IEEE 802.11 Wireless LANs

IEEE Standard 802.11 specifies the physical and MAC layers for operation of
WLANs and addresses the direct sequence spread spectrum (DSSS) and
frequency-hopping spread spectrum (FHSS) access methods for the radio
medium. It also allows for a third option for the infrared medium, which is still
under development. The standard provides for data rates of 1 and 2 Mb/s—the
latter being optional. It specifies support for asynchronous as well as synchronous data transfers—again the latter being optional. The asynchronous data transfer applies to applications that are not time sensitive

The synchronous mode supports time-bounded applications like
video and packetized voice. The two main architectures supported by IEEE
802.11 standard are illustrated

In the independent BSS (basic service set), the LAN stations (STAs) communicate with each other without any supporting infrastructure, and the
topology is useful for such applications as file sharing in a limited geographic
area (like a conference room). The second topology makes use of an access
point (AP), which allows the LAN stations in a BSS to connect to a backbone
local area network (wireline or wireless LAN) or distribution system (DS),
and to extend its reach beyond the range of the BSS. The stations within the
BSS can still communicate with each other directly without involving the AP,
but a station cannot use another station in the BSS to route packets to the AP.

Physical Layer.

 For the direct sequence spread spectrumscheme in IEEE 802.11, the original data signal is modulated by means of a single predefined spreading code, (unlike multiple codes in the IS-95 CDMA cellular system). Knowing the spreading code, the receiver can recover the original signal by despreading the received signal. The processing gain (bandwidtH)of the spread spectrum signal) of 11 (10.4 dB) provides adequate protection against narrow-band interference and allows the available frequency band (83.5 MHz in the United States) to be segmented into a number of direct
sequence center frequencies. IEEE 802.11 specifies 11 such frequencies.

MAC Layer.

 The MAC layer is responsible for providing functions
like channel allocation and access procedures, protocol data unit addressing,
frame formatting, error checking, and packet segmentation and reassembly.
The IEEE 802.11 MAC layer protocol is designed as a flexible
protocol that can support the range of potential operating environment
choices in terms of:
• physical layers (direct sequence or frequency-hopping spread spectrum)
• service types (asynchronous, time-bounded, contention-free)
• network topologies (access point, independent BSS)
• power management (with infrastructure, without infrastructure)