self-healing. AWP ensures that the mesh network is not disruptive and provides consistent coverage. The wireless network is a very dynamic environment. When there is interference, or if access points are added or removed, AWP reconfigures the path back to the rooftop access point. AWP uses a stickiness factor to mitigate route flaps. This approach ensures that a loss of connection, which causes a temporary disruption, does not allow the mesh to change unnecessarily. Enterprise outdoor wireless applications include the following :
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.1 Unlicensed Frequency Bands There are three unlicensed bands: 900 MHz, 2.4 GHz, and 5.7 GHz. The 900-MHz and 2.4-GHz bands are referred to as the Industrial, Scientific, and Medical (ISM) bands, and the 5-GHz band is commonly referred to as the Unlicensed National Information Infrastructure (UNII) band. Frequency ranges for these bands are as follows: Figure shows WLAN frequencies. Next to the WLAN frequencies in the spectrum are other wireless services such as cellular phones and Narrowband Personal Communication Services (NPCS). The frequencies used for WLANs are ISM bands and UNII bands. Unlicensed frequency bands do not require a license to operate wireless equipment. However, there is no exclusive use of a frequency for a user or a service. For example, the 2.4-GHz band is used for WLANs, video transmitters, Bluetooth, microwave ovens, and portable phones. Unlicensed frequency bands offer best-effort use, and interference and degradation are possible. Radio frequencies are radiated into the air by antennas that create radio waves. When radio waves are propagated through objects, they may be absorbed by some objects (for instance, walls) and reflected by other objects (for instance, metal surfaces). This absorption and reflection causes areas of low signal strength or quality. The following factors influence the transmission of radio waves: The following rules apply for data transmission over radio waves:
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.2 WLAN Regulation and Standardization Regulatory agencies control the use of the RF bands. With the opening of the 900-MHz ISM band in 1985, the development of WLANs started. New transmissions, modulation schemes, and frequencies depend on the approval of the regulatory agencies. A worldwide consensus is truly required. Regulatory agencies include the Federal Communications Commission (FCC) for the United States (http://www.fcc.gov) and the European Telecommunications Standards Institute (ETSI) for Europe (http://www.etsi.org). The IEEE defines numerous standards. 802.11 is part of the 802 networking standardization. You can download ratified standards from the IEEE website (http://standards.ieee.org/getieee802).The Wi-Fi Alliance offers certification for interoperability between vendors of 802.11 products. This certification provides a comfort zone for the users who are purchasing the products. It also helps to market the WLAN technology by promoting interoperability between vendors. Certification includes all three 802.11 RF technologies and Wi-Fi Protected Access (WPA), a security model released in 2003. WPA is based on the new security standard IEEE 802.11i, which was ratified in 2004. The Wi-Fi Alliance promotes and influences WLAN standards. Ratified products can be found on the Wi-Fi website (http://www.wi-fi.org). Web Links http://www.fcc.gov http://www.etsi.org http://standards.ieee.org/getieee802 http://www.wi-fi.org
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.3 IEEE 802.11b Standard The IEEE 802.11b standard, ratified in 1999, is the most commonly deployed WLAN standard. Products were actually introduced into the market before the standard was ratified, and it became the unwritten but accepted standard for wireless and was adopted rapidly. It operates in the 2.4-GHz ISM band that is available worldwide. The standard specifies one RF transmission: Direct Sequence Spread Spectrum (DSSS). It provides four data rates up to 11 Mbps: 1, 2, 5.5, and 11 Mbps. There are 11 channels available in the United States. However, only three of these channels are non-overlapping. The ETSI domain has 13 available channels, but again there are only three non-overlapping channels. In Japan, a fourteenth channel located at the upper end of the band is available, and it is possible to use this along with three other channels for a total of four non-overlapping channels. Figure lists the 14 channels. The channels are known by their center frequency. The figure also shows the lowest and highest frequency used by each 22-MHz wide channel. Different countries have different regulatory bodies and may have as many as 14 channel sets available. In some countries, the number of non-overlapping channels is reduced to one.Regulatory domain information is subject to change. An up-to-date listing of the countries that correspond to these regulatory domains is available at Wireless LAN Compliance Status. In the 2.4-GHz frequency band there are three non-overlapping channels for the 802.11b standard that do not share any frequency. The existence of these channels means that three access points could operate in the same cell area without sharing the media. An access point on channel 1 does not share frequencies with an access point on channel 6 because they do not have any common frequencies. There is no degradation in throughput when three access points are in the same wireless cell area if the access points are each on a non-overlapping channel. Three access points in the same cell on three non-overlapping channels (example of 1, 6, and 11) provide an aggregated data rate for the cell of 33 Mbps (3 x 11 Mbps), with an aggregated throughput of about 16 Mbps (half of the data rate). If the same three access points shared the same channel, the aggregate data rate would be 11 Mbps (shared between the three access points), and the aggregated throughput would be lower than 6 Mbps. Each access point shares the same media (same frequency