Ethernet; Wireless local area network; Spread spectrum; Safety; mobile ip
I. Introduction
With the wide application of wireless communication technology, the traditional local area network can no longer meet people's needs, so the wireless local area network came into being and developed rapidly. Although WLAN is not completely independent of wired network at present, its products have gradually matured in recent years, and it is playing an increasingly important role in network applications with its superior flexibility and convenience.
Wireless local area network is the product of the combination of wireless communication technology and network technology. From a professional point of view, WLAN is to realize the communication between network devices through wireless channels, and realize the mobility, personalization and broadband of communication. Generally speaking, WLAN provides Ethernet interconnection function without using network cable.
Broad application prospects, broad market demand and technical realizability have promoted the perfection and industrialization of WLAN technology, which has also been confirmed by the commercial 802.438+05438+0B network. With the continuous development of commercial 802. 1 1a network and other wireless LAN technologies, wireless LAN will usher in a golden age of development.
Two. Overview of wireless LAN
The historical origin of wireless network can be traced back to World War II 50 years ago. At that time, the US Army developed a set of radio transmission technology, which used radio signals to transmit data. This technology has inspired many scholars. 197 1 year, researchers at the university of Hawaii created the first radio communication network called ALOHNET. This network consists of seven computers, which are connected in a two-way star topology and span four islands in Hawaii. The central computer is on Oahu. Since then, the wireless network was officially born.
1. Advantages of WLAN
(1) Flexibility and mobility. In wired network, the location of network equipment is limited by the location of the network, while wireless LAN can access the network anywhere within the coverage of wireless signals. Another biggest advantage of WLAN is mobility. Users connected to WLAN can stay connected to the network while moving.
(2) Convenient installation. Wireless LAN can avoid or minimize the workload of network wiring. Usually, as long as one or more access point devices are installed, a local area network covering the whole area can be established.
(3) It is convenient for network planning and adjustment. For wired networks, the change of office location or network topology usually means the reconstruction of the network. Reconnecting is an expensive, time-consuming, wasteful and trivial process, and WLAN can avoid or reduce the above situation.
(4) Fault location is easy. It is often difficult to find the physical faults of wired networks, especially the network interruption caused by poor line connection, which will cost a lot to repair the lines. Wireless network is easy to locate faults, and network connection can be restored only by replacing faulty equipment.
(5) Easy to expand. Wireless local area network (WLAN) has a variety of configurations, which can rapidly expand from a small LAN with only a few users to a large network with thousands of users, and can provide functions that wired networks cannot achieve, such as roaming between nodes.
Due to the above advantages, wireless local area network has developed very rapidly. In recent years, WLAN has been widely used in enterprises, hospitals, shops, factories and schools.
2. The theoretical basis of WLAN
At present, there are two main transmission media used in WLAN, namely infrared and radio waves. According to different modulation methods, wireless local area networks using radio waves as transmission media can be divided into spread spectrum methods and narrowband modulation methods.
(1) infrared (IR) LAN
Compared with radio wave mode, infrared communication mode can provide extremely high data rate, high security and relatively cheap and simple equipment. However, due to the poor transmission and diffraction ability of infrared rays to obstacles, the transmission distance and coverage are greatly limited. Usually, the coverage of IR LAN is limited to one room.
(2) Spread spectrum LAN.
If spread spectrum technology is used, the network can work in ISM (industrial, scientific and medical) frequency band. Its theoretical basis is that information is transmitted in broadband by spread spectrum in exchange for the improvement of signal-to-noise ratio. Spread spectrum communication has the characteristics of strong anti-interference ability and concealment, good confidentiality and strong multi-address communication ability. Spread spectrum technology is mainly divided into frequency hopping technology (FHSS) and direct sequence spread spectrum (DSSS).
The so-called direct sequence spread spectrum is to spread the signal with high-speed spread spectrum sequence at the transmitter and despread it with the same spreading code sequence at the receiver, thus restoring the spread spectrum signal to the original signal. Frequency hopping technology is different from direct sequence spread spectrum technology. The carrier frequency of frequency hopping is controlled by pseudo-random code, and its frequency changes constantly according to random law. The frequency of the receiver also changes randomly, which is consistent with the changing law of the transmitter. The level of frequency hopping directly reflects the performance of frequency hopping system. The higher the frequency hopping, the better the anti-jamming performance, and the military frequency hopping system can reach tens of thousands of hops per second.
(3) Narrow-band microwave LAN
This kind of LAN uses microwave radio frequency band to transmit data, and the bandwidth is just enough to accommodate signals. However, this network product usually needs to apply for a radio spectrum license, and other ways can use the ISM band without a license.
