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4 AirPcap User’s Guide
A Brief Introduction to 802.11
Terminology
The terms Wireless LAN or WLAN are used to indicate a wireless local
area network, e.g. a network between two or more “stations” that uses
radio frequencies instead of wires for the communication.
All components that can “connect” to a WLAN are referred to as stations.
Stations fall into one of two categories: access points or wireless clients.
Access points transmit and receive information to/from stations using
radio frequencies. As we shall see later, the particular choice of a radio
frequency determines a wireless “channel.” An access point usually acts as
a “gateway” between a wired network and a wireless network.
Wireless clients can be mobile devices such as laptops, personal digital
assistants (PDAs), IP phones or fixed devices such as desktops and
workstations that are equipped with a wireless network interface card.
In some configurations, wireless devices can communicate directly with
each other, without the intermediation of an access point. This kind of
network configuration is called peer-to-peer or ad-hoc.
A Basic Service Set (BSS) is the basic building block of a WLAN. The
“coverage” of one access point is called a BSS. The access point acts as
the master to control the stations within that BSS. A BSS can be thought
of as the wireless version of an IP subnet. Every BSS has an id called the
BSSID, which is the MAC address of the access point servicing the BSS,
and a text identifier called the SSID.
802.11 Standards
802.11 is a standard that defines the physical layer and the data-link layer
for communication among wireless devices. The original 802.11
specification was ratified in 1997, uses the 2.4 GHz frequency band, and
allows transmission rates of 1 or 2 Mbps.
802.11a, ratified in 1999, is an extension of 802.11 that operates at 5 GHz.
It supports 8 additional transmission rates: 6, 9, 12, 18, 24, 36, 48 and 54
Mbps.
802.11b, ratified in 1999, is an extension of 802.11 that uses the same 2.4
GHz frequency band, and supports two additional transmission rates: 5.5
and 11 Mbps.
802.11g, ratified in 2003, is backward compatible with 802.11b, and
supports the same additional transmission rates found in 802.11a: 6, 9, 12,