【 Weak current College 】 detailed interpretation of wireless local area network (WLAN) technology---Power By 【 China power house network 】

A wireless LAN can be extended as a wired local area network to use, can also be independent as wired LAN alternative facilities, wireless local area network provides strong networking flexibility. Wireless LAN (WLAN) technology development began in the mid-1980s, it was made by United States Federal Communications Commission (FCC) for industrial, scientific and medical (ISM) band application provides authorization for the public. This policy enables corporations and end users do not need to obtain FCC license, you can use wireless products, thus promoting the WLAN technology development and application. And wired LAN through a copper or fiber-optic, conductor transmission, wireless local area network using the electromagnetic spectrum to deliver information. With the wireless radio and TV, wireless LAN use channel (Airwave) send information. Transmission through the use of wireless microwave or infrared, but requires the use of effective frequency and send power level standards in government bodies allowed range. WLAN technology WLAN is a wireless channel as the transport medium of the computer local area network, a computer network and wireless communication technology of products with wireless multiple access channel as the transmission medium, providing traditional wired LAN function, enables the user to the actual realization of at any time, anywhere, free broadband Internet access. WLAN technologies enable Internet computer with mobility, can quickly and easily solve hard wired network channel connectivity problems. WLAN using electromagnetic waves in the air to send and receive data without cable media. Compared with the wired network, WLAN has the following advantages: ease of installation: Wireless LAN installation work is simple, it does not require construction permit, without the need for wiring or excavation Groove. It's time to install a wired network is the only time of the change. Coverage: wired network, the network device placement by network information point location restrictions. And wireless LAN communication range, is not affected by environmental conditions, network transmission range significantly widen, and the maximum transmission range up to tens of kilometres. Economy: due to the lack of flexibility of the cable network, which requires network planners, to the extent possible, to consider the future development needs, so often lead to a large number of preset with lower utilization information point. Once the development is beyond the network design and planning, and to take a lot of expenses for network transformation. WLAN is not subject to restrictions on the location of the routing nodes, with traditional LAN incomparable flexibility to avoid or reduce the above circumstances occur. Easy to scale: there are several configuration mode WLAN capable of flexibly select as needed. In this way, the WLAN can be anything from a few users of small networks to large network of thousands of users, and offers like "roaming" (Roaming), and other wired network not available. Transmission rate: WLAN data transfer rate is now able to reach 11Mbit/s, the transmission distance can be far more than 20km. Applied to the orthogonal frequency division multiplexing (OFDM) technology of WLAN, you can even reach 54Mbit/s. In addition, the wireless LAN of anti-interference, network confidentiality. For wired LAN many security issues in wireless LAN basically can be avoided. But relative to a wired network, wireless local area network set up, configure, and maintain more easily, and general computer staff can be qualified for network management. WLANs have many advantages, its development very rapidly. In recent years, WLAN is already in the hospital, stores, factories, schools, and so is not suitable for the network cabling occasions has been widely used. WLAN topology in WLAN there are two main kinds of topology from the organization network (that is a peer-to-peer network, that is, people often called ad-hoc networks) and infrastructure network (InfrastructureNetwork). Self-organization of WLAN is a peer-to-peer model of network, it was created to meet the needs of service temporarily. Ad hoc network is a set of wireless interface card wireless terminal, particularly mobile computer. These wireless terminal to the same workgroup name, extended service set identifier (ESSID) and password, and other peer-to-peer way direct-attached, in the WLAN coverage, a point-to-point or point-to-multipoint communication. Formed self-organizing network does not require adding any network infrastructure, mobile nodes and only need to configure a common agreement. In this topology, do not need to have a central controller coordination. Therefore, ad hoc networks using non-centralized MAC protocols, for example, CSMA/CA. But since this agreement every node has the same functionality, implement complex and expensive. Self-organizing WLAN another important aspect is that it cannot use the connection topology. The reason is for mobile nodes, a node may temporarily in another node transmission range, it receives no other node of the transmission signal and, therefore, unable to both establish communication directly between nodes. Underlying structural WLAN utilizes high-speed wired or wireless backbone transmission network. In this topology, the mobile node in the base station (BS), under the coordination of access to the wireless channel. The base station of another role is to move the node to an existing wired network connection. When the base station carrying out this task, it is called an access point (AP). Infrastructure network while also uses non-centralized MAC protocols, such as based on competition 802.11 Protocol can be used for infrastructure topology, but most of the infrastructure networks use centralized MAC protocols, such as polling mechanism. Since most of the protocols are implemented by the access point, the mobile node only need to perform a small part of the function, so greatly reduce their complexity. In the infrastructure network, there are many base station and base station coverage of mobile node forms of cellular. Base station in the community can achieve full network coverage. In current practice, the majority of wireless WLAN are based on the infrastructure network. A user moves from one location to another location, should be determined to leave one access point, enter another access point, this situation is called a "roaming". Roaming feature requires that the community must have a reasonable overlap, so that the user will not beInterrupt communicating link connection. Access point also needs to be coordinated, so that the user transparently roaming from one community to another community. Occurs when roaming, you must perform the switch operation. Toggle either through Exchange Bureau, a focused way to control, or by mobile nodes, monitoring node signal strength to achieve control, non-centralized switching. In the underlying structural networks, community size is generally smaller. Community, means the radius of decreasing the transmission range of the mobile node is reduced, which can reduce power consumption. Also, small honeycomb community can adopt frequency multiplexing technology, thereby enhancing the system of spectrum utilization. Currently, improve spectrum utilization of common policy: a fixed channel assignment (FCA), dynamic channel allocation (DCA) and power control (PC), etc. In the use of FCA strategy, each community, assign a fixed resource, but it has nothing to do with the mobile node number. This strategy is that it does not take full account of the distribution of mobile users. In sparsely populated areas, and also assign the same number of bandwidth resource to the community, the community may only contain few or simply does not contain any mobile node, allowing resources to be wasted. Therefore, in this case, spectrum utilization and is not optimal. In the mobile node using DCA, PC technology, or integrated DCA and PC technology, you can increase the capacity of the cellular system, reduce channel interference, and to reduce the transmission power. DCA technology will all available channels are placed in a public channel pool, and community current load, will these channel dynamically assigned to the community. Move the node to the base station reports its interference, the base station to the minimum interference means channel multiplexing. Send PC programmes by reducing power to reduce system noise and reduce the mobile node's battery energy consumption. When a disruption in the community increases, PC programme by increasing the power of the sending node, to improve the reception of the signal to noise ratio (SIR). When a node is reduced, the interference of sending node by lowering the TX power to save energy. In addition to these two broader application of topology, there is another theory research stage is in the topology, which is a fully distributed network topology. The structural requirements, the relevant node in the data transfer process to complete certain functions, similar to the concept of packet radio network. On each node, it may only know part of the topology of the network (you can also install specialized software gets all of topology knowledge), but it can be used with a neighboring node as a way to share knowledge on topology, to complete distribution routing algorithm, the routing on the network, each node should assist each other, so that the data transmitted to the destination node. Distributed architecture to damage property, mobile ability, can form a Multihop networks, suitable for a lower rate of small and medium-sized networks. For the users node, its complexity and cost than other topologies, and multipath interference and "far-near" effect. At the same time, with the expansion of the scale of the network, its performance index declined more rapidly. But distributed WLAN in the military field with good prospects.