【正文】 easier to move than ripping up all of the cables that a wired system would have snaked throughout the of these advantages also translate into monetary savings. 4. History of WLAN(Negus amp。 Petrick, 2009) The wireless local area network (WLAN) is today everywhere device often taken for granted as a default interface for networked devices by users and manufacturers alike. But not very long ago, it was most definitely not so. In the early 1990’s WLANs found almost no success in selling to enterprise or campus environments as wired LAN replacements or enablers of mobility. The WLAN products of that day were far too slow, too expensive, too bulky, and too power hungry. Furthermore, mobile network connectivity was simply not yet a killer application. The “survivor” panies of that age were the ones who focused on adapting WLAN technology to specialty niches such as retailing, hospitality, and logistics. Organizations that went after the “big” market of enterprise networking, and there were many that did, either went bankrupt or became largely scaled back divisions of large panies. By the middle of the 1990’s the WLAN industry had mainly consolidated into 4 players, But in the late 1990’s the first significant market opportunity for WLANs emerged and it was quite unlike what the WLAN industry to date had largely envisioned. The opportunity was the sharing of a broadband Internet connection within the home amongst multiple networked devices such as PCs initially, but inevitably also voice over Internet protocol (VoIP) phones, gaming consoles, media streamers and home automation appliances. Consumers, not enterprise IT managers, became the ones to choose what WLAN technology and products would achieve the de facto standard for the decade to follow.Advantages of Wireless Networks Wireless LANs designed to operate in licensefree bands making their operation and maintenance costs less than contemporary cellular and PCS networks. The use of licensefree spectrum, however, increases the risk of network security and inband interference. The key advantages of wireless networks as opposed to wired networks are mobility, flexibility, ease of installation and maintenance, and reduced cost. (Aziz, 2003) According to (Symantec , 2002) wireless LANs are less expensive and less intrusive to implement and maintain, as user needs change. Simple implementation and maintenance, extended reach, increased worker mobility and reduce total cost of ownership and operation. Emerging Developments Fundamental step forward in information theory, which first emerged during the time of the early development of wireless LANs, have now reached a level of maturity and acceptance that is allowing them to drive the quest for higher spectral efficiencies and data rates. Another important development in wireless LAN technology is the emergence of mesh networking. Mesh networks have the potential to dramatically increase the area served by a wireless network. Mesh networks even have the potential, with sufficiently intelligent routing algorithms to boost overall spectral efficiencies attained by selecting multiple hops over high capacity links rather than single hops over low capacity links (Holt, 2005).5Wireless LAN Technologies When making a decision about the best protocol or standard to use. We need to consider its features and our needs. Weight the features and pare the advantages and disadvantages of each one to make the final decision. There are several wireless LAN solutions available today, with varying levels of standardization and interoperability. Many solutions that currently lead the industry, IrDa, Bluetooth, HomeRF and IEEE . These technologies enjoy wider industry support and targeted to solve Enterprise, Home and public wireless LAN needs.? Infrared (IrDa) The appearance of portable information terminals in work and living environments is increase the introduction of wireless digital links and local area networks(LAN’s). Wireless LANs can use either radio frequencies or infrared light to transmit signals. While it is considerably cheaper to install infrared networks, as many devices already have infrared (IrDA) ports (Franklin, 2010). Portable terminals should have access to all of the services that are available on highspeed wired networks. Unlike their wired counterparts, portable devices are subject to severe limitations on power consumption, size and weight. The desire for inexpensive, highspeed links satisfying these requirements has motivated recent interest in infrared wireless munication (Gfeller amp。 Bapst, 1979). Wireless infrared munications refers to the use of freespace propagation of light waves in the near infrared band as a transmission medium for munication (Carruthers, 2002). The Infrared Data Association (IrDA) is another trade association, which defined standards for infrared munication for many years. It has some advantages。 notably that it is cheap and there are many devices which already include infrared including most laptops and PDAs as well as some printers. Before the advent of radio frequency LANs people were building infrared LANs, with some success. (, 2011) The wavelength band between about 780 and 950 nm is presently the best choice for most applications of infrared wireless links, due to the availability of lowcost LED’s and laser diodes (LD’s), and because it coincides with the peak responsively of inexpensive, lowcapacitance silicon photodiodes (Rancourt, 1993). It provide a useful plement to radiobased systems, particularly for systems requiring low cost, light weight, moderate data rates, and only requiring short ranges (Carruthers, 2002).However, this radiation cause problem relates to eye safety。 it can pass through the human cornea and focused by the lens onto the retina, where it can potentially induce thermal damage (Kahn amp。 Barry, 1997). To achieve eye safety with an LD user can employ a