Page 410 - From GMS to LTE

396 From GSM to LTE-Advanced Pro and 5G

As many companies produce WLAN products today, interoperability between them
is of paramount importance. This is why the Wi‐Fi Alliance was founded in 1999, by a
number of companies manufacturing 802.11‐compatible devices. Being a non‐profit
organization its aim is to ensure WLAN product interoperability with a testing and
certification program. Today, hundreds of companies have joined the Wi‐Fi Alliance
and use the Wi‐Fi Alliance certification program to validate their products and to obtain
the ‘Wi‐Fi certified’ marketing logo for their device. This is also why 802.11‐based
WLAN is often also referred to as Wi‐Fi.

6.3 WLAN Configurations: From Ad Hoc to Wireless
Bridging

All devices that use the same transmission channel to exchange data with each other
form a basic service set (BSS). The definition of the BSS also includes the geographical
area covered by the network. There are a number of different BSS operating modes.

6.3.1 Ad Hoc, BSS, ESS and Wireless Bridging
In ad hoc mode, also referred to as Independent Basic Service Set (IBSS), two or more
wireless devices communicate with each other directly. Every station is equal in the
system and data is exchanged directly between two devices. The ad hoc mode therefore
works just like a standard wireline Ethernet, where all devices are equal and where data
packets are exchanged directly between two devices. As all devices share the same
transport medium (the airwaves), the packets are received by all stations that observe
the channel. However, all stations except the intended recipient discard the incoming
packets because the destination address is not equal to their hardware address. All par-
ticipants in an ad hoc network have to configure a number of parameters before they
can join the network. The most important parameter is the service set identity (SSID),
which serves as the network name. Furthermore, all users have to select the same fre-
quency channel number (some implementations select a channel automatically) and
ciphering key. While it is possible to use an ad hoc network without ciphering, it poses
a great security risk and is therefore not advisable. Finally, an individual IP address has
to be configured in every device, which the participants in the network have to agree on.
Owing to the number of different parameters that have to be set manually, WLAN ad
hoc networks are not very common.
One of the main applications of a WLAN network is access to a local network and the
Internet. For this purpose, the infrastructure BSS mode is much more suitable than the
previously described ad hoc mode. In contrast to an ad hoc network, it uses an access
point (AP), which takes a central role in the network, as shown in Figure 6.2.
The AP can be used as a gateway between the wireless and the wireline networks for
all devices of the BSS. Furthermore, devices in an infrastructure BSS do not communi-
cate directly with each other. Instead, they always use the AP as a relay. If device A, for
example, wants to send a data packet to device B, the packet is first sent to the AP. The
AP analyzes the destination address of the packet and then forwards the packet to
device B. In this way, it is possible to reach devices in the wireless and wireline networks
without knowledge of where the client device is. The second advantage of using the AP