Overview of Wireless LAN

A wireless local area network can be used as the extension of the wired local area network, it can also be independent as the substitute of the wired LAN, so the wireless LAN provides a strong network flexibility.

The development of wireless local area network (WLAN) technology began in the mid 1980s, and it was created by the Federal Communications Commission (FCC) for public applications in the industrial, scientific and Medical (ISM) band. This policy facilitates the development and application of WLAN technology by enabling companies and end users to apply wireless products without the need for FCC licenses.

Unlike a wired LAN, which is transmitted via copper or fiber, the wireless LAN uses the electromagnetic spectrum to transmit information. Like radio and television, wireless LANs use radio waves (Airwave) to send information. Transmission can be achieved by using a wireless microwave or infra-red, but requires the effective frequency used and the transmit power level standard within the limits permitted by government agencies.

Advantages of WLAN Technology

WLAN refers to the wireless channel as the transmission medium of the computer local area network, is the computer network and wireless communication technology combination of the product, it is the wireless multiple access channel as a transmission medium, to provide the traditional cable LAN functions, can enable users to truly achieve at any time, anywhere, random broadband network access.

WLAN technology makes the computer on the Internet mobile, and can quickly and conveniently solve the network channel connectivity problem which is not easy to realize by Wired way. WLANs use electromagnetic waves to send and receive data in the air without the need for cable media. Compared to wired networks, WLANs have the following advantages:

Easy installation: Wireless LAN installation work is simple, it does not require construction permits, do not need wiring or trench excavation. Its installation time is only a fraction of the time the wired network was installed.

Wide coverage: In the wired network, the placement of network equipment is limited by the location of network information points. And the wireless local area network communication range, is not limited by the environment condition, the network transmission scope greatly widens, the maximum transmission range may reach dozens of kilometers.

Economic savings: Because of the lack of flexibility in wired networks, this requires network planners to consider the needs of future development as much as possible, so it often leads to a large number of preset information points with low rates. And once the network development beyond the design planning, but also spend more money for network transformation. WLAN is not limited by the location of wiring contacts, with the flexibility of traditional LAN can not be compared to avoid or reduce the occurrence of the above.

Easy to scale: WLANs are configured in a variety of ways and can be flexibly chosen as needed. In this way, the WLAN is capable of a small network of only a few users to a large network of thousands of users, and can provide features such as "roaming" (roaming) that wired networks cannot provide.

High transmission rate: the data transmission rate of WLAN is now able to reach the 11mbit/s, transmission distance can be as far as 20km. The WLAN, which is applied to orthogonal frequency division Multiplexing (OFDM) technology, can even reach the 54mbit/s.

In addition, the wireless local area network Anti-interference Strong, the network confidentiality is good. For many security problems in wired LAN, it can be avoided in wireless local area network basically. And compared with the wired network, the establishment, configuration and maintenance of WLAN is easy, general computer workers can be competent network management work.

Because WLAN has many advantages, its development is very fast. In recent years, WLANs have been widely used in hospitals, shops, factories and schools that are unsuitable for network cabling.

The topological structure of WLAN

There are two main topologies of WLANs, namely self-organizing networks (i.e. Peer-to-peer networks, commonly known as AD-HOC networks) and infrastructure networks (infrastructure network).

Self-organized WLAN is a peer-to-peer model of the network, which is built to meet the temporary needs of the service. The self-organizing network is made up of a set of wireless terminals with wireless interface cards, especially mobile computers. These wireless terminals are directly connected to each other in the same workgroup name, extended Service Set identification Number (ESSID), and password, in the context of WLAN coverage, point-to-point, or point-to-point communication, as shown in Figure 1.


Fig. 1 Self-organizing network structure

There is no need to add any network infrastructure to a self-organizing network, only mobile nodes and a common protocol are configured. In this topology, there is no need for central controller coordination. Therefore, the Self-organizing network uses a non centralized MAC protocol, such as CSMA/CA. However, because all nodes of the protocol have the same functionality, the implementation is complex and expensive.

