Advantages of the standard IEEE802.11g Wireless LAN technology

This article mainly discusses the technical advantages of the wireless LAN Standard IEEE802.11g.

Data Transmission Based on OFDM technology

With the wide application of wireless LAN technology, users have higher and higher requirements on data transmission rates. However, in the indoor environment, the complex electromagnetic environment, the multi-pass effect, frequency selective fading, and other interference sources make it more difficult to achieve high-speed data transmission in wireless channels than in Wired channels, IEEE802.11g uses OFDM modulation technology to achieve high-speed data transmission.

The OFDM technology is actually a kind of MCM (Multi-Carrier Modulation, Multi-Carrier Modulation). Its main idea is to divide the channel into many orthogonal subchannels, narrowband modulation and transmission are performed on each sub-channel to reduce mutual interference between sub-channels. The signal bandwidth of each sub-channel is smaller than the relevant bandwidth of the Channel. Therefore, the frequency selective fading of each sub-channel is flat, greatly eliminating inter-symbol interference.

In the OFDM system, the carriers of subchannels are orthogonal to each other, and their spectrum overlaps with each other. This not only reduces the mutual interference between subcarriers, but also improves the spectrum utilization. In each sub-channel, IFFT and FFT methods can be used for orthogonal modulation and decoding. With the development of large-scale integrated circuit technology and DSP technology, IFFT and FFT are very easy to implement. The introduction of Fast Fourier Transform FFT greatly reduces the implementation complexity of OFDM and improves the system performance.

Generally, wireless data services have asymmetry, that is, the amount of data transmitted in the downlink is much larger than the amount of data transmitted in the uplink. Therefore, both the user's high-speed data transmission needs and the wireless communication itself require the physical layer to support asymmetric high-speed data transmission, however, OFDM can easily achieve different transmission rates in the uplink and downlink by using different numbers of subchannels.

Due to the frequency selectivity of wireless channels, all sub-channels are not in the fading State at the same time. Therefore, we can use Dynamic Bit Allocation and dynamic sub-channel allocation methods, fully utilizes sub-channels with high signal-to-noise ratio to improve system performance. Because narrowband interference can only affect a small number of subcarriers, the OFDM system is able to resist this interference to some extent.

The OFDM technology has broad development prospects and has become the core technology of the fourth band mobile communication. IEEE802.11a/g standard uses OFDM modulation technology to support high-speed data transmission. Currently, OFDM integrates space-time encoding, diversity, and interference, including inter-symbol interference (ISI) and adjacent channel interference (ICI) suppression and Smart Antenna Technology to maximize the reliability of the physical layer. For example, the performance can be further optimized by combining technologies such as adaptive modulation, adaptive encoding, dynamic sub-carrier allocation, and dynamic bit allocation algorithms.

Standard IEEE802.11g frame Combination Method

From the network logic structure, 802.11 only defines the physical layer and the Media Access Control (MAC) Sub-layer. The MAC layer provides competitive and non-competitive use of shared wireless media, and features such as wireless media access, network connection, data verification, and confidentiality.

The physical layer provides physical connections for the data link layer to achieve transparent transmission of bit streams. The unit of transmitted data is bit ). The physical layer defines the mechanical, electrical, and process features of communication equipment and interface hardware to establish, maintain, and release physical connections. The physical layer consists of three parts: the physical layer management layer, the physical layer convergence process Sub-layer PLCP), and the physical media dependent sub-layer PMD ).

The Physical Frame Structure of 802.11g can be divided into the Preamble signal, header and payload ). Preamble: it is mainly used to determine when data is sent and received between the mobile station and the Access Point. Other mobile stations are notified during transmission to avoid conflicts and transmit synchronous signals and frame intervals at the same time. The front signal is complete, and the receiver starts to receive data. Header: After Preamble, it is used to transmit important data such as the load length, transmission rate, and service information. Payload: because of the data rate and the number of bytes to be transferred, the length of the load package varies greatly, which can be very short or very long. During the transmission of a single frame of signal, the more transmission time Preamble and Header occupy, the less transmission time used by Payload, and the lower transmission efficiency.

Based on the features of the above three modulation technologies, 802.11g uses key technologies such as OFDM to ensure its superior performance, and modulated Preamble, header, and payload, respectively, this frame structure is called the OFDM/OFDM method. In addition, the draft standard of 802.11g specifies options and mandatory options. To ensure compatibility with 11b, the optional modulation modes of si-/ OFDM and si-/ PBCC can also be used.

