Manual Site Survey Tools
This section discusses the popular survey tools that are
available for performing manual WLAN surveys, how they operate, and what they
offer the survey engineer. Most WLAN vendors offer some type of client device
for taking at least a signal-strength reading, whereas some go into much more
detail. Standalone site survey software tools are also available, intended to be
used with any 802.11 radio device and offering a wide variety of information and
capabilities.
Cisco Systems Aironet Client Utility
The Cisco Aironet Client Utility
(ACU) has long been used for manual surveys and is well known. However, it is
usable only with the Cisco 802.11b radio devices. It does not function with the
newer Cisco 802.11a/g radio clients.
As with many tools, the signal-strength readings obtained with
the Cisco ACU are in a percentage value rather than an exact RF level. One
helpful feature of this Cisco tool is that it displays the actual dBm values rather than
percentages, providing the experienced survey engineer a much better evaluation
of the RF signal. The tool also enables the monitoring of noise floor, which in
turn enables a signal-to-noise ratio (S/N or SNR)
reading, both of which are critical to proper communications.
When using the Cisco ACU tool, the first thing you must do is
set up the client to communicate to an AP. With the Cisco ACU site survey tool,
the only thing that must be configured is the
proper SSID. However, it is recommended that other parameters be configured when
performing the actual survey. Whereas Chapter 11 discusses these parameters and settings,
this chapter concentrates just on the features of the survey tools
themselves.
Starting at the main screen of ACU, shown in Figure 10-1, notice the two key areas on the top toolbar.
One is used to launch the site survey tool, and the other sets the preferences
for reading values.
Change the default setting showing percentages to dBm via the
Preferences button. (See Figure 10-2.) In
the box titled Signal Strength Display Units, click the dBm (decibels per milliwatt) button.
From this screen, you can also choose to have seconds displayed
on the clock in the status bar on the main screen as well as how often the
screen updates. By default, the screen update parameter is set to one second
(the lowest available setting) to ensure that the information read is as
accurate as possible.
Next select the site survey utility from the toolbar. The ACU
site survey tool offers two modes: active and passive. In passive mode, the ACU
does not initiate any RF network traffic; it only listens to any other RF
network traffic that the Cisco WLAN adapter hears (from the associated AP). The
active mode is actually transferring packets between the AP and the client
devices and evaluating packet performance. Active is more representative of an
actual application because it has directed packets to and from the AP, and
therefore is recommended for use in surveys.
To set up active mode, you must first click the Setup button at
the bottom of the screen (see Figure
10-3).
In the Site Survey Active Mode Setup screen, the destination
MAC address should be that of the AP you want to test coverage to (see Figure 10-4). This keeps the client from
roaming to another AP. When using a Cisco client to survey a non-Cisco AP,
uncheck the Destination Is Another Cisco Device field.
Set the packet size to a size representative of the packet size
required for the applications used at the site. Pick the largest packet size
used for any of the applications to be used on the WLAN. The larger the packet,
the more likely the packet will become corrupted in noisy environments. Set the
data rate to the lowest data rate desired by the customer. The higher the data
rate, the smaller the size cell for the modulation type.
One helpful setting here is the number of times the client will
send packets. (Inherent in this is the ability to continually run a survey test
by selecting Continuous Link Test.) When satisfied with the settings, click the
OK button to return to the Site Survey screen.
Use the Start button (shown in
Figure 10-5) to start the site survey in
active mode.
After the survey starts, it will provide the following
information:
-
Percent
Complete Shows the percentage of packets that have been sent. If
Continuous Link Test has been selected, it shows the percentage of the number of
packets that have been sent until it reaches 100 percent, and then starts over
again.
-
Percent
Successful Shows the number of packets that have been successfully sent
and received. Notice the threshold line. If the percentage drops below this
line, the bars will become yellow.
-
Lost to
Target Shows the number of packets that were lost in the transmission
from the client and the AP.
-
Lost to
Source Shows the number of packets that successfully reached the AP but
did not reach the client.
To stop the survey, click Stop
or OK.
When performing the site survey, monitor the signal level and
S/N ratio and stay above a
predetermined value, which is based on data rate and application usage (as
discussed more fully in Chapter 11).
The signal and the noise levels might fluctuate, but if the S/N ratio remains at
or above the given level, the signal can usually be considered reliable.
