Wireless LAN protocol (802.11b) detailed
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Cheap laptop, mobile phones, and hand-held devices, and the rapid development of Internet applications and e-commerce, users need to interconnect online. To meet these needs, two methods can be used to connect portable devices to the network without the inconvenience caused by the cable. These two standards are IEEE 802.11b and Bluetooth. IEEE802.11b is a 11MB / s wireless standard that provides full network services for laptops or desktop users.
IEEE802.11b characteristics and applications
Speed 2.4GHz direct sequence spread spectrum, the maximum data transmission rate is 11Mb / s, no linear propagation. Dynamic rate conversion When the RF condition is deteriorated, the data transfer rate can be reduced to 5.5 Mb / s, 2Mb / s and 1 Mb / s. The range supported by the range is 300 meters outdoors, and in the office environment is 100 meters. Reliability Use connection protocols and packets similar to Ethernet to provide efficient use of reliable data transfer and network bandwidth. The interoperability only allows a standard signal transmission technology, and WECA will authenticate the interoperability of the product. The power management network interface card can go to sleep mode, and the access point bursts the information to the customer, extending the battery life of the laptop. Roaming Support When the user moves between the building or company departments, seamlessly is allowed to be seamless between the access points. Load balance NIC changes to access points to improve performance. The maximum scalability can be positioned simultaneously in the effective use range to support hundreds of users. Safety built-in identification and encryption. IEEE802.11b application range:
The area that is not easy to wiring provides network services in areas that are not easy to wiring or wiring cost; the flexible working group reduces total cost of ownership for the workspace that often changes network configuration; networking room users can be from a conference room Move to another meeting room for network connection to get the latest information, and can communicate with each other during decision; fast installation and compatibility software for special network live consultants and small working groups can improve work efficiency; subsidiary network is remote or The sales office provides a network that is easy to install, used and maintained; the department-wide network mobile roaming function allows companies to establish easy-to-use wireless networks to overwrite all sectors. Comparison of two technologies
Support for standard support IEEE802.11B has a support of Wireless Ethernet Compatibility Alliance (WECA), Bluetooth has the support of Bluetooth Special Interest Group (SIG). The working frequency band IEEE802.11b and Bluetooth are working on the 2.4GHz band. In technology IEEE 802.11, only the physical layer and MAC layer of the open system interconnection reference model (OSI / RM) are specified, and the MAC layer uses carrier monitoring multiple access / conflict avoidance (CSMA / CA) protocol, and in physical layer, 802.11 Define three different physical media: infrared, frequency hopping sputum mode (FHSS), and direct expansion (DSSS). 802.11 supports the data rate of 1 ~ 11MB / s, but it only supports data communication, wireless data communication, the data device must be installed with a wireless network card. Bluetooth technology has a complete set of new agreements that can be applied to more occasions. The frequency hopping in Bluetooth technology is faster, and it is more stable, and it also has the characteristics of low power, low cost and flexibility. The IEEE802.11b is implemented is a tangible, specific network, and the network formed by Bluetooth is invisible, invisible, Bluetooth technology is a mainstream technology in the Ad hoc network. In the application, IEEE802.11b is long and fast, which can meet the network operation of users run a large amount of bandwidth, just like in wired local domain. The Bluetooth technology is a small range of connectivity between mobile devices, so it is an essential that it is a technology that replaces the cable. Bluetooth, suitable for simple data transfer in mobile phones, palms, etc.; 802.11b at 11MB / s is more suitable for high-speed wireless transmission such as images, and the effective distance is 100 meters. IEEE802.11b is more suitable for enterprise wireless networks in the office, making it suitable for high-speed wireless transmission such as images, and the effective distance is 100 meters; the speed of less than 1MB / s can be applied to any wirelessly alternative line. The cable of the cable is suitable for simple data transmission such as mobile phones, palms. The development trend is currently in a coexistence state. Since IEEE802.11b and Bluetooth carrier frequency band use 2.4GHz band, when transmitting and receiving data of these two specifications, it is possible to cause electric wave interference such as data package conflicts; from the long run Look, as the products and the market continue to evolve, they will be integrated, and the most competitive is Bluetooth technology. US Mobilian has introduced chipsets with wireless LAN and Bluetooth. This chipset composed of two chips includes wireless transmission and delivery functions and Bluetooth features of the wireless LAN IEEE802.11b. Mobilian's chipset developed by Mobilian, through the use of eliminating radio wave interference, two specification data communication is achieved. The IEEE 802.15 Committee proposed by 10M near-distance wireless communication technology has recently adopted technical proposals that can be coexist of Bluetooth and IEEE802.11b. This adopted program of the United States Mobilian Corp. and the United States Symbol Technologies, Inc. and NIST, US NIST. The proposal is scheduled to be announced in the second half of 2001. Intersil, Silicon Wave works to develop Bluetooth and IEEE 802.11b Dual-Mode WLAN solutions that enable laptops and other devices to link company LAN or other similar components through Bluetooth wireless communication. The goal of interacting with Silicon Wave is to develop a series of dual-mode solutions that will be compatible with Bluetooth wireless devices, and build on the same platform with the IEEE 802.11b wireless devices of Wi-Fi. Estimation will launch both of the CardBus32 and MiniPCI, and then the goal is to make the two devices to use a general ordinary antenna by dynamic switched technology. This dual-mode radio will be smaller than previous wireless devices. IEEE802.11 standard structure
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 Services Basic Service Group (BSS) is the basic unit of the wireless LAN, which features distributed coordinated function (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.
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