3. Disadvantages of WLAN
Wireless LAN can bring convenience and practicality to network users, but it also has some defects. The disadvantages of WLAN are as follows:
(1) performance. Wireless LAN is transmitted by radio waves. These radio waves are transmitted by wireless transmitters, and obstacles such as buildings, vehicles and trees may hinder the transmission of electromagnetic waves, thus affecting the performance of the network.
(2) speed. The transmission rate of wireless channel is much lower than that of wired channel. At present, the maximum transmission rate of WLAN is 54Mbit/s, which is only suitable for personal terminals and small-scale network applications.
(3) Safety. In essence, radio waves do not need to establish a physical connection channel, and wireless signals are divergent. Theoretically, it is easy to monitor any signal within the broadcast range of radio waves, resulting in communication information leakage.
Thirdly, wireless LAN protocol standard.
Wireless LAN technology (including IEEE802. 1 1, bluetooth technology and HomeRF, etc.). ) will be one of the most promising major technologies in the wireless communication field in the new century. A number of research institutions represented by IEEE (Institute of Electrical and Electronics Engineers) have formulated a series of protocol standards for different applications, which has promoted the practicality of wireless local area networks.
1.ieee802.11series protocols.
As a globally recognized authority on local area networks, the standards formulated by IEEE 802 working group have been widely used in local area networks. These protocols include 802.3 Ethernet protocol, 802.5 token ring protocol and 802.3z 100BASE-T fast Ethernet protocol. 1997, IEEE released the first internationally recognized wireless LAN protocol-802.11protocol. 1September 1999, IEEE proposed the 802. 1 1b protocol, which supplemented the 802. 1 1 protocol, and then introduced 802.1/a, and. The specific agreements made by the IEEE802. 1 1 working group are as follows:
( 1)802. 1 1a
802. 1 1a uses orthogonal frequency division (OFDM) technology to modulate data, and uses 5GHz frequency band. OFDM technology divides the wireless channel into sub-frequencies with low data rate for parallel transmission, and then puts these frequencies back to the receiver, which can provide 25Mbit/s wireless ATM interface, 10Mbit/s Ethernet wireless frame structure interface and TDD/TDMA air interface. To a great extent, it can improve transmission speed, improve signal quality and overcome interference. The physical layer rate can reach 54Mbit/s and the transport layer can reach 25Mbit/s, which can meet indoor and outdoor applications.
(2)802. 1 1b
802. 1 1b, also known as Wi-Fi technology, adopts CCK modulation mode and uses 2.4GHz frequency band. Its greatest contribution to wireless LAN communication is that it can support two rates-5.5 Mbit/s and11mbit/s. The multi-rate media access control mechanism can ensure that when the distance between workstations is too long or the interference is too large and the signal-to-noise ratio is below a certain threshold, the transmission rate can be automatically changed from11mbit according to the direct sequence spread spectrum technology. Under the premise of not violating FCC regulations, frequency hopping technology cannot support higher rates, so it is necessary to choose DSSS as the only physical layer technology of this standard.
(3)802. 1 1g
The draft of 200 1,1,802. 1 1 g standard was formed at the 802.1/IEEE conference, in order to reach 802./in the 2.4GHz band. The standard will be approved in early 2003. 802. 1 1g adopts PBCC or CCK/OFDM modulation, and uses 2.4GHz frequency band to be backward compatible with the existing 802. 1 1b system. It can not only adapt to the traditional 802. 1 1b standard (the data transmission rate provided by 2.4GHz is 1 1Mbit/s), but also conform to the 802. 1 1a standard (the data transmission rate provided by 5GHz is 50. Users can also configure multimode wireless local area networks compatible with 802. 1 1a, 802. 1b and 802.1g, which is beneficial to promoting the development of wireless network market.
(4) Other relevant agreements.
The IEEE802 working group will continue to discuss the 802. 1 1 series protocols in the future, and plans to launch a series of protocols to improve the application of wireless local area networks, including 802. 1 1e (defining service quality and service type) and 802.1/kloc-0. Authentication), 802. 1 1j (Japanese 4.9GHz specification), 802. 1 1k (advanced wireless/network measurement specification) and related protocols of Qualcomm Quantity Research Working Group.