The packet-loss count is also a key parameter to watch when
performing the survey. Sometimes the signal level or S/N might be acceptable,
but packets are still being lost. Typically, this loss results from interference
in the area; without some type of communication monitoring tool, such as packet
count or retry count, such loss can be hard to account for. For these reasons, a
survey tool that does not provide a packet-loss count is not as effective as one
that provides these details.
Cisco Systems Aironet Desktop Utility
When Cisco released the 802.11a/g radio card, it was based
on a different internal chip set and therefore required totally new drivers and
utilities. Cisco decided to provide only a limited (and comparable to most other
vendors) survey tool in the client utilities, relying instead on standalone,
third-party site survey tools such as AirMagnet (discussed later in this
chapter). In its basic reading mode, signal strength in the Aironet Desktop Utility (ADU) is displayed in a
relative mode (see Figure 10-6). To get
more details, use the Advanced selection.
Compared to the Cisco ACU, the Cisco ADU provides only
signal-strength and noise-floor readings, as shown in Figure 10-7. The actual transfer of data between the AP
and the client is not available, and therefore packet-loss counts are also not
available. This puts the ADU tool at a disadvantage for surveying, when compared
to the ACU tool for the 802.11b cards.
An additional feature of the Cisco ADU is its capability to
scan and report other 802.11 systems in the area, which makes it a good tool for
searching for interfering systems and rogue APs. Figure 10-8 shows a list of systems that have been found,
the SSID associated with them, whether security is used, signal strength,
channel numbers, and the frequency band.
The Cisco ADU provides the signal strength and noise level
for any given AP, but does not have provide any method of viewing data retries
or packet loss, making it useful for checking installed systems and verifying
signals but minimally useful as a site survey tool.
Intel Centrino Utility
Upon the introduction of the Intel Centrino program, many PC
vendors moved quickly to adopt it. Today a vast majority of the laptops that are
802.11 equipped use a Centrino-based radio, and hence the utilities that come
with it. As shown in Figure 10-9, the
main page of the Centrino utility displays a simple graphical indication of
signal strength, as well as a few statistics about the association
parameters.
The Centrino utility Statistics page offers both
signal-strength and noise-floor readings, as well as transmitter retries,
beacons missed, and throughput, as shown in Figure 10-10. These values do provide a minimum level of
usefulness as a true site survey tool.
The Centrino utility also enables you to scan for multiple WLAN
systems, as shown in Figure 10-11.
Selecting an AP out of the list and then clicking Connect causes an attempt to
associate with that AP.
ORiNOCO Survey Utility
The ORiNOCO products come with a client utility offering two
features to assist with site readings: Client Manager and Site Monitor. The
Client Manager, as shown in Figure 10-12,
offers a graphical scale of signal strength, indicating five different levels of
signal strength, as well as a radio connection description (based on signal
strength). Like many of the other utilities available, the ORiNOCO Client
Manager Site Monitor function enables you to view signal statistics of different
APs in the area, as shown in Figure
10-13.
Similar to the Cisco ADU, this utility, does provide the
signal strength and noise level for any given AP, but also like the Cisco
utility, the ORiNOCO survey utility does not enable you to view transmit retries
or packets lost, making it useful for checking installed systems and verifying
signals but minimally useful as a site survey tool.
Netgear Clients
It is not unusual to find some of the inexpensive client cards,
initially designed for the consumer market, in the corporate world. The Netgear
client is one such device, and its utilities are very similar in looks and
function to many others found on the market. From its initial screen, shown in
Figure 10-14, the Netgear client utility
shows signal strength as well as association parameters.
When you select the site survey
utility, as shown in Figure 10-15, a list
of all APs that have been found displays. You have an option to select and
connect to any of the listed APs, much like with the Centrino utility. This
screen provides the signal strength for any of the APs from which the client can
hear beacons, but little else. As with many of the utilities, this is fine for
troubleshooting WLANs that are already installed, but for performing surveys it
lacks some key features.
Wireless 802.11 Phones
The adoption of WLANs in the enterprise and several other areas
is bringing with it the demand for wireless, noncellular phones. Several 802.11b
phones are available today. These include products from Spectralink, Symbol, and
Cisco, which come with some type of survey tool as well. However, these phones
do not have a tool that is what most survey engineers would consider adequate.
