Subject: [NEXTEL-1] How to Design a Wireless LAN

From: [email-masked]
Date: Tue, 16 Sep 2003 02:35:40 -0400

How to design a wireless LAN
Key issues: RF coverage,
 sufficient capacity and
accounting for RF signal-loss factors.

by Michelle McLean
September 2003

Designing enterprise wireless LANs (WLANs) is a new craft, even for many
experienced network architects. When designing a wired network, most IT
managers are familiar with the steps to ensure sufficient capacity for the
users and applications. With IEEE 802.11 wireless LANs, a new factor comes
into play: the tradeoff between radio-frequency (RF) coverage and bandwidth
capacity per user. Planning for both capacity and coverage is one of the key
design issues for WLANs. In short, if you plan for sufficient capacity,
adequate coverage will follow.

The biggest change in mindset when designing WLANs is that they provide
shared, not switched, connections. Many WLAN designers mistakenly focus only
on providing adequate coverage, rather than sufficient bandwidth, for their
users. While coverage may be the design point if you are deploying a single
access point (AP)-based network for the conference room or a workgroup, the
application demands of an enterprise make bandwidth the real design
criteria. Quite simply, you will not deliver enough bandwidth if you plan
for coverage alone.

Key issues to consider when deploying a WLAN include determining RF coverage
from a single AP, ensuring sufficient capacity to support the user
population and accounting for RF signal-loss factors.

The distance at which a particular throughput can be achieved will vary with
802.11a or 802.11b WLANs. 802.11b has a data rate of 11 Mbps inside a radius
of 100 feet when indoors. Similarly, 802.11a should deliver 36 Mbps within a
75-foot radius.

Many APs will automatically decrease their data rates as the RF signal
degrades because a lower-frequency signal is more likely to get through when
there is interference. One user associated at 1 Mbps will slow the entire
group, since the AP will take longer to communicate with that user, taking
bandwidth from all other connected users.

To avoid degrading the performance for all users associated to an AP,
consider setting minimum association rates that force users to associate
with a new AP once their throughput falls below the minimum rate. By
designing smaller cells with higher throughput, you can create an
enterprise-quality experience.

The number of users and their applications are major drivers of bandwidth
requirements. The network architect must account for the number of users
within the AP's cell diameter. In a large office or where user density is
high, you should design smaller cells to achieve a higher data rate, since
walls and other objects will not naturally create the cells by attenuating
or blocking the RF signal. With smaller cells, you will need to re-use
frequencies more often and thus ensure that the channels do not overlap.

Determining how much bandwidth each user will need is critical, as your
calculations will define the user experience, as well as the number of APs
required. A good rule of thumb for an 802.11a network is to allow for 2 Mbps
downstream and upstream per user, which delivers about the same user
experience as being on a wired LAN. For an 802.11b network, a rule of thumb
is to allow for 500 Kbps each way, which delivers a user experience similar
to a DSL connection.

The 802.11 specification includes 802.11a, 802.11b and 802.11g. 802.11b
features a raw data rate of 11 Mbps and provides three non-overlapping
channels. 802.11a offers a peak throughput of 54 Mbps and provides eight to
12 non-overlapping channels. 802.11g offers a peak throughput of 54 Mbps,
provides the same three non-overlapping channels available in 802.11b, and
is backward-compatible with 802.11b clients.

Because of several factors, actual throughput on a wireless system is much
lower than the technology's specified data rate. For instance, with even a
one-way transmission on a 54-Mbps system, the best possible throughput is
approximately 30 Mbps. For an 802.11b network, the best possible throughput
is 4 Mbps to 6 Mbps.

For an enterprise environment with high-throughput needs, 802.11a is likely
the better choice, as it has higher throughput and more non-overlapping
channels. Including an overlay of either 802.11b or 802.11g will provide
support for older devices that were purchased prior to the availability of
802.11a.

A major difference between designing for wired and wireless LANs is the
impact of objects on RF signals. Walls, doors, windows and other fixed
objects in the building will absorb RF signals, causing signal loss. The
building construction also has an impact: Concrete absorbs more signal than
wood. Look for tools that automate these calculations.

CALCULATING AP LOCATIONS

The next step is to define the size of the area where high-speed coverage is
desired by calculating its width and length. An office building can be
divided into multiple areas for planning. For example, you may plan for the
engineering organization separately, as engineering applications are more
bandwidth-intensive than the office applications used by sales and
marketing.

Plan for a hotspot area, such as a conference room, separately from the rest
of the enterprise, as it will have different access and quality-of-service
requirements. Calculate the expected total bandwidth needed to serve the
area by multiplying the average bandwidth needed per user by the number of
users in that coverage area.

Once you know the expected total bandwidth, you can calculate the number of
APs needed to provide that capacity. Simply divide the total needed
throughput by the average throughput per AP. For example, if you need to
provide 50 Mbps total, you would divide by 5 Mbps for the number of 802.11b
APs needed and by 30 Mbps for the number of 802.11a APs needed.

Once you have computed the number of APs required based on capacity, you
need to calculate how many APs are required for adequate coverage. Use the
benchmark of a 100-foot radius for 802.11b APs and a 75-foot radius for
802.11a APs. The right design will use the higher number of APs generated by
determining the AP count for capacity and then coverage to ensure that both
requirements will be met.

Once you know the number of APs required, you can place them appropriately
in the building and provide their channel assignments. When allocating
channels to the cells, be sure that adjacent cells use non-overlapping
channels. 802.11b provides three non-overlapping channels, while 802.11a
offers 12. Be sure to consider the vertical cell overlap between floors if
you are designing for a multistory building.

Network managers should demand enterprise-quality design and management
tools for their WLANs-the same quality tools that they have for their wired
networks. WLAN design tools should assist network managers with the design
parameters, including building size and topology, obstacles, throughput per
user, country of operation and choice of 802.11 technologies. The tool
should also automatically assess how many APs are needed, where they should
be located and what their settings should be.

McLean is product marketing manager for Trapeze Networks

www.trapezenetworks.com

www.communicationsnews.com

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