802.11a802.1a802.1/g: three standards of wireless local area network.

IEEE wireless network standard stipulates that the maximum data transmission rate is 54Mbps and the working frequency band is 5GHz.

802. 1 1a standard is the follow-up standard of 802. 1 1b wireless networking standard, which has been widely used in offices, families, hotels, airports and other occasions. Working in 5 GHz-NII frequency band, the physical layer rate can reach 54Mb/s and the transport layer can reach 25Mbps. Can provide 25Mbps wireless ATM interface and 10Mbps Ethernet wireless frame structure interface, as well as TDD/TDMA air interface; Support voice, data and image services; A sector can access multiple users, and each user can take multiple user terminals.

802. The second branch of11is designated as 802. 1 1 a. Desperately, 802. 1 1 was brought into a different frequency band-5.2 GHz-NII frequency band, and was assigned a data rate as high as 54Mbps. Different from single carrier system 802. 1 1b, 802. 1 1a adopts orthogonal frequency division multiplexing (OFDM) multicarrier modulation technology, which improves the frequency channel utilization. Because 802. 1 1a uses the radio frequency spectrum of 5.2GHz, it cannot interoperate with 802. 1 1b or the original 802.1/WLAN standard.

IEEE 802. 1 1b

The bandwidth of IEEE 802. 1 1b WLAN can reach up to 1 1Mbps, which is five times faster than the standard of IEEE 802. 1 1 just approved two years ago, and expands the application field of WLAN. In addition, the bandwidth of 5.5Mbps, 2 Mbps and 1 Mbps can be used according to the actual situation, and the actual working speed is about 5Mb/s, which is almost at the same level as the ordinary wired LAN with 10Base-T specification. As the company's internal facilities, it can basically meet the use requirements. IEEE 802. 1 1b uses an open 2.4GB frequency band and can be used without application. It can be used not only as a supplement to wired network, but also as an independent network, so that network users can get rid of the shackles of network cables and realize real mobile applications.

The principle of IEEE 802. 1 1b wireless local area network is very similar to the familiar IEEE 802.3 Ethernet, which uses carrier sense to control the transmission of information in the network. The difference is that Ethernet adopts CSMA/CD (carrier sense/Collision Detection) technology. All workstations on the network monitor whether there is any information sent in the network, and send their own information when the network is idle. Just like rushing to answer, only one workstation can rob the building, and the other workstations have to wait. If two or more workstations send messages at the same time, there will be conflicts in the network, and these conflicting messages will be lost after the conflict, and each workstation will continue to grab the right to speak. 802. 1 1b WLAN introduced CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) technology and RTS/CTS (Request-to-Send/Clear-to-Send) technology, which avoided the occurrence of conflicts in the network and greatly improved the network efficiency. The principle of CSMA/CA technology here is different from CSMA/CD technology under normal circumstances. The principle is that the station waits for an acknowledgement frame (ACK) from the access point AP (basic mode) or another station (peer-to-peer mode) after sending a message. If the acknowledgement frame is not received within a certain time, it is considered that a collision has occurred and the data will be transmitted. If the station notices the activity on the channel, it will not send data. RTS/CTS works like a modem. Before sending data, the station sends a request frame to the destination station. If there is no activity on the channel, the target station will send a clear frame back to the source station. This process becomes "preheating" other websites, thus preventing unnecessary conflicts. RTS/CTS is only used for very large data packets, and there may be serious bandwidth problems when retransmitting data.

Functions and advantages

Speed: 2.4ghz direct sequence spread spectrum radio station provides the maximum data transmission rate 1 1mbps, which is wireless.

Dynamic rate conversion: when the RF situation becomes worse, reduce the data transmission rate to 5.5mbps, 2mbps and 1mbps.

Scope of use: 802.11bSupport 100 meters (300 meters outdoors; In the office environment, the longest 100m).

Reliability: Ethernet-like connection protocols and packet acknowledgements provide reliable data transmission and effective use of network bandwidth.

Interoperability: Unlike previous standards, 802.438+0 1b only allows one standard signal transmission technology. Weca will certify the interoperability of products.

