The history of wireless LANs
When it comes to the historical origin of wireless networks, it may be earlier than you think. The initial application of wireless networks can be traced back to fifty years ago during World War II, when the U.S. Army used radio signals to transmit information. They developed a set of radio transmission technology and used strong encryption. This technology was widely used by both the U.S. Army and the Allied Forces. This technology has inspired many scholars, in 1971, the University of Hawaii (University of Hawaii) researchers created the first packet-based radio communication network, which is called ALOHNET network, can be regarded as a fairly early wireless local area network (WLAN). This earliest WLAN consisted of seven computers in a bi-directional star topology spanning four Hawaiian islands, with a central computer placed on Oahu Island. From this point on, wireless networks can be said to be officially born. Although almost all local area networks (LANs) are still wired, in recent years the use of wireless networks has been increasing, mainly in academia (such as college campuses), health care, manufacturing and warehousing, etc., and the technology has been advancing to the enterprise to switch to wireless networks is also easier and cheaper.
Technical characteristics of wireless LANs
Wireless LANs use electromagnetic waves to send and receive data over the air without the need for wires. The data transfer rate of wireless LAN is now able to reach 11Mbps, and the transmission distance can be as far as 20km or more. It is a complementary and expansion of the wired networking method, so that the online computer with mobility, can quickly and easily solve the use of wired methods are not easy to realize the problem of network connectivity.
1. Advantages of wireless LAN
Compared with wired networks, wireless LANs have the following advantages:
Convenient installation
Generally in the construction of the network, the construction cycle of the longest, the greatest impact on the surrounding environment, is the network cabling construction project. In the construction process, often need to break the wall digging, threading rack tube. The biggest advantage of wireless LAN is that it eliminates or reduces the workload of network wiring, generally as long as the installation of one or more access point AP (Access Point) devices, you can establish a local area network covering the entire building or area.
Flexible use
In the wired network, the placement of network equipment by the network information point location restrictions. Once the wireless LAN is built, any location within the signal coverage area of the wireless network can access the network.
Economic Savings
The lack of flexibility in wired networks requires network planners to take into account future growth as much as possible, which often results in a large number of underutilized points of information being preconfigured. And once the network development beyond the design planning, and to spend more money for network transformation, and wireless LAN can avoid or reduce the occurrence of the above situation.
Easy to expand
Wireless LAN has a variety of configurations that can be flexibly selected according to needs. In this way, the wireless LAN can be able to handle from only a few users of the small LAN to thousands of users of the large network, and can provide like "roaming (Roaming)" and other wired networks can not provide the characteristics. Wireless LANs are growing rapidly because of their many advantages. In recent years, wireless LANs have been widely used in hospitals, stores, factories, and schools where network cabling is not appropriate.
2. Related Technologies of Wireless LAN
1). IEEE 802.11 standard
IEEE 802.11 was adopted in 1997 by a large number of LAN and computer experts, and IEEE 802.11 specifies that WLANs operate in the 2.4 GHz band, which is defined by the global radio regulatory entities as the Spread Spectrum Usage Band.
In August 1999, the 802.11 standard was further refined and revised, including the replacement of the original OSI protocol-based MIB with an SNMP-based MIB, and two additions, 802.11a, which expanded the physical layer of the standard, with a 5GHz band, QFSK modulation, and transmission rates of 6Mb/s - 54Mb/s. It expanded the physical layer of the standard, with a 5GHz band, QFSK modulation, and transmission rates of 6Mb/s - 54Mb/s. It was also expanded to include the following new features -It adopts the unique spread spectrum technology of Orthogonal Frequency Division Multiplexing (OFDM), which can provide 25Mbps wireless ATM interface and 10Mbps Ethernet wireless frame structure interface, and support voice, data and image services. Such a rate can fully meet the indoor and outdoor applications. However, products using this standard have not yet entered the market. The other is the 802.11b standard, in the 2.4GHz band, using direct sequence spread spectrum (DSSS) technology and compensated coded keying (CCK) modulation. The standard provides a data rate of 11 Mb/s and is also capable of automatically switching between different rates of 11 Mbps, 5.5 Mbps, 2 Mbps, and 1 Mbps depending on the situation. It fundamentally changes the status quo of wireless LAN design and application, and expands the application areas of wireless LAN. Now, most manufacturers produce wireless LAN products based on the 802.11b standard.
2). Concepts Related to Wireless LANs
In a typical wireless LAN environment, there are a number of devices, called access points (APs), that perform data transmission and reception. Typically, an AP is capable of connecting multiple wireless users within a range of tens to hundreds of meters. In the case of both wired and wireless networks, the AP can be connected to a traditional wired network via a standard Ethernet cable to act as a connection point between the wireless and wired networks. End-users of the wireless LAN can access the network through a wireless network card, for example.
