User Manual

www.moxa.com [e-mail]35
Understanding Industrial WLAN – IEEE 802.11
2.5 Mobile Optimization
In mobile applications that involve multiple access points (APs), the speed and roaming (handover) mechanism can
be crucial to a project’s success. In the world of wireless, roaming refers to when a client moves between two or
more access points. As a result, seamless connection is required for the client to roam from one AP to another.
Simply put, as the client physically moves from one AP to another, the signal from the first AP will drop while the
signal strength from the other AP will increase. By the time the signals from the first AP drop below the signals from
the second AP, the client will have roamed to the second AP.
There are a number of factors that will affect the smoothness of roaming. These include the topology of the access
points, gain and coverage of the antennas, and the roaming threshold settings on the client. To ensure smooth
roaming, we first have to take into consideration the routes of the moving object and carefully plan the wireless AP
deployment configuration.
Different applications encounter different roaming conditions, but we will use linear roaming as an example to
illustrate how roaming is conducted and what factors we need to pay special attention to.
Roaming Under Linear Movement
A client is moving from left to right across three different APs. As the client moves, the signal from the first
AP drops and the signal strength from the second AP increases. Most of the commercial wireless clients only
monitor the communication quality as the basis for roaming decisions. That is to say, when the signal from
first AP drops and frames can not be transmitted, the client, in an IEEE 802.11b application, will first drop
the communication speed from 11 Mbps to 5.5 Mbps and then to 2 Mbps and 1 Mbps. If the communication
quality is still poor and frame transmission continues to fail, the client will decide to roam from the first AP to
the second AP.
A roaming mechanism of this sort might be able to satisfy many non-critical applications. However, this type
of mechanism severely impairs the smoothness of data transmission for video or audio applications, which
require higher quality data transmission.
Limitations of High Speed Roaming
The first limitation of the Threshold-based Handover Algorithm is wireless resource consumption because the
client is constantly scanning for APs that also need to respond in due time. To solve this problem, the network
topology and system configuration needs to be carried out in greater detail.
The second limitation is that there is currently no unified high speed roaming standard so roaming among
different APs from different manufacturers might not be possible.