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Wireless Network

In ad-hoc mode, wireless user devices such as laptop computers and PDAs communicate directly with each other in a peer-to-peer manner without the benefit of access points.

Ad-hoc mode is generally used to form very small spontaneous networks. For instance, with ad-hoc mode, laptop users in a meeting can quickly establish a small network to share files.


Infrastructure mode uses wireless access points to enable wireless devices to communicate with each other and with your wired network. Most networks use infrastructure mode.

The basic components of infrastructure mode networks include:


  • The radios embedded or installed within the wireless devices themselves. Many notebook computers and other Wi-Fi-compliant mobile devices, such as PDAs, come with the transmitters built in. But for others, you need to install a card-type device to enable wireless communications. Desktop PCs may also need an ISA or a PCI bus adapter to enable the cards to work.
  • The access point, which acts as a base station that relays signals between the 802.11 devices.


One or many access points?
Access points are standalone hardware devices that provide a central point of communication for your wireless users. How many you need in your application depends on the number of users and the amount of bandwidth required by each user. Bandwidth is shared, so if your network has many users who routinely send data-heavy multimedia files, additional access points may be required to accommodate the demand.


A small-office network with fewer than 15 users may need just 1 access point. Larger networks require multiple points. If the hardware supports it, you can overlap coverage areas to allow users to roam between cells without any break in network coverage. A user's wireless device picks up a signal beacon from the strongest access point to maintain seamless coverage.


How many access points to use also depends on your operating environment and the required range. Radio propagation can be affected by walls and electrical interference that can cause signal reflection and fading. If you're linking mobile users indoors-where walls and other obstructions impede the radiated signal-the typical maximum range is 45 meter. Outdoors, you can get greater WLAN range-up to 600 meter (depending on your antenna type) where there's a clear line of sight!


For optimal speed and range, install your wireless access point several meter above the floor or ground and away from metal equipment or large appliances that may emit interference.


Battle of the bands.
In addition to sharing bandwidth, users also share a band. Most IEEE 802.11 or 802.11b devices function in the 2.4-2.4835-GHz band. But these frequencies are often congested, so you may want to use devices that take advantage of the IEEE 802.11a 5.725-5.825-GHz band.


No matter what frequency you use, you'll want to isolate your users from outsiders using the same frequency. To do this, assign your users a network identifier, such as an Extended Service Set Identifier (ESSID), as well as distinct channels.


Web and wired network links.
The access point links your wireless network to your wired network, enabling your wireless users to access shared data resources and devices across your LAN enterprise. Some access points even feature capabilities for routing traffic in one or both directions between a wired and wireless network.


For Internet access, connect a broadband router with an access point to an Internet connection over a broadband service such as DSL, cable modem, or satellite.

For connecting network printers, you can dedicate a computer to act as a print server or add a wireless print server device; this enables those on your wireless network to share printers.


When to use external antennas.
If you plan to install access points, you can boost your signal considerably by adding external antennas. Various mounting configurations and high- and low-gain options are available.

You can also use add-on antennas to connect nodes where the topology doesn't allow for a clear signal between access points. Or use them to link multiple LANs located far apart.


Additional external antennas are also useful to help overcome the effects of multipath propagation in which a signal takes different paths and confuses the receiver. It's also helpful to deploy antennas that propagate the signal in a way that fits the environment. For instance, for a long, narrow corridor, use an antenna that focuses the RF pattern in one direction instead of one that radiates the signal in all directions.


Plan ahead with a site survey.
A site survey done ahead of time to plot where the signal is the strongest can help you identify problem areas and avoid dead spots where coverage isn't up to par or is unreliable. For this, building blueprints are helpful in revealing potential obstructions that you might not see in your physical site walkthrough


To field test for a clear signal path, attach an antenna to an access point or laptop acting as the transmitter at one end. Attach another antenna to a wireless device acting as a receiver at the other end. Then check for interference using RF test equipment (such as a wireless spectrum analyzer) and determine whether vertical or horizontal polarization will work best.


