This paper discusses the IEEE802.11g wireless LAN standard main technical advantages.
Data transmission based on OFDM Technology
As wireless LAN technology increasingly wide range of users for data transfer rates have become increasingly demanding. But indoors, the more complex electromagnetic environment, and more by the effects of frequency selective fading and other interference sources in the presence of wireless channel to achieve high-speed data transmission than the cable channel difficulties, IEEE802.11g standard with OFDM modulation technology high-speed data transmission.
OFDM technology is MCM (Multi-Carrier Modulation, multi-carrier modulation) is one of the main idea is: to channel into many orthogonal subchannels, each subchannel on narrowband modulation and transmission, thus reducing the quantum channel mutual interference. The signal bandwidth of each subchannel is less than the relevant channel bandwidth, so each sub-channel frequency selective fading is flat, substantially eliminating the ISI.
Since each sub-channel OFDM system each orthogonal carrier, their spectrum is overlapping, it will not only reduce interference between subcarriers, at the same time improve the efficiency of spectrum. In each sub-channel in quadrature modulation and demodulation this IFFT and the FFT method can be used to achieve, as large scale integrated circuit technology and DSP technology, IFFT and FFT are very easy to achieve. Fast Fourier Transform (FFT) of the introduction, implementation greatly reduces the complexity of OFDM to improve the system performance.
The existence of wireless data services are generally non-symmetry, that is, downlink data transmission is much larger than the uplink data throughput. Therefore, both from the user demand for high-speed data transmission services, or from the wireless communication itself to consider, all hope that the physical layer to support non-symmetric high-speed data transmission, and easily by using a different number of OFDM sub-channel to achieve uplink and downlink in different transfer rate.
As the existence of frequency selective radio channel, all the sub-channel will not decline while the situation in more depth, so you can dynamically allocate bits and dynamic subchannel allocation, take full advantage of the high SNR subchannels, improve system performance . As the narrowband interference can only affect a small portion of sub-carriers, the OFDM system in a way to resist such interference.
OFDM technology has very broad prospects for the development, has become the fourth mobile communication with the core technology. IEEE802.11a / g standard to support both high-speed data transmission using OFDM modulation techniques. Currently, OFDM combining space-time coding, diversity, interference (including inter-symbol interference ISI and adjacent channel interference ICI) suppression, and smart antenna technology for maximum improvement of the reliability of the physical layer. If combined with adaptive modulation, adaptive coding and dynamic subcarrier allocation, the dynamic bit allocation algorithm technology, its performance can be further optimized.
IEEE802.11g standard way of framing
Logical structure from a network point of view, 802.11 defines the physical layer and media access control (MAC) sublayer. MAC layer provides the competition on the shared wireless medium to use and non-competitive use of a wireless media access, network connectivity, data authentication and confidentiality functions.
Physical layer data link layer provides a physical connection, to achieve a transparent bit stream transmission, the transmission of data units of bits (bit). Physical layer defines the hardware interface, communication equipment and machinery, electrical function and the characteristics of the process to establish, maintain and release physical connections. Physical layer consists of three parts: the physical layer management, physical layer convergence procedure sublayer (PLCP), and physical medium dependent sublayer (PMD).
802.11g physical frame structure is divided into a leading signal (Preamble), the letter head (header) and load (payload). Preamble: mainly used in mobile stations and access points to determine when to send and receive data between the transmission in progress to other mobile stations in order to avoid conflict, simultaneous transmission and frame sync interval. Leading to complete signal, the receiver had just started to receive data. Header: In the Preamble, the used to transmit the load of some important data such as length, data rate and service information. Payload: As the data rate and different number of bytes to transfer the load of the packet size vary widely, can be very short can be quite long. In a signal transmission process, Preamble and Header share of transmission time more, Payload less time with the transmission, the lower the efficiency of transmission.
Comprehensive features of the three modulation techniques, 802.11g uses OFDM and other key technology to secure their superior performance, respectively Preamble, header, payload modulation, frame structure such as OFDM / OFDM mode. In addition, 802.11g draft standard provides for optional and will be options, in order to protect compatible with 11b also can be used CCK / OFDM and CCK / PBCC optional modulation.
1.OFDM modulation options to protect the transmission rate will reach 54Mbps.
2. Using CCK modulation for backward compatibility must-protection.
3.CCK/PBCC and CCK / OFDM as an option.
