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无线上网原理(Principles of wireless internet access)

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无线上网原理(Principles of wireless internet access)无线上网原理(Principles of wireless internet access) 无线上网原理(Principles of wireless internet access) Wireless networks are divided into two configurations: point-to-point and master-slave. Point to point configuration is used to connect PC or portable computers, al...

无线上网原理(Principles of wireless internet access)
无线上网原理(Principles of wireless internet access) 无线上网原理(Principles of wireless internet access) Wireless networks are divided into two configurations: point-to-point and master-slave. Point to point configuration is used to connect PC or portable computers, allowing the computer to move within the coverage of the wireless network and automatically establish point-to-point connections and transmit data directly between different systems. In the master-slave configuration, all wireless nodes are connected with the access node, and the access node takes the management of wireless communication and the connection with the wired network. Using master-slave configuration, wireless users do not need to consume electricity for finding other nodes when they work within the coverage area of the nodes. Therefore, saving resources is the most ideal way to work with low power consumption. Wireless network transmission technology is mainly divided into "radio frequency technology" and "infrared technology" two kinds. Among them, infrared technology is only suitable for short-range wireless transmission (usually less than one meter), and radio frequency technology has a wide range of coverage, and it is a more common wireless transmission technology. Agreement 802.11 standard Wireless LAN products appear on the market for about 990 years. In 1997, the IEEE 802.11 wireless LAN standard was a milestone in the development of wireless networking technology. At present, there are many standards of wireless LAN, the most important one is IEEE 802.11 standard. The IEEE 802.11 standard specifies the standards for two types of wireless LAN types, "point-to-point access" and "master slave structure", and defines wireless specifications such as 2.4GHz frequency hopping, 2.4GHz direct sequence and infrared ray respectively. The wireless LAN protocol announced by IEEE specifies three standards in the physical layer protocol: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS) and infrared, and the three physical layer protocols can be flexibly chosen. Besides the advantages and various performances of WLAN, the ----802.11 standard also enables wireless products of different manufacturers to be interconnected. The promulgation of the 802.11 standard enables wireless LANs to be widely accepted in a variety of mobile environments, including educational, medical, storage, and other industries. Recently, the 802.11 standards have been further improved and revised, and become a joint standard of IEEE/ANSI and ISO/IEC. ISO/IEC sets the standard as ISO 880211. The revision of the 802.11 standard includes the use of a SNMP based MIB to replace the original MIB based OSI protocol. In addition, two new items have been added, 802.11a and 802.11b. ----802.11a, which extends the standard physical layer, requires that the layer use the 5GHz band. This standard uses orthogonal frequency division modulation data, the transmission rate range is 6Mbps ~ 54Mbps. This kind of speed can satisfy the indoor application, and can also satisfy the outdoor application. ----802.11b is another extension of the 802.11 standard, which specifies the use of 2.4GHz frequency bands, and the modulation method uses CKK. CKK comes from the direct sequence spread spectrum technology, medium access control multi rate mechanism (MAC) to ensure that when the distance between the station is too long or too much interference, the SNR is below a certain threshold, the transmission rate can be automatically from the 11Mbps down to 5.5Mbps, or to 2Mbps and 1Mbps based on direct sequence spread spectrum technology adjustment. 802.11 advantages of Technology Reliable communication, this technology has strong anti radio interference performance, and has ideal receiving sensitivity. The range antenna can provide powerful and reliable wireless transmission; Low cost use of wireless LANs avoids the high cost of installing cables, the monthly rental costs of leased lines, and the additional costs associated with the equipment being moved; Flexibility, because there is no cable limit, users can increase workstations or reconfigure workstations wherever they like; Mobile wireless LAN settings allow users to access network data at any time and at any point, without having to specify a specific location of access, so that users can roam in the network; High throughput WLAN can also achieve the data transmission rate of 11Mbps, This number is higher than the speed of the T1 and E1 lines; Quick installation of wireless LAN is very simple, without construction permits, the need for wiring or excavation trenches, installation time is only installed cable network time change. In addition to the wireless LAN standard IEEE 802.11, the 802.11HR (High, Rate, high-speed