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Client Roaming Decision
IEEE 802.11 clients typically decide to roam when the connection to the current AP becomes degraded. Roaming necessarily has some impact on client traffic because a client scans other IEEE 802.11 channels for alternative APs, reassociates, and authenticates to the new AP. Prior to roaming, a client may take some actions to improve its current connection without necessitating a roam:
Data retries—The IEEE 802.11 MAC specifies a reliable transport. Every unicast frame sent between a wireless client and an AP is acknowledged at the MAC layer. The IEEE 802.11 standard specifies the protocol used to retry the transmission of data frames for which an acknowledgment was not successfully received.
Data rate shifting—IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g each support a variety of possible data rates. The data rates supported for a given frequency band (such as 2.4GHz or 5GHZ) are configured on the WCS/WLC and are pushed down to the APs using that frequency band. Each AP in a given WLAN then advertises the supported data rates in its beacons. When a client or AP detects that a wireless connection is becoming degraded, it can change to a lower supported transmission rate (lower transmission rates generally provide superior transmission reliability).
Although the roaming algorithms differ for each vendor or driver version (and potentially for different device-types from a single vendor), there are some common situations that typically cause a roam to occur:
Maximum data retry count is exceeded—Excessive numbers of data retries are a common roam trigger.
Low received signal strength indicator (RSSI)—A client device can decide to roam when the receive signal strength drops below a threshold. This roam trigger does not require active client traffic in order to induce a roam.
Low signal to noise ratio (SNR)—A client device can decide to roam when the difference between the receive signal strength and the noise floor drops below a threshold. This roam trigger does not require active client traffic in order to induce a roam.
Proprietary load balancing schemes—Some wireless implementations have schemes where clients roam in order to more evenly balance client traffic across multiple APs. This is one case where the roam may be triggered by a decision in the WLAN infrastructure and communicated to the client via vendor-specific protocols.This section reviews the events that cause a client to roam. The roaming process is described in the Fast Secure Roaming section. Roaming is always initiated by the client, and is not defined by IEEE standards. For Cisco clients, roaming is caused by one of the following events.
Maximum data retry count is exceeded
Missed too many beacons
Datarate shift
Initial startup
Periodic client interval (if configured)