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400 From GSM to LTE-Advanced Pro and 5G

Overlapping area in which both
access points can be received
Access point

6 + 11 11 + 1 1 + 6

… Channel 6 Channel 1
Channel 11 Channel 6 ….

Figure 6.5 Overlapping coverage of access points forming an ESS.

APs close to each other. To be able to keep the three APs at least five channels apart
from each other, channels 1, 6 and 11 should be used.
In practice, channels 12 and 13 are only allowed for use in Europe. Unfortunately,
some WLAN drivers do not ask during software installation about the country in which
the device is going to be used and block these channels by default. If it is unclear during
the installation of a new AP as to which devices will be used in the network, channels 12
or 13 should not be selected, to enable all client devices to communicate with the AP.
802.11a and 11n systems use the spectrum in the 5 GHz range in Europe, between
5.170 and 5.350 GHz, and between 5.470 and 5.725 GHz for data transmission. In this
455 MHz bandwidth, 18 independent networks can be operated. This is quite signifi-
cant, especially when compared to the three independent networks that can be oper-
ated in the 2.4 GHz band.
On a client device, the basic configuration for joining a BSS or ESS network is usually
straightforward. To join a new network, the device automatically searches for active
APs on all possible frequencies and presents the SSIDs it has discovered to the user as
shown in Figure 6.6. The user can then select the desired SSID of the network to join.
Selecting a channel is not necessary, as the client device will always scan all channels for
the configured SSID during power up. If more than one AP is found with the same SSID
during the network search procedure, the client device assumes that they belong to the
same ESS. If the user wants to join such a network, the device then selects the AP on the
channel on which the beacon frames are received with the highest signal strength.
Further details about this process can be found in the Section 6.4.
In addition to selecting the SSID, activating encryption for the air interface is the
second important step while setting up a BSS or an ESS. Fortunately, fewer and fewer
APs are shipped with encryption disabled.

6.4 Management Operations

In a wired Ethernet, it is usually sufficient to connect the client device via cable to the
nearest hub or switch to get access to the network. Physically connecting a wireless
device to a WLAN network is of course not possible, as there is no cable. Also, a WLAN
device has the ability to automatically roam between different APs of an ESS and is able