Wireless LAN
1.the history of the wireless LAN
Speaking of the historical origins of the wireless network may be even earlier than you imagined. Preliminary application of wireless networks can be traced back 50 years ago during World War II, when the U.S. Army using radio signals as data transmission. They developed a set of radio transmission technology, and using high strength encryption. U.S. and allied forces have had extensive use of the technology. This technology has been inspired for many scholars, in 1971, the University of Hawaii (University of Hawaii) researchers created the first packet-based technology based on radio communication network, called ALOHNET network, can be very early wireless local area network (WLAN). This is the first WLAN includes seven computers, which use two-way star topology (bi-directional star topology), across four Hawaiian islands, central computer placed in Oahu (Oahu Island) on the. From this point onwards, the wireless network can be officially born. Although almost all of the local area network (LAN) architecture are still wired, but wireless network applications in recent years have been increasing, mainly used in academia (such as college campuses), medical industry, manufacturing and warehousing industries, and related technology has been in progress for businesses to convert to a wireless network is also easier, more cheaper.
2.the technical features of wireless LAN
Wireless local area network using electromagnetic waves in the air to send and receive data without the need of cable media. Wireless LAN data transfer rates now can reach 11Mbps, the transmission distance can be far more than 20km. It is the way the cable network as a complement and extension to the Internet computer has a removable Xing, can quickly and easily resolved using the wire difficult to realize the Wangluo Unicom problem.
2.1 The advantages of wireless LAN
Compared with wired networks, wireless local area network has the following advantages:
1.installation easy. Generally in the network construction, the longest construction period, the greatest impact on the surrounding environment is the network cabling construction project. During the construction process, often require Poqiang digging, pipe threading framework. The biggest advantage of the wireless LAN is waived or reduced the workload of network cabling, installation normally as long as one or more access point AP (Access Point) device, you can set up covering the whole building or local area network.
2.the use of flexible. In the cable network, network equipment, network information placed in position by the positions of the restriction. Once completed the wireless local area network, wireless network coverage area at any one location can access the network.
3.economical. Lack of flexibility as cable networks, which requires the network planners to consider possible future development needs, which often leads to lower utilization of information by default a lot of points. Once beyond the development of the network design and planning, we have to spend more for the network transformation, and wireless local area network to avoid or reduce the occurrence of the above.
4.easy to expand. There are many configurations of wireless local area network can be flexibly chosen. In this way, the wireless local area network can handle only a few customers from small local area network to thousands of users of large networks and can provide such as "Roaming (Roaming)" and other cable networks can not provide the features. As the wireless LAN has some advantages, it has developed very rapidly. In recent years, wireless local area network has been in the hospital, shops, factories and schools, places not suitable for network cabling has been widely used.
2.2Wireless LAN related technologies
1.IEEE 802.11 standard. IEEE 802.11 in 1997 by a large number of local area networks and computer experts, approval of standards. Provides IEEE 802.11 wireless local area network operates in the 2.4GHz band, the band was defined as a global entity of radio spectrum use regulations bands.
August 1999, the 802.11 standard has been further improved and revised, including the use of an SNMP-based MIB to replace the original OSI based protocol MIB. It also adds two elements, one 802.11a, which extends the standard physical layer, the band 5GHz, using QFSK modulation, transmission rate 6Mb/s-54Mb/s. It uses orthogonal frequency division multiplexing (OFDM) unique spread spectrum technology to provide wireless 25Mbps 10Mbps Ethernet interfaces and ATM frame structure of wireless interfaces, and supports voice, data and video traffic. This rate can satisfy the indoor and outdoor range of applications. However, using the standard product has not yet entered the market. The other is the 802.11b standard, in the 2.4GHz frequency band, direct sequence spread spectrum (DSSS) technology and the compensation code keying (CCK) modulation. The standard provides 11Mb / s data rates, but also changes in circumstances, in 11 Mbps, 5.5 Mbps, 2 Mbps, 1 Mbps auto-switch between the different rates. It fundamentally changed the design and application of wireless local area network to expand wireless LAN applications, and now, most manufacturers of wireless LAN products are based on the 802.11b standard.
2.Wireless LAN concepts. In a typical wireless LAN environment, there are some data to send and receive the equipment, known as the access point (AP). Typically, a first AP to tens of meters in the context of connecting multiple wireless users. In both wired and wireless networks case, AP via a standard Ethernet cable and associated traditional wired networks, as wireless networks and wired network connections. Wireless LAN end-users can access the network through a wireless network card and so on.
3.Wireless LAN outdoor mainly the following structure: peer-based, multipoint, multi-point type and mixed type. Point type. The type used in a fixed position to the interconnection between the two is a common way of wireless networking, built using this interconnection network, the advantage of the long transmission distance, high transfer rate, less affected by the external environment. Multipoint type. The type used in a central point where multiple remote points. The biggest advantage is the formation of networks of low cost, easy maintenance; Secondly, since the center using the omnidirectional antenna, equipment is relatively easy to debug. Disadvantage of this type of network is also because the use of omnidirectional antenna, beam spread allows the full power to much attenuation, the network transmission rate is low, for a more long-distance remote point, network reliability can not be guaranteed. Mixed. This type applies to the construction of networks in long-distance point, close the point, there are buildings or mountains obstruct the point. In the formation of this network, the integrated use of the several types of networks, is critical for long-distance point-point method used at close range by a number of point-to-multipoint mode, there are stop points using relay mode.