1. OFDM modulation is required to ensure the transmission rate reaches 54 Mbps.

2. It is required to ensure backward compatibility by using KNN modulation.

3. as an option, the content of the left-side Navigation Pane and the content of the left-side Navigation Pane are the same as that of the left-side Navigation Pane.

OFDM/OFDM: The frontend, header, and load are all modulated by OFDM, and the transmission rate can reach 54 Mbps. One feature of OFDM is that it has a short leading signal. The frame header of the modulation signal is 72 s compared with that of the content. The frame header is an important part of a signal. It takes less time to transmit data. OFDM allows shorter headers to transmit data for more time, with high transmission efficiency. Therefore, it is a good choice for the 11 Mbps transmission rate, but the OFDM modulation technology must be used to continue to increase the speed. The maximum transmission rate is 54 Mbps. The OFDM/OFDM method in 802.11g protocol can also coexist with WIFI, but it needs to use the RTS/CTS protocol to solve the conflict problem.

The UDF/OFDM protocol is a hybrid modulation method that is optional for the 802.11g protocol. The signal header and the leading signal are transmitted in the modulation mode using the Supplementary Code-keying (ASR), and the load is transmitted using the OFDM technology. Because the OFDM technology is separated from the content of the content. Therefore, the conversion between the PREAMBLE and PAYLOAD requires the conversion of the cosine-and OFDM.

802.11g uses the technique of KNN/OFDM to ensure coexistence with 802.11b. 802.11b cannot demodulated the data in the OFDM format, so it is inevitable that there will be a data transmission conflict. 802.11g can transmit the header and the leading Signal by using the content-sharing technique to make 802.11b compatible, enable it to receive 802.11g headers to avoid conflicts. This ensures backward compatibility with 802.11b WIFI devices. However, because the Preamble/header is implemented using the KNN modulation, the overhead increases and the transmission rate is lower than that of OFDM/OFDM.

The same as that of the UDF/PBCC and the UDF/OFDM systems, PBCC is also a hybrid waveform. the header is modulated by the UDF and the load is modulated by PBCC, so that it can work at a high rate and be compatible with 802.11b. The highest data transmission rate of PBCC modulation is 33 M, which is lower than the transmission rate of OFDM or KNN/OFDM.

IEEE802.11g standard with backward compatibility

The IEEE802.11g standard works in the 2.4GHz band with the 11b standard, while the 11g standard can be compatible with the 11b standard. Compatibility means that the 11g device can communicate with the 11b standard device in the same AP node network .) One of the biggest features of the 802.11g protocol is to ensure compatibility with 802.11b and Wi-Fi systems. 802.11g can receive OFDM and KNN data, but the traditional Wi-Fi system can only receive the information of the content. This creates a problem, that is, how to solve the conflict problem caused by the inability of 802.11b to describe the Information Frame header in the OFDM format in the environment where 11g and 11b coexist. The 802.11g protocol uses the RTS/CTS technology to solve the above problems.

Initially, 802.11MAC introduced the RTS/CTS mechanism to solve the problem of the hidden station, that is, the sender station cannot detect the data sent by the other station, so the receiving station has a collision ), workstation B is within the signal transmission range of workstation A and workstation C, while workstation C is outside the signal transmission range of workstation. When workstation A accesses the media from each station under this mechanism, the station that obtains the media control does not directly send data frames but sends the RTS frame (Ready to send) to the receiving station ), the receiving site replies to the CTS frame clear to send). other sites that are not the target site of the RTS can read the reserved information of the transmission time after receiving the RTS frame, that is, the network allocation vector NAV ), update the local NAV accordingly. The non-CTS destination station that receives the CTS frame also reads the NAV and updates the local NAV, in this way, both the station in the transmission station signal transmission range and the station in the receiving station signal transmission range can understand the idle media.

The hybrid operation of 802.11b and 802.11g is very similar to that of concealed stations. 11b devices cannot receive 11g message frame headers in OFDM format. Therefore, they can be solved using the RTS/CTS mechanism. In an environment where 802.11g and 802.11b work together, there are both 11g devices and 11b devices in the AP service area of the same access point.) before transmitting data, each worker Node, an RTS frame must be sent to the AP. Data Transmission starts when the AP returns a CTS frame. The workbench sends the RTS to the AP node to return the CTS signal, so that all the workstations can receive the signal, thus avoiding the collision between hybrid sites, solve the compatibility problem between 11b AND 11g. The RTS and CTS signals both adopt the KNN signal ). The RTS/CTS mechanism also brings additional overhead to the system, so the data rate is lower than the IEEE802.11a system that only uses OFDM, but for the IEEE802.11b system that is backward compatible and will replace at GHz, the data rate has been greatly improved. The compromise shows that IEEE802.11g has a great advantage. For the current IEEE802.11g system, every AP monitors the mobile devices next to it. If there is no IEEE802.11b device, the system will automatically cancel the RTS/CTS mechanism and increase the system throughput accordingly. In the future, when IEEE802.11g system completely replaces IEEE802.11b and only uses OFDM modulation technology, it will be more advantageous than IEEE802.11a system.

