Introduction to Wireless Local Area Network (WLAN)

Wireless LAN:
Wireless local area network (Wireless Local Area Networks; WLAN) is a very convenient data transmission system. It uses radio frequency (Radio Frequency; RF) technology to replace the old-fashioned twisted pair copper wire (Coaxial) local area network, making wireless The local area network can use a simple access structure to allow users to use it to achieve the ideal state of "information is carried around and the world is convenient."

Why use wireless LAN:
For one of the main tasks of local area network management, the time-consuming work of laying cables or checking whether the cables are disconnected is very irritating and it is not easy to find the location of the disconnection in a short time. In addition, due to the continuous update and development of the enterprise and application environment, the original enterprise network must be re-layed and the network lines need to be reinstalled. Although the cable itself is not expensive, it is very expensive to hire a technician to wire it. Especially for old buildings, the cost of wiring works is even higher. Therefore, setting up a wireless local area network becomes the best solution.

What situation needs wireless LAN:
The wireless local area network is never used to replace the wired local area network, but to make up for the shortcomings of the wired local area network to achieve the purpose of network extension. Limited by the environment • As a backup system for wired LAN

Wireless LAN access technology:
At present, when designing wireless LAN products, manufacturers have quite a variety of access design methods, which can be roughly divided into three categories: narrow-band microwave (Narrowband Microwave) technology, spread spectrum (Spread Spectrum) technology, and infrared (Infrared) technology Each technology has its advantages, disadvantages, limitations, and comparisons. The following is a detailed discussion of these technical methods.
Spread-spectrum technology Spread-spectrum technology wireless LAN products are based on the ISM (Industrial ScienTIfic, and Medical) prescribed by the FCC (Federal CommunicaTIons Committee; US Federal Communications Commission). Frequency band, so there is no restriction on the so-called use authorization. Modulation technology is mainly divided into "frequency hopping technology" and "direct sequence" (OFDM).
1. Frequency Hopping Technology (FHSS)
Frequency-Hopping Spread Spectrum (FHSS) accepts a specific type of narrow-band carrier to transmit signals at both ends in synchronization and at the same time. For an unspecified receiver, the hopping signal pair generated by FHSS In terms of it, it is only impulse noise. The signals developed by FHSS can be specially designed to avoid noise or One-to-Many non-repetitive channels, and these frequency hopping signals must comply with FCC requirements, use more than 75 frequency hopping signals, and hop to the next one The maximum time interval of frequency (Dwell TIme) is 400ms.
2. Direct Sequence Spread Spectrum Technology (DSSS)
Direct Sequence Spread Spectrum (DSSS) uses the original signal "1" or "0" to use more than 10 chips to represent the "1" or "0" bits, making the original higher power, more powerful The narrow frequency becomes a lower power frequency with a wider frequency. The number of chips used per bit is called Spreading chips. A higher Spreading chips can increase the noise immunity, while a lower Spreading RaTIon can increase the number of users.
Basically, Spreading Ration in DSSS is quite small. For example, Spreading Ration used in almost all 2.4GHz wireless LAN products is less than 20. In the IEEE802.11 standard, the Spreading Ration is about 100.
Third, FHSS VS DSSS modulation differences The difference in performance and capabilities of the wireless local area network mainly depends on whether FHSS or DSSS is used to implement and the modulation method used. However, the choice of modulation method is not completely arbitrary. For example, FHSS does not impose a specific modulation method, and most of the existing FHSS use some different forms of GFSK. However, the IEEE 802.11 draft Requires the use of GFSK. As for DSSS, the use of variable phase modulation (eg, PSK, QPSK, DQPSK) can achieve the highest reliability and high data rate performance.
In terms of anti-noise capability, the DSSS using QPSK modulation and the FHSS using FSK modulation can find the advantages of these two different technologies of wireless local area networks. The reason why the FSK modulation method is selected for the FHSS system is because of the simplicity of the internal architecture of the FHSS and FSK. The FSK wireless signal can use a non-linear power amplifier, but this sacrifices the scope of action and the ability to resist noise. The DSSS system requires a slightly more expensive linear amplifier, but it can get more feedback.

Fourth, the WLAN technology standard situation • 802.11a
54Mbps wireless LAN standard operating in the 5GHz band • 802.11b
11Mbps rate wireless LAN industry standard working in the 2.4GHz band • 802.11e
Defines the quality-of-service (QOS) of the wireless LAN, for example, support for voice-over IP • 802.11g
The successor of 802.11b, providing a data transmission rate of up to 54Mbps on the same 2.4GHz band used by 802.11b • 802.11h
Supplement to 802.11a to ensure that it complies with European standards for 5GHz wireless LANs 802.11i
Wireless security standard, WPA is a subset • 802.11j
The protocol equivalent to 802.11h adopted in Japan • 802.11n
It is expected to adopt the recommended specification in 2006, which will double the transmission rate of 802.11a / g wireless LAN
Bluetooth-based personal area networks (personal area networks) standard • 802.16
A set of communication standards for high-capacity metropolitan area networks (Metropolitan area networks, Mans), also known as WirelessMan or WiMAX
• 802.20
The draft of a wireless metropolitan area network that provides a 1Mbps rate in the range of up to 15km is planned to work in the licensed frequency band at 3.5GHz. 802.1x
Authentication scheme based on Extensible Authentication Protocol (EAP) • Access Point (AP)
The access point acts as a hub for wireless users. There are indoor (100 and 200mW) and outdoor (500mW), supporting local and remote power supply.
• Bluetooth
Bluetooth, short-range (10 to 100m) wireless connection technology, as an alternative to cables for connecting mobile phones, handheld PCs, and other personal digital devices. Another alternative technology is infrared

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