wireless local area network design

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ess Loca rea NeDesignlhis time we'll move toward the practicalz i 1 considerations that need to be consideredwhen building a wireless network. In the

past, this column has discussed the design of sucha network, but now I want to offer some specificsuggestions as to hardware and configuration.We'll also take a look at a great book I recentlyreceived, a professional text on site surveying for802.1 1 systems.Some HistorvBack in the days of numerous packet networks,we used to assemble stacks of Terminal NodeControllers (TNCs) using diode matrix boards orcomputers with multi-port RS-232 cards to createnodes, which were used as network switches toroute connections across the network. Today, wecan leverage the equipment available in the com-mercial sector to perform these functions at amuch lower cost. he network model I'm sug-gesting looks like a wired network but uses wire-less bridges to replace the ethernet wires thatwould normally connect together a number of net-work switches or hubs. The end result looks andacts much like a wired network.

et WiredTo build a wired network (fig. I , we simply run tothe computer or office supply store and pick up awired ethernet switch or hub, which generally costs*P.O. Box 114, Park Ridge, NJ 07656e-mail: cn2irz @cq-amateur-radio.com>

under $30. switch uses the full bandwidth avail-able for each single connection, while a hub sharesthe total bandwidth across all connections. Thinkof a hub as a radio channel with all users on thesame frequency, while a switch is like each userhaving his or her own frequency, the switch con-necting the audio between users as necessary.We can also connect multiple switches to oneanother in order to make our wired network larg-er. I might use a 4-port switch to connect togeth-er four 8-port switches, allowing up to 28 com-puters (4 8 32 ports, but one port per switchis used to connect back to the 4-port switch, so32 4 28 available ports) to be on the same net-work at one time. Of course, like any peer-to-peernetwork, you must know how to find the othercom-puter(s) with which you wish to communicate, andthose other computers must be set up to allowsuch communications.

oing WirelessWe can use consumer-grade wireless devices toreplace that piece of wire connecting an 8-portswitch to the 4-port switch, resulting in a wirelesslink. At each networkfacility, add a wireless routerto allow users to connect in, and you have a wire-less data network. Yes, it's that simple.You may have heard the terms hub, switch,router, and bridge used before (at least if you'rea ham you might have ). I already explained whata hub and switch are. router (which usually alsohas an integrated multi-port switch) allows you tocommunicate to another network (such as througha cable or DSL modem). The big advantage of a

Cat 5 Ethernet

4-Port Switch

Fig. 1- A typical wired network, with a 4-port switch as its center and two 8-port switches to connectup to 16 client computers only six are shown for simplicity) together in a wired Local Area Network,or LAN.CQ December 2 6 Visit Our Web Site


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Cat 5 Ethernet

Wireless Bridge Wireless Bridge2- The same network as in fig. 1 but with one switch connected via a wireless bridge link as well as wireless routershown providing service to a wireless client This forms a basic wireless network which can be expanded as necessary.

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router is that it has DHCP; this meansit can dynamically assign a TCPIIPaddress to a user joining the networkwhich is essential where you cannot ordo not want to pre-configure all the net-work addresses.A bridge in contrast is just a deviceto extend a network connection. It iscompletely transparent to the networkjust like a piece of wire. Professional-quality wireless bridges can cost afew thousand dollars but consumer-grade models can be found for under100 each.

I also want to mention that some wire-less routers and access points have abridge mode which makes them act likea bridge. If you can find one it makes afine alternative to a dedicated bridge.Also some of the modified firmwareavailable from the resources listed inthe sidebar with this article can addbridge capability to some of the reallyinexpensive consumer-grade wirelessrouters making for an even biggerbargain.Therefore we simply replace thewires between the switches in our

24 3 MHzt 0 MHz 2473 MHz

WiFi 802 11big Channel llocationsI

Fig. 3- WiFi2.4-GHz channel allocations. Note that only channels 1 6 and 1 1do not overlap. Channel6 is a common default and tends to have a lot of Part 15activity so it should be avoided where possible.

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wired network with a pair of wireless nas as far as possible from one anoth-bridges, and off we go. Add some user er vertically-one above the other on aaccess via a wireless router, and we tower, for example-is also effective.have the high-speed wireless networkshown in fig. 2. Equipment

Equipment is much less of an issue. AReducing Interference Google search for wireless routerWe need, however, to take actions to revealed many choices, such as theeliminate nterference. With multiple RF LinkSys WET54G for under $90, or thedevices operating on 2.4 GHz, some TrendnetTEW-43OAPBforaround 40.interference can be expected. It should Of course, the Cisco Aeronet 1400 cango without saying that co-located RF be had for a mere $3200-a gooddevices need to be on different chan- choice for a large corporation morenels, but we must also use standard RF interested in range, reliability, anddiversitytechniques o minimizeorelim- throughput than cost.inate interference. Note that only WiFi Speaking of range, the consumer-channels 1, 6, and 11 do not overlap grade (meaning inexpensive) models(see fig. 3). are usually in the 10- to 30-milliwattFM repeaters use expensive duplex- power range (10-1 5 dBm), while moreers, which are very sharp but efficient expensive units can have 200 mw (23filters, to eliminate interference dBm) output. If power becomes anbetween the output and input frequen- issue, find a bi-directional amplifiercies. This should also work on 2.4 GHz, (BDA), which is a quick (but not neces-but duplexers are expensive and need sarily cheap) way to get into the wattto be tuned. More simply, onecan mere- range. I ound many 1 watt BDAs in thely separate the antennas sufficiently, under-$200 price range, but an amplifi-especially if they are directional Yagis. er isn't always necessary.Pointing the Yagis in different directions However, what we save on outputhelps a lot, as does physical separation power can sometimes be made up byand shielding them from one another (a antennas. Once nice thing about bigwindow screen works well). Placingver- antennas is that they hear better, too.tically-polarized omnidirectional anten- On 2.4 GHz, big is a relative erm, with

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Resources: A website offering open-source alternative firmware for variousrouters, some of which can greatly enhance networking functionality to profes-: etailed s not related to LinkSys (a division of Cisco, Inc.), but is a valu-

wireless local area network design

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