what is the disadvantage of wireless LAN?

As the number of computers using the network increases, the data transfer rate to each computer will decrease accordingly.

As standards change, it may be necessary to replace wireless cards and/or access points.

Lower wireless bandwidth means some applications such as video streaming will be more effective on a wired LAN.

Security is more difficult to guarantee and requires configuration.

Devices will only operate at a limited distance from an access point, with the distance determined by the standard used and buildings and other obstacles between the access point and the user.

A wired LAN is most likely to be required to provide a backbone to the WLAN; a WLAN should be a supplement to a wired LAN and not a complete solution.

Long-term cost benefits are harder to achieve in static environments that require few moves and changes.

Wireless networks are much more susceptible to unauthorized use. If you set up a wireless network, be sure to include maximum security. You should always enable WEP or WPA which will improve security and help to prevent virtual intruders and freeloaders.

Interference: Because wireless networks use radio signals and similar techniques for transmission, they are susceptible to interference from lights and electronic devices.

Inconsistent connections: How many times have you hears “Wait a minute, I just lost my connection?” Because of the interference caused by electrical devices and/or items blocking the path of transmission, wireless connections are not nearly as stable as those through a dedicated cable.

Power consumption: The wireless transmitter in a laptop requires a significant amount of power; therefore, the battery life of laptops can be adversely impacted. If you are planning a laptop project, be sure to have power plugs and/or additional batteries available.

Speed: The transmission speed of wireless networks is improving; however, faster options are available via cables. In addition, if set up a wireless network at home, and you are connecting to the Internet via a DSL modem your wireless access to the Internet will have a maximum of 3 Mbps connection speed.

It is like a hub, so if more than 1 device and/or the router try to communicate at the same time, data can collide and packets may need to be resent. Even for a single wireless client it is best to have your wireless speed at least 3 times your Internet speed, judging from some testing I did at work with 50/10 Mbps cable.

I don't have that issue at home where my WiFi speed is just wireless-g @ 54 Mbps and Internet download speed is 5.1 Mbps (speedtest is the same as Ethernet).

It is slower than wired, it is less secure than wired, it is subject to radio interference. Other than those it has the major advantage of not needing ethernet to connect to the LAN. (All wifi is a LAN connection even if it is used for internet.) Wifi is only a replacement for needing ethernet.

It's slower then wired connection.