Power management: 802. 1 1b The network card can enter the sleep mode, and the access point buffers the information to the customer, thus prolonging the battery life of the notebook computer. Roaming support: allows seamless connection between access points when users move between buildings or company departments.

Load balancing: 802. 1 1b network card improves performance by changing the access point connected to it (for example, when the current access point is congested or sends out low-quality signals).

Scalability: Up to three access points can be located at the same time within the effective use range, supporting hundreds of users to support voice and data at the same time.

Security: Built-in authentication and encryption

Basic operation mode:

802. 1 1b operation modes are basically divided into two types: ad-hoc mode and infrastructure mode, as shown in figure 1. Point-to-point mode refers to the communication mode between sites (such as wireless network cards). As long as a PC is plugged in with a wireless network card, it can be connected to another PC with a wireless network card. For small wireless networks, this is a convenient way to connect up to 256 PCs. The basic mode refers to the communication mode when the scale of wireless network expands or wireless and wired networks coexist, and it is the most commonly used 802+0438+0b mode. At this time, a PC with a wireless network card needs to be connected to another PC through an access point (AP). The access point is responsible for the command of frequency band management and roaming, and an access point can connect at most 1024 PCs (wireless network cards). When the number of wireless network nodes increases, the network access speed will slow down with the expansion of the range and the increase of nodes. At this time, adding access points can effectively control and manage bandwidth and frequency bands. When the wireless network needs to be interconnected with the wired network, or when the nodes of the wireless network need to connect and access the resources and servers of the wired network, the access point can serve as a bridge between the wireless network and the wired network.

App application

Functional advantage

Difficult wiring areas provide network services in areas where wiring is difficult or expensive (such as historical buildings, buildings with asbestos and classrooms). Flexible workgroups reduce the total cost of ownership of workplaces that frequently change network configurations. Networked conference room users can connect to the network to get the latest information when moving from one conference room to another, and can

Communicate with each other in decision-making

Specialized web site consultants and small working groups can quickly install and compatible software to improve work efficiency.

The subsidiary network provides a network for remote offices or sales offices that is easy to install, use and maintain.

The mobile roaming function of the whole department network enables enterprises to establish an easy-to-use wireless network, which can cover all departments.

Generally speaking, 802. 1 1b allows the use of any existing application or network service running on a wired network.

Multi-access point solution

When the network scale is larger than the coverage radius of a single access point, multiple access points can be used to connect with wired networks respectively, thus forming a multi-access point wireless network with wired networks as the backbone. All wireless terminals can access the network through the nearest access point and access the resources of the whole network, which breaks through the limitation of the coverage radius of the wireless network.

Wireless relay solution

Another purpose of wireless access devices is to act as an extension of wired networks. For example, in a factory workshop, there is a network interface connected to a wired network, but many information points in the workshop are far away, and the cost of network wiring is high. Some information points cannot be wired because of the harsh surrounding environment. Because the distribution of these information points exceeds the coverage radius of a single access point, we can use two access points to realize wireless relay to expand the coverage of the wireless network.

Wireless redundancy solution

For the application environment with high network reliability requirements, such as finance and securities, once the access point fails, the whole wireless network will be paralyzed, which will bring great losses. Therefore, two access points can be placed in the same location, thus realizing the scheme of wireless redundant backup.

Multi-cell roaming working mode

When deploying a wireless network in a building or a large plane, multiple access points can be arranged to form a microcellular system, which is very similar to the microcellular system of a mobile phone. Micro-cellular system allows users to roam freely in different access point coverage areas. As the location changes, the signal will automatically switch from one access point to another. The whole roaming process is transparent to users. Although the access point providing connection service has been switched, the service to users will not be interrupted.

