Factory Line - Wireless LAN
1-12
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7190_en_02
Infrastructure mode and especially the use of ESS enables the operation of large networks
according to IEEE 802.11 and is also known as Wireless Ethernet.
Figure 1-8
Extended Service Set – Wireless Ethernet
1.3.3
Roaming
Roaming was defined in IEEE 802.11r to enable the transfer of a communication connection
from one access point to another. The following sequence was specified for roaming:
–
If a device determines that the signal from an access point is too weak, it searches for
an access point with a stronger signal (scanning). A distinction is made between active
scanning and passive scanning:
Passive scanning: Tapping the medium to determine whether another access point is
available.
Active scanning: Transmitting a request to all channels. If a response is transmitted
from an access point, it contains all the information required to transfer the
communication.
–
The device uses the signal strength to select the most suitable access point and sends
it an association request, which requests that it takes over the communication.
–
If the response to the association request is positive, roaming was successful. If not, the
sequence starts again.
Please note that in applications such as that shown in Figure 1-8 all access points must
have the same SSID.
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