Wireless LANs have the following main structures outdoors: point-to-point, point-to-multipoint, multi-point-to-point, and hybrid.
● Point-to-point type
This type is commonly used between two fixed locations to be networked, and it is the common way of wireless networking. The advantages of the network built by this type of networking are the long transmission distance, high transmission rate, and less influence by the external environment.
● Point-to-multipoint type
This type is often used in the case of a center point, multiple remote points. Its biggest advantage is that the formation of the network cost is low, simple maintenance; secondly, because of the use of omni-directional antennas in the center, equipment commissioning is relatively easy. The disadvantage of this network is also because of the use of omnidirectional antennas, the omnidirectional spread of the beam makes the power greatly attenuated, the network transmission rate is low, for the farther distances of the remote end point, the reliability of the network can not be guaranteed.
● Hybrid
This type is suitable for the construction of the network there are long-distance points, close to the point, as well as buildings or mountains blocking the point. When forming this kind of network, the above types of network methods are used comprehensively, using the point-to-point method for the long-distance points, the point-to-multipoint method for the short-distance points, and the relay method for the blocked points.
The indoor application of wireless LAN has the following two types of situations
● Independent wireless LAN
This refers to the situation where the entire network uses wireless communication. In this case, APs can be used or not, and when APs are not used, the users are directly connected to each other via wireless. However, the disadvantage is that the communication distance between the users is close and the performance is poor when the number of users is large.
● Non-independent WLAN
In most cases, wireless communication is used as a complement and extension to wired communication. We refer to this situation as a non-standalone wireless LAN. In this configuration, multiple APs are connected to the wired network via cables to enable wireless users to access various parts of the network.
Other Related Concepts
● Microcells and Wireless Roaming
Radio waves attenuate as they propagate, resulting in the communication range of an AP being limited to a certain range, which is called a microcell. When there are multiple TAPs in the network environment, and their micro-units overlap with each other within a certain range, wireless users can move throughout the WLAN coverage area, and the wireless card is able to automatically discover the nearby AP with the highest signal strength, and send and receive data through this AP to maintain an uninterrupted network connection, which is called wireless roaming.
● Spread Spectrum
Most wireless LAN products use spread spectrum technology. Spread Spectrum was originally a broadband wireless communications technology used in military communications. The use of spread spectrum allows data to be transmitted wirelessly with integrity and reliability, and ensures that simultaneous transmissions in different frequency bands do not interfere with each other.
● Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSBS) is the use of a spread spectrum sequence with a high bit rate to expand the spectrum of a signal at the transmitter end, and then de-expanding it at the receiver end with the same spread spectrum code sequence to reduce the expanded spread spectrum signal to the original signal.
● Frequency Hopping Spread Spectrum
Frequency Hopping is a completely different spread spectrum technology than Direct Sequence Spread Spectrum. The carrier frequency of the frequency hopping is controlled by a pseudo-random code, in its operating bandwidth range, its frequency according to a random law constantly changing frequency. The frequency of the receiving end is also according to the random law changes, and to maintain the same rule of change with the transmitting end.
The high and low frequency hopping directly reflects the performance of the frequency hopping system, the higher the frequency hopping, the better the anti-jamming performance, the military frequency hopping system can reach tens of thousands of hops per second. In fact, the mobile communication GSM system is also a frequency hopping system. For cost considerations, commercial frequency hopping system hopping speed are slower, generally below 50 hops / sec. Because the slow hopping frequency hopping system is simple to realize, so low-speed wireless LANs often use this technology.
Applications of wireless LAN
Based on the many advantages of wireless LAN, it can be widely used in the following areas:
1. Access to network information systems: e-mail, file transfer and terminal emulation.
2. Difficult-to-wire environments: old buildings, open-air areas where wiring is difficult or expensive, urban complexes, campuses, and factories.
3. Frequently changing environments: retailers and manufacturers who frequently change workplaces and locations, as well as field surveys, tests, military, public security, and banks.
4. Using portable computers and other removable devices for fast network connections.
5. Used for long-distance information transmission: such as in the forest area for fire, pests and diseases, and other information transmission; public security traffic management department for traffic management.
6. Temporary LANs required for specialized projects or peak hours: schools, commercial exhibitions, construction sites and other places with strong personnel mobility; the use of wireless LANs for the exchange of information; additional workstations required at peak hours for retailers, air and shipping companies, and so on.
7. Areas where information is available to mobile workers: Doctors, nurses, retailers, white-collar workers who need to be informed while moving through hospitals, retail stores, or office areas.
