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Intro to Wireless LAN

Wireless LAN is the fastest growing technology nowdays. The most tangible benefit of wireless implementations is cost reduction -- wireless LAN use air as the communication media, saving substantial cost on copper wires purchasing and installing!

Wireless LAN (WLAN) can be either ad hoc WLAN or infrastructure WLAN.

ad hoc WLAN is also known as Independent Basic Service Sets, where wireless devices communicate with each other directly.

On the contrary, in Infrastructure WLAN, all wireless devices must communicate through a wireless access point (WAP or AP). There are two kinds of infrastrue WLANs.

  • Basic Service Set (BSS) is a topology created by mobile clients using a single AP to connect to each other or to wired networks. The Basic Service Set Identifier (BSSID) is the MAC address of the access point's wireless card.

  • Basic service area (BSA): The physical area of radio frequency coverage provided by an access point define the basic service area (BSA).

  • Extended Service Set (ESS) with multiple APs. The wireless topology is extended with two or more BSSs connected by a distribution system or a wired infrastructure and usually share a common SSID.

  • Extended service area (ESA): The physical area of radio frquency coverage provided by overlapping basic service area define the extended service area (ESA). When implementing extended service areas, 10 to 15 percent overlap is suggested. When wireless voice is needed, an overlap of 15 to 20 percent is recommended. Cells are overlapping areas of wireless coverage created by multiple APs. For adjacent cells, different nonoverlapping channels should be used for best performance.

Note that BSS and ESS refer to the topology building block of the WLAN, BAS and ESA refer to the actual physical pattern of wireless coverage.

WLAN allows travelling laptops to access wireless network seaminglessly.  One of the challenges for ESS is how to associate the moving client to the APs when it across the cells bondary. While signal may get weak near the point of overlapping, the AP that have the strongest signal will be associate with the client. The clients are also able to shift data rates -- for example, when transmission errors and transmission retries occur, or signals at the receiver gets low, the WLAN clients tend to shift to lower data rates.

Difference between WLAN and LAN

Wireless LANs differ from wired LANs in several ways. In wireless LANs, radio frequencies are used instead of wires as the physical layer of the network. Wireless LANs use CSMA/CA (or Carrier Sense Multiple Access with Collision Avoidance) instead of CSMA/CD (Carrier Sense Multiple Access with Collision Detection) used by Ethernet LANs. Collision detection is not possible because a sending station cannot receive at the same time it transmit and, therefore, cannot detect a collision. Instead, the Request to Send and Clear to Send protocols are used to avoid collisions. Wireless LANs use a different frame format than wired Ethernet LANs. Additional information for wireless LANs is provided in the Layer 2 frame header.

  • WLANs use CSMA/CA (Collision Avoidance) instead of CSMA/CD (Collision Detection) for media access.

  • Two-way radio (half-duplex) communication -- everyone is sharing the same bandwidth and only one user is communicating at a time.

  • Radio waves have problems that are not found on wires such as coverage problem, interference, noise and privacy issues.

  • Access points are shared devices similar to Ethernet hub for shared bandwidth.

  • WLANs must meet country-specific RF regulations.

Radio Frequency Transmission

  • Radio frequencies range from the lower AM radio band to higher frequencies used by cell phones. Radio frequencies are radiated into the air via an antenna, creating radio waves. 

  • Objects can affect radio wave propagation resulting in reflection, scattering and absorption.

  • Higher frequencies allow higher data rates; however, they have a shorter range. 

  • Higher transmit power results in greater range. To double range, power must increase by a factor of 4.

Organizations That Define WLAN

  • International Telecommunication Union-Radiocommunication Sector (ITU-R) -- regulate the radio frequency (RF) used in wireless. The ITU-R members include: Federal Communications Commission (FCC) -- Regulates the use of wireless devices in the U.S. European Telecommunications Standards Institute (ETSI) -- Chartered to produce common standards in Europe

  • Institute of Electrical and Electronic Engineers (IEEE) -- Creates and maintains 802.11 wireless standards

  • Global nonprofit industry trade association (Wi-Fi Alliance) -- grants certification for interoperability among 802.11 products offered by various vendors. This certification provides a comfort zone for users purchasing products.

The FCC has released three unlicensed bands for public use

  • 900MHz

  • 2.4GHz

  • 5GHz

The 900MHz and 2.4GHz bands are refereed to as the Industrial, Scientific, and Medical (ISM) bands, and the 5 GHz band is known as the Unlicensed National Information Infrastructure (UNII) band.


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