With wireless systems, it’s very difficult to predict the propagation of radio waves and detect the presence of interfering signals without the use of test equipment. Even if the system implements omni-directional antennas, radio waves don’t really travel the same distance in all directions. Instead walls, doors, elevator shafts, people, and other obstacles offer varying degrees of attenuation, causing the RF (radio frequency) radiation pattern to be irregular and unpredictable. As a result, it’s absolutely necessary to perform a wireless site survey to fully understand the behavior of radio waves within the operating environment before installing access points.
Site survey concepts
The ultimate goal of a wireless site survey is to determine the number and placement of access points (or mesh nodes) that provides adequate signal coverage throughout a facility or city area. With most implementations, “adequate coverage” means support of a minimum data rate or throughput. In order to perform a successful survey, you’ll need to relate the required performance to a value that survey tools measure, such as SNR. A wireless site survey also detects the presence of RF interference coming from other sources that could degrade the performance of the wireless LAN.
The need and complexity of a wireless site survey will vary depending on the facility. For example, a small three-room office may not require a site survey. This scenario can probably get by with a single Wi-Fi access point (or router) located anywhere within the office and still maintain adequate coverage. If the access point encounters RF interference from another nearby wireless LAN, you can likely choose a different channel and eliminate the problem.
A larger facility, such as an office complex, airport, hospital, or warehouse, or an outdoor area like a city, generally requires an extensive wireless site survey. Without a survey, users will probably end up with inadequate coverage and suffer from low performance in some areas.
When conducting a wireless site survey, consider the following steps and tips:
1. Understand the wireless requirements. In order to identify optimum locations for access points or mesh nodes, you must have a good understanding of specific requirements for the network that impacts signal coverage. For example, maximum range between a client device and the access point decreases as data rate and resulting performance increases. Thus, you need to know the target data rates (and throughput) to correctly interpret survey results. Also, client devices may have relatively low transmit power, which must be taken into consideration when using most site survey tools (see related tutorial).
2. Obtain a facility diagram. Before getting too far with the site survey, locate a set of building blueprints or city maps. If none are available, prepare a drawing that depicts the location of walls, walkways, etc. Site survey tools import diagrams in various image formats. Of course mapping software is a good source for outdoor city surveys. If all else fails for in-building surveys, consider taking a digital photograph of the fire escape diagram, which is usually present on hallway walls.
3. Visually inspect the facility. Walk through the facility before performing any testing to verify the accuracy of the facility diagram. This is a good time to note any potential attenuation barriers that may affect the propagation of RF signals. For example, a visual inspection will uncover obstacles to signals such as metal racks and partitions, items that blueprints generally don’t show. Also, note possible locations for mounting access points, such as above ceiling tiles or on pillars. For outdoor city environments, you should carefully assess the locations and availability of street lights and water towers for mounting mesh nodes and backhaul equipment. These actions will make the later testing efforts go much more smoothly.
4. Assess existing network infrastructure. Determine the capacity of any existing wired networks that can interface the access points or mesh nodes. Most buildings have Ethernet and in some cases optical fiber networks. Check on how much of the existing networks can be made available for supporting the wireless network. This will aid designers later on in the deployment when defining the architecture and bill of materials for the wireless network.
5. Identify coverage areas. On the facility diagram or city map, indicate all areas where coverage is needed, such as offices, hallways, stairwells, utility rooms, bathrooms, break rooms, patios, parking garages, and elevators. Also, identifying where users will not wireless coverage is important to avoid wasting time surveying unnecessary areas. Keep in mind that you might get by with fewer access points and lower equipment costs if you can limit the roaming areas.
6. Determine preliminary access point locations. By considering the location of wireless users and range estimations of the wireless LAN products you’re using, approximate the locations of access points that will provide adequate coverage throughout the user areas. Plan for some propagation overlap (generally 25 percent) among adjacent access points, but bear in mind that channel assignments for access points will need to be far enough apart to avoid inter-access point interference.
Be certain to consider mounting locations, which could be vertical posts or metal supports above ceiling tiles. Recognize suitable locations for installing the access point, antenna, and data / PoE cable. Also think about different antennas when deciding where to position access points. An access point mounted near an outside wall, for example, could be a good location if you use a patch antenna with relatively high gain oriented within the facility.
7. Verify access point locations. This is when the site survey testing begins. Most wireless LAN vendors provide wireless site survey software that identifies the associated access point, data rate, signal strength, and signal quality. You can load this software on a laptop and test the coverage of each preliminary access point location. Alternately, you can use a third party site survey tool available from several different companies, such as AirMagnet, Berkeley Varitronics Systems, and Ekahau.
