BlackBerry Wi-Fi access point roaming triggers and thresholds

Article ID: KB25119

Type: Support Content

Last Modified: 03-23-2012

 

Product(s) Affected:

  • BlackBerry Bold 9780
  • BlackBerry Bold 9700
  • BlackBerry Bold 9650
  • BlackBerry Bold 9000
  • BlackBerry Curve 9330 (3G)
  • BlackBerry Curve 9300 (3G)
  • BlackBerry Curve 8900
  • BlackBerry Curve 8500 Series
  • BlackBerry Curve 8300 Series
  • BlackBerry Pearl 9100 Series
  • BlackBerry Pearl 8200 Series
  • BlackBerry Pearl 8100 Series
  • BlackBerry Storm 9550
  • BlackBerry Storm 9530
  • BlackBerry Storm 9520
  • BlackBerry Storm 9500
  • BlackBerry Style 9670
  • BlackBerry Torch 9860
  • BlackBerry Torch 9850
  • BlackBerry Torch 9800
  • BlackBerry Tour 9630
CollapseEnvironment
  • BlackBerry® Bold™ 9780
  • BlackBerry® Bold™ 9700
  • BlackBerry® Bold™ 9650
  • BlackBerry® Bold™ 9000
  • BlackBerry® Curve™ 9330 (3G)
  • BlackBerry® Curve™ 9300 (3G)
  • BlackBerry® Curve™ 8900
  • BlackBerry® Curve™ 8500 Series
  • BlackBerry® Curve™ 8300 Series
  • BlackBerry® Pearl™ 9100 Series
  • BlackBerry® Pearl™ 8200 Series
  • BlackBerry® Pearl™ 8100 Series
  • BlackBerry® Storm™ 9550
  • BlackBerry® Storm™ 9530
  • BlackBerry® Storm™ 9520
  • BlackBerry® Storm™ 9500
  • BlackBerry® Style™ 9670
  • BlackBerry® Torch™ 9800
  • BlackBerry® Tour™ 9630
  • BlackBerry smartphones with 802.11 Wi-Fi capability (A, B, G  or N bands)
CollapseOverview
BlackBerry smartphone devices are often used in large distributed Wi-Fi® networks, where inter-access point device roaming is required to maintain a connection. As a user moves from one zone of coverage to another, the device will select and associate itself with alternate access points to maintain a continuous connection.

Proper roaming with well engineered Wi-Fi networks are critical with Voice over Wireless Lan (VoWLAN) style network applications, like the Mobile Voice System (MVS) Wi-Fi BlackBerry product.

Wireless network engineers may wish to further understand the roaming logic BlackBerry devices use, in further designing or troubleshooting their own networks interoperability with BlackBerry devices.
CollapseAdditional Information
The BlackBerry with its Wi-Fi roaming manager will scan periodically on a limited number of channels at a time building an internal list of candidate access points it can roam to when required.

The tracking scan data for sites already stored in the table are used to next select scan channels, allowing the device to scan channels similar to the following example pattern -  1, 2 and 3 in the first cycle, then 4, 5 and 6 in the next cycle, and so on.

This continuous scan above either takes place every 20 seconds if the current AP RSSI (Relative Signal Strength Indicator) level is above -63 dBm and will be more frequent at every 4 seconds if it is below. Beacons or probe responses received with a RSSI level below -87 dBm are discarded and not included in the devices internal scan data for those associated AP's.

A number of different triggers will cause the BlackBerry to attempt to roam to another known access point, which are listed below.


  • Low Quality trigger (set at -73 dBm) - In this case the candidates must be above the current AP by 4 dB; if for some reason the current AP signal is unknown then the candidates must be above the AP quality threshold set at -80 dBm

  • Maximum Transmission Retries - This trigger (set at 40 consecutive retries) activates once 40 attempted transmission attempts to the AP have failed without an 802.11 ACK received. The candidates in the devices scan inventory must be better than the low RSSI threshold as well (-87 dBm)

  • For any other trigger which is of a more immediate and disruptive nature any candidate in the table is eligible for roaming to:
    • Base Station Signal Loss - The Wi-Fi radio no longer senses an existing signal to maintain a connection with the AP
    • Access Point Disconnection Event - The access point has sent an 802.11 disconnection signal to the BlackBerry device

  • If more than one candidate is available the AP with the highest RSSI will be selected by the BlackBerry under most circumstances

NOTE:  Further selection rules apply depending on the category of the candidate. These include neighbor AP's, previously authenticated or regular candidates, AP Blacklisting features and other features. This KB is not intended as a complete scope of W-Fi Roaming logic within BlackBerry devices, but covers the most common factors in roaming logic used in device.


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