IEEE802.11g standard upgraded to dual-band multimode Application Mode

Introduction of dual-band multimode WLAN

The 802.11 Working Group has successively released the 802.11a, 802.11b, and 802.11g physical layer standards. The wide variety of standards improve the performance of the wireless LAN and bring new problems. As mentioned above, 802.11a and 802.11b work at 5 GHz for 802.11a in different frequencies, while 802.11b works at 2.4 GHz respectively. 802.11a uses OFDM for different modulation modes, while 802.11b uses the PCP mode ). A workstation using 802.11b standard equipment enters an 802.11a standard community and its AP nodes use 802.11a standard equipment. Therefore, it must be replaced with a year-on-year standard network device to work properly. This is the network compatibility problem caused by different physical layer standards.

To solve the above problems and make the network devices of different standards more freely move, there is an optimized method of Wireless LAN: "dual-band multimode. Like the development of wired networks, wired networks are mainly working in multi-mode. For example, 10 Mbps/Mbps hybrid LAN accelerates the development of wired networks and has become the main working method of wired LAN. WLAN has also begun to develop towards "multimode.

Dual-band multimode WLAN

The so-called "dual-band" product refers to an adaptive product that can work at 2 to 4 GHz and 5 GHz. That is to say, it supports two standard products: 802.11a and 802.11b. Because 802.11b and 802.11a are not compatible with each other, you must change the wireless network adapter along with the location when accessing public wireless access networks that support 802.11a and 802.11b, this brings great inconvenience to users, and the use of wireless LAN products that support 802.11a/B dual-band adaptive can solve this problem well. Dual-band products can automatically identify 802.11a and 802.11b signals and support roaming connections, so that users can stay connected in any network environment. The 802.11a standard of 54Mbps and 11Mbps802. the 11b standard has its own advantages and disadvantages, but from the user's point of view, this dual-band adaptive wireless network product is undoubtedly a solution that organically integrates the two wireless network standards, large investment is required.

With the emergence of the 802.11g standard, dual-band products subsequently integrated the standard into it, becoming a comprehensive wireless network solution. This product, also known as Dual-Band and Multimode WLAN, can be connected to three standard products ). As the name suggests, "dual-band and three-mode" is a product that runs in two frequency bands and supports three standard modes. This product also supports three standard adaptive wireless products, 802.11a, B, and g, it can achieve interconnection and compatibility with most of the current wireless LAN standards, allowing users to smoothly and quickly roam in 802.11a, B, and g standard wireless networks, spanning over three standards, this type of product is still rare on the market, but it is the development direction of the "dual-band" product and has a good prospect.

Application of dual-band multimode WLAN

With the continuous integration of 802.11 standard B, a and g, dual-band multimode Wireless LAN is becoming more and more advantageous. First, as described above, standards B, a, and g have their own advantages and characteristics as well as their work environments. The dual-band multimode mode uses different standards based on different environments to maximize the advantages and features of the 802.11 standard. Second, in hot areas such as stations, airports, warehouses, supermarkets, etc., the density of Wireless LAN is high, and inter-zone switching is frequent. The dual-band multimode working method is also a good idea to solve the seamless switching problem between cells.

Summary

As a new standard, compared with the previous 802.11 protocol, IEEE802.11g Wireless LAN has many advantages. First, the short-term advantage, that is, compatibility with IEEE802.11b products, makes the wireless LAN smooth transition to high-speed, prolongs the service life of IEEE802.11b products, and reduces user investment; second, long-term advantages, in the future, wireless LAN products can use dual-band multimode, that is, the GHz and 5 GHz frequencies support the 11b, 11a, and 11g physical layer standards at the same time. OFDM modulation technology is used in both frequencies, increase data transmission rate. To sum up, the IEEE802.11g protocol standard is a wireless LAN Standard with great development potential, and will surely make the wireless LAN develop rapidly towards a higher data rate.

In recent years, wireless LAN products in have gradually matured, prices have also gradually declined, and corresponding software has become increasingly mature. In addition, wireless LAN has been able to provide mobile Internet multimedia services by combining with Wan. Undoubtedly, the 802.11g standard will play an important role with its high transmission rate and network flexibility.