the wireless client wakes up and sees its bit set in the TIM, it issues a poll request to the AP for packets stored for it. The AP sends them to the wireless client and the wireless client goes back to sleep. A DTIM field, also called a countdown field, informs wireless clients of the next window for listening to broadcast and multicast messages. The AP sends the messages following the nth beacon where n is the DTIM interval defined in the AP. When the AP has buffered broadcast or multicast messages for associated wireless clients, it sends the next DTIM with a DTIM Interval value. This value decreases by '1' with each successive beacon. The AP sends broadcast and multicast messages immediately following the beacon where the DTIM value is '0.' To prevent a PSP-mode wireless client from sleeping through a DTIM notification, select a PSP mode value less than or equal to the DTIM value. PSP-mode wireless clients hear the beacons and awaken to receive the broadcast and multicast messages.