8. Office and home office (SOHO) users, as well as users who need to easily and quickly install a small network.
Structure of Wireless LAN
According to the different LAN application environments and needs of different LANs, wireless LANs can take different network structures to achieve interconnection. Commonly used in the following specific types:
1, bridge connection type: different LAN interconnection, due to physical reasons, if you take the wired way is not convenient, you can use the wireless bridge way to realize the two point-to-point connection, wireless bridge not only provides the physical and data link layer connection between the two, but also for the users of the two networks to provide a higher level of routing and protocol conversion.
2, base station access type: when using mobile cellular communication network access to form a wireless LAN, the communication between the stations is through the base station access, data exchange to realize the interconnection. Each mobile station not only through the switching center to form their own network, but also through the wide area network and distant sites to form their own working network.
3, HUB access type: the use of wireless Hub can form a star structure of the wireless LAN, with wired Hub networking with similar advantages. On the basis of the structure of the WLAN, can be used in a similar way to the work of the switched Ethernet, the Hub is required to have a simple network switching function.
4, no center structure: the requirements of any two sites in the network can communicate directly. This structure of the wireless LAN generally use the public broadcast channel, the MAC layer using CSMA type of multiple access protocol.
Wireless LAN can be realized on the basis of ordinary LAN through wireless Hub, wireless access station (AP), wireless bridge, wireless Modem and wireless network card, etc., which is the most common and most used wireless network card. Wireless LAN key technologies, in addition to infrared transmission technology, spread spectrum technology, network synchronization technology, there are a number of other technologies, such as: modulation technology, plus descrambling technology, wireless diversity reception technology, power control technology and energy-saving technology.
Specific realization of wireless LAN
I have had more profound contact with wireless LAN equipment and technology realization through the actual work. The following to Guangzhou Kai Chuang company (Enterasys Networks) RoamAbout 802.11 series of wireless LAN equipment on the specific realization of the wireless LAN to be briefly introduced:
1. RoamAbout802.11 device introduction:
Enterasys introduced the RoamAbout Enterasys introduces the RoamAbout wireless networking solution, which is used to quickly, easily and cost-effectively establish a wireless LAN, which can provide users with reliable performance similar to that of Ethernet. The RoamAbout 802.11 series of products consists of two parts: a full-featured switching access point and a 2.4GHz direct-sequence spread-spectrum wireless Ethernet PC card. The former enables quick and easy wired LAN connections over unshielded twisted-pair cables, while the latter functions like all standard wired Ethernet cards, but uses RF rather than cables to establish a LAN connection. As users roam throughout the network, the RoamAbout PC Card seamlessly switches to different access points so that it is always connected to the network.
2. Project Specific Realization Examples:
Example 1: A wired LAN has been built in a tax branch building, and there are seven offices outside the branch building that need to be connected to the wired network in the building through a wireless LAN. Branch building outside the seven, to the branch of the farthest distance of 15km, the latest 3km, two of which are in a building has been built in a small wired LAN, each generally have two to four workstations. The wireless LAN product we use works in the frequency range of 2.4GHz to 2.4835GHz, which requires that there is no object blocking between the antennas of the two communication points, but because the building is in the busy area, so we choose a higher floor of the institute as the center site of the wireless LAN. In the central site access to a wireless access point AP-10D, the other institutes through the access to a station adapter SA-40D and the central site of the AP-10D for communication, the wired LAN in the branch building through the access to a wireless bridge WB-10D and the central site of the AP-10D for communication. So that all the sites of the Institute and the branch of the access to the wireless LAN through the control of the central site to achieve, they **** enjoy the central site AP-10D 3M bandwidth.
Example 2: A group of companies distributed in different buildings within the office, according to the conventional design must be connected to a dedicated line, pay expensive monthly fees and maintenance costs, and can not solve the mobile site access and access to the company's online information. The use of 2.4GHz band wireless LAN products, can be more flexible to form an integrated enterprise network, to achieve the same performance with the dedicated line, and to solve the problem of mobile sites, and easy to install and maintain, do not need to pay the frequency usage fee. Specific method is to use the wireless access point (AP) bridge function, one end of the antenna is connected to the building, one end is connected to the wired network Hub, so that the two buildings are connected to each other instead of the leased line function. Around the mobile site through the wireless access point and the company wired network interconnection, access and access to company information.
Conclusion
The emergence of wireless networks is to solve the wired network can not overcome the difficulties. Although wireless networks have many advantages, there are also many shortcomings of wireless LANs compared to wired networks. Wireless networks are slower and more expensive, so it is mainly for users with specific needs. At present, wireless LAN can not be completely detached from the wired network, wireless network and wired network is complementary relationship, rather than competition; at present, it is only a wired network to supplement, rather than replace. However, it should also be seen that in recent years, as the price of products suitable for wireless LAN is gradually decreasing, the corresponding software is also gradually mature. In addition, wireless LAN has been able to provide multimedia services of mobile Internet by combining with WAN. I believe that in the future, wireless LAN will play a more important role with its high-speed transmission capability and flexibility!