standards) has also been approved recently. Compared with the past, the wireless LAN speed can reach 11Mbps, but the past can only reach 2Mbps, roaming function, the security of the wireless LAN (through encryption) and management function (based on SNMP) have a great breakthrough, applied more widely, interoperability has been further guaranteed. Important provisions of the IEEE802.11 agreement - because of the wireless local area network transmission medium (microwave, infrared) non limited cable, there are some new technical problems, the objective for the IEEE 802.11 protocol provides some important technical mechanism. ----CSMA/CA protocol We know that the standard protocol for linear LANs in the MAC layer is CSMA/CD, that is, carrier sense multiple access / collision detection. However, because the wireless products adapter is not easy to detect the channel conflict, the new protocol has been defined by 802.11, that is carrier sense multipoint access / collision avoidance (CSMA/CA). On the one hand, the carrier sensing -- Viewing medium is free; on the other hand, avoid collision -- through random time to wait, the probability of signal conflict to a minimum, when the medium is idle to listen, send priority. Not only that, but in order to make the system more stable, 802.11 also provides CSMA/CA with the confirmation frame ACK. A signal conflict may occur once the other noise is disturbed or because the listener fails, and the ACK operating at the MAC layer can provide a fast recovery capability. ----RTS/CTS protocol The ----RTS/CTS protocol, called the request / send protocol, is equivalent to a handshake protocol that is primarily used to solve the hidden terminal problem. - hidden terminals occur in large units (usually in outdoor environments), which results in loss of efficiency and requires error recovery mechanisms. When large capacity files need to be transmitted, it is particularly necessary to prevent the occurrence of hidden terminal phenomena. WaveLAN 802.11 provides the following solution: in the parameter configuration, if the RTS/CTS protocol is used, the upper bound bytes are set at the same time. Once the data to be transmitted is greater than this limit, the RTS/CTS handshake protocol is started. First, the base station A B to send the RTS signal, show that the A is sent to the B number of data, B received the RTS, send a CTS signal to all base stations, that is ready, A can send the rest of the base station, then temporarily halt the troops and wait; A to send data to B, finally, B received after the data, i.e. to all the base station broadcasts ACK acknowledgement frame, so that all of the base again can equal interception, channel competition. Envelope reforming When retransmission frames are seriously disturbed, they must be retransmitted. So if a packet is larger, which is consumed (retransmission time, control signal, recovery mechanism) is greater; then, if reduce the frame size - the package is divided into several small large packets, even if retransmission, only a small packet retransmission, the cost is relatively small many. This can greatly improve the anti-jamming capability of wireless networks in noise jamming areas. Multichannel roaming The human pursuit of freedom is unlimited, and with the growing popularity of mobile computing devices, there is a truly seamless network access device. WaveLAN802.11 is developing in this direction. The transmission frequency band is set on the access device AP (Access Point), and the base station does not have to set a fixed frequency band, and the base station has automatic identification function. The base station is dynamically tuned to the frequency band set by the AP, called scanning. IEEE802.11 defines two modes: passive scan and active scan. Passive scanning means that the base station listens to an indication signal sent by AP and switches to a given frequency band; Active scanning station refers to propose a heuristic request access point AP sends back a response packet containing band information, change to a given frequency band. Thus, when the base station in the coverage area of the access point AP (A) is roaming to the access point AP (B), the base station can adaptively update the AP (B) as the current access point. Wireless spread spectrum technology Summary Below the MAC layer, 802.11 specifies three transmit and receive technologies: spread spectrum (Spread Spectrum) technology, infrared (Infared) technology, and narrowband (Narrow Band) technology. Spread spectrum is divided into direct sequence (Direct, Sequence, DS) spread spectrum technology (referred to as DSSS) and frequency hopping (Frequency, Hopping, FH) spread spectrum technology. Spread spectrum system can be regarded as the two modulation process, the first step, the effective use of conventional signal modulated; the second step, using spread spectrum encoding modulation carrier, which is extended to a very large bandwidth, the spectrum broadening. Spread spectrum coding: - (1) direct sequence spread spectrum technology ---- the principle of direct sequence spread spectrum technology is the modulation of the RF carrier signal using binary bit rapid changes in flow, the binary bit stream appears to