In the continuously changingworld of wireless networks, newstandards for 802.11 are constant-

ly evolving. The most popular stan-dards currently are 802.11a, 802.11 b ,and 802.11g.

Each technology has its own prosand cons in regards to successful net-work deployment. Table 1 shows asummary of these pros and cons.Rows marked with a * rank the tech-nologies from 1-3, 1 being the best.802.11a and 802.11g both have a 5 to1 edge in maximum throughput withthe ability to transmit up to 54 Mb/s.802.11b tops out at 11 Mb/s.

802.11a has an advantage of oper-ating in the cleaner 5 GHz band whichcontains less RF interference than theheavily trafficked 2.4 GHz band.802.11b and 802.11g operate in thisband, which contains interferencefrom microwave ovens, cordlessphones, BLUETOOTH™ devices,etc. Some newer cordless phoneswork up in the 5 GHz band.

Also, the RF sensitivity for 802.11bis approximately 9 dB better than for802.11a receivers.

802.11b equipment, being on themarket the longest, is the most inex-pensive of the trio. 802.11g and8 0 2 . 11a equipment escalate up theprice scale from the base of 802.11b.A main reason is that there is muchmore competition from 802.11 bequipment manufacturers, therebylowering the cost to the consumers.

802.11a equipment cost is high fora couple of reasons. First, the actualequipment is higher-priced on themarket. Secondly, due to the smallercoverage distance for 802.11a propa-gation, more access points would berequired over the same area than foranother technology.

Of course, for a home office net-work, usually only one access point isneeded to acceptably cover the aver-age size home.

8 0 2 . 11b and 802.11g access pointscover a greater area than 802.11a A P ’s .

8 0 2 . 11a and 802.11g allow more usersto access the network than 802.11b atthe same time. Finally, 802.11a doesnot penetrate clutter (such as off i c ewalls) as well as 802.11b and 802.11 gdue to the higher frequency band inwhich it operates.

These are some of the major com-parisons used by network administra-tors to make the decision on whichtechnology to use when deploying awireless interface into the networkarchitecture.

This discussion will focus on the8 0 2 . 11a technology and measurementtechniques to verify network availabil-i t y, security, and throughput. W h e ndeploying a wireless network and latermaintaining it during use, several fac-tors must be taken into account. T h e s einclude such factors as:

1. The network must have complete coverage over the intended client domain.

2. The network must be at least as secure as an equivalent wired network.

3. The network must process transactions in a timely manner such that it maintains a data rate that is satisfactory to meet productivity goals.

In order to meet these requirements,a wireless network test tool is very use-ful in becoming a network administra-t o r’s aide in detecting and trou-bleshooting various problem areas.

This tool will be needed over thelifetime of the wireless network. Thereason for this is that there will beconstant changes in the RF environ-ment over time. These changes are notseen in a wired network (with theexception of network load) because

the environment is contained withinwires and (hopefully) contains a fire-wall. This firewall protects againstexternal anomalies (such as viruses)and intruders.

The RF environment W I L L c h a n g e ,h o w e v e r, due to such actions as:

1. Building construction (more cubicles, less walls, etc.)

2. Cordless phone usage3. Microwave oven usage4. Other new sources of interference

This is where the correct testequipment becomes extremely impor-tant. The test tool we will use for reference in this discussion is the newBerkeley Varitronics Systems, Inc.YellowJacket™ 802.11a tool. T h i stool contains a proprietary 802.11 aRF module (high-speed I&Q A / Dconverters, DSP, memory, and I/O)that connects to an iPAQ™ displayand user-input device through the CFserial input.

One important feature of this tool,which other test tools do not have, isthe ability to do a spectrum analysison the RF environment. It is as neces-sary to be able to see non 802.11a traf-fic in the 5GHz band to determine ifthe network is being compromised byinterference.

Using a directional antenna, onecould determine the actual direction ofthe interfering party. A d i r e c t i o n a lantenna is also useful in detecting thelocation of unauthorized MAC(Media Access Control) addresses thatare interfering with the network.

The unauthorized MAC addressesof a network may be trying to attackthe network with a denial of serviceattack. This is an attack where theunauthorized addresses log into thenetwork illegally and then create a lotof unnecessary traffic to bring downthe throughput of the network.