Again, the phones just provide the ability to report signal strength from the AP
and do not test any type of communications or link between the two devices with
actual data (or voice packets). The Cisco 7920 phone does enable you to view the
overall loading of the RF channel, referred to as channel utilization (CU). Although this feature proves
useful when reviewing overall performance of a working system, it does not help
in the task of performing surveys for yet-to-be-installed WLANs. The utilities
on the phones are best used as troubleshooting tools, to be used after the
system is fully installed, up, and running.
AirMagnet
The AirMagnet utility surfaced in the WLAN market in 2002.
It was introduced as a WLAN sniffer tool and quickly became a tool of choice by
many WLAN administrators for troubleshooting and monitoring WLANs and detecting
rogue APs. The site survey portion of this tool is adequate for most
requirements, but the overall cost of the tool makes it prohibitive for many
smaller companies.
The AirMagnet sniffer tool, as shown in Figure 10-16, is available for use on both a laptop
(called AirMagnet Laptop) as well as the Compaq Ipaq (AirMagnet Handheld), and
according to AirMagnet will be made available on other PDA devices as well. The
handheld version makes it a convenient tool for carrying around a site and
performing surveys with a PCMCIA card.
However, with the onslaught of 802.11a/g cards, which use a Card-bus interface
rather than a PCMCIA interface, and the lack of Card-bus support in Compatible Extension (CE) devices, this is not always a
feasible solution.
The AirMagnet tool provides many useful troubleshooting and
surveying tools. Figure 10-17 shows the
channel monitor mode, in which it can view a single channel and display signal
strength of all APs heard on that given channel. This capability enables the
engineer to view given channels to determine whether the signal from nearby APs
(or neighboring systems) will be a problem. It also helps to determine where
rogue APs might be located.
As shown in Figure
10-18, when selecting the infrastructure mode, you can select a single AP
from the list on the left side (all APs that are heard), and you can view the
S/N ratio in both a graphical and text display.
With the AirWISE selection, AirMagnet can display security
information concerning the AP and its selections. Figure 10-19 shows the utility displaying an AP that has
been set with no encryption, as explained in the text box on the upper right. To
the left is a list of all APs and their basic settings. On the lower right you
again see the S/N ratio.
A useful troubleshooting tool, the 802.11 packet-tracing
capability, shown in Figure 10-20,
enables you to trace problems with the WLAN. You can use this to locate
malicious RF, to troubleshoot authentication and association issues, and to view
retries and other RF communication problems.
The site survey tool provides configuration for the AP Media
Access Control (MAC) address in which the test will be run, the packet
size for data transfer, and the delay between packet transmissions. Figure 10-21 illustrates the survey in
action, using the AirMagnet Handheld utility. This provides a display for signal
strength, noise level (and hence S/N ratio) packet loss, as well as basics about
the RF communication link (SSID, channel, and so on).
AirMagnet Site Survey Utility (SiteViewer)
Because of the lack of overall site
survey tools available and the higher cost of the AirMagnet sniffer tool, the
company decided to develop a standalone site survey tool, AirMagnet SiteViewer.
With assistance from a few select companies in the WLAN industry, the AirMagnet
Site survey tool was developed into one of the best available today.
The cost of the AirMagnet SiteViewer is reasonable, and the
features it brings to a survey and troubleshooting engineer are excellent. The
ability to not only survey but also document the results makes it exceptional
for delivering the required survey reports. Some of the screens in the GUI look
similar to the original AirMagnet Laptop (sniffer) tool, but do not be misled.
AirMagnet SiteViewer provides a much-improved tool for surveys.
AirMagnet SiteViewer is used as a walkabout survey tool, as are
most client-side tools. From the information collected during the walkabout,
AirMagnet can develop several views.
The SiteViewer opening screen shown in Figure 10-22 presents the three main areas of the
workspace. On the lower toolbar are the control buttons to toggle between the
Survey (data collection) mode and the display (data presentation) mode. The
upper left is the data catalog area, where different sets of survey data are
listed and can be individually enabled or disabled in display mode. The lower
left is the Survey Tool Control area, where depending on the mode, the data
collection or data manipulation controls display. The main area on the right is
the window for displaying the floor or building plan graphics, with overlays for
the AP locations and survey data rendering.
