The mechanism to determine when to roam is not defined by the IEEE 802.11 specification and is, therefore, left to vendors to implement. Although this issue posed an interoperability challenge early on with the first 802.11 products, vendors work together today to ensure basic interoperability. The fact that the algorithms are left to vendor implementation provide vendors an opportunity to differentiate themselves by creating new and better performing algorithms than their competitors. Roaming algorithms become a vendor's "secret sauce," and as a result are kept confidential.
Determining Where to Roam
Finding an AP to roam to is another mechanism that is vendor-specific. In general, there are
two mechanisms for finding APs:
- Preemptive AP discovery
- Roam-time AP discovery
Each mechanism can employ one or both of the following mechanisms:
- Active scanning— The client actively searches for an AP. This process usually involves the client sending probe requests on each channel it is configured to use (channels 1 to 11 in North America) and waiting for probe responses from APs. The client then determines which AP is the ideal one to roam to.
- Passive scanning— The client does not transmit any frames but rather listens for beacon frames on each channel. The client continues to change channels at a set interval, just as with active scanning, but the client does not send probe requests.
With passive scanning, the client iterates through the channels slower than active scanning because it is listening for beacons that are sent out by APs at a set rate (usually 10 beacons per second). The client must dwell on each channel for a longer time duration to make sure it receives beacons from as many APs as possible for the given channel. The client looks for different information elements such as SSID, supported rates, and vendor proprietary elements to find an AP. Although it can be a faster mechanism to scan the medium, some elements are not transmitted, depending on AP configuration. For example, an administrator might block the SSID name in the SSID IE from being transmitted in beacons, so the passive scanning client is unable to determine whether the AP is in the same roaming domain.
There is no ideal technique for scanning. Passive scanning has the benefit of not requiring the client to transmit probe requests but runs the risk of potentially missing an AP because it might not receive a beacon during the scanning duration. Active scanning has the benefit of actively seeking out APs to associate to but requires the client to actively transmit probes. Depending on the implementation for the 802.11 client, one might be better suited than the other. For example, many embedded systems use passive scanning as the preferred method, whereas 802.11 Voice over IP (VoIP) phones and PC client cards rely on active scanning.
Preemptive AP Discovery
Preemptive AP Discovery is the function that provides the client the ability to roam to a predetermined AP after the client has made the decision to roam. This process allows for minimal total roaming time, which reduces application impact from roaming. Preemptive roaming does not come without a penalty, however.
Figure 5-3. Preemptive AP Discovery
Roam-Time AP Discovery
The other option for AP discovery is to look for an AP after the decision to roam has been made. This process is similar to the process a client goes through on initiation power up, except that the association message the client sends to the new AP is actually a reassociation frame.
Roam-time AP discovery does not have the overhead of preemptive roaming during on-roaming times, but because the client does not know which AP to reassociate to, there ca be a larger time penalty during the roaming process. Figure 5-4 shows roam-time AP discovery.
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