User`s guide

Aironet Wireless Communications, Inc. 3-2 Confidential and Proprietary
The MAC entity provides the basic access mechanisms to the RF medium and is responsible for
functions such as encryption and fragmentation.
The MAC layer management is responsible for the synchronization functions, power management and
roaming coordination.
The PHY layer management is primarily responsible for channel tuning and coordination of the radio
functions.
The PLCP sublayer is responsible for clear channel assessment of the RF channel.
The PMD sublayer is responsible for the modulation and coding of the radio signals.
The station management entity interacts with both the PHY management and the MAC management to
coordinate the desired overall station operation.
As identified in Chapter 1, the 802.11 specification supports three basic network topologies: the
Independent Basic Service Set (IBSS), the Basic Service Set (BSS) and the Extended Service Set (ESS).
• The IBSS accomplishes peer to peer communication in which all stations desiring to communicate
must establish a direct one to one connection.
• The BSS configuration uses an access point to provide communication coordination among
multiple stations. The BSS typically has allows for a larger range than an IBSS as all parties are
not required to be in direct communication with each other but are required to be in range of the
access point.
• An ESS configuration is used to extend the wireless coverage area and consists of multiple BSS
cells. Each cell can be linked via a wired backbone or the ESS can consist entirely of wireless
communication paths.
IEEE 802.11 specifies the access point (AP) functionality required to coordinate the wireless station
communication, however, it does not specify the functionality required to perform bridging functions
(between a wired and the wireless network) or what to do with stored messages when a mobile client
roams from one BSS to another. Aironet provides a consistent bridging procedure as well as message
forwarding for power save and mobile stations.
There are many uncertainties which exist due to the nature of wireless communications such as
interference and noise, hidden node problems and variations in the wireless link reliability. The 802.11
protocol provides several mechanisms to overcome these issues:
• Each frame is protected by a CRC to ensure data integrity.
• MAC layer acknowledge frames are used to indicate the successful delivery of a frame.
• RTS/CTS reservation can be used to ‘clear the way’ for a particular data exchange.
• Collision avoidance mechanisms are used to randomize subsequent delivery attempts of a failed
frame.
IEEE 802.11 also provides a framework for power save operation which allows for a device to disable
the radio functionality in order to conserve resources. Aironet provides several enhancements by which a
user can establish the performance of power save operation by adjusting several competing parameters.
The 802.11 protocol also defines a mechanism through which the wireless transmissions can be
encrypted as well as specifying an authentication procedure in order to protect against unauthorized
network access. Aironet provides a higher level of encryption than what is specified by 802.11 and also
provides end to end encryption functionality by offering a means for exchanging encryption keys.