Using the YellowJacket, a list canbe set up using the MAC addresses ofauthorized access points and clients onthe network. When the Ye l l o w J a c k e tsees a MAC address that is not author-ized, an alarm is set off, warning of apotential intruder and threat to the

FEATURE ARTICLEPAGE 48 • NOVEMBER 2003 MICROWAVE PRODUCT DIGEST

802.11a Measurement Techniques and Network Issuesby Herb Petrat, Senior Software Engineer, Berkeley Varitronics Systems, Inc.

802.11a 802.11b 802.11gMaximum Data Rate 54 Mb/s 11 Mb/s 54 Mb/sFrequency Band 5 GHz 2.4 GHz 2.4 GHzRF Interference Concerns * 1 3 3Modulation OFDM DSSS OFDMEquipment Cost High Low MediumCoverage Distance * 3 1 1Clutter Interference Concerns * 3 1 1Number of Users * 1 3 1

Table 1: 802.11 Technology Summary

B V S , Con’t on pg 57

Figure 1: BVSYellowjacket™ for 802.11a

security of the network.Going into the AP Search screen,

the unauthorized intruder can belocated with the directional antenna.By fanning the antenna back andforth, the unauthorized MAC addresswould have a stronger RSSI valuewhen in the direct path of the antenna.Once the direction is found, proceed-ing towards the MAC address wouldbe produce even higher readings untilthe user is right on top of the intruder.

A game that the network hackersplay is another denial-of serviceattack. Two clients set up within rangeof the network in question. They log

into their own network. They transferhigh amounts of data as quickly asthey can. This interferes and reducesthroughput of any legal network traf-fic in the same area.

These culprits can be found in thesame manner as the other ones. Use adirectional antenna and the AP searchscreen to zero in on the intruder.

Performing site surveys is anotherimportant step in maintaining a wire-less network.

An associated software packagethat is an option with the YellowJacketis BirdsEye Site Initiator, SiteSupervisor, and Site Investigator. Thisapplication package performs site sur-

veys and analyzes coverage issues incurrent network environments. T h eresult is a printable color report of sitecoverage and interference.

There are three applications associ-ated with the BirdsEye™ package.The first is the Site Initiator applica-tion which runs on any Wi n d o w sdesktop or laptop. This programimports bitmapped floor plans for usein the site survey. Associated land-marks may be added to the drawing.These landmarks include AP’s, cord-less phones, microwaves, and textmessages for different areas of thefloor plan.

The finished site is saved and

imported into the Site Supervisorapplication. This application runs onthe iPAQ Pocket PC that controls theYellowJacket hardware. The site ispulled up on the iPAQ. Then the userwalks around the site, tapping the cur-rent point in the floor plan with theattached stylus. The YellowJacket per-forms a quick scan of all 8 channels ateach point, recording any accesspoints that are found. The user onlyhas to make sure that enough samplepoints are taken throughout the site. Agood rule of thumb is taking a pointevery 40-60 square feet.

After the survey has been complet-

B V S , Con’t from pg 48

Figure 2: Yellowjacket™Spectrum Screen

Figure 3: Yellowjacket™ AP Screen

Figure 4: Yellowjacket™ APSearch Screen

B V S , Con’t on pg 60

Figure 5: Yellowjacket™Security Screen

FEATURE ARTICLEPAGE 60 • NOVEMBER 2003 MICROWAVE PRODUCT DIGEST

ed, the resulting data is transferred offof the iPAQ back onto the desktop orlaptop. Here is where Site Investigatoris used. This application will plot outthe data from the Site Survey and pre-pare a visual and/or printed report ofthe coverage for the site in question.In the figure shown for SiteI n v e s t i g a t o r, a typical analysis isshown. The different colors representdifferent access points.

As you can see from the diagram,access point markers were placed onthe site using Site Initiator. The colorsfor the RSSI (Received SignalStrength Indicator) data for the associ-ated access points get noticeably dark-er as they get closer to the markers ofthe actual location of the accesspoints. The shade of the colors will getdarker as the RSSI values increase.For example, a value of -40 dBm willresult in a darker shade than a value of-80 dBm.

BirdsEye™ software withYellowJacket™ hardware combines toprovide a network administrator witha tool to constantly monitor the wire-less network environment. Coverageholes would show up in the resultingreports as colorless (white). Then the

user could turn around and use theYellowJacket™ to determine whythere is a network hole.

There could simply be a need foranother access point. If it seems that anearby access point should have cov-ered the hole, outside RF interferencecould be the culprit. The user can takethe YellowJacket™ spectrum screento see if that is indeed the case.

