WAN Basics Tsunami MP'11 QB'11 QuickBridge 60250 Tsunami MP'11 Roaming

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WAN Basics Tsunami MP.11 - QB.11 QuickBridge
60250 Tsunami MP.11 Roaming
April 2008
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Definition

• Roaming is the process in which an SU terminates
  the session with the current BSU and starts the
  registration procedure with another BSU when it
  finds the quality of the other BSU to be better.
  
• Advantages
• Mobility
• Redundancy
• Roaming requires the new BSUs to be configured
  with the same
• Network name
• Shared secret
• Encryption Key

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Roaming How it works

• When Roaming is enabled, BSU can shorten the
  announcement period.
• Shortening the announcement period speeds up
  roaming to enable faster handoff
• When Roaming is enabled, SU monitors the local
  SNR of every received frame
• The SU will not scan other channels for a better
  BSU as long as
• Average local SNR gt Slow Scanning Threshold and
  
• Number of retransmitted frames lt Slow Scanning
• Average calculated over the last 128 WORP frame
  and checked every 200 ms
• All of the above are configurable
• In order to speed up the roaming process, the
  list of channels to be scanned can be reduced
  and prioritized.
• Each BSU propagates the scanning list and order
  to its associated SU

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Roaming SU Operation mode

• Normal Operation mode
• SU exchange data with its associated BSU
• Slow Scanning mode
• SU scans one channel for a maximum 100 ms
• If it hears a BSU announcement
• Then check if new BSU SNR gt old BSU SNR roam
  threshold
• SU roams to the new BSU
• Else increase channel counter and resume normal
  operation
• Fast Scanning mode
• SU scans one channel for a maximum 100 ms
• If it hears a BSU announcement
• Then check if new BSU SNR gt old BSU SNR roam
  threshold
• SU roam to the new BSU
• Else increase channel counter and start the
  scanning process again
• Emergency mode
• SU hasn't heard from the BSU for 1s, it scans
  every channel for 100 ms
• As soon as SU hears a BSU announcement it roams

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Roaming Forwarding Tables

• When SU roams from BSU1 to BSU2
• BSU2 sends a who has this SU request
• BSU1 replies by sending the entries on Port 2
  (WORP) of its forwarding table
• BSU2 then populates Port 2 of its own forwarding
  table with these entries
• At the moment when SU gets registered with BSU2,
  forwarding table in BSU2 is correct and traffic
  flows for both the SU and all its attached
  devices
• In Emergency Roaming (when BSU1 dies), BSU2 can
  get the forwarding table directly from the SU

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High Speed Roaming optimization

• For High Speed Roaming several parameters need to
  be tuned
• Fixed modulation only (DDRS disabled)
• BSU announcement period set to 30 ms
• BSU only announces channel for neighboring BSU
• SU optimized for slow roaming
• Slow SNR threshold 20 dB
• Fast SNR threshold 10 dB
• Slow retransmission threshold 10
• Fast retransmission threshold 50
• SU only goes for much stronger BSU
• Roaming threshold 6 dB
• Spacing between BSU depend on link budget
• Throughput to achieve (modulation)
• Antenna gain

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How to enable Roaming

• Roaming must be enabled at both the BSU and SU in
  order to work properly

From 1 to 100 msec Same value is optimum
Active High Active Inactive