802. 1 1g

In recent years, the IEEE802. 1 1 working group began to define a new physical layer standard IEEE 802.1/g. Compared with the previous IEEE802. 1 1 protocol standard, the draft IEEE 802.1g has the following two characteristics: orthogonal frequency division multiplexing (OFDM) modulation technology is used in the 2.4GHz band, so that the data transmission rate is increased to more than 20Mbit/s; It can be interconnected with the Wi-Fi system of IEEE802. 1 1b and stored in the same AP network, ensuring backward compatibility. In this way, the original WLAN system can smoothly transition to high-speed WLAN, which prolongs the service life of IEEE802. 1 1b+0B products and reduces the investment of users. In July 2003, the working group of IEEE802. 1 1 approved the draft of IEEE 802.1/g, which became a new focus of attention.

Key technologies of IEEE 802. 1 1 wireless LAN implementation.

With the wide application of wireless local area network technology, users demand higher and higher data transmission rate. However, in the indoor complex electromagnetic environment, multipath effect, frequency selective fading and other interference sources make it more difficult for wireless channels to achieve high-speed data transmission than wired channels, so WLAN needs to adopt appropriate modulation technology.

IEEE 802. 1 1WLAN is a self-managing computer local area network which can support high data transmission rate (1 ~ 54 Mbit/s), and adopts microcellular and picocellular structures. There are three key technologies: direct sequence spread spectrum modulation (DSSS) and complementary code keying (CCK), block binary convolutional code (PBCC) and orthogonal frequency division multiplexing (OFDM). Each technology has its own characteristics. At present, spread spectrum modulation technology is becoming the mainstream, and OFDM technology has become a new focus because of its superior transmission performance.

1.DSSS modulation technology

There are three modulation techniques based on DSSS. Initially, the IEEE802. 1 1+0 standard adopted differential binary phase shift keying (DBPSK) at the data rate of1mbit/s. If the data rate of 2 Mbit/s is to be provided, DQPSK (Differential Quadrature Phase Shift Keying) can be used, which processes two bits at a time and becomes double bits. The third is QPSK based on CCK, which is the basic data modulation method adopted by IEEE802. 1 1b standard. It adopts complement sequence and direct sequence spread spectrum technology, which is a single carrier modulation technology. It transmits data through phase shift keying (PSK), and the transmission rates are divided into 1, 2, 5.5 and11mbit/s. CCK cooperates with the Pake receiver at the receiving end to effectively overcome the multipath effect while transmitting data efficiently. IEEE802. 1 1b adopts CCK modulation technology to improve the data transmission rate, and the highest rate can reach11mbit/s. But when the transmission rate exceeds1/mbit/s, CCK needs more complicated equalization and modulation. Therefore, the IEEE802. 1 1 working group has introduced new modulation technology to promote the development of WLAN.

2.PBCC modulation technology

PBCC modulation technology was put forward by Texas Instruments (TI) Company and has been adopted as an option of IEEE 802.11g. PBCC is also a single carrier modulation, but unlike CCK, it uses a more complex signal constellation. PBCC uses 8PSK, CCK uses BPSK/QPSK；; ; Furthermore, PBCC uses convolutional codes, while CCK uses block codes. Therefore, their demodulation processes are very different. PBCC can achieve higher data transmission rate, and its transmission rate is 1 1, 22,33 mbit/s.

3.OFDM technology

OFDM technology is actually a kind of multi-carrier modulation (MCM). The main idea is to divide the channel into many orthogonal subchannels, and carry out narrowband modulation and transmission on each subchannel, thus reducing the mutual interference between subchannels. The signal bandwidth on each subchannel is smaller than the correlation bandwidth of the channel, so the frequency selective fading on each subchannel is flat, which greatly eliminates the inter-symbol interference.

Because the carriers of each subchannel in OFDM system are orthogonal to each other, their spectra overlap each other, which not only reduces the mutual interference between subcarriers, but also improves the spectrum utilization. Orthogonal modulation and demodulation in each subchannel can be realized by inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). With the development of large-scale integrated circuit technology and DSP technology, IFFT and FFT become very easy to realize. The introduction of FFT greatly reduces the complexity of OFDM implementation and improves the performance of the system.

