Interpretation IEEE802.11 Standard Article Source: CCID
Chu mad (2000-12-21)
IEEE802.11 is one of the first generation wireless local area network standards. This standard defines the specification of the Physical Layer and Media Access Control (MAC) protocol, allowing wireless LAN and wireless device manufacturers to establish interoperability network devices within a certain range. IEEE 802.11 Working Committee sets the following basic requirements for the business and application environment, functional conditions of the wireless LAN: 1 to 20 mbit / s, including 1 to 20 mbit / s, including the components provided by the wireless media; fixed or available within the local area Mobile station; free communication between arbitrary two stops; meets the functional conditions of 802.1, 802.2 and 802.10; packet loss rate ≤ 4 × 10-5. Compared to 802.3, 802.11 has the next new feature: the signal characteristics and modulation methods of data transmission are defined in the physical layer, and two radio frequency (RF) transmission methods and an infrared transmission method are defined. The RF transmission standard is direct sequence spread spectrum (DSSS) and frequency hopping spread (FHSS). Since conflict detection is difficult in wireless networks, the Media Access Control (MAC) layer adopts the Avoiding Conflict (CA) protocol, not conflict detection (CD), but can only reduce conflicts. 802.11 Physical Layer Wireless Media (WM) determines that it is different from the MAC of the existing wired local area network, which has a unique media access control mechanism to share wireless media in CSMA / CA. Mac structure and service content
Basic Services (BSS) is the basic unit of WLAN, which features distributed coordination (DCF) and point coordination function (PCF). Coordination function is a station that determines working within BSS, when the WM is allowed to send and possibly receive the logical function of the protocol unit. DCF is the basic media access method of 802.11MAC protocol, which acts on basic service groups and basic network structures, which can be implemented in all stations, which supports competitive asynchronous services. PCF supports no competitive time limit business and no competitive asynchronous business. The basic structure is as shown in the figure above. Wireless LAN MAC provides services with: secure services, MSDU reordering services and data services. The services provided by the security services in 802.11 are limited to data exchange between station and stations. The content is: encryption, verification, access control associated with the layer management entity. The MAC structure provides the possibility of successful delivery, and the Mac provides reordering services. Only the station under power saving mode and is not in the active state, before you first activate the station, then burst out, reordering the cache data. The MAC data service enables the peer LLC entity to exchange data units. Local MAC uses the underlying service to pass an MSDU to a peer-to-peer MAC entity, and then transmit it to the peer LLC entity. When channel characteristics limit the reliability of long frame transfer, the reliability can be increased by increasing the possibility of successful transmission of MSDUs. CSMA / CA protocol
In order to minimize data transmission collision and retry transmission, prevent each site from disorderly contention channels, and the wireless LAN CSMA / CA (carrier monitor multi-channel access / conflict prevention) protocol is used in the wireless LAN CSMA / CD. . The CSMA / CA communication method is closely linked to the frame format to ensure that only one site is sent at a certain time, and the centralized control of the network system is implemented. Due to the different transmission media, CSMA / CD and CSMA / CA have different methods. The CSMA / CD detects by a change in the voltage in the cable. When the data collides, the voltage in the cable will change, while CSMA / CA uses energy detection (ED), carrier detection (CS), and energy carrier mixing detection. Three types of detection channels are free.
