Wi-Fi Roaming - What You Need to Know to Save

by Parks Associates | Jul. 31, 2012

Roaming occurs when a wireless client device moves outside the usable range of one wireless access point and connects to another AP. There is no “roaming” issue as far as the AP is concerned. As long as the APs are setup properly, client devices can roam seamlessly from one AP to another. The issue and challenge comes in the actual “handoff ” process, which, according to the IEEE 802.11 standard [1], is dictated by the client device. The handoff is the process of the client device disconnecting from one AP and then re-associating with another. This process consists of 3 phases: 1) Scanning, as the device moves away from the AP to which it is connected and the RSSI values begin to drop below certain levels, the client device sends out probe packets to identify AP alternatives. Upon discovery of accessible APs, the device then selects its next AP based on certain criteria, as defined by the device itself; 2) Authentication, uring this phase, the client device sends an authentication request to the new AP and waits for a response from the AP to approve or reject the request; and 3) Re-association, upon approval by the new AP, the client sends a re-association request and waits for a response. Once the re-association is complete, the new AP sends out a disassociation packet to the old AP so that the routing tables can be updated. The handoff process is now complete.

What are the technology alternatives for minimizing handoff delays? Given that roaming is a client dependent function, how can uninterrupted Wi-Fi service be achieved? Under current 802.11 standards, the reliable method is to keep devices from executing a handoff. What are the alternatives for minimizing handoff delays? There are various approaches and technologies on the market: 1) Non-Controller Multiple AP Approach, installers have traditionally used multiple APs set to the same SSID and security level; 2) Configuration Controllers, used in conjunction with multiple vendor-specific low powered APs and typically does little more than optimize the AP setup by pushing the settings out to the APs on the network and ensuring they are all set to the same SSID and security levels; 3) Management Controllers, to optimize the handoff process between APs. While the handoff process may become somewhat faster, the client must still determine at what point to move from one AP to another, which typically takes more time than the handoff process itself; and 4) Wireless Network Virtualizatio, while still employing the use of a controller, this method is significantly more sophisticated than the aforementioned controller solutions. In this scenario, the controller actively monitors and “listens” to all APs on the network, selecting the best one for transmitting data to the client. Roaming is eliminated because the client sees all APs as a single AP.

What is the right solution? Find out more.

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