have limitations and are not suitable for every situation.
Graeme K. Le Roux describes when and how it can be best
characteristic of the IT industry is that it tends to
over hype 'new' technology while ignoring the applicable
lessons taught by older technologiesmany of which
were over hyped in their day. Wireless LAN (WLAN) technology,
specifically the 802.11 family of standards, is a current
case in point.
Yes, it can be very useful and when deployed properly,
very convenient. A wireless network can be used to solve
all sorts of tricky sites and situations, but it has
its limitations. These limitations are often the same
ones that older technology, such as shared Ethernet,
had. Of course, limitations of new technology often
get lost in the hype.
The first limitation of a WLAN is often overlookedWLANs
transmit data via radio waves. In the case of 802.11b
and 802.11g, they use the 2.4GHz ISM (Industrial, Scientific
and Medical) band and 5GHz band respectively.
There are things which stop radio waves, like metal
boxesmany industrial buildings act like metal
boxes. In such cases, you can use wireless transmission
either inside the building or outside it, but not both
unless you install either a dual antenna access point
(with one antenna inside the building and one outside)
or two access points. Certain types of equipment, like
X-Ray machines, are also metal boxes. Bank and/or document
vaults are, in effect, metal boxes too.
Metal boxes are not the only thing which can stop a
radio wave, especially a low power signal of the sort
used by WLANs. Distance, ordinary walls, thick stone
walls, trees and vegetation, all interfere with the
transmission of a microwave signal.
Naturally the people writing the standards for WLANs
take all these situations into account, but in practice
one has to physically survey a proposed WLAN site with
at least a WLAN access point and one or more portable
clients. Since the signal characteristics can change
from acceptable to unusable within a meter or two, such
surveys have to be done very carefully and in great
The next problem when deploying a WLAN, especially in
the unlicensed 2.4GHz ISM band used by 802.11b and 802.11g,
is competition for the spectrum. Since the 2.4GHz band
is unlicensed, there are a lot of other devices which
use the same band. Two common examples of this are digital
cordless telephones and devices using Bluetooth. If,
for example, a company chose to use Bluetooth telephone
handsets, then it is very likely that WLANs operating
in the 2.4GHz band would be unreliable. The company
would have to restrict itself to a WLAN in the 5GHz
bandi.e. 802.11awhich would be more expensive
than the 2.4GHz option. A 5GHz WLAN may also incur a
license fee depending on local regulations, and would
almost certainly require more access points as higher
frequencies do not propagate as well as lower ones in
a physically cluttered environment. Changing from 2.4GHz
units to 5GHz would require a complete resurvey of the
WLAN site, which would further add to the cost of the
Another significant consideration for would-be WLAN
builders is that the 802.11 standards family provide
shared bandwidth; access points are bridges not switches.
For this reason all the limitations of shared bandwidth
in copper Ethernet environments apply to WLANs, but
unfortunately the most common solutionsegmentationis
much harder to apply.
In a wired network you segment the system by simply
breaking the network in half and adding a bridge or
a switch, but you can't break a wireless link in half.
You can add more access points to a given area, but
not without limit as they are all using the same set
of channels. The 802.11 Task Groups are working to ensure
that WLAN channels are used as efficiently as possible,
but the fact remains that copper-based LANs are always
going to permit greater client density than WLANs.
Is Bluetooth the answer?
Bluetooth has the potential to provide some degree of
frustration to WLAN owners simply because it is intended
for use in mobile devices.
For example, consider the situation of a public access
WLAN, several of which have been proposed and/or implemented
around the world. The idea is to provide WLAN 'hotspots'
in public areas to allow users to access the Internet.
The sort of public areas proposed as hotspots include
airport lounges, shopping malls, etc.
The problem with these situations is that they may also
attract dense clusters of Bluetooth users. Consider
an airport lounge. It is not uncommon to see a number
of business travelers making last minute phone calls
prior to boarding an aircraft, nor is it uncommon to
see a number of people using laptops.
Now, if a significant proportion of the people making
phone calls are using Bluetooth headsets, then laptop
users who are trying to use WLANs may find that Bluetooth
users are creating a 'dead spot' in the WLAN coverage.
Both the Bluetooth and WLAN users may suffer interference,
and if they complain the WLAN and cellular phone network
service providers will be expected to correct the "fault".