There could be co-channel interfer-ence. BirdsEye™ can map the area bychannel and it can be seen whether ornot there are two adjoining accesspoints that are using the same channel.

It could also be that certain clutteris preventing an access point signalfrom reaching the designated area.Clutter such as copper-lined wallscould cause a signal to not propagateand simply reflect.

Combining BirdsEye™ with the

YellowJacket™ is one of the moreeffective tools in the battle againstconstantly changing RF environmentsfor 802.11a networks.

There are a number of issues thatmust be considered when decidinghow and when to deploy an 802.11awireless network for home or corpo-rate use. A test tool such as theYellowJacket™ is extremely useful innetwork setup and troubleshootingand can make an IT manager’s dailywork less strenuous as well provide abaseline archive of a wireless net-work’s performance.

The key is being able to maintainyour wireless network amidst con-stantly evolving security and environ-mental concerns. The right test toolhelps reduce the amount of labor costinvolved with network maintenance.

1. BLUETOOTH is a trademark owned by Telefonaktiebolaget L MEricsson, Sweden.

2. YellowJacket and BirdsEye are trademarks of Berkeley VaritronicsSystems, Inc. of Metuchen, NJ 732-548-3737 www.bvsystems.com

3. iPAQ is a trademark of Compaq Corporation

Figure 6: Yellowjacket™Access Point MAC List

Figure 7: Birdseye™ Site Initiator

Figure 8: Birdseye™ SiteInvestigator

B V S , Con’t from pg 57

A WLAN system’s RF environment challengesdesigners, installers and administrators with planning re-usepatterns, interference detection and system coverage in bothinstallation and maintenance. These challenges are alsofound in seemingly more complicated cellular phone sys-tems. Frequency re-use patterns, coverage mapping, inter-ference from neighbors, locating unauthorized users andlocating stolen equipment must be considered in addition tothe standard PER and throughput metrics. Installing andmaintaining a large WLAN system can rival the complexity ofa cellular phone system. The author presents a comparisonof equipment that is available to designers, installers andadministrators to measure and overcome these challenges.Spectrum analyzers, standard WLAN cards and RF equip-ment specifically designed to measure 802.11 on the air areexamined. Examples are drawn from the author’s experiencein designing WLAN test equipment; parallels are also drawnto methods and equipment that are found in the relativelymature cellular phone industry.

Introduction

The installation of an 802.11b Wireless LAN systemto cover a large office setting can be very challenging andtechniques found in cellular system engineering are oftenrequired. WLAN systems have the added complexity ofoperating in an unlicensed band where interference may notbe under control of the WLAN manager, and the WLAN oftenoperates in a harsher indoor RF environment.

Large WLAN installations will in many ways resem-ble a cellular phone system. Access Points (APs) are anal-ogous to Base Stations. APs connect to the clients withintheir coverage area. APs have a "backhaul", Ethernet, whichties them together and into the network. Adjacent APs mustbe channelized so that they do not interfere with each other.

TTake the Wake the W-LAN T-LAN Test Challengeest Challenge

Figure 1 illustrates a frequency sweep of an 802.11b channel with several types of common interference.We see a frequency hopper (left), an 802.11b access point (center) and a microwave oven (right).

The most basic and readily available test tool is alaptop with an 802.11b card. The measurements vary withmanufacturer, but the ability to measure signal strength andsignal quality is common. These measurements are oftenrelative, scaled 1–10. Specialized test tools are availablethat measure to traceable quantities, dBm, and have moretypes of measurements available. The tool or set of meas-urement tools should detect and measure interference,measure AP signal strength and Packet Error Rate (PER)from an AP.

Measuring the Existing RF Environment for Interference

The place to start an 802.11 installation is to meas-ure the existing interference. Microwave ovens, 2.4 GHzcordless telephones, other 802.11b WLANs, 802.11 frequen-cy hoppers and bluetooth devices can all interfere with anddegrade the performance of an 802.11b system.

Figure 1 illustrates a frequency sweep of an 802.11bchannel with several types of common interference. A spe-cialized WLAN test tool or spectrum analyzer is used tomeasure the interference and is moved throughout the cov-erage area of the WLAN system. A peak hold or logging ofthe data is essential to establish the "noise floor" that will beinterfering with the WLAN in different areas. A more rigorouscheck would leave the instrument measuring for perhaps aday or more with data logging and then moved to differentlocations in the coverage area. The tool or spectrum analyz-er should have a sensitivity of at least –90dBm.