Install an access point at each preliminary location, and monitor the site survey tool readings by walking varying distances away from the access point. There’s no need to connect the access point to the distribution system because the survey tests merely ping the access point or read the beacon signal strength. Very important: Definitely consider the SNR range boundary and uplink signal strength when interpreting the results. To make the access point easy to move about the facility, you can mount it on a pole attached to a cart with a battery and DC/AC converter. Otherwise, you’ll need to haul around an extension cord and always be looking for where to plug in for power (not recommended).
Take note of performance or signal readings at different points as you move to the outer bounds of the access point coverage. In a multi-floor facility, perform tests on the floor above and below the access point. Keep in mind that a poor signal quality reading could indicate that RF interference is affecting the wireless LAN. This would warrant the use of a spectrum analyzer to characterize the interference, especially if there are no other indications of its source. Based on the results of the testing, you might need to reconsider the location of some access points and redo testing for the affected locations.
8. Document findings. Once you’re satisfied that the location of access points you’ve identified will provide adequate signal coverage, document your findings on the facility diagrams by depicting the location of each access point. The installers will need this information.
These steps will point you in the right direction, but experience really pays off. If you’re new to wireless LANs, you’ll begin to build an odd intuition about the propagation of radio waves after accomplishing several wireless site surveys.
For more of Jim Geier’s tutorials, read “Deploying Voice over WLANs,” Troubleshooting 802.1x Missing Supplicant Problems (Part I),” and “802.11a/b Site Survey: A Testimonial.”
For more on AirMagnet, read “AirMagnet’s Wireless LAN Handheld Analyzer,” “Review: AirMagnet’s OQO Analyzer/Survey Bundle,” and “Simplified WLAN Analysis: The AirMagnet Attraction.”
Jim Geier provides independent consulting services and training to companies developing and deploying wireless networks for enterprises and municipalities. He is the author of a dozen books on wireless topics, with recent releases including Deploying Voice over Wireless LANs (Cisco Press) and Implementing 802.1x Security Solutions (Wiley).
Pamoja na mifumo ya wireless, ni vigumu sana kutabiri kuenea kwa mawimbi ya redio na kugundua uwepo wa ishara za kuingiliana bila matumizi ya vifaa vya mtihani. Hata kama mfumo kutekeleza antennas omni-directional, mawimbi ya redio si kweli kusafiri umbali sawa katika pande zote. Kwa upande mwingine, kuta, milango, shimo la lifti, watu, na vizuizi vingine hutoa viwango tofauti vya kupunguzwa, na kusababisha muundo wa mnururisho wa masafa ya redio kuwa usio wa kawaida na usioweza kutabiriwa. Kama matokeo, ni muhimu kabisa kufanya uchunguzi wa tovuti ya wireless kuelewa kikamilifu tabia ya mawimbi ya redio ndani ya mazingira ya uendeshaji kabla ya kufunga pointi za upatikanaji. Lengo la mwisho la uchunguzi wa tovuti ya wireless ni kuamua idadi na uwekaji wa pointi za upatikanaji (au nodes za mesh) ambazo hutoa chanjo ya kutosha ya ishara katika kituo au eneo la jiji. Pamoja na utekelezaji wengi, "ufuniko wa kutosha" ina maana ya msaada wa kiwango cha chini cha data au throughput. Ili kufanya utafiti wa mafanikio, unahitaji kuhusiana utendaji required kwa thamani kwamba zana za utafiti kipimo, kama vile SNR. Utafiti wa tovuti ya wireless pia hugundua uwepo wa kuingiliana kwa RF kutoka vyanzo vingine ambavyo vinaweza kudhoofisha utendaji wa LAN ya wireless. Uhitaji na utata wa uchunguzi wa tovuti ya wireless itatofautiana kulingana na kituo. Kwa