But there is no fault! Points to note
Because of their limitations, WLAN designers need to
qualify sites and situations with regard to their suitability
for WLAN deployment. This must be done before going
ahead with the time consuming and relatively expensive
process of doing a site survey, let alone attempting
to design a WLAN for a given site.
Fortunately the process of qualification is reasonably
straight forwardit is basically about answering
a few simple questions. For example, are there a large
number of devices which might compete for spectrum?
Does the site contain a significant number of obvious
impediments to signal propagation? Is the user density
within the capacity of the WLAN?
A simple rule of the thumb for the last question is
20 concurrent users per access point. Note that WLANs'
bandwidth is not the governing factor here, but rather
the number of available channels and the number of users
which can be supported per channel. 802.11a can provide
almost five times the bandwidth of 802.11b, but does
not provide five times the number of channels.
802.11a can potentially support more users per access
point than earlier standards because transmission and
reception of a given amount of data will take less time,
but the increase in the number of users per access point
is likely to be around 25 percent rather than several
WLAN designers also need to assess the type of traffic
to be transferred. As is the case with shared Ethernet
systems, WLANs work well with bursty traffic consisting
of short data transfers. Web page access would be one
example, short text-based e-mail another. However, WLANs
supporting a few users would not be appropriate when
large files are routinely being transferred across the
In summary, WLANs are best used as an addition to a
copper-based network in situations where a percentage
of users require mobility and/or where it is physically
difficult, impossible or extremely expensive to deploy
a copper solution.
WLAN technology can also be effectively used as a short
haul backbone link between buildings on a single campus
or across a road. WLANs are particularly appropriate
when it is necessary to set up a small LAN quickly,
either as a temporary or permanent solution. Standalone
WLANs can also be used for public Internet access, but
as discussed above this needs some careful thought.
a, b and g of wireless LANs
original 802.11 standard was approved back in
1997, 802.11a and 802.11b in late 1999, and 802.11g
just this year. In practice all these standards
support the same distance limits (about 400 feet
maximum with a standard antenna), and all of them
allow a variety of different antenna for different
situations. The 802.11, 802.11b and 802.11g standards
operate in the 2.4GHz ISM band which now requires
no license in most countries. 802.11a operates
in a 5GHz band which theoretically requires no
license in most placesbut its worth checking
before you buy.
WLANs use Direct Sequence Spread Spectrum (DSSS),
Frequency Hopping Spread Spectrum (FHSS) or Orthogonal
Frequency Division Multiplexing (OFDM), all of
which are mutually exclusive. They can however
co-exist in a given location.
The 802.11a standard permits data rates of up
to 54 Mbps and is not compatible with access points
and clients supporting the other standards because
it uses the 5GHz band. 802.11g offers speeds of
up to 54 Mbps in the 2.4GHz band and is interoperable
with both 802.11 (speeds up to 2 Mbps) and 802.11b
(speeds up to 11 Mbps). 802.11g achieves this
by using DSSS to 11 Mbps and then switching to
OFDM at higher speeds. 802.11g does not support
FHSS. In fact, FHSS is only used by 802.11; clients
using FHSS will not interoperate with 802.11b
equipment. The various standards use a variety
of modulation types in various combinations. All
standards except 802.11a have an 83.5MHz frequency
window; 802.11a has a 300MHz window.
Most of the equipment shipping now supports the
802.11b standard. At present, OFDM is relatively
expensive to implement which limits the availability
of 802.11a and 802.11g equipment. But prices will
inevitably drop, and availability will then improve.
Perhaps the best course of action for WLAN buyers
is to select an access point which supports PCMCIA
transceivers. Such an access point can be fitted
with 802.11b modules now, and these can be replaced
with 802.11a or 802.11g units if and as required.
As far as clients are concerned, most PCI WLAN
adaptors are implemented as a PCI carrier for
a PCMCIA modulewhich can be upgraded simply
by changing modules. The same goes for laptops
that use PCMCIA modules.
with built in WLAN NICs will need to be replaced,
or the updated WLAN environment will have to accommodate
a mix of equipmentwhich would point to 802.11g
as a favorable option.
K. Le Roux is the director of Moresdawn (Australia),
a company which specializes in network design and consultancy.
Got more on WLAN limitations? E-mail at email@example.com