OFDM / OFDM: leading, letter head and the load are transmitted using OFDM modulation, the transmission rate up to 54Mbps. A characteristic of OFDM is its short leader signal, compared with CCK modulation signal frame header is 72 s, however, the frame header OFDM modulated signal is 16 s. Frame header is an important component of a signal, frame header occupies less time to improve the signal transmission of data. OFDM allows a shorter letter head to the more time for transmission of data, with high transmission efficiency. Therefore, the transmission rate of 11Mbps, CCK modulation is a good choice, but to continue to enhance the rate of OFDM modulation techniques must be used. Its maximum transfer rate of up to 54Mbps. 802.11g protocol OFDM / OFDM mode and WIFI can co-exist, but it need to use RTS / CTS protocol to resolve the conflict.
CCK / OFDM: a mixed modulation, is the optional 802.11g. The letter signals the first and leading complementary code keying (CCK) modulation transmission, OFDM technology transfer load. Because, OFDM technology and technology is the separation of CCK. Therefore, in the PREAMBLE and PAYLOAD between CCK and OFDM have the conversion.
802.11g using CCK / OFDM technology to protect and 802.11b coexistence. 802.11b can not demodulate OFDM formats, so the conflict will inevitably occur in data transmission, 802.11g CCK technology transfer using the letter head and leading the signal can be compatible to 802.11b, 802.11g can receive a letter to head to avoid conflict. This protection device with 802.11b WIFI backward compatibility, but because of Preamble / header using CCK modulation, spending increased transmission speed than the OFDM / OFDM way down.
CCK / PBCC and CCK / OFDM, like, PBCC is also a mixed waveform, Baotou using CCK modulation and load using PBCC modulation, so that it can work in a high bit rate and is compatible with 802.11b. PBCC modulation maximum data transfer rate is 33M, so it is more than OFDM or CCK / OFDM transmission rate low.
IEEE802.11g standard with backward compatibility
11b standard IEEE802.11g at work in the same 2.4GHz frequency band, and 11g and 11b can be standards-compliant equipment. (Compatibility refers to 11g and 11b devices can be standard equipment on the same node in the network interconnection AP.) 802.11g one of the biggest features of the agreement is to protect the system compatible with 802.11b and WIFI. 802.11g OFDM and CCK can receive data, but traditional systems can only receive CCK WIFI information. This poses a problem, that co-exist in the 11g and 11b to solve because the environment can not be demodulated OFDM 802.11b frame header format of information arising from the conflict. 802.11g protocol with RTS / CTS technology to solve these problems.
Initially, 802.11MAC introduced the RTS / CTS mechanism is to solve the hidden station problem (that is not detected sending station to send data to another stand, which collided in the station), workstation Workstation A and Workstation B in the signal transmission range of C within the workstation C workstations A signal transmission in scope. When workstation A to stand in this mechanism to access each medium to obtain media control station does not send data directly to the receiving station to send RTS frame but the frame (Ready to send), CTS frame back terminal (clear to send ), other non-RTS destination station receives the RTS frame the site after reading one of the reserved transmission time information, that is, the network allocation vector (NAV), and accordingly update the local NAV, received the non-CTS CTS frame destination station One of the NAV also read and update the local NAV, so whether it is in the sending station signal transmission range of station or in the receiving station signal transmission range of stations can understand busy medium condition.
802.11b and 802.11g mixed work of the problem is very similar with the hidden station, 11b devices can not receive the information 11g OFDM frame header format, so you can use RTS / CTS mechanism to resolve. Mixed 802.11g and 802.11b in the work environment (ie, access point AP in the same service area in both devices have 11b 11g equipment), each working node in the transmission of data information is required before sending a RTS frame to the AP, from AP returns a CTS frame, we start transferring data. Send RTS to the AP node table to return CTS signal, so that all of the tables can receive the signal, thus avoiding collisions between the mixed site, solve the compatibility issue 11b and 11g (RTS and CTS signals are used CCK signal). RTS / CTS mechanism to bring the system overhead, and thus data rate than the IEEE802.11a OFDM systems use only low, but for backward compatibility and will be replaced on the 2.4GHz IEEE802.11b systems, data rate there are much improved, still has a very eclectic view of the advantages of IEEE802.11g. IEEE802.11g systems for now, next to it for each AP to monitor the movement of equipment, when no IEEE802.11b devices, the system will automatically cancel RTS / CTS mechanism, the corresponding increase in system throughput. IEEE802.11g future system, when fully replace the IEEE802.11b products, only use OFDM modulation technology, compared with IEEE802.11a system to another advantage.
