What is a Site Survey?
A site survey is an in-depth examination and analysis of a proposed wireless LAN
site. The purpose of a site survey is to determine the number of access points
needed,
the types of antennas needed, and the best placement for those access points and
antennas.
Although the goal of a site survey is simple, the means of arriving at that
goal are not. Some of the steps involve taking measurements, but most
involve experience, trial and error, and a little guesswork rather than
numbers and figures.
When to Perform a Site Survey
Intel recommends that you perform a site survey prior to installing a wireless
LAN. Site surveys are especially important at these three times:
- Before installing a new site - Evaluate the placement of the access points and antennas throughout the
proposed site.
- Before changing an existing site - When modifying or extending an existing network structure, re-evaluate the
placement of the access points and antennas. If you need a different level
of coverage in some areas, you may need to move, replace, or supplement
access points and antennas.
- After physical changes to the site - Remodeling may introduce new sources of interference, such as motors and
metal structures within the coverage area of the access point, even if it
does not directly effect the sites where the access points are located.
Elements of an Effective Site Survey
An effective site survey requires four elements. Failure to commit the
appropriate time, money, and energy to accomplish a proper site survey in
advance may result in greater expenditures of money and time later, when
problems arise that require repeated adjustments to the wireless configuration.
The four elements of an effective site survey are:
- Examine the network usage problems the wireless LAN solves.
How many clients need a wireless LAN connection? What areas of the site
require wireless LAN connectivity? How many hours each day is wireless LAN
connectivity required? Which locations are likely to generate the largest
amount of data traffic? Where is future network expansion most likely?
- Study blueprints of the proposed wireless LAN site.
A site blueprint provides a map of the site as well as the location of
objects such as walls, partitions, and anything else that could effect the
performance of a wireless LAN. Examining the site blueprint prior to
conducting the physical walk-through helps you identify areas where wireless
equipment is likely to perform well and areas where it is not. Many
obstructions are not readily visible and, in some cases, a room originally
built for a specific purpose, such as a radiology lab, might have been
converted into something completely different, such as a conference room.
The blueprint may also show areas proposed for future building expansion.
To prepare for the next step of the site survey, mark possible wireless
device locations on the blueprint and refer to the marked blueprint during
the physical walk-through and inventory.
- Conduct a physical walk-through and inventory.
The primary purpose of the physical walk-through is to document any items or
materials near a proposed device location that may interfere with reception
or transmission and effect network performance. Document stock and inventory
levels, current environmental conditions and any materials that may
interfere with wireless LAN transmissions.
- Measure the radio frequency (RF) transmissions.
Intel® PRO/Wireless LAN Site Survey provides the means to measure the
signal strength, the transmission rate and the number of successfully
transmitted packets. This step in the site survey involves mounting access
points and antennas in the proposed locations and measuring the RF signal
strength, transmission rate and packet throughput.
Network Usage Requirements
When designing a site, consider all of the possible uses for the wireless
network. Important factors include mobility, performance, security, redundancy,
and system interface requirements.
Mobility Requirements
Determine whether the customers are in motion continually, such as in a
warehouse or hospital, or whether the customers work from different fixed
locations throughout the site.
Performance Requirements
Consider whether the people using wireless devices need higher data rates for
time-bound transmissions, such as audio and video, or whether the system most
often handles burst or typical knowledge worker data traffic. If computers
require higher data rates, put access points closer together, because the higher
transmission rates can only be obtained near the access points.
The number of computers per access point varies with the customer
applications and other system factors. Typically, an access point readily
accommodates 10 to 30 computers. Areas with a high density of computers, such as
conference rooms, may require additional access points.
Security Requirements
Because wireless LANs use unsecured radio waves to transmit data, security is an
important consideration for network administrators designing wireless networks.
Network administrators should consider whether encryption such as Wired Equivalent Privacy (WEP)
is necessary to protect sensitive data transmitted over wireless networks or if
the customer can afford a lower degree of security.