The first step for creating a new project is to import the
floor or building plan graphics file(s) and specify the scale. Many popular
graphics file formats are supported (BMP, JPEG, and so on). After importing the
graphic, you must place the overall dimensions into the tool. This provides a
very good approximation of the dimensions within the site. This screen also
allows the setting of packet size and delay between packets for the testing (see
Figure 10-23).
Figure 10-24 shows an
example of importing graphics for a typical office building.
There are two modes for collection of survey data:
-
Active
mode The SiteViewer network interface card
(NIC) associates to an AP and sends round-trip data packets to the AP. Active
mode most closely simulates a real WLAN environment, so it is strongly
recommended that active mode be used for initial AP surveys. Active mode allows
the resulting survey data to be displayed in several different views, including
by the link speed achieved at each location. Active mode supports two connection
methods:
Specific
AP The NIC will associate only to a specific AP, which is selected in the
AP drop-down menu.
SSID
The NIC will associate to the "best" AP it can observe, the one that matches the
SSID chosen in the drop-down menu. In this mode, the NIC roams between APs that
have the same SSID, in the case where more than one AP is active in the area
being surveyed. This mode most closely simulates the actual behavior of a client
in the real network. The Survey tool status panel always displays the SSID and
MAC of the AP that is currently associated.
-
Passive
mode Where the SiteViewer NIC measures the received signal strength
indicator (RSSI) of the packets coming from the AP and records this
signal strength data. This mode is most often used for auditing the coverage of
an existing network, but can also be used in conjunction with active mode in an
initial survey.
Figure 10-25 shows a
completed walkabout path taken for this coverage area. The path includes a
section outside of the perimeter walls of the building. This is done to measure
the possible propagation outside of the building, where is it usually desired
for the signal level to be as low as possible without compromising connectivity
to the interior areas, making any unauthorized connection to the WLAN as
unlikely as possible.
If you select the display mode toggle button on the lower
toolbar, SiteViewer can display data collected from the survey in different
manners (see Figure 10-26). Two new
panels appear in this mode: the filter area in the center left panel, and the
map zoom box in the lower left. Selection of one or more survey data sets to
display is accomplished by clicking the appropriate check boxes in the data
catalog area in the upper left of the screen.
To display parameters for the data sets, select from the
drop-down menu in the upper-right corner of the floor plan display area. The
choices are as follows:
-
Signal
-
Noise
-
Speed
-
S/N ratio
You can also enable appropriate filters to the data based on
several different parameters, providing a wide variety of display options. The
survey data displays as color-coded zones tied to the legend shown. For signal
level, shown in Figure 10-26, the
different colors represent 10-dBm levels.
DOS and Other Systems That Do Not Support Standard
Utilities
It is not uncommon to find clients
that offer few or no utilities to assist with site surveys, especially in the
markets where devices such as bar code scanners, devices with custom operating
systems, PDAs, and so on are common. For these situations, it is recommended
that an alternative device, such as a PDA that supports a survey utility or a
laptop, be used for the survey. However, this might not provide the best
possible survey results, because the actual clients will have different
performances from those of the PDA or laptop used for the survey. Because of
this difference, some coverage comparisons must be made before surveying. Using
the PDA or laptop, do a sample survey of an area, noting where coverage edges
are. Then using the actual client, attempt some type of connection. Most often a
DOS window is available, and you can use a ping
command for this.
From the command interface of the client, verify the IP address
of the client. It should be static and in the IP address range of the AP. The
client must be associated to the AP to run pings over the RF to the AP.
Use the ping options that allow setting multiple transmits of
the ping packet and setting the packet size to a value that is representative of
the customer application. An example of a ping command is as follows:
ping t l 256 192.168.200.1
Where:
When the ping command ends,
the success rate of completed packets and other statistics such as TX/RX
(transmit and receive) packets and approximate round-trip times display in
milliseconds.
The size of the packet defined to send to the AP from the
client is echoed back to the client by the AP. To evaluate the performance,
record the ping times in milliseconds while near the AP and compare those times
as the client is moved about the cell. As the ping times start to increase, this
indicates that the communication link between the AP and the client is starting
to have problems. This method provides a good comparison between a laptop or PDA
with a survey utility and the specific client device.
Figure 10-27 shows an
example of the ping command.
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