Upgraded to dual-band IEEE802.11g standard way of multi-mode applications
The introduction of multi-mode dual-band WLAN
IEEE802.11 Working Group has launched the 802.11a, 802.11b and 802.11g physical layer standard. Variety of standards to enhance the performance of wireless local area network, also brings new problems. As noted earlier 802.11a and 802.11b are working in different frequency bands (802.11a working in 5GHz, and 802.11b work in the 2.4GHz), using different modulation (802.11a uses OFDM, while 802.11b uses CCK way). A standard device with 802.11b 802.11a standard workstation into a district in (the AP node using 802.11a standard equipment), the node can not contact the AP. Therefore, the need to replace the standard network equipment for the same period last year, to work properly. This is caused by different physical layer standards for network compatibility issues.
In order to address these problems so that different standards of network equipment can be more free to move, there is a wireless local area network optimization approach: "dual-band multi-mode" of work. The development process, as cable networks, cable networks are now the main way of working in multi-mode, for example, a mixture of local area network 10Mbps/100Mbps accelerated the development of cable networks, a wired LAN's main work. WLAN also began to "multi-mode" trends.
Dual-band multi-mode WLAN brief
The so-called "dual-band" products, is available in 2.4GHz and 5GHz work of adaptive products. In other words, can support both 802.11a and 802.11b standards. Since 802.11b and 802.11a standard equipment two are not compatible, the user support of 802.11a and 802.11b access public wireless access network, the location must be replaced with wireless cards, giving users a great inconvenience, but with support for 802.11a / b dual-band adaptive wireless LAN products can be a good solution to this problem. Dual-band 802.11a products can automatically identify and 802.11b signals and to support roaming connectivity, allowing users in any network environment can remain connected. 54Mbps in 802.11a standard and 11Mbps802.11b standard advantages and disadvantages, but from the user point of view, this dual-band adaptive wireless networking products, is undoubtedly a standard two kinds of organic integration of wireless networking solutions, needed investment are also high.
With the birth of 802.11g standard, the standard dual-band products into which subsequently became a full range of wireless networking solutions. Three criteria which can be connected with the product called "dual-band tri-mode" products, also known as dual-band multi-mode (Dual Band and Multimode WLAN). "Dual-band tri-mode," the name suggests, is to run in two frequency bands to support the three modes (standard) products, which also supports the three standard adaptive 802.11a/b/g wireless products through the product, can be realized with Most wireless LAN standard and compatible with Internet, enable users to seamlessly roam in the high-speed 802.11a, b, g standard wireless networks across the three criteria above, these products are now available in the market is still relatively rare, but it is a "dual" product development direction and have good prospects.
Multi-mode dual-band WLAN applications
With 802.11 b, a and g continuous integration, dual-band multi-mode wireless LAN has increasingly shown its advantages. First, as noted above b, a and g standards have their own advantages and characteristics and suitable for their working environment. Dual-band multi-mode approach depending on the environment, using a different standard, the 802.11 standard maximize their advantages and characteristics. Secondly, in hot spots such as railway stations, airports, warehouses, supermarkets, etc., wireless local area network density, frequent switching between communities. Dual-band multi-mode works also address the issue of seamless handoff between cell a good idea.
Summary
As a new standard, compared with the previous 802.11, IEEE802.11g wireless LAN has many advantages. One short-term advantage, that is compatible with IEEE802.11b products, so that a smooth transition to high-speed wireless local area network to extend the life of IEEE802.11b products, reducing the user's investment; second, long-term advantage, the future of wireless LAN products can be used dual-band multi-mode approach, that is 2.4GHz and 5GHz frequency bands simultaneously support 11b, 11a and 11g physical layer standards are used in the two band OFDM modulation technology to increase the data transfer rate. In summary, IEEE802.11g protocol standards is a huge potential for development of wireless LAN standards, will enable wireless local area network towards higher data rates to flourish.
In recent years, IEEE802.11 Wireless LAN products have matured, prices have decreased, the corresponding software matures. In addition, wireless local area network and wide area network has been able to form the combination of mobile Internet, multimedia services. No doubt, 802.11g standard will be its high transmission rate and the flexibility to play an important role in networking.