Wireless data services are generally asymmetric, that is, the amount of data transmitted downstream is much larger than that transmitted upstream. Therefore, the physical layer is expected to support asymmetric high-speed data transmission, whether it is the demand of users' high-speed data transmission service or the demand of wireless communication itself, and OFDM can easily achieve different transmission rates in uplink and downlink by using different numbers of subchannels.

Because of the frequency selectivity of wireless channels, all subchannels will not be in deep fading at the same time, so dynamic bit allocation and dynamic subchannel allocation can make full use of subchannels with high signal-to-noise ratio, thus improving system performance. Because narrowband interference can only affect a few subcarriers, OFDM system can resist this interference to some extent.

OFDM technology has a very broad development prospect and has become the core technology of the fourth generation mobile communication. In order to support high-speed data transmission, the IEEE 802.11A/G+0A/G standards all adopt OFDM modulation technology. At present, OFDM combines space-time coding, diversity, interference suppression (including inter-symbol interference (ISI) and inter-channel interference (ICI)) and smart antenna technology to maximize the reliability of the physical layer. If adaptive modulation, adaptive coding, dynamic subcarrier allocation and dynamic bit allocation algorithms are combined, its performance can be further optimized.

Frame structure and technical details of 4.4. IEEE802. 1 1g+0G protocol.

From the logical structure of the network, IEEE 802.5438+05438+0 only defines the physical layer and MAC sublayer. The MAC layer provides competitive and non-competitive use of wireless media, and has the functions of wireless media access, network connection, data verification and confidentiality.

The physical layer provides physical connection for the data link layer, realizing the transparent transmission of bit stream, and the data unit of transmission is bit. The physical layer defines the mechanical and electrical functions and process characteristics of communication equipment and interface hardware, and is used to establish, maintain and release physical connections. Physical layer consists of three parts: physical layer management layer, physical layer convergence protocol (PLCP) and physical medium related sublayer (PMD).

The physical frame structure of IEEE802. 1 1g is divided into preamble, header and payload. The preamble is mainly used to determine when to send and receive data between a mobile station and an access point, notify other mobile stations to avoid collisions during transmission, and transmit synchronization signals and frame intervals. The preamble is completed and the receiver begins to receive data. The header is used to transmit some important data after the preamble, such as load length, transmission rate, service, etc. Because the data rate and the number of bytes to be transmitted are different, the packet length of the payload varies greatly, which can be very short or very long.

In the transmission process of frame signal, the more transmission time is occupied by preamble and header, the less transmission time is occupied by payload and the lower transmission efficiency is.

Combining the characteristics of the above three modulation technologies, IEEE802. 1 1g adopts key technologies such as OFDM to ensure its superior performance, and modulates the preamble, header and payload respectively. This frame structure is called OFDM/OFDM mode.

In addition, the draft IEEE802. 1 1g standard stipulates optional and mandatory items. In order to ensure compatibility with IEEE802. 1 1b, CCK/OFDM and CCK/PBCC optional modulation methods can also be adopted. Therefore, OFDM modulation is a necessary option to ensure the transmission rate reaches 54 Mbit/s; CCK modulation is a necessary guarantee for backward compatibility; CCK/PBCC and CCK/OFDM are optional. See table 11g for the comparison of frame structures of IEEE 802.1g.

( 1)OFDM/OFDM

The preamble, header and payload are all modulated by OFDM, and its transmission rate can reach 54 Mbit/s. A good feature of OFDM is that the preamble is short. The frame header of CCK modulation signal is 72μs, while the frame header of OFDM modulation signal is only16 μ s. The frame header is an important part of the signal, and the reduction of the time occupied by the frame header improves the signal's ability to transmit data. OFDM allows shorter headers to provide more time for data transmission and has higher transmission efficiency. Therefore, CCK modulation is a good choice for the transmission rate of 1 1Mbit/s, but OFDM modulation technology must be used to continuously improve the rate. Its highest transmission rate can reach 54 mbit/s, OFDM OFDM in IEEE802. 1 1g protocol can also work with Wi-Fi***, but RTS/CTS protocol is needed to solve the conflict problem.

