Consideraciones sobre la red inalámbrica a tener en cuenta al momento de implementar una solución de Vocollect

1. Wireless LANs: Configurations and Considerations
THE WIRELESS LAN ENVIRONMENT
Vocollect Talkman® terminals are specifically designed to provide reliable and
forgiving voice operation under a wide variety of operating conditions. However,
just as all wireless terminals, the Talkman terminal relies upon the underlying
wireless communications infrastructure to deliver data in a reliable and timely
fashion. A properly installed and operating wireless communications system is
critical to successful deployment and use of the Talkman® terminal. Based on
Vocollect’s experience in integrating products into hundreds of wireless
environments, this document discusses recommended equipment and best practices
for problem-free implementation.
Basic wireless LAN requirements:
• IEEE 802.11b or IEEE 802.11g (802.11b compatible) wireless LAN
• Must be WiFi certified for interoperability
WiFi certification can be verified at the WiFi web site at
http://www.wirelessethernet.com/OpenSection/certified_products.asp?TID=2 or
request a copy of the WiFi certificate from the wireless LAN manufacturer.
ACCESS POINT HARDWARE
Due to the popularity of the 802.11b/WiFi wireless, and the subsequent economies
of scale, the cost of wireless LAN Access Points (APs) have dropped dramatically in
recent years. But before shopping based on price alone, be aware that there are two
classes of APs: SOHO and Commercial.
SOHO APs are designed for small office/home office environments (hence the
name) and are the least expensive. However, since home/small offices rarely
require more than one AP and have very few users, SOHO APs place little emphasis
on factors like roaming or user capacity. On the other hand, commercial APs are
designed for high speed roaming, large numbers of users, and add other important
features such as remote management, power management, dual antennas, Power
over Ethernet, network security, etc.
Bottom line: don’t be tempted to go cheap since everything else in your system will
use the wireless network as a foundation.
ACCESS POINT CONFIGURATION
There are many options when configuring a commercial Access Point and there are
many manufacturer specific parameters, so no attempt is made for comprehensive
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2. coverage in this section. The following table summarizes Vocollect’s
recommendations for specific configuration settings. Note that none of these
settings are absolutely required, but from experience, they are strongly
recommended. A more detailed discussion of each setting follows the table.
Data Rate Restriction of data rates to use 2Mbps and 5.5Mbps only
Antenna Diversity Dual antenna diversity enabled
Output Power 30mW-50mW
Traffic Filtering Filter out extraneous broadcast traffic
Security WEP at minimum, WPA or better recommended
DATA RATE
WiFi APs are designed to operate at varying data rates. An 802.11b AP uses data
rates of 11, 5.5, 2, and 1 Megabit(s) per second (Mbps). When a mobile terminal
device is communicating with an AP with a strong signal, the fastest data rate
(11Mbps) is used. As the mobile terminal moves away from the AP and the radio
signal diminishes, a lower data rate is used to minimize data errors. This lowering
of data rates continues until a reliable signal cannot be maintained at the minimum
data rate.
APs can be configured to either operate at all of the available data rates or be
restricted to only specific data rates. Due to the roaming nature of mobile terminals,
the inefficiencies in the AP when shifting data rates and the low data volume
to/from mobile terminals in general, overall performance can often be enhanced by
restricting data rates to those which are statistically dominant. This results in better
effective response times for the user. Consequently, Vocollect recommends
configuring the AP such that only 5.5Mbps and 2Mbps are used.
ANTENNA DIVERSITY
Commercial Access Points generally support two antennas. The purpose of the two
antennas is to address a RF problem called multi-path interference where radio
signals at high frequencies (such as those used by 802.11b) can bounce off RF
reflective surfaces and cancel themselves out. Using two antennas allow for two
separate receive paths so that a signal null is unlikely at both antennas
simultaneously. This is especially important in mobile environments or where metal
structures exist in the AP coverage area.
Of course in order for antenna diversity to work, the AP software configuration must
be set to use two antennas. AP manufacturers often refer to this feature as antenna
diversity.
OUTPUT POWER & MUTUAL INTERFERENCE
Most commercial APs allow the configuration of output power. The temptation
exists to simply set the output power at maximum for a large RF coverage area.
