CWNA Chapter 16 – Site Survey Systems and Devices

My Notes from chapter 16 of the CWNA study guide

Site survey defined

  • Protocol and spectrum analysis
    • Ten years ago, if you wanted to plan and design a wireless network, the likelihood of interference from other networks or wireless devices was much less than it is now.
    • A spectrum analyser will help identify whether there is any type of RF interference from 802.11 devices or other devices that could interfere with your WLAN
    • Wi-Fi-based protocol analysers can examine 802.11 frames and identify SSID and BSSID information along with packet and security information.
    • Some Wi-Fi protocol analysers are specifically designed for performing site surveys.
  • Standalone
    • Wi-Fi cards and spectrum analyser cards go about seeing the RF world in slightly different ways
    • The Wi-Fi card can see frames and modulated bits going across the RF medium. Protocol analysers take the data received by the Wi-Fi cards and provide packet analysis of that data
    • Spectrum analysers monitor the RF signal itself.
    • Because the Wi-Fi receiver and the spectrum analyser receiver are separate devices that monitor different pieces of information, historically they have been standalone devices, each performing a dedicated task.
  • Integrated
    • By correlating the raw RF with the data from the Wi-Fi card, you can better understand the effects of various scenarios on your wireless network
    • Look to spectrum and protocol analyser vendors to be adding more integration between both the spectrum analysis cards and Wi-Fi cards
  • Spectrum analysis
    • are frequency domain measurement devices that can measure the amplitude and frequency space of electromagnetic signals
    • To conduct a proper 802.11 spectrum analysis survey, the spectrum analyser needs to be capable of scanning both the 2.4 GHz ISM band and the 5 GHz U-NII bands
    • A true spectrum analyser picks up RF energy regardless of the source.
    • If the background noise level exceeds –85 dBm in either the 2.4 GHz ISM band or the 5 GHz U-NII bands, the performance of the wireless network can be severely degraded
    • A noisy environment can cause the data in 802.11 transmissions to become corrupted
    • Interfering devices might also prevent an 802.11 radio from transmitting
    • It is a recommended practice to conduct spectrum analysis of all frequency ranges, especially in the 2.4 GHz ISM band
    • The following are potential sources of interference in the 2.4 GHz ISM band:
      • Microwave ovens
      • 2.4 GHz cordless phones, DSSS and FHSS
      • Fluorescent bulbs
      • 2.4 GHz video cameras
      • Elevator motors
      • Cauterizing devices
      • Plasma cutters
      • Bluetooth radios
      • Nearby 802.11, 802.11b, 802.11g, or 802.11n (2.4 GHz) WLANs
    • A common everyday interfering item that should be documented during the site survey interview is the location of any microwave ovens
    • Current potential sources of interference in 5 GHz U-NII bands include the following:
      • 5 GHz cordless phones
      • Radar
      • Perimeter sensors
      • Digital satellite
      • Nearby 5 GHz WLANs
      • Outdoor wireless 5 GHz bridges
    • The 802.11-2012 standard defines dynamic frequency selection (DFS) and transmit power control (TPC) mechanisms to satisfy regulatory requirements for operation in the 5 GHz band to avoid interference with 5 GHz radar systems
    • Using a 5 GHz spectrum analyser during a site survey may help determine in advance whether radar transmissions exist in the area where the WLAN deployment is planned.
    • After locating the sources of interference, the best and simplest solution is to eliminate them entirely.
    • If interfering devices cannot be eradicated in the 2.4 GHz bands, consider moving to the less crowded 5 GHz U-NII bands.