(2)CCK/OFDM

It is a mixed modulation method and an option of IEEE 802.11g. Its header and preamble are transmitted by CCK modulation, and the load is transmitted by OFDM technology. Because OFDM technology and CCK technology are separated, there should be conversion between CCK and OFDM between preamble and payload.

IEEE802. 1 1g adopts CCK/OFDM technology to ensure the storage with IEEE 802. 1 1b * *. IEEE802. 1 1b can't demodulate data in OFDM format, so data transmission conflicts will inevitably occur. IEEE802. 1 1g can make IEEE 802.1/b compatible by using CCK technology to send headers and preambles, so that it can receive IEEE 802. 168. This ensures backward compatibility with IEEE802. 1 1bWi-Fi devices, but because the preamble/header uses CCK modulation, the overhead increases and the transmission rate is lower than that OFDM OFDM.

(3)PBCC CCK Company

Like CCK/OFDM, CCK/PBCC is a mixed waveform. CCK modulation is adopted at the head end and PBCC modulation is adopted at the load, so it can work at high speed and is compatible with IEEE 802.11B. The highest data transmission rate of PBCC modulation technology is 33Mbit/s, which is lower than OFDM or CCK/OFDM.

Performance analysis of IEEE802. 1 1g

The draft IEEE802. 1 1g, which has not been formally standardized, has become the focus of attention because of its different characteristics. The compatibility between IEEE802. 1 1g and IEEE802. 1 1b, the * * * storage capacity of co-frequency equipment and the problems of OFDM technology itself will become research hotspots.

Compatibility of1.ieee802.11g

IEEE802. 1 1g compatibility means that IEEE802. 1 1g devices can be interconnected with IEEE 802.1b devices in the same AP node network. The biggest feature of IEEE802. 1 1g is to ensure compatibility with IEEE802. 1 1bWi-Fi system. IEEE802. 1 1g can receive OFDM and CCK data, while the traditional Wi-Fi system can only receive CCK information, which leads to a problem, that is, how to solve the conflict caused by the inability of IEEE802. 1 1b to demodulate the header of OFDM format information. To solve the above problems, IEEE802. 1 1g adopts RTS/CTS technology.

Initially, IEEE802. 1 1 introduced RTS/CTS mechanism to solve the problem of hidden stations, that is, the sending station cannot detect that another station is sending data, so there is a conflict at the receiving station.

The mixed work of IEEE802. 1 1b and IEEE802. 1 1g is very similar to the problem of hidden stations. The IEEE802. 1 1b device cannot receive the information frame header of IEEE 802.1g in OFDM format, so RTS/CTS is adopted.

IEEE 802. 1 1n

IEEE 802. 1 1n: using 2.4GHz band and 5GHz band, the transmission speed is 300Mbps, and the maximum is 600Mbps, but it is backward compatible with 802.1/b, 802.11.

191October 19, Broadcom launched a new WLAN chipset strength-fi series, which is the first solution compatible with the IEEE 802. 1 1n standard (draft). Intensity-Fi technology provides excellent performance and powerful wireless connection in home or office, and becomes the next generation of Wi-Fi.

Intensi-fi-Fi-fi technology integrates all mandatory components of IEEE 802. 1 1n standard (draft), and the software can be upgraded as soon as the standard is completed. Loyalty to standards is the focus of Broadcom's work, because it does not need to consider the compatibility and performance problems of non-standard products that annoy users. Broadcom works closely with other leading manufacturers in the industry. When the draft 802. 1 1n product becomes a reality, the real interconnection will be demonstrated in the branch. Broadcom also provides technical resources to the Wi-Fi Alliance to accelerate the 802. 1 1n interconnection test plan.

Intensi-fi-Fi-Fi technology supports the simultaneous transmission of multiple data (or "space") streams on multiple transmitting and receiving antennas, providing a data rate of up to 300Mbps, which is wider than the previous 802. 1 1 product (which uses a transmitter and a receiver to support a single data stream). It provides sufficient bandwidth, range and reliability. Provide high-definition video for every room in the family. In order to provide a perfect multimedia experience, Intensi-fi-Fi-Fi technology extends traditional PC and network devices to consumer electronics and entertainment devices, and provides basic devices for sending movies, photos, music, voice calls and data in cable /DSL/ satellite set-top boxes, personal video recorders, DVD players, game systems, audio equipment cameras, mobile phones and other handheld devices.