2.3 Wireless LAN indoor application of the two categories
1.Independent wireless LAN. This refers to the use of wireless communications throughout the network to the case. In this way you can use the AP, you can not use AP. AP when not in use, all users directly via the wireless Internet. But the drawback is the short distance communication between users, and when the number of users is large, poor performance.
2.Non-independent wireless LAN. In most cases, wireless communication as a supplement to wired communications and expansion. We call this situation as non-independent wireless LAN. In this configuration, the number of AP by cable connected to the wired network, so that wireless users can access the various parts of the network.
2.3.1 Other Concepts - micro cell and wireless Roaming
Radio wave propagation process will continue to decay, resulting in AP's communication range is limited to a certain range, this range is known as micro-units. When there are multiple network environment TAP, and their micro-units have a range of overlap with each other, the wireless user can move throughout the coverage area of wireless local area network, wireless network card can automatically find the largest signal strength near the AP, and send and receive data through the AP to maintain uninterrupted network connection, which is called wireless roaming.
Spread Spectrum. Most of the wireless LAN products use spread spectrum technology. Spread-spectrum technology was originally used in military communications in broadband wireless communication technology. Use of spread spectrum technology enables wireless transmission of data integrity and reliability and to ensure that while data transmitted in different frequency bands will not interfere with each other.
Direct Sequence Spread Spectrum. The so-called direct sequence spread spectrum, is the use of high bit-rate of the spreading sequence, the expansion of the signal transmitter spectrum, and in the spread spectrum receiver using the same despreading code sequences, the start of the spread spectrum signal restore the original signal.
Frequency hopping spread spectrum. Frequency hopping and direct sequence spread spectrum technology is completely different, is another spread spectrum technology. Frequency-hopping carrier frequency by a pseudo-random code control within its operating bandwidth, the frequency of changing frequency of random rules. The frequency of the receiver based on the random rule changes, and maintain the same variation of the transmitter.
Directly reflect the level of frequency hopping FH system performance, the higher the frequency hopping, the better the performance of interference, frequency hopping system of military thousands of jumps per second. In fact GSM mobile communications system is also frequency hopping system. Because of cost considerations, commercial frequency hopping systems are slower jump rate, generally at 50 beats / sec. As the slow jumping frequency system is simple and low-speed wireless local area network so often used this technology.
3. wireless LAN applications
3.1 The field of Wireless Local Area Network
1.Access to network information system: e-mail, file transfer and terminal emulation.
2.Hard wiring of the environment: the old buildings, difficult or expensive wiring open areas, city buildings, schools and factories.
3.Frequently changing environment: frequent change of duty station and to change the location of retailers, manufacturers, and field investigation, testing, military, police and banks.
4.The use of portable computers and other mobile devices for fast network connection.
5.For the long-distance transmission of information: if carried out in forest fires, pests and other information transmission; police traffic management department of traffic management.
6.Special projects or peak time being required for the LAN: schools, trade shows, construction sites and other places where a strong movement; use of wireless local area network for information exchange; retailers, air and shipping time required for peak additional workstations.
7.Mobile workers can get information of the region: the need for hospitals, retail stores or office access to information when the regional flow of doctors, nurses, retailers, white-collar workers.
8.'Office and home office (SOHO) users, and the need to quickly and easily install a small network of users.
3.2 Structure of Wireless LAN
According to different LAN environments and needs of different wireless local area network can take different network structures to achieve interconnection. There are several commonly used specific:
1.Bridge connection types: interconnection between different local area network, due to physical causes, if not convenient to wire, can be achieved using a wireless bridge between the two point to point connection, wireless bridge not only provided between the physical and data link connection, the two networks also provide a higher level of routing and protocol conversion.
2.base station access type: When using the mobile cellular communication network set up wireless LAN access methods, the communication between the various sites through the base station access, data exchange method to achieve interconnection. The mobile station switching center can not only its own network, but also through wide area network and remote Web site set up their own work.
3.HUB access type: Wireless Hub to set up joint topology using wireless LAN, wired Hub network formation with similar advantages. Based on the structure, WLAN, Ethernet can be used similar to the exchange of working methods, requirements Hub has a simple exchange of capabilities within the network.
4.no central structure: Requires network can be any two sites to communicate directly. The structure of the general use of public wireless LAN broadcast channel, MAC layer uses CSMA type of multiple access protocol.
Wireless LAN can be based on the general wireless LAN Hub, wireless access point (AP), wireless bridges, wireless Modem and wireless network cards to achieve, which is most common in the wireless network card, using up. The key technology of wireless local area network, in addition to infrared transmission technology, spread spectrum technology, network synchronization technology but also have a number of other technologies, such as: modulation techniques, plus descrambling technology, wireless technology diversity reception, power control and energy efficiency technologies.
3.3 The concrete implementation of wireless local area network
Practical work by the author on wireless LAN equipment and technology have had a more profound engagement. Here in Guangzhou Kaichuanggongsi (Enterasys Networks) of the RoamAbout 802.11 family of wireless LAN equipment on the concrete implementation of wireless local area network to be brief.
RoamAbout802.11 equipment profile. Enterasys RoamAbout wireless network launched in solution for fast, easy and economic to set up a wireless LAN, it can provide reliability similar to Ethernet. RoamAbout 802.11 family of products consists of two parts: the exchange of full-function access point and 2.4GHz direct sequence spread spectrum wireless Ethernet PC card. The former can be unshielded twisted pair right, quickly and easily connect wired LAN; latter feature is similar to all standard wired Ethernet card, but it uses radio frequency instead of cables to create a LAN connection. When users roam throughout the network, RoamAbout PC card can seamlessly switch to a different access point, thereby always maintaining the network.
