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DTUS065 rev A.7 – June 27, 2014
V.7
Fast roaming features
In order to keep network connectivity when a client product is installed in a
quickly moving vehicle, you can adjust some configuration parameters.
V.7.1
Mono-channel vs. multichannel roaming
The WLn client can either look for APs on one channel only, or it can scan
several channels. Each way has its pro’s and con’s.
Mono-channel
All the APs compete for the air media, so that the available bandwidth is
reduced for all clients and APs. But the client is informed of the APs
presence and condition at all times, and can communicate with its current
AP at all times. Also, if one of the APs is near a source of interference on
the selected channel, all APs must be switched to another channel.
Multi-channel
You can arrange for APs which are in radio range of each other to use
different channels. In this way they will not compete for air bandwidth. You
should not choose channels which are too close to each other, since they
might interfere.
The WLn client must scan each chosen channel in its turn. For this it must
go “off-channel” for a small time, leaving the channel of its currently
associated AP; during this time it cannot exchange data. The data is then
buffered under certain limits. This reduces data throughput for the client.
Configuration
After activating the proactive roaming feature, you must adjust the list of
channels scanned by the WLn client. You can select one or several channels.
If proactive roaming is not activated, all channels allowed in the country are
scanned; this maximizes the chance of finding a matching AP, but slows
down data transfers.
V.7.2
Proactive roaming vs. reactive roaming
Reactive
Reactive roaming takes place when the client can no more communicate
with its AP. When too many failures take place, the client disconnects from
its current AP and begins to search a new one. Reactive roaming is the
default mode, because there is nothing to configure in this case. In this
mode, channel scanning; also called “foreground scan”, does never take
place during data transfers, leaving all the bandwidth available for data
transfers. But the roaming process is slow (it must wait for the end of the
scan) and data cannot be transmitted during this time. Whenever a WLn
client cannot associate to any AP, it enters reactive roaming.
Proactive
Proactive roaming means that the client will search, select and switch to
another AP before signal level is so low that a lot of errors can happen. By
selecting appropriate parameters, the change from one AP to another will
take place before data throughput is affected, and the reassociation process
will be quick if the new AP is in sufficient radio range. Hence few data (if
any) will be lost.