be random, in fact is in accordance with the specific algorithm generated by digital circuits, called pseudo random code (Pseudo noise). In the pseudo random code modulation, the carrier phase changes from 0 degrees to 180 degrees, and the modulated carrier wave is mixed with the effective information and transmitted by the transmitter. The same pseudo random code can be generated in the corresponding receiver, and the valid information signal can be analyzed according to the inverse process of the emission. - (2) frequency hopping spread spectrum technology The frequency hopping spread spectrum technology is through the pseudo random code modulation, causes the carrier work the central frequency to jump unceasingly changes, but the noise and the interference signal central frequency can not change. Thus, as long as the same pseudo digital code is generated between the receiver and the transceiver in accordance with a fixed digital algorithm, synchronization can be achieved, and noise and other interference signals can be eliminated. DSSS in IEEE802.11 - - we know that spread spectrum technology takes advantage of the open ISM 2.4GHz band. This is the 2.4 ~ 2.484GHz band without a permit (but transmit power, thus restricted) this band is very crowded, the microwave noise, taken by the sending and receiving technology, will directly affect the rate and quality of microwave transmission. In contrast, direct expansion takes active ownership, and frequency hopping is a passive adaptation. At the same time using the technology of DSSS signal is sub band expansion times without loss; the frequency hopping technology is continuously jumps using multiple frequency points, when the jump to a certain frequency, the frequency to judge whether the noise interference, without transmission of signal, if there is on the basis of the algorithm to the next hop frequency continue to judge. DSSS, a digital signal processor, produces 11 bit random codes that provide the "three strong" of DSSS products": Strong anti-interference ability; Spread spectrum communication systems expand the wider spectrum, the higher the processing gain, the stronger the anti-interference ability. Simply, if the signal spectrum broadening 0 times, then the interference in the wider frequency band to interference, scattered interference power, resulting in the total power under the same conditions, the interference intensity is only the original 1 / 10. In addition, because the receiver of spread spectrum sequence correlation detection, even if there is a similar air signal interference, if cannot detect the sequence of useful signal interference also can not play a big role, so the strong anti-jamming performance is the most prominent advantages of spread spectrum communication. 2. code division multiple access capability; Due to existence of spread spectrum sequence spread spectrum modulation in spread spectrum communication, make full use of all kinds of different code sequence excellent self correlation and cross correlation properties, despread at the receiver by using the correlation detection technology are assigned to different users with different code type, signal system can distinguish between different users. So in the same frequency band for many users can communicate simultaneously without interference. Three High speed and scalability. Because of exclusive channel and code division multiple access, the rate is very high. Because of the IEEE802.11 standard, only 1 bits of 11 bit random symbols are used to transmit data, so throughput is very scalable. Compared with the phase change DQPSK/DPSK modulation technique adopted in general standards, the enhanced mode adopts the direct sequence / pulse position modulation (DS/PPM) technique. PPM technology uses 3 bits of data in a preset 8 bit symbol to transmit data, which makes the transmission rate leap. Channel usage of spread spectrum technology Spread spectrum signal is used, the carrier frequency extended random sequence -- pseudorandom modulation of RF signal or continue to jump and get, so spread spectrum system is different from the traditional communication system, it can greatly share the same channel resources. Each system has extended sequence out of the ordinary to reduce interference from other devices, the receiver only with the same emission can be extended or compressed recombinant sequence spread spectrum transmission signal to obtain the effective information loading. Even if a plurality of spread spectrum devices use the same channel for signal transmission in the same area, they can not interfere with each other as long as different spread spectrum sequences are adopted. The advantage of this channel is that it is the best choice for the spectrum resource of the big city. Application range - wireless spread spectrum communication as a mature high-tech technology, can be used to: (1) in rural areas and communication were diluted to secluded and underdeveloped areas; (2) cable infrastructure already saturated downtown; (3) for the surge and the cable infrastructure lag the new area; (4) the main user / backup communication network the public telecommunications network, supplement.
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