    • If your WLAN is being used for either data or voice or for both, a proper and thorough spectrum analysis is mandatory in an enterprise environment.
  • Coverage analysis
    • The next step is the all-important determination of proper 802.11 RF coverage inside your facility
    • During the site survey interview, capacity and coverage requirements are discussed and determined before the actual site survey is performed
    • In certain areas of your facility, smaller cells or co-location may be required because of a high density of users or heavy application bandwidth requirements
    • RF measurements must be taken to guarantee that these needs are met and to determine the proper placement and configuration of the access points and antennas.
    • Proper coverage analysis must be performed using some type of received signal strength measurement tool or planning tool
    • how do you conduct proper coverage analysis?
    • One mistake that many people make during the site survey is leaving the access point radio at the default full-power setting.
    • A good starting point for a 2.4 GHz access point is 25 mW transmit power.
    • The hardest part of physically performing a coverage analysis site survey is often finding where to place the first access point and determining the boundaries of the first RF cell.
    • It is important to avoid excessive overlap because it can cause frequent roaming and performance degradation.
    • The shape and size of the building and the attenuation caused by the various materials of walls and obstacles will require you to change the distances between access points to ensure proper cell overlap
    • WLAN design guides and white papers from various WLAN vendors often reference 15 percent to 30 percent coverage cell overlap for roaming purposes.
    • Coverage overlap is really duplicate coverage from the perspective of a Wi-Fi client station
    • A proper site survey should be conducted to make sure that a client always has proper duplicate coverage from multiple access points
    • The SNR is an important value because, if the background noise is too close to the received signal, data can be corrupted and
    • retransmissions will increase
    • Many vendors recommend a minimum SNR of 18 dB for data networks and a minimum of 25 dB for voice networks.
  • AP placement and configuration
    • Coverage analysis also determines the proper placement of access points and power settings.
    • When the site survey is conducted, all the cell edge measurements will be recorded and written on a copy of the floor plan of the building.
    • An entry with the exact location of each access point must also be recorded.
    • The location of all the wiring closets will also be noted on the floor plan, and care should be taken to ensure that the placement of any access point is within a 100-meter (328-foot) cable run back to the wiring closet because of copper Ethernet cabling distance limitations
    • A good site survey kit should have a variety of antennas, both omnidirectional and semidirectional.
    • Do not be afraid to provide coverage in a building by using a combination of both low-gain omnidirectional antennas and indoor semidirectional antennas
  • Application analysis
    • With the proliferation of Wi-Fi networks along with the importance of these networks in the enterprise, capacity planning has become an integral part of the site survey process
    • This takes into account not only the user capacity but the bandwidth capacity as well.
    • Software tools exist that can perform application stress testing of a WLAN
    • Several companies offer 802.11a/b/g/n/ac multistation emulation hardware that can simulate multiple concurrent virtual wireless client stations
    • Roaming performance can also be tested.