2. Bluetooth mode (Bluetooth)
Bluetooth profile (Bluetooth Profile) is a public and license-free specification formulated by SIG (Special Interest Group), and its purpose is to realize short-distance wireless voice and data communication. Bluetooth technology works in ISM band of 2.4GHz, the data rate of baseband part is 1Mbit/s, and the effective wireless communication distance is10 ~100 m. It adopts time division duplex transmission scheme to realize full duplex transmission. Bluetooth technology adopts automatic seeking technology and fast frequency hopping technology to ensure the reliability of transmission, and has omni-directional transmission ability, but it does not need to orient the connected equipment. It is an improved wireless LAN technology, but its equipment size is smaller and its cost is lower. At any time, as long as Bluetooth technology products enter the effective range of the other party, they will immediately transmit address information and form a network. All these tasks are done automatically by the equipment, without the participation of users.
3.HomeRF standard
Before the Federal Communications Commission (FCC) formally approved the HomeRF standard, the HomeRF Working Group formulated a specification in 1998 to realize wireless communication of voice and data in the home, namely * * * Enjoy Wireless Access Protocol (SWAP). This protocol is mainly aimed at home wireless LAN, and its data communication adopts the simplified IEEE 802.438+05438+0 protocol standard. After that, the HomeRF working group formulated the HomeRF standard, which is an open standard combining IEEE802. 1 1 and the pan-European digital cordless telephone standard (DECT). HomeRF standard adopts spread spectrum technology, works in 2.4GHz band, can support four high-quality voice channels at the same time, has the advantage of low power consumption, and is suitable for notebook computers.
4.HyperLAN/2 standard
In February, 2002, ETI's Broadband Wireless Access Network (BRAN) team released the HiperLAN/2 standard. HiperLAN/2 standard was developed and formulated by the Global Forum (H2GF), which works in 5GHz band and adopts OFDM modulation mode. The highest rate of the physical layer can reach 54Mbit/s, which is a high-performance LAN standard. HyperLAN/2 standard defines a variety of signaling and measurement methods, such as dynamic frequency selection, wireless cell handover, link adaptation, multi-beam antenna and power control, to support the functions of wireless networks. The network based on HyperRF standard has its specific application, which can be used in the last segment of enterprise LAN to support the IP mobility of users between subnets. In hot spots, it provides long-distance high-speed access to the Internet for business people, and as a supplement to the W-CDMA system, it is used for 3G access technology, so that users can move between two networks or automatically switch services without affecting communication.
5. Comparison of WLAN standards
802. 1 1 series protocols are formulated by IEEE and are the dominant wireless LAN standards at present. HomeRF is mainly designed for home network, which is a combination of 802. 1 1 and DECT. HomeRF and Bluetooth both work in ISM band of 2.4 GHz, and both adopt frequency hopping spread spectrum (FHSS) technology. So there is almost no interference between HomeRF products and Bluetooth products. Bluetooth technology is suitable for loose networks, allowing devices to establish connections for single data, while HomeRF technology is not as casual as Bluetooth technology. Before setting up HomeRF network, it is necessary to determine a unique identification code for each network member in advance, which is safer than Bluetooth technology. 802. 1 1 adopts TCP/IP protocol, which is suitable for more powerful networks, and its effective working distance far exceeds Bluetooth technology and HomeRF.
Fourthly, the architecture of WLAN.
1. Main components of wireless local area network
(1) wireless network card. Provide system interfaces as rich as wired network cards, including PCMCIA, Cardbus, PCI and USB. In wired LAN, the network card is the interface between the network operating system and the network cable. In WLAN, they are the interfaces between the operating system and the antenna, which are used to create a transparent network connection.
(2) Access point. The access point acts as a LAN hub. It receives, buffers, stores and transmits data between wireless LAN and wired network to support a group of wireless user equipment. Access points are usually connected to wired networks through standard Ethernet cables and communicate with wireless devices through antennas. When there are multiple access points, users can roam and switch between access points. The effective range of an access point is 20~500m. According to the technology, configuration and usage, one access point can support 15~250 users. By adding more access points, WLAN can be easily expanded, thus reducing network congestion and expanding network coverage.
2. Configuration mode of WLAN
(1) point-to-point mode. The application of Ad-hoc mode includes multiple wireless terminals and a server, all of which are equipped with wireless network cards, but they do not connect access points and wired networks, but communicate with each other through wireless network cards. It is mainly used to build WLAN quickly and conveniently where there is no infrastructure.
(2) Infrastructure mode. Infrastructure model. This mode is the most common architecture at present, including an access point and multiple wireless terminals. Access points are connected with wired networks through cable connections and wireless terminals through radio waves, which can realize communication between wireless terminals and between wireless terminals and wired networks. By replicating this model, a larger wireless network with multiple access points can be realized.
The future research direction of verb (abbreviation of verb)
As mentioned above, the research and application of WLAN technology is in the ascendant, and it is a research hotspot in the field of wireless communication and even the whole communication industry. From the further popularization and application of WLAN, the future research direction mainly focuses on security, mobile roaming, network management and the relationship with other mobile communication systems such as 3G.
