E-DOC-CTC-20060609-0001 v2.0
Chapter 1
Introducing Wireless Networking
8
Beacon frame
Beacon frames are transmitted periodically by APs to let wireless stations identify the wireless APs nearby.
They inform the wireless stations in the BSSs (and thus a possible ESS) about the existence of a wireless
network.
Beacon frames are transmitted on all channels (in the regulatory allowed spectrum) and contain among other
things the BSSID, the SSID and a set of capabilities, e.g. the supported data rate, the supported security
mechanism,...
Standards overview
The 802.11 standard defines a set of different physical layer technologies to be used in combination with
802.11 MAC. The standard has evolved over the years. The different technologies primarily vary in frequency
bands and applied modulation techniques (resulting in different transmission rates). A short overview:
> 802.11
The first standard was released in 1997. It operated at a data transmission rate of 1 or 2 Mbps, which was
much too slow for most applications, and was transmitted at 2.4 GHz.
This standard is now often referred to as 802.11 legacy.
> 802.11b
This standard was ratified in 1999. It uses the same frequency band as the original 802.11, but uses a
different modulation technique, so that a transmission rate of 11 Mbps is achieved.
> 802.11a
At the same time that 802.11b was ratified, 802.11a was ratified.This standard uses the 5 GHz band and
has a data transmission rate up to 54 Mbps.
> 802.11g
This standard was ratified in 2003. 802.11g is backward compatible with 802.11b and also operates in the
2.4 GHz band. Because of the use of a different modulation technique, data transmission rate can go up to
54 Mbps.
For further details on each of these standards, please refer to “2 802.11 Standards” on page 11.
The following list contains an exhaustive overview of all existing 802.11 standards:
Standard Description
802.11 Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: the
original 1 Mbps and 2 Mbps, 2.4 GHz RF and IR standard (1997).
802.11a High-speed Physical Layer in the 5 GHz band: 54 Mbps, 5 GHz standard (1999).
802.11b Higher speed Physical Layer extension in the 2.4 GHz band: enhancements to 802.11 to
support 5.5 and 11 Mbps (1999).
802.11c Bridge operation procedures; included in the IEEE 802.1D standard (2001).
802.11d Specification for Operation in Additional Regulatory Domains (2001).
802.11e Enhancements: QoS, including packet bursting (2005).
802.11F Recommended Practice for Multi-Vendor Access Point Interoperability via an Inter-Access
Point Protocol Across Distribution Systems Supporting IEEE 802.11 (2003). Withdrawn in
February 2006.
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