mfano, ofisi ndogo yenye vyumba vitatu huenda isiwe na uhitaji wa kuchunguza eneo hilo. Hii hali ya uwezekano wa kupata kwa njia ya moja Wi-Fi hatua ya upatikanaji (au router) ziko mahali popote ndani ya ofisi na bado kudumisha chanjo ya kutosha. Ikiwa kituo cha upatikanaji kinakabiliwa na kuingiliwa kwa RF kutoka kwa LAN nyingine isiyo na waya iliyo karibu, unaweza kuchagua kituo tofauti na kuondoa tatizo. Kituo kikubwa, kama vile jengo la ofisi, uwanja wa ndege, hospitali, au ghala, au eneo la nje kama jiji, kwa ujumla inahitaji uchunguzi mkubwa wa tovuti ya wireless. Bila uchunguzi, watumiaji pengine mwisho na chanjo inadequate na kuteseka kutoka utendaji wa chini katika maeneo fulani. Wakati wa kufanya uchunguzi wa tovuti ya wireless, fikiria hatua zifuatazo na vidokezo: 1. Kuelewa mahitaji ya wireless. Ili kutambua maeneo bora kwa ajili ya pointi za upatikanaji au nodes mesh, lazima kuwa na uelewa mzuri wa mahitaji maalum kwa ajili ya mtandao kwamba athari ishara chanjo. Kwa mfano, upeo wa umbali kati ya kifaa mteja na hatua ya upatikanaji hupungua kama kasi ya data na matokeo ya utendaji huongezeka. Hivyo, unahitaji kujua lengo data viwango (na throughput) kwa usahihi kutafsiri matokeo ya utafiti. Pia, vifaa mteja inaweza kuwa na nguvu ya chini ya kutuma, ambayo lazima kuchukuliwa katika kuzingatia wakati wa kutumia wengi tovuti uchunguzi zana (tazama tutorial kuhusiana). 2. Uwezekano wa kupata Kupata mchoro wa kituo. Kabla ya kwenda mbali sana na uchunguzi wa eneo hilo, tafuta ramani za majengo au ramani za jiji. Ikiwa hakuna, andika picha inayoonyesha mahali ambapo kuta, vijia, na kadhalika viko. Vifaa vya uchunguzi wa tovuti huingiza michoro katika muundo mbalimbali wa picha. Bila shaka programu ya ramani ni chanzo kizuri cha uchunguzi wa nje wa jiji. Ikiwa yote mengine hayana mafanikio, fikiria kuchukua picha ya dijiti ya ramani ya njia ya kutoroka moto, ambayo kwa kawaida iko kwenye kuta za ukumbi. 3. Uwe na mtazamo wa kweli Chunguza kituo hicho kwa macho. Kutembea kwa njia ya kituo cha kabla ya kufanya upimaji wowote kuthibitisha usahihi wa mchoro kituo cha. Hii ni wakati mzuri wa kumbuka vizuizi vyovyote vya kupunguza ambayo yanaweza kuathiri kuenea kwa ishara za RF. Kwa mfano, ukaguzi wa kuona utafunua vikwazo kwa ishara kama vile racks chuma na partitions, vitu kwamba blueprints kwa ujumla si kuonyesha. Pia, angalia mahali ambapo unaweza kuweka vituo vya kuingia, kama vile juu ya vigae vya dari au kwenye nguzo. Kwa mazingira ya nje ya mji, unapaswa kuchunguza kwa uangalifu maeneo na upatikanaji wa taa za barabara na minara ya maji kwa ajili ya kufunga nodes mesh na vifaa vya backhaul. Hatua hizi zitafanya juhudi za baadaye za kupima ziende vizuri zaidi. 4. Uwe na uhakika Tathmini ya miundombinu ya mtandao iliyopo. Kuamua uwezo wa yoyote ya sasa wired mitandao ambayo inaweza interface pointi ya upatikanaji au mesh nodes. Majengo mengi yana Ethernet na katika baadhi ya kesi mitandao ya fiber optic. Angalia juu ya kiasi gani cha mitandao iliyopo inaweza kufanywa inapatikana kwa ajili ya kusaidia mtandao wireless. Hii itasaidia wabunifu baadaye katika kupelekwa wakati wa kufafanua usanifu na orodha ya vifaa kwa ajili ya mtandao wireless. 5. Uwe na mtazamo mzuri. Tambua maeneo ya kufunika. Katika ramani ya kituo au ramani ya mji, onyesha maeneo yote ambapo chanjo inahitajika, kama vile ofisi, vijia, stairwells, vyumba vya huduma, bafu, vyumba vya mapumziko, patios, maegesho ya magari, na lifti. Pia, kutambua ambapo watumiaji si wireless chanjo ni muhimu ili kuepuka kupoteza muda kuchunguza maeneo yasiyo ya lazima. Kumbuka kwamba unaweza kupata kwa pointi chache za ufikiaji na gharama za chini za vifaa ikiwa unaweza kupunguza maeneo ya kuzurura. 