Page 413 - From GMS to LTE

Wireless Local Area Network (WLAN) 399

can be used. In practice, this limits the maximum coverage area substantially because
IP subnets are only suitable for covering a very limited area, like a building or several
floors.
All APs have to use the same BSS service ID, also called an ‘SSID’. More about SSIDs
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can be found in Section 6.3.2.
The APs have to transmit on different frequencies and should stick to a certain
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frequency repetition pattern, as shown in Figure 6.5.
Many APs use a proprietary protocol to exchange user information with each other if
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the client device switches to a new AP. Therefore, all APs of an ESS should be from
the same manufacturer. To allow the use of APs of different manufacturers, the IEEE
released the 802.11f standard (Recommended Practice for Multi‐Vendor Access Point
Interoperability) at the beginning of 2003. However, this standard is optional and by
no means binding for manufacturers.
The coverage area of the different APs should overlap to some extent so that client
●
devices do not lose coverage in border areas. As different APs send on different fre-
quencies, the overlapping poses no problem.

Another WLAN mode is wireless bridging, sometimes also referred to as a wireless
distribution system. In this mode, the APs of an ESS can wirelessly forward packets they
have received from client devices between each other. In practice, this mode is used if
only one connection to the wired network exists but a single AP is unable to cover the
desired area on its own. Usually, a wireless bridging AP also supports simultaneous BSS
functionality. Therefore, only a single AP is required to offer service at a certain location
to users and to backhaul the packets to the AP connected to the Internet.

6.3.2 SSID and Frequency Selection
When an AP is configured for the first time, there are two basic parameters that should
be set.
The first parameter is the basic SSID. The SSID is periodically broadcast over the air
interface by the AP inside beacon frames, which are further discussed in Section 6.4.
Note that the 802.11 standard uses the term ‘frame’ synonymously for ‘packet’ and this
chapter also makes frequent use of it. The SSID identifies the AP and allows the opera-
tion of several APs at the same location for access to different networks. Such a
configuration of independent APs should not be confused with an ESS, in which all APs
work together and have the same SSID. Usually, the SSID is a text string in a human
readable form because during the configuration of the client device the user has to
select an SSID if several are found. Many configuration programs on client devices also
refer to the SSID as the ‘network name’.
The second parameter is the frequency or channel number. It should be set carefully
if several APs have to coexist in the same area. The ISM band in the 2.4 GHz range uses
frequencies from 2.410 to 2.483 MHz. Depending on national regulations, this range is
divided into a maximum of 11 (United States) to 13 (Europe) channels of 5 MHz each.
As a WLAN channel requires a bandwidth of 25 MHz, different APs at close range
should be separated by five ISM channels. As can be seen in Figure 6.5, three infrastruc-
ture BSS networks can be supported in the same area or a single ESS with overlapping
areas of three APs. For infrastructure BSS networks, the overlapping is usually not
desired but cannot be prevented when different companies or home users operate their