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IEEE 802 glossary

802.11 - This early wireless standard provides speeds of up to 2 Mbps. Because 802.11 supports two entirely different methods of encoding - Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS) - there is often incompatibility between equipment. 802.11 has also had problems dealing with collisions and with signals reflected back from surfaces such as walls.


802.11a - This is an extension of the 802.11 standard and uses a different band than 802.11b and 802.11g - the 5.8-GHz band called Unlicensed National Information Infrastructure (U-NII) in the United States. Because the U-NII band has a higher frequency and a larger bandwidth allotment than the 2.4-GHz band, the 802.11a standard achieves speeds of up to 54 Mbps.


802.11b - This extension of the original 802.11 standard boosts wireless throughput from 2 Mbps to 11 Mbps. It can transmit up to 100 m under good conditions, although this distance may be reduced considerably by obstacles such as walls. This upgrade has dropped FHSS


in favor of the more reliable DSSS. Settling on one method of encoding eliminates the problem of having a single standard that includes two kinds of equipment that aren't compatible with each other. 802.11b devices are compatible with older 802.11 DSSS devices but are not compatible with 802.11 FHSS devices. 802.11b is currently the most widely used wireless standard.


802.11g - 802.11g is an extension to 802.11b and operates in the same 2.4-GHz band. It brings data rates up to 54 Mbps using Orthogonal Frequency-Division Multiplexing (OFDM) technology. Because 802.11g is backward compatible with 802.11b, an 802.11b device can interface directly with an 802.11g access point. You may even be able to upgrade some newer 802.11b access points to be 802.11g compliant via relatively easy firmware upgrades


802.11i - 802.11i addresses many of the security concerns that come with a wireless network by adding Wi-Fi Protected Access (WPA) and Robust Security Network (RSN) to 802.11a and 802.11b standards. WPA uses Temporal Key Integrity Protocol (TKIP) to improve the security of
keys used with WEP, changing the way keys are derived and adding a message-integrity check function to prevent packet forgeries. RSN adds a layer of dynamic negotiation of authentication and encryption algorithms between access points and mobile devices. 802.11i is backwards compatible with most 802.11a and 802.11b devices, but loses security if used with non-802.11i devices.


802.11n - The next standard in development is IEEE 802.11n. This new standard offers far higher speeds than current standards. Speed projections are at least 100 Mbps, but they could go up to 320 Mbps. The standard isn't expected to be ratified until November 2006.


802.11X - This refers to the general 802.11 wireless standard - b, g, or i. It is not to be confused with 802.1X, a security standard.

802.15 - This specification covers how information is conveyed over short distances among a Wireless Personal Area Network (WPAN). This type of network usually consists of a small networked group with little direct connectivity to the outside world. It is compatible with Bluetooth 1.1.


802.16 - IEEE 802.16, was ratified in January 2001. It enables a single base station to support many fixed and mobile wireless users. It is also called the Metropolitan
Area Network (MAN) standard. 802.16 aims to combine the long ranges of the cellular standards with the high speeds of local wireless networks. Intended as a - last-mile - solution, this standard could someday provide competition for hard-wired broadband services such as DSL and cable modem. 802.16 operates in the 10- to 66-GHz range and has many descendants.


802.16d - This recent standard - also called the IEEE 802.16-2004 standard or WiMax - can cover distances of up to 30 miles. Theoretically, a single base station can transmit hundreds of Mbps with each customer being allotted a portion of the bandwidth. 802.16d may use either the licensed 2.6- and 3.5-GHz bands or the unlicensed 2.4- and 5-GHz bands.


802.16e - This is based on the 802.16a standard and specifies mobile air interfaces for wireless broadband in the licensed bands ranging from 2 to 6 GHz.

802.20 - Specifies mobile air interfaces for wireless broadband in licensed bands below 3.5 GHz.


802.1X - 802.1X is not part of the 802.11 standard. It is a sub-standard designed to enhance the security of an 802.11 network. It provides an authentication framework that uses a challenge/response method to determine if a user is authorized. See EAP.


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