Redundancy Requirements
It is recommended that you design your wireless system so that each access point
has redundancy from another access point. This provides not only for redundancy,
but improved overall system performance.
System Interface Requirements
Consider the degree to which the wireless LAN (WLAN) will interact with the
conventional wired LAN. Some legacy systems, such as older mainframe networks,
may not support WLANs without additional hardware or software.
Roaming from one access point located on one IP subnet to an access point
located on another IP subnet causes loss of IP connectivity, unless special
arrangements are made.
Channel Selection Considerations
The exact number of available channels is dependent on local regulation. To
avoid inter-channel interference between access points, configure your access
points to local clear channels or their equivalents in other countries. Arrange
your access points to avoid having same channel units adjacent
to one another.
You cannot adhere to this recommendation in some situations, such as in a
building with several access points on multiple, adjacent floors, where the
access points transmit through many floor structures. Adjacent access points
operate properly when they use the same channels. Each access point
"hears" the other, and takes its turn transmitting. This situation may
cause a reduction in data throughput if it occurs in a very active area, so
design your channel layout accordingly.
Preparing for an Intel® Wireless LAN Site Survey
Installing an Intel® PRO/Wireless LAN begins with
conducting a site survey. A site survey involves using the Site Survey Utility to determine the physical
requirements for a site-specific Intel PRO/Wireless LAN. A site survey
analyzes the installation environment and provides recommendations for equipment
and placement.
Use the Site Survey Utility to determine the
placement of access points and antennas, as well as the number of devices
necessary to provide optimal service. The facility can be a warehouse,
manufacturing plant, office building or retail store.
In the installation of an Intel PRO/Wireless LAN, complete radio coverage
could require optional antennas.
Inspecting the Survey Area
During the planning stages of the site survey, a network administrator or
equivalent visits the proposed radio
coverage site. As a standard practice in the site survey consultation, the network
administrator gathers facility drawings and completes a Site Survey
Requirements document and a site survey questionnaire. The network administrator
documents the wiring used within the facility (Category 5 for 10 Mbps Ethernet
or 10/100 switched Ethernet, fiber optic cabling, etc.) and assesses its
applicability to Intel PRO/Wireless LAN components.
Select several trial installation areas. The
site survey team analyzes each proposed installation area to document radio
transmission constraints and to develop preliminary access point placement
alternatives to be tested during the actual site survey. Document the findings
from the initial site inspection in a Site Survey Request Form and use that form
as the outline of the site survey.
Consider the following variables in the site survey requirements definition:
- RF systems already in use
- Availability of lifts for mounting access points
- Location of host system(s).
- Available AC power
- Interfering metal fire breaks and wall structures
- Availability of customer technical personnel to answer questions during
the survey
- Doorways and passages effecting RF propagation
The completion of the RF Site Survey Requirements document is a coordinated
effort between the network administrator and the customer management team.This document does not identify potential
installation constraints within the customer site, nor does it recommend access
point and antenna placement location. The document
represents a preliminary overview of the customer site used as a baseline for
refining site survey requirements.
Assessing Environmental Radio Coverage
The network administrator selects trial component installation areas away from
transformers, heavy-duty motors, fluorescent lights, microwave ovens,
refrigerators and other industrial equipment. Areas with excessive moisture,
heat and dust are inappropriate for staging a wireless network.
Signal loss can occur when metal, concrete, walls or floors block access
point transmission areas. Access point antennas are trial-mounted in open areas
or added to an existing access point to boost the Intel PRO/Wireless LAN
coverage area.
The positioning of an access point depends on the floor plan of the site. The
network administrator makes access point and antenna placement recommendations based on the following
installation site variables:
- Outdoor or indoor installation site
- Large or small proposed radio coverage area
- Wide or narrow proposed coverage area
- Number of additional devices operating in the proposed coverage area
- Open coverage area or area with documented obstructions
General Guidelines
Site surveys for wireless devices require experimentation
with different antenna coverage at various angles. Placing a directional
antenna in a vertical position can often minimize multi-path problems.
 |
NOTE: Do not
locate access points near corners, against walls, against metal walls or inside
metal conduits. |
Preventing Channel Interference in a 2.4 GHz environment
Intel PRO/Wireless LAN access points require careful survey area testing to
ensure radio transmissions do not overlap.
