802.11WLAN

    Basically is kind of a region network (LAN) , Because its transmission medium is invisible (like Ethernet Network), Establishes when does not have the problem of wiring, May call it Wireless Lan  (abbreviation is WLAN), namely for wireless second grade too network (Wireless Ethernet Network).

Wireless medium has many kinds of uses, mainly may divide into the optical wave (to have infrared ray and -Laser) and the radio wave (has narrow frequency microwave, DSSS- direct sequence unfolds frequency, the FHSS-frequency-jump unfolds frequency, Home RF, Hyper Lan and Bluetooth technology). The optical wave medium shortcoming is, it unable to penetrate the obstacle, the server disconnected when it meets the communications barrier , But the radio wave does not have this problem.

In the present, the WLAN specification is IEEE 802.11, It has the many kinds of editions, the semicolon was a standard, which is 802.11a, 802.11b and 802.11g, 802.11g was recently only then it appears. But 802.11b is at present broadly the standard which uses, but the 802.11a merit is the frequency width is bigger (its specification 802.B is new, Also is comparatively late promotes compared to 802.11b) But the relativity of  its transmitting range is shorter, If uses 802.11a WLAN in 2 buildings. Possibly it is unable to connect in 3 buildings on WLAN, but 802.11g was a specification which recently promoted. It uses the frequency band and 802.11b are same (all is 2.4GHZ). But because it has improved the wrapped data package form the communication, therefore its speed compares to 802.11b is faster. In the synthesis said, 802.11a/B/G respectively has its good and bad points, Therefore the intelligent manufacturer has promoted the comprehensive edition, Is similar to when supports 802.11b/A,Or simultaneously supports 802.11a/G,Or simultaneously supports 802.11b/G,Or simultaneously supports 802.11a/B/G and so on. Many kinds of specifications coexist product, on as soon as starts like the star shape Ethernet network only to have 10 Mbps 10Base-T, Afterwards it appeared 100Mbps 100base-T, in order to have the same reason of 10/100Mbps hub mix edition net card and the concentrator.

Strengths

1.   Good mobility :May deposit and withdraw the material in the mobile , Largely enhances the productive forces。

2.    Deployed easy, establishes fast: Compares to the wired Ethernet network, visible line material is not easy to obsolete the buildings. But does not have this problem with wireless network。

3.    High flexibility: No wiring problems when reconstructs a WLAN, high mobility, especially in the popular scenic spot (for example station, net cafe, dining room and hotel and so on) may become popularly selling spot。

4.     Cost reasonable: Compared to rents of the telecommunication company special line is cheaper。

Restrictions

1.    WLAN Is unable to substitute for the traditional type of LAN。

2.    For the frame wireless of medium transmission（Frame）,its reliability of transmission is stable.  For the correct and unmistakable transmission and receive, It should match other agreements（for example WEP and 802.1x）to guarantees its integrity and the accuracy。

3.    WLAN will have several ways of  disturbance（Multi path interference）and also dead angle（shadows）problems。

4.    Due to the WLAN transmission medium is invisible（Light or wireless radio wave）it is sprayed in the air，therefore when it has to be in one Service Area，could cause people to eardrop or intercept（Wire Tapping）.  Causes WLAN user to confirm and register the mechanism and the encryption function（like WEP and 802.1x）is mostly important，unfortunately，WEP and 802.1x has already been breached by others。

Other different medium transmit WLAN

1.   Blue Tooth： Blue tooth suites the essence of uses for the network installment the mutual transmission。

2.    3G Cell Phone：It is known as million bps of the transmission speed as well as XDSL and Cable Modem as an necessary segment service。

At the present, the mainstream wireless standard（802.11b）

In the present, IEEE 802.11b’s specification is the majority. The international characteristic organizes WECA (Wireless Ethernet Compatibility Alliance)  proposed WiFi (Wireless Fidelity)authenticates standard, so as long as the manufacture’s wireless product conforms to this WiFi standard (also is conforms to IEEE the 802.11b specification)it is certainly can be allowed to help to supply each other with the different manufacturer WiFi product mutually operates. For example I have bought the A factory non- wire card, But actually used B factory Access Point (transmission receive base station or calls wireless bridge joint), but as long as both gone through WiFi authentication, it will guaranteed the correct mutual operation and unmistakable。

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