The spectrum scan detects energy present in theband from all sources and is best to scan for all types of inter-ference. A test tool or 802.11b card should also be used todemodulate any existing 802.11b interference on the air.Testing for 802.11b interference via demodulation yieldsmore information about these interferers and is more sensi-

tive.A directional antenna can be used to help locate

sources of interference, shown in figure 2. As many interfer-ers should be eliminated as possible. Microwave ovens maybe moved, and bluetooth devices and cordless phones canbe banned from the office. Some interference may beimpractical to eliminate. A neighboring business may alsohave an 802.11 system or some other interfere. In theseareas, plan to have a higher density of APs, spend more timeplanning channelization and use some special techniquesthat are discussed later.

Measuring the Coverage of an Access Point

The coverage area of an Access Point is experimen-tally found by simply locating the AP at a candidate site andmeasuring the signal strength and PER with a test tool. Aspectrum analyzer could be used, but it can only measuresignal power in the channel; and this measurement may alsoinclude interference, other Access Points other than the onebeing measured or even energy from a different AP on anoverlapping adjacent channel. A test tool with demodulationis desirable because it can measure the signal strength of theAP coverage under test and the signal to noise is indirectlymeasured by the PER.

A typical signal level required for adequate coverageis around -80 dBm or stronger. This level includes somemargin for typical interference and signal fading. The signalstrength required will be greater in areas with interferencelevels greater than -90 dBm. Figure 3 tabulates typical sig-nal strengths required with varying amounts of interference.

A directional antennacan be used to helplocate sources ofinterference, figure 2.

Measuring Coverage and Co-Channel System Interference

After testing AP coverage in several locations, theAPs required for adequate coverage and overlap can belocated. This may require an educated estimate or special-ized propagation software. Neither method is perfect, soadditional APs may need to be added or locations adjustedwith testing.

Channelization is required so that neighboring APsdo not interfere with each other. Figure 4 depicts a typicalhexagonal frequency reuse pattern for three frequencies.For continuos coverage, APs must overlap, and the frequen-cies in these overlap areas should be different for each AP toavoid interference in these overlap areas.

The installation site must be surveyed to insure thatall areas have sufficient signal strength, good PER from atleast one AP and is without significant co-channel interfer-ence. Co-channel interference is when energy on the samechannel is received from different APs. Co-channel interfer-ence should be at least 15dB weaker than the stronger AP.This survey requires that 3 frequencies be measured for a re-use pattern of 3, figure 5.

Fixing Problem Areas

A site survey or the operation of the WLAN itself willprobably reveal areas where coverage is not adequate.Adding or moving an AP closer to an area with high interfer-ence will boost the signal level. Directional antennas such

as corner reflectors can direct more energy to your coveragearea and reduce the amount of received interference. This isa useful technique for existing with a neighboring WLAN sys-tem; use directional antennas on the AP to direct coverageinto the coverage area and to reduce interference receivedfrom outside the coverage area. A directional antenna mayalso be used at the client.

Keep the tools ready for Problems

Periodic checks and monitoring can avoid problems,but be prepared for a client with little or no throughput. Toolscan quickly verify the RF link for signal level, PER and inter-ference. The tools can help debug a new interfering AP aneighbor has set-up or to find the antenna that has beenknocked over.

A directional antenna and receiver are very useful forlocating interference or even an unauthorized user or stolenequipment. The directional set-up shown in figure 2 can beused to direction find a node with a specific MAC address.

Summary of tools and techniques

A microwave oven, a client with slightly weak signalstrength or another interferer with short transmissions shouldnot bring down the network or significantly degrade perform-ance. The guidelines presented here are conservative, aimfor them when planning and setting up a network. An outageor a drop in performance will justify the cost of WLAN RF testtools.

A laptop with an 802.11b card is a good test tool andmay be adequate for small installations and maintenance ofsmall systems where interference is not significant. For larg-er installations and maintenance or in areas with significantinterference, a tool or set of tools with the following charac-teristics is recommended:

Spectrum Analysis: sensitivity at least -90dBm, with peakhold, and data logging is a plus. Best used to check for andmeasure interference of any type.

Demodulation Analysis: Signal strength (measured in dBm

is a plus), PER, and scanning of multiple channels is a plus.Best used to measure 802.11b interference, coverage ofAPs, channelization and co-channel interference.

Directional antenna: Can be used to locate interference oran unauthorized user.