Another important aspect of self-organizing WLANs is that it cannot take a fully connected topology. The reason is that for two mobile nodes, one node may be temporarily outside the transmission range of another node, it can not receive the transmission of another node, so it is not possible to establish communication between these two nodes directly.

Infrastructure-type WLANs utilize high-speed wired or wireless backbone transport networks. In this topology, the mobile node is connected to the wireless channel under the coordination of the base station (BS), as shown in Figure 2.


Another function of the base station is to connect the mobile node to the existing wired network. When the base station performs this task, it is called an access point (AP). Infrastructure networks also use a centralized MAC protocol, where competitive 802.11 protocols can be used for infrastructure topologies, but most infrastructure networks use central MAC protocols, such as polling mechanisms. Since most protocol processes are performed by access points, mobile nodes need only perform a small fraction of the functionality, so their complexity is greatly reduced.

In the infrastructure network, there are many cellular cells formed by mobile nodes in the base station and base station coverage area. Base station in the community can achieve full network coverage. In today's applications, most wireless WLANs are based on infrastructure networks.

A user moving from one location to another should be identified as leaving an access point and entering another access point, a situation called roaming. Roaming features require a reasonable overlap between the cells so that the user does not interrupt the link connection that is being communicated. There is also a need to coordinate access points so that users can transparently roam from one community to another. When roaming occurs, you must perform a toggle operation. Switching can be controlled either through a switching station, in a centralized manner, or through a mobile node, monitoring the signal strength of the node to achieve control, that is, the switch is not centralized.

In the infrastructure network, the community is generally relatively small. The reduction of cell radius means that the transmission range of mobile node is shortened, which can reduce power loss. Furthermore, the frequency multiplexing technique can be used in small cellular cell to improve the spectrum utilization of the system. At present, the common strategies for improving spectrum utilization are: Fixed channel allocation (FCA), Dynamic channel allocation (DCA) and power Control (PC).

When using the FCA policy, each cell is assigned a fixed resource, but it is independent of the number of mobile nodes. The problem with this strategy is that it does not fully consider the distribution of mobile users. In sparsely populated areas, the same amount of bandwidth is allocated to the community, but the community may contain only a few or no mobile nodes at all, making resources wasteful. Therefore, in this case, the utilization of the spectrum is not optimal.

The use of DCA, PC technology, or the integration of DCA and PC technologies on mobile nodes can improve the capacity of the entire cellular system, reduce channel interference, and reduce transmission power.

DCA places all available channels in a common channel pool and assigns these channels dynamically to the community based on the current load of the community. The mobile node reports its interference level to the base station, and the base station realizes the channel multiplexing with the minimum interference.

The PC scheme reduces the interference in the system by reducing the transmission power, and reduces the battery energy consumption of mobile nodes. When the interference in a certain cell increases, the PC scheme increases the signal to noise ratio (SIR) by increasing the power of the transmitting node. When the interference of the node is reduced, the sending node saves energy by reducing the transmit power.

In addition to the above two widely used topologies, there is another topological structure which is in the theoretical research stage, that is, the complete distributed network topology structure. This structure requires that the relevant nodes perform certain functions in the process of data transmission, similar to the concept of packet radio network. For each node, it may only know part of the topology of the network (or you can get all the topology knowledge by installing specialized software), but it can be shared with neighboring nodes in some way to understand the topology, to complete the distributed routing algorithm, that is, each node on the routing network to assist each other in order to transfer data to the destination node. (Computer science)

The distributed structure has good resistance to damage, strong mobile ability, and can form Multi-hop network, which is suitable for small and medium sized networks with low speed. For user nodes, its complexity and cost are higher than other topologies, and there is multipath interference and "far-near" effect. At the same time, with the expansion of the network scale, the performance index decreases quickly. But distributed WLAN will have a good application prospect in military field.