Intel PRO/Wireless LAN access points supporting different coverage areas in
close proximity to one another should be configured using direct sequence
channels as far apart as possible, such as channels 1, 6, and 11.
Supplying Power from Site Electric Circuits
Intel PRO/Wireless LAN access points draw power from wall
outlets. Access point performance is subject to degradation due to inherent or
random electrical problems or site-specific disturbances.
Access point electrical installation
alternatives are listed below in order from most to least preferred for an
installation area.
- Isolated ground circuit with
an online, uninterruptible power supply (UPS) that also acts as a filter and
surge suppressor.
- Isolated ground circuit with a surge
suppressor.
- Dedicated circuit with a UPS.
- Dedicated circuit with a surge suppressor.
- Non-dedicated circuit with a UPS.
- Non-dedicated circuit with a surge suppressor.
 |
CAUTION:
When using a Network Controller Unit, items one through four are recommended. The use of nondedicated circuits for configurations five and six could
cause data loss and serious transmission problems. By its nature, a nondedicated
circuit contains open receptacles. The load and type of use of open receptacles
in a nondedicated circuit cannot be predicted after installation. |
If you must use a non-dedicated circuit, Intel recommends that the circuit not
support any of the following:
- Hard-wired devices.
- Devices with components intended or known to produce heat such as space
heaters, laser printers, heat guns and soldering irons.
- A single device drawing more than 20% of the rated value of the circuit.
- Devices drawing more than 60% of the rated circuit value.
 |
NOTE:
Ensure that each access point has power available 24 hours per day. It is
recommended access point power never be provided
from an Energy Management System (EMS). |
Recommended Procedure for Performing a Site
Survey
Using the Site Survey Utility to perform a site survey involves selecting possible locations for the
access points from a review of the floor plan layout, based on estimated of the
range of an access point within the site. Range estimates are based on previous
experience, physical test and measurement or calculation from the rated hardware
specifications.
Proposed locations are validated by temporarily installing one access point
and using the Site Survey Utility to test the
actual range and coverage area of that access point in that location. Adjust the
position of the access point as required until you validate the coverage area.
After you validate the first proposed access point position, annotate it on
the floor plan layout, move the access point to the next adjacent location, and
test the access point coverage from that location. Repeat this procedure of
testing, adjusting positions, and relocating the access point until the entire
proposed coverage area and all of the proposed locations have been validated.
It is recommended that you accomplish the site survey using the continuous Internet Control Message Protocol (
ICMP ) "ping" test with a block size of 1024 bytes.
Helpful Hints and Tips
- Floor to floor interaction between access points is usually not a problem,
unless the floor is wood. If interaction can occur, ensure that the channel
selections are appropriate for vertically adjacent access points. Floor to
floor interaction can be a good thing, as it provides for redundancy.
- Close all office and room doors before beginning the survey. This puts
more material between the transmitter and receiver, allowing you to assess
reception at its lowest everyday level.
- Conference rooms may have a high density of computers, so ensure adequate
access points for satisfactory throughput. Consider redundancy for these
areas.
- In high security areas where access may be limited, such as boardrooms and
labs, consider placing access points adjacent or straddling the area, rather
than directly in the area. When maintenance is required on the access
points, access to the restricted area is not required.
- Cafeteria areas can be a high usage and demand area, as informal employee
meetings are often conducted there.
- Computer to access point ratios are typically maintained at a maximum of
approximately 20-to-1, depending on the type of applications and data.
- Areas with multiple conference rooms in close proximity are often given
additional access points to provide redundancy and ensure adequate
throughput.
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