Intensi-fi-Fi-Fi solution includes MAC/ baseband chip and radio chip, which can be configured for various high-speed wireless applications. Broadcom also provides two kinds of network processors, which enable users to optimize the cost performance of wireless router design. The complete product family includes all the following CMOS devices:

BCM432 1: The industry's first MAC and baseband compatible with the 802. 1 1n standard (draft), which provides a PHY rate of over 300Mbps and can interface with PCI, Cardbus and host PCI-Express.

BCM2055: Broadcom's fifth generation 802. 1 1 radio integrates multiple 2.4GHz and 5GHz radios, supports synchronous spatial data stream of 802. 1 1n products, and has 2x2, 3x3 or 4x4 antenna configurations. BCM2055 is the best performance 802. 1 1 radio, with smaller chip size, lower power consumption, lower phase noise and error vector amplitude (EVM). These are very important for the Qualcomm 802. 1 1n (draft) system.

BCM 4704: The fifth generation wireless network processor verifies BCM4704:Broadcom and provides advanced routing/bridging functions, which can meet the target performance of 802. 1 1n (draft) chipset and is used for the design of routers and gateways.

Bcm4705: the 6th generation wireless network processor of Broadcom, which supports 2.4GHz and 5GHz radios. The integrated Gigabit Ethernet MAC enables the throughput between 802. 1 1n (draft) and Ethernet to exceed 200Mbps.

Now, we can provide samples of Intensi-fi-Fi-Fi chipset and reference design.

Atheros Company of the United States held a press conference in Japan on February 6, 2006, and launched its wireless network chipset "AR5008" conforming to IEEE 802. 1 1n specification. This chipset was launched in the United States on February 24th, 65438.

Atheros Company calls its product group for IEEE 802. 1 1n "XSPAN". This AR5008 maintains the continuity of its original product corresponding to IEEE 802. 1 1a/b/g, and the highest wireless transmission speed reaches 300Mbps. However, this is only the theoretical maximum speed. In the actual communication process, the actual speed should be about 60% of this speed after loading TCP and other protocols. Even so, the efficiency of 802. 1 1n is much higher than the fastest 802.1/g at present. The actual speed of11n is expected to be 8-9 times higher than that of 802.11g.

According to Atheros Communications, the AR5008 chipset is the first product based on the 802. 1 1n draft specification confirmed by the International Institute of Electrical and Electronics Engineers (IEEE) on October 20th. These next-generation WLAN solutions will make full use of the potential of MIMO technology, give play to breakthrough performance and interoperability with the industry. AR5008 solution will have a wider coverage and better reliability, reaching 6 times the data transmission capacity of 802. 1 1g and 802. 1 1a/g products. Since the draft specification of 802. 1 1n has been formulated, consumers can finally enjoy the interworking technology of MIMO in various devices and applications at home, office and on the move.

Atheros' innovation XSPAN introduces the signal persistence technology (SST), which greatly enhances the signal reliability and data transmission within the coverage, and fully releases the potential of MIMO. All this is achieved through the world's first single-chip three-RF design. The physical data rate of AR5008 is 300 Mbps (megabits per second), while the actual end-user data transmission can reach 150 to 180 Mbps, which is 50% more than the coverage of 2x2 MIMO system.

The signal persistence technology transmits simultaneously through different spatial signal paths, and combines the information from three receivers at the same time when the receiver processes the signal, thus greatly increasing the connection strength and data transmission capacity. If the transmitter only switches between additional antennas at the same time, it is impossible to achieve such strength. Atheros integrates three complete sets of RF transmitter and receiver chains into a single chip. Through the built-in SST baseband processing, it achieves unparalleled coverage and strength at a price close to the uncompetitive 2x2 MIMO scheme, while the latter has poor strength.