However, in order for two-way communications to function properly, the coverage
area of the AP should approximate the coverage area of the mobile terminal. This
ensures that each can “hear” each other without drowning out other RF devices
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3. within transmission range. In general, a lower power output setting of 30mW or so
in the AP better approximates the range of a mobile device and limits mutual
interference.
Mutual interference can also be minimized by careful placement of the AP when
installed. In general, APs should be installed such that the RF coverage area of one
AP has a low percentage overlap into the adjacent coverage area of a neighboring
AP. Lower power settings can make overlap more manageable. Your WLAN
installer can best advise you of the optimum power setting through a site survey.
BROADCAST TRAFFIC
When deploying Wireless LANs, it is especially important to eliminate extraneous
broadcast traffic from being passed from the wired LAN. Broadcasts are widely
used by Microsoft, Novell, and other enterprise network operating systems. By
design, broadcast packets require all receiving devices to process the data they
contain; even if the data is not applicable to the receiving device. This unnecessary
packet processing on battery powered mobile devices results in poor user response
and poor battery performance.
Broadcast traffic can be filtered by using settings in the AP, or through the design of
the wired LAN (e.g. VLANs). Consult with your WLAN vendor to determine the
best method for your architecture.
SECURITY
No recommendation would be complete without some discussion on wireless LAN
security. A comprehensive discussion is beyond the scope of this document, but it is
important to mention that at least some security is strongly recommended. At the
very minimum, WEP (Wired Equivalent Privacy) security should be enabled. Even
though WEP encryption can be cracked, it is only vulnerable when enough data is
passed while on a single AP (roughly 500,000 uniquely keyed packets). Most mobile
terminal applications do not pass enough data while not roaming and under the
coverage of a single AP to allow cracking. The very mobile nature of the terminal is
a substantial barrier against WEP cracking.
Of course, better security is always more desirable. The next step up from WEP is
the WPA/PSK (Wireless Protected Access/Pre-Shared Key) protocol developed by
the WiFi organization. Deployment of WPA/PSK is strongly recommended, as it
overcomes the vulnerability of WEP but does not require a security server and the
associated overhead costs.
Other more advanced security methods are available and may be more desirable for
your organization. The more advanced the security, the more overhead that is
required for additional equipment and administration, so do not automatically
assume that the most sophisticated security is best for you. The right amount of
security for your organization will be a balance between the burden that your data
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4. security system imposes in terms of expense and administration, the value of the
data being protected, and the likelihood of an incursion.
SITE SURVEY
RF Site Surveys are the recommended method to optimize & verify WLAN coverage
and ensure that the WLAN environment is clear of any unintentional or outside
interference.
In recent years the cost of WLAN APs has fallen dramatically. This has precipitated
a tendency within IT departments to skip the site survey process entirely and simply
install APs at regular intervals within a building. This method is not recommended.
APs spaced too closely can cause them to interfere with each other. APs spaced too
far apart will result in coverage gaps.
Engagement of a reputable network integrator with WLAN expertise is highly
recommended. The integrator should be able to provide a spectrum analysis of your
site to identify any potential sources of interference, be able to design your wireless
network, recommend antenna options, guarantee needed RF coverage, perform a
physical site survey, and provide comprehensive documentation once the WLAN
installation is complete.
Site survey related recommendations:
• Assuming a spherical coverage area, the coverage of one AP should ideally
overlap an adjacent AP’s coverage area by 25% or less unless non-interfering
channels usage can be guaranteed. Since antennas radiate in three dimensions,
radiation patterns that extend above and below onto adjacent floors must be
considered.
• Since each type of mobile terminal have different radio and antenna performance
characteristics, surveys should be performed using the mobile terminal type
being deployed. If this is not possible, the site surveyor should approximate
mobile terminal performance as closely as possible by using a similar terminal, or
by adjusting power as appropriate.
• Ideally, site surveys should be performed with the installation environment as
close to its final configuration as possible. For example, when installing in an
environment where inventory is stored, the inventory should be in place in the
storage locations prior to performing the RF survey. This ensures that radio
signals will not be blocked by the final state of the environment.
ADDITIONAL INFORMATION
There are a number of excellent resources for additional information. Here are some
links to articles and books:
• http://www.oreillynet.com/pub/a/wireless/2005/05/02/80211myths.html
• http://www.wi-fiplanet.com/tutorials/article.php/1116311
• 802.11 Wireless Networks: The Definitive Guide, Second Edition by Matthew Gast,
ISBN 0-596-10052-3
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