Spectrum assignments and operational limitations are not consistent worldwide; most of Europe allows for an additional 2 channels beyond those permitted in the US; Japan has one more on top of that - and some countries, like Spain, prohibit use of the lower-numbered channels. Furthermore some countries, such as Italy, used to require a 'general authorization' for any WiFi used outside an operator's own premises, or require something akin to an operator registration. For Europe; consult for an annual report on the additional restrictions each European country imposes.
Power consumption is fairly high compared to some other standards, making battery life and heat a concern.

The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be breakable even when correctly configured (* WiFi Access Points typically default to an open (encryption-free) mode. Novice users benefit from a zero configuration device that works out of the box but are unaware they are allowing unsecured wireless access to their LAN. WPA Wi-Fi Protected Access fixes those problems, began shipping in 2003, and is now generally available, but adoption rates remain low.
Many 2.4 GHz 802.11b and 802.11g Access points default to the same channel, contributing to congestion on certain channels.
Wi-Fi networks have limited range. A typical Wi-Fi home router using 802.11b or 802.11g with a stock antenna might have a range of 45 m (150 ft) indoors and 90 m (300 ft) outdoors. Range also varies with frequency band, as WiFi is no exception to the physics of radio wave propagation. WiFi in the 2.4 GHz frequency block has better range than WiFi in the 5 GHz frequency block, and less range than the oldest WiFi (and pre-WiFi) 900 MHz block. Outdoor range with improved antennas can be several kilometres or more with line-of-sight.
Wi-Fi pollution, meaning interference of a closed or encrypted access point with other open access points in the area, especially on the same or neighboring channel, can prevent access and interfere with the use of other open access points by others caused by overlapping channels in the 802.11g/b spectrum as well as with decreased signal-to-noise ratio (SNR) between access points. This is a widespread problem in high-density areas such as large apartment complexes or office buildings with many Wi-Fi access points.

WiFi networks can be monitored and used to read and copy data (including personal information) transmitted over the network when encryption is not enabled.

Interoperability issues between brands or deviations from the standard can disrupt connections or lower throughput speeds on other user's devices within range. Wi-Fi Alliance programs test devices for interoperability and designate devices which pass testing as Wi-Fi CERTIFIED.
Use of the 2.4 GHz Wi-Fi band does not require a license in most of the world provided that one stays below the local regulatory limits and provided one accepts interference from other sources, including interference which causes your devices to no longer function. It is sometimes claimed that Amateur Radio operators have permission to boost the power on their WiFi transmitters up to the legal maximum for their Amateur Radio license class under some conditions; this is not permitted in the US, nor in most locations.
Legislation/regulation is not consistent worldwide; most of Europe allows for an additional 2 channels over those allowed for b and g ; Japan has one more on top of that - and some countries, like Spain, prohibit use of the lower-numbered channels. Furthermore some countries, such as Italy, used to require a 'general authorization' for any WiFi used outside the owned premises; or required something akin to operator registration.
The 802.11b and 802.11g flavors of Wi-Fi use the 2.4 GHz spectrum, which is crowded with other equipment such as Bluetooth devices, microwave ovens, cordless phones (900 MHz or 5.8 GHz are, therefore, alternative phone frequencies one can use to avoid interference if one has a Wi-Fi network), or video sender devices, among many others. This may cause a degradation in performance. Other devices which use these microwave frequencies can also cause degradation in performance.

Power consumption is fairly high compared to other standards, making battery life and heat a concern.
The most common wireless encryption standard, Wired Equivalent Privacy or WEP, has been shown to be easily breakable even when correctly configured. Although newer wireless products are slowly providing support for the Wi-Fi Protected Access (WPA) protocol, many older access points will have to be replaced to support it. The adoption of the 802.11i (aka WPA2) standard in June 2004 makes available a rather better security scheme for future use — when properly configured. In the meantime, many enterprises have had to deploy additional layers of encryption (such as VPNs) to protect against interception.
Wi-Fi networks have limited range. A typical Wi-Fi home router using 802.11b or 802.11g might have a range of 45 m (150 ft) indoors and 90 m (300 ft) outdoors. 802.11a equipment, using a higher frequency, has reduced range, all other things being equal.
Interference of a closed or encrypted access point with other open access points on the same or a neighboring channel can prevent access to the open access points by others in the area. This can pose a problem in high-density areas such as large apartment buildings where many residents are operating Wi-Fi access points.
WiFi equipment could be used to steal personal information (passwords, financial information, identity information, ...) transmitted from Wi-Fi users, if sensible protections are not used. WEP is not sensible protection for it is easily evaded. The 802.11i standard (partially implemented in some equipment as W(ireless)P(rotected)A(ccess) will be a substantial improvement if properly configured at both ends. Free access points (or improperly configured access points) may be used by a hacker to anonymously initiate an attack that would be impossible to track beyond the owner of the accespoints

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