Verifying the Design

 
Verifying the Design
After selecting a technology and products, the next step before installing a wireless LAN system
is to verify that the solution does indeed satisfy requirements. It would be ideal to verify
the selected technology’s capability to satisfy each requirement; however, this is not always
cost effective. The main idea is to verify those elements that are not clear and create significant
risk if the solution doesn’t operate as expected. Most often it’s necessary to verify the capability
of the technology to support performance and range requirements.
When verifying the chosen technology, consider the following approaches:
• Simulation
• Physical prototyping
• Pilot testing
Simulation
Simulation uses software models that artificially represent the network’s hardware, software,
traffic flows, and use. You can run simulations and check results quickly; days of network
activity go by in minutes of simulation runtime. There are simulation tools on the market that
can assist a designer in developing a simulation model. Most simulation tools represent the network
using a combination of processing elements, transfer devices, and storage devices.
Simulation tools are generally costly, with prices in the tens of thousands of dollars. You might
be better off hiring a company that already owns a simulation tool.
The main attributes of using simulation to verify the technologies are the following:
• Results are only as accurate as the model; in many cases you’ll need to estimate traffic
flows and utilization.
• After building the initial model, you can easily make changes and rerun tests.
• Simulation does not require access to network hardware and software.
• It does not require much geographical space, just the space for the hardware running the
simulation software.
• Simulation software is fairly expensive, making simulation not economically feasible for
most one-time designs.
• The people working with the simulation program will probably need training.
Consider using simulation for the following situations:
• When developing a type of wireless LAN product that doesn’t yet exist.
• When it’s not feasible or possible to obtain applicable wireless LAN hardware and associated
software for testing purposes.
• When testing performance requirements based on predicted user activity (it’s often not
practical to do this with physical prototyping).
• When it’s cost effective to maintain a baseline model of a product or system to test
changes to the baseline.
Physical Prototyping
A physical prototype is a part of the product or system you want to verify through construction
and testing. It consists of the actual hardware and software you may eventually deploy.
Prototyping generally takes place in a laboratory or testbed.
The main attributes of physical prototyping are as follows:
• Yields accurate (real) results because you’re using the actual hardware and software,
assuming you can include applicable user utilization loads.
• Relatively inexpensive as part of a system installation because you can obtain components
under evaluation from vendors.
• Takes time to reconfigure the prototype to reflect changes in requirements.
• Requires access to network components, which can be a problem if you don’t have easy
access to vendors.
• Requires space to lay out the hardware and perform the testing.
Consider using physical prototyping for the following situations:
• When initially testing the design of a new wireless LAN product before going into mass
production.
• When testing the system design of a wireless LAN solution prior to vendor selection,
especially when the operating environment may have a high degree of signal impairment
(such as multipath distortion and RF interference).
Typically, you don’t need to physically prototype the entire system, especially those parts that
other companies have implemented without encountering problems. Consider prototyping any
solutions that have not been tested before, especially those elements dealing with performance
and range.
When performing physical prototyping, consider the following tests:
• Performance Tests Performance tests determine associated throughputs of sending
information across the wireless LAN. To perform these tests, configure a test network
consisting of an access point and multiple radio-equipped user devices. Install application
software on each user device that transmits information packets of the size and
repetitiveness stated in the requirements. If possible, include the number of user devices
indicated in the requirements. Use the monitoring capability that is part of the vendorsupplied
management software for the access point and radio cards. This software generally
provides a measure of utilization and delay on the network, which can also be used
for comparing throughputs of various vendors. Most of these utilities will indicate the
time (average, maximum, and minimum) it takes to receive a response from the destination.
You can also connect a protocol analyzer to the access point to monitor performance
if vendor-supplied monitoring software is not available.
• Range Tests To test the range capability of a wireless LAN, use a laptop equipped
with a radio card and vendor-supplied wireless link test software. The wireless link test
function determines the quality of transmissions between the radio card in the laptop and
the access point. This test sends special control packets to a particular destination, which
echoes another packet to the sending station. Based on the sequence numbers of the
packets being sent back and forth, the station running the link test knows whether the
link corrupts packets on the forward or return leg of the link.
Implementing a Wireless LAN
CHAPTER 8
8
IMPLEMENTING A
WIRELESS LAN
265
Link test software generally enables the tester to set test parameters such as the type of
frames sent, target station address, frame size, and number of frames to send. The link
test generally indicates the percentage of frames sent successfully on both forward and
return paths, received signal strength at both ends of the link, the number of retries the
source accomplishes before actually sending the test frame, and which access point the
radio card is associated with.
To test the worst-case scenario, set up the link test software to transmit frames at the
required data rate and continuously send full-size frames. The test will begin near an
active access point (with other access points turned off or disabled), then the tester will
walk away from the access point while monitoring the signal level and association status.
Once the association with the access point is lost, the tester measures and records the distance
from the access point. This identifies the fringe area (maximum operating distance)
of the access point, which indicates the density of access points needed to provide full
coverage of an area the size of a plant.
• Roaming Tests Most wireless LAN standards don’t specify a protocol for handling
wireless user devices roaming from one access point to another. Roaming tests will
ensure that the access points will properly hand off radio card connections to other
access points as the user moves from one part of the facility to the other. This testing is
especially important when the radio card and access point vendors are different.
To accomplish the roaming tests, equip a laptop with a radio card, stand near an access
point, and make sure that the radio card is associated with that access point. The tester
will walk out of range of the associated access point and closer to another active access
point. The radio card should disassociate from the initial access point and reassociate
with the next access point. The association information can generally be found via the
vendor-supplied management software.
In addition, consider performing this test while the laptop periodically transmits a file to
a laptop or PC connected to the Ethernet switch. After completing this test, the management
software should indicate that no packets were lost during the hand-off to the second
access point.
• Multivendor Interoperability Tests A problem with mixing vendors in a wireless
LAN solution is that most vendor-enhanced features are not usable. As a result, the functionality
of the network is reduced to the least common denominator, which is the functions
specified only by the standard. Test the use of these enhanced features, such as
network monitoring tools and performance enhancements, as part of the multivendor
testing. This can be done by noting the enhanced features of each vendor and testing
whether these features work properly when using a radio card of a different vendor.