1. Security issues
The IEEE802. 1 1 protocol standard proposes two security solutions. One is the Robust Security Network (RSN), a security framework built by IEEE 802.1/security task force (TGi). The network uses IEEE 802. 1x to provide port-based access control, authentication and key management. This standard uses Extensible Authentication Protocol (EAP) to authenticate users. Remote authentication dial-in user service protocol (RADIUS) is used for communication between authentication server and users, and RADIUS protocol is widely used for authentication, authorization and accounting (AAA) of network access. Because IEEE 802. 1x is mainly designed for wired LAN, there are inevitably loopholes in using IEEE 802. 1x in wireless LAN. Therefore, although the security performance of WLAN is greatly improved, the combination of 802. 1x and 802. 15438+0 still cannot provide sufficient security.
Another way is the virtual private network (VPN) security technology widely used in local area network and remote access. Different from the security technology adopted in the 802. 1 1b standard, in the IP network, VPN mainly adopts IPSec technology to ensure the security of data transmission. For users with high security requirements, combining the existing VPN security technology with 802. 1 1b security technology is an ideal security solution for WLAN at present.
2. Roaming handover problem
The roaming problem of WLAN is another important problem after the security problem. In the wireless network, if the access location is moved when using the wireless LAN access service, once the mobile terminal goes beyond the subnet coverage, the IP packet cannot reach the mobile terminal, and the ongoing communication will be interrupted. Therefore, IETF has formulated a series of standards to extend the mobility of IP networks. The so-called mobile IP refers to the technology that multiple subnets on an IP network can use the same IP address. This technology is realized by using special routers called home agent and foreign agent to manage the network where the network terminal is located. In the mobile IP system, the user's mobile terminal can always use a fixed IP address for network communication, and TCP connection can be established without interruption no matter how it moves. In the wireless LAN system, the wide application of mobile IP technology can break through the geographical limitation of the network and overcome the communication interruption and authority change caused by the use of dynamic host configuration protocol (DHCP) across network segments.
3. Wireless network management issues
Compared with wired network, wireless LAN has very unique characteristics, so it is necessary to establish a corresponding wireless network management system. In addition to the system structure, user needs and typical applications, a good wireless network management system must also consider the following factors:
(1) standard network management communication mode. The network management subsystem is usually connected to a central host. The network management subsystem must be based on industrial standard management protocols (such as SNMP), so as to monitor the status information on each link between the host and the subsystem, and quickly analyze and solve problems according to the status information.
(2) Network monitoring and reporting. The host must be able to monitor all units in the wireless network system. Considering that the connectivity of wireless network is not as stable as wired network, the wireless network management system must monitor and report the changes of wireless signals, service types and loads of access points, and can also automatically discover new devices entering the wireless network architecture.
(3) Effective use of bandwidth. Although the available bandwidth of wireless network is gradually increasing with the development of new technology, it is still far less than wired LAN. Therefore, the rational use of bandwidth must be considered in practical applications.
4. Wireless LAN and 3G
Whether WLAN will pose a threat to the third generation mobile communication system is a concern of the industry in recent years. In fact, WLAN and 3G use two completely different technologies to meet different needs. Different from 3G, WLAN is not a complete whole network solution, but only used to meet the needs of small user groups. WLAN and 3G can complement each other, so it will not pose a threat to 3G operators, and operators can also benefit from WLAN and 3G storage. The research of NorthStream shows that the combination of WLAN with 3G and GPRS can improve user satisfaction and business volume, thus increasing the profits of mobile operators. As an important supplement to 3G, WLAN can establish wireless Internet connection in airport lounge, hotel lounge, coffee shop and other places.
Ending of intransitive verbs
After more than 10 years of development, wireless local area network (WLAN) is becoming more and more mature and widely used, and it will enter the mainstream application from a small scale. It is estimated that the global sales of WLAN access points will increase steadily from 500,000 in 2000 to 4.5 million, with an annual growth rate of 55%. The sales volume of wireless network cards will increase from about 3 million in 2000 to 34 million in 2005, with an annual growth rate of 53%. In the next few years, WLAN technology will be more mature, product performance will be more stable, the market will continue to grow, prices will continue to decrease, large equipment providers will enter this market, and most enterprises and companies will adopt WLAN for internal network construction.
Faced with such a good development prospect, China should vigorously promote the research and practical application of WLAN technology and seize the opportunity of WLAN development. This will not only greatly promote the development of national informatization, but also provide a good opportunity for China's information industry and communication market to enter the international market.