The 802.11 standard states that each conformant wireless LAN must provide nine services.
These services are divided into two categories: five distribution services and four station
services. The distribution services relate to managing cell membership and interacting with
stations outside the cell. In contrast, the station services relate to activity within a single cell.
The five distribution services are provided by the base stations and deal with station mobility as they enter and leave cells, attaching themselves to and detaching themselves from base stations. They are as follows.
1. Association. This service is used by mobile stations to connect themselves to base
stations. Typically, it is used just after a station moves within the radio range of the
base station. Upon arrival, it announces its identity and capabilities. The capabilities
include the data rates supported, need for PCF services (i.e., polling), and power
management requirements. The base station may accept or reject the mobile station. If the mobile station is accepted, it must then authenticate itself.
2. Disassociation. Either the station or the base station may disassociate, thus breaking
the relationship. A station should use this service before shutting down or leaving, but
the base station may also use it before going down for maintenance.
3. Reassociation. A station may change its preferred base station using this service. This
facility is useful for mobile stations moving from one cell to another. If it is used
correctly, no data will be lost as a consequence of the handover. (But 802.11, like
Ethernet, is just a best-efforts service.)
4. Distribution. This service determines how to route frames sent to the base station. If
the destination is local to the base station, the frames can be sent out directly over the
air. Otherwise, they will have to be forwarded over the wired network.
5. Integration. If a frame needs to be sent through a non-802.11 network with a
different addressing scheme or frame format, this service handles the translation from the 802.11 format to the format required by the destination network.
The remaining four services are intracell (i.e., relate to actions within a single cell). They are used after association has taken place and are as follows.
1. Authentication. Because wireless communication can easily be sent or received by
unauthorized stations, a station must authenticate itself before it is permitted to send
data. After a mobile station has been associated by the base station (i.e., accepted into
its cell), the base station sends a special challenge frame to it to see if the mobile
station knows the secret key (password) that has been assigned to it. It proves its
knowledge of the secret key by encrypting the challenge frame and sending it back to
the base station. If the result is correct, the mobile is fully enrolled in the cell. In the
initial standard, the base station does not have to prove its identity to the mobile
station, but work to repair this defect in the standard is underway.
2. Deauthentication. When a previously authenticated station wants to leave the
network, it is deauthenticated. After deauthentication, it may no longer use the
network.
3. Privacy. For information sent over a wireless LAN to be kept confidential, it must be
encrypted. This service manages the encryption and decryption. The encryption
algorithm specified is RC4, invented by Ronald Rivest of M.I.T.
4. Data delivery. Finally, data transmission is what it is all about, so 802.11 naturally
provides a way to transmit and receive data. Since 802.11 is modeled on Ethernet and transmission over Ethernet is not guaranteed to be 100% reliable, transmission over 802.11 is not guaranteed to be reliable either. Higher layers must deal with detecting and correcting errors.
An 802.11 cell has some parameters that can be inspected and, in some cases, adjusted. They relate to encryption, timeout intervals, data rates, beacon frequency, and so on.
Wireless LANs based on 802.11 are starting to be deployed in office buildings, airports, hotels,
restaurants, and campuses around the world. Rapid growth is expected. For some experience about the widespread deployment of 802.11 at CMU, see (Hills, 2001).
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