Site survey tools

  • Indoor site survey tools
    • Here are some of the tools that you might use for an indoor site survey:
      • Spectrum Analyser
        • Is needed for frequency spectrum analysis.
      • Blueprints
        • Are needed to map coverage and mark RF measurements.
      • Signal Strength Measurement Software
        • For RF coverage analysis.
      • 802.11 Client Card
        • Used with the signal measurement software
      • Access Point
        • At least one AP is needed, preferably two
      • WLAN Controller
        • If autonomous access point is not available, controller will be required to manage the survey AP
      • Battery Pack
        • Necessity because the site survey engineer does not want to have to run electrical extension cords to power the access point while it is   temporarily mounted for the site survey
      • Binoculars
        • Can be very useful in tall warehouses and convention centre
      • Flashlight
        • A powerful, directional flashlight can come in handy in a dark corner or in a ceiling.
      • Walkie-Talkies or Mobile Phones
        • Walkie-talkies or cell phones are typically preferred over yelling across the room
      • Antennas
        • A variety of both indoor omnidirectional and indoor directional antennas historically has been common in indoor Wi-Fi site survey kits.
        • If the internal antennas do not meet your design needs, most enterprise AP vendors also have AP models that support external antennas.
      • Temporary Mounting Gear
        • you will be temporarily mounting the access point—often high up, just below the ceiling
      • Digital Camera
        • used to record the exact location of the access point placement.
      • Measuring Wheel or Laser Measuring Meter
        • A tool is needed to make sure the access point will in fact be close enough for a 100-meter cable run back to the wiring closet.
      • Coloured Electrical Tape
        • The coloured tape can be used to leave a trail back to where you want to mount the access points
      • Ladder or Forklift
        • Ladders and/or forklifts may be needed to temporarily mount the access point to the ceiling.
  • Outdoor site survey tools
    • The following list includes some of the tools that you might use for an outdoor bridging site survey:
      • Topographic Map
        • A map that outlines elevations and positions will be needed.
      • Link Analysis Software
        • Point-to-point link analysis software can be used with topographic maps to generate a bridge link profile and also perform many of the necessary calculations, such as Fresnel zone and EIRP.
      • Calculators
        • Software calculators and spreadsheets can be used to provide necessary calculations for link budget, Fresnel zone, free space path loss, and fade margin.
        • Other calculators can provide information about cable attenuation and voltage standing wave ratio (VSWR).
      • Maximum Tree Growth Data
        • Trees are a potential source of obstruction of the Fresnel zone, and unless a tree is fully mature, it will likely grow taller
        • A chainsaw is not always the answer, and planning antenna height based on potential tree growth might be necessary.
      • Binoculars
        • Visual line of sight can be established with the aid of binoculars.
      • Walkie-Talkies or Cell Phones
        • 802.11 bridge links may span up to (or at times exceed) a mile. Two site survey engineers working as a team will need some type of device for communicating during the survey.
      • Signal Generator and Wattmeter
        • Also known as a Bird meter, to test cabling, connectors, and accessories for signal loss and VSWR
        • This testing gear can be used for testing cabling and connectors before deployment
      • Variable-Loss Attenuator
        • A variable-loss attenuator has a dial that enables you to adjust the amount of energy that is absorbed.
        • Used simulate different cable lengths or cable losses.
      • Inclinometer
        • Used to determine the height of obstructions.
        • Doing so is crucial when you need to ensure that a link path is clear of obstructions.
      • GPS
        • Recording the latitude and longitude of the transmit sites and any obstructions or points of interest along the path is important for planning
      • Digital Camera
        • Used to take pictures of outdoor mounting locations, cable paths, grounding locations, obstructions, and so on.
      • Spectrum Analyser
        • Used to test ambient RF levels at transmit sites.
      • High-Power Spotlight or Sunlight Reflector
        • In the case of a wireless bridge, you will need to make sure you are surveying in the right direction. As the path gets farther away, the ability to identify a specific rooftop or tower becomes harder and harder.
        • Because light travels so well, it can be used to narrow in on the actual remote site and ensure that the survey is conducted in the right direction

Coverage analysis

  • Manual
    • Used to find the cell boundaries.
    • There are two major types of manual coverage analysis surveys:
      • Passive
        • The radio collects RF measurements, including received signal strength (dBm), noise level (dBm), and signal-to-noise ratio (dB).
        • Although the client adapter is not associated to the access point during the survey, information is received from radio signals that exist at layer 1 and layer 2.
      • Active
        • The radio is associated to the access point and has layer 2 connectivity, allowing for low-level frame transmissions.
        • If layer 3 connectivity is also established, low-level data traffic such as Internet Control Message Protocol (ICMP) pings are sent in 802.11 data frame transmissions.
        • Layer 1 RF measurements can also be recorded during the active survey.
    • Most vendors recommend that both passive and active manual site surveys be conducted.
  • Predictive
    • Applications that provide RF simulations and modelling design capabilities
    • Is accomplished using an application that creates visual models of RF coverage cells, bypassing the need for actually capturing RF measurements.
    • Predictive applications are an excellent tool to use with blueprints of buildings that have yet to be built.
  • Dynamic RF
    • Radio resource management (RRM), where access points can dynamically change their configuration based on accumulated RF information gathered from the access points radios.
    • RRM can address isolated WLAN capacity needs by utilizing dynamic load balancing of clients between the access points
    • When implemented, RRM provides automatic cell sizing, automatic monitoring, troubleshooting, and optimization of the RF environment, which can best be described as a self-organizing wireless LAN
    • RRM cannot make up for a poorly planned network, but it can help adjust and adapt to periodic or isolated surges in network usage and demand.
  • Wireless network validation
    • Immediately after a wireless network has been installed, it is important to audit or validate the installation
    • This validation allows you to verify the RF coverage and data rates that are being provided by the installed network; you can then compare the actual values with expected values from your network design plans.
    • A wireless network validation is typically performed by systematically walking through the building or coverage area of the wireless network and taking RF and network measurements.
    • These measurements are then documented on the floor plan or map.
    • This information should help you to identify where and why your problem exists.
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s