6. Uwe na uhakika Kuamua maeneo ya awali ya pointi ya upatikanaji. Kwa kuzingatia eneo la watumiaji wireless na makadirio ya mbalimbali ya bidhaa wireless LAN wewe ni kutumia, takriban maeneo ya pointi za upatikanaji ambayo itatoa chanjo ya kutosha katika maeneo ya mtumiaji. Mpango kwa baadhi ya propagation overlap (kwa ujumla asilimia 25) kati ya pointi za upatikanaji jirani, lakini kukumbuka kwamba channel utoaji kwa pointi za upatikanaji itabidi kuwa mbali kutosha mbali ili kuepuka kuingiliana kati ya pointi za upatikanaji. Hakikisha kwamba unafikiria mahali pa kuweka, ambayo yaweza kuwa nguzo za wima au nguzo za chuma juu ya vigae vya dari. Kuamua mahali pa kuunganisha antena, pointi ya ufikiaji, na data ya PoE Pia fikiria antena tofauti unapochagua mahali pa kuweka vituo vya ufikiaji. Njia ya kuingia iliyowekwa karibu na ukuta wa nje, kwa mfano, inaweza kuwa mahali pazuri ikiwa unatumia antenna ya patch na faida ya juu inayolenga ndani ya kituo. 7. Uwe na mtazamo gani? Angalia maeneo ya pointi za ufikiaji. Hii ni wakati majaribio ya uchunguzi wa tovuti huanza. Wauzaji wengi wa LAN ya wireless hutoa programu ya uchunguzi wa tovuti ya wireless ambayo hutambua hatua ya upatikanaji inayohusiana, kiwango cha data, nguvu ya ishara, na ubora wa ishara. Unaweza kupakia programu hii kwenye kompyuta ndogo na mtihani chanjo ya kila eneo la awali upatikanaji uhakika. Kwa mfano, unaweza kutumia chombo cha uchunguzi wa tovuti ya mtu wa tatu inayopatikana kutoka kwa makampuni kadhaa tofauti, kama vile AirMagnet, Berkeley Varitronics Systems, na Ekahau. Kuweka hatua ya upatikanaji katika kila eneo la awali, na kufuatilia tovuti uchunguzi chombo kusoma kwa kutembea umbali tofauti mbali na hatua ya upatikanaji. Hakuna haja ya kuunganisha hatua ya upatikanaji kwa mfumo wa usambazaji kwa sababu vipimo vya uchunguzi tu ping hatua ya upatikanaji au kusoma nguvu ya ishara ya beacon. Muhimu sana: Hakika kuzingatia SNR mbalimbali mpaka na uplink nguvu ishara wakati wa kutafsiri matokeo. Ili kufanya kituo cha upatikanaji rahisi kusonga juu ya kituo, unaweza kuweka juu ya pole kushikamana na gari na betri na DC-AC converter. Vinginevyo, unahitaji kuvuta karibu kamba ya upanuzi na daima kuwa kuangalia ambapo plug katika kwa ajili ya nguvu (haipendekezi). Angalia utendaji au ishara ya kusoma katika pointi tofauti kama wewe hoja kwa mipaka ya nje ya upatikanaji pointi chanjo. Katika kituo cha ghorofa nyingi, fanya vipimo kwenye ghorofa juu na chini ya hatua ya upatikanaji. Kumbuka kwamba poor signal ubora kusoma inaweza kuonyesha kwamba RF kuingiliwa ni kuathiri wireless LAN. Hii ingehakikishia matumizi ya mchambuzi wa wigo ili kuelezea kuingiliana, haswa ikiwa hakuna dalili nyingine za chanzo chake. Kulingana na matokeo ya majaribio, unaweza kuhitaji kufikiria tena eneo la baadhi ya pointi za ufikiaji na kufanya majaribio tena kwa maeneo yaliyoathiriwa. 8. Uwe na mtazamo gani? Upatikanaji wa hati. Mara baada ya wewe ni kuridhika kwamba eneo la pointi upatikanaji umebaini itatoa kutosha ishara chanjo, nyaraka matokeo yako juu ya vifaa michoro kwa kuelezea eneo la kila pointi upatikanaji. Wasanifu watahitaji habari hii. Hatua hizi zitakuelekeza katika mwelekeo sahihi, lakini uzoefu unalipa kweli. Kama wewe ni mpya kwa LAN wireless, wewe kuanza kujenga intuition ajabu kuhusu kuenea kwa mawimbi ya redio baada ya kukamilisha kadhaa wireless tovuti uchunguzi. "Kwa zaidi ya mafunzo ya Jim Geier, soma ""Kupeleka Sauti juu ya WLANs"" ""Kuchunguza matatizo ya 802.1x ya kukosa Supplicant"" na ""Utafiti wa Tovuti ya 802.11a: Ushuhuda"" ""Kwa zaidi juu ya AirMagnet, soma ""AirMagnet's Wireless LAN Handheld Analyzer"" ""Review: AirMagnet's OQO Analyzer"" na ""Simplified WLAN Analysis: The AirMagnet Attraction"" ""Jim Geier hutoa huduma za ushauri wa kujitegemea na mafunzo kwa makampuni ya kuendeleza na kupeleka mitandao kwa makampuni na manispaa." Yeye ni mwandishi wa vitabu kadhaa juu ya mada wireless, na releases hivi karibuni ikiwa ni pamoja na Deploying Voice juu ya Wireless LANs (Cisco Press) na Implementing 802.1x Security Solutions (Wiley).