rant, or other local place with a wireless connection. At home, it is pure
pleasure to do any computer work
from your deck, patio, or easy chair
without dragging a long, twisted pair
cable. Flexibility, mobility, and freedom made WLANs the reigning LAN
So, as companies expand their
Ethernet LANs, they are going wireless. There is no messy and expensive
wiring to add and no reconfiguration
hassles if people move offices, and it
adds the mobility that people love.
But the real revolution has been the
development and deployment of public access points called hot spots.
They first showed up in airports and
now most major airports have one or
more such access points. Hotels
began adding hot spots about the
same time and now most major
chains offer this as a guest benefit.
Coffee shops, like Starbucks, also
have wireless access. Some
McDonald’s also offer this service.
And it is becoming more popular than
ever in convention centers. Today, we
have over 12,000 hot spots to choose
from, but some projections say that
we can expect over 100,000 by 2007.
Thanks to technology, you can take it
called the access point (AP). Also
known as a wireless gateway or
router, this is the unit that services all
the remote transceiver nodes. Figure
2 shows a typical home networking
In a large enterprise or small
company LAN, the AP is connected to
the existing wired Ethernet LAN by
the usual twisted pair line. The AP is
mounted high on a wall or ceiling so
that its antenna has a good "view" of
the area it is to cover. Then, many
radio modem equipped PCs or laptops in that area can communicate
with the LAN via the gateway. The
radius of the coverage is from 100 to
300 feet, depending upon the environment, especially if walls, ceilings, and
floors are involved.
The connection at a hot spot has
the gateway or router connected back
to an Internet service provider (ISP),
usually by way of a leased T1 or T3
line. At home, the gateway or router is
usually connected to a high speed
broadband Internet source, such as a
cable TV or DSL modem. The gateway usually provides coverage for the
How it Works
Wireless Ethernet is implemented
with sophisticated microwave radio
transceivers. Every PC or server contains a digital two-way radio that
sends and receives Ethernet formatted data. Also known as radio
modems or wireless adapters, these
interfaces appear in several forms.
They can be plug-in cards for regular
desktop PCs or PCMCIA cards for laptops. See Figure 1.
External transceivers that connect via a USB port are also available.
Many of the newer laptops come with
Wi-Fi radios built in. Intel's widely promoted Centrino chip set is an example of an embedded Wi-Fi interface.
Each of these transceivers forms one
node in the LAN. Finally, there is the
transceiver that is part of what is
The 802.11 standard is not just
one standard, but many. By far, the
most popular is the 802.11b standard
that almost everyone uses today.
802.11b defines what we call the PHY
and MAC layers of a multilayer protocol scheme that Ethernet uses. PHY
means physical layer, which, in this
case, is radio. The standard defines
what we call the air interface. The
MAC, or media access control layer,
takes care of all the addressing,
formatting, and transmission between
the air interface and the network.
The PHY in 802.11b uses the
unlicensed 2.4 GHz industrial-scientif-ic-medical (ISM) band defined in the
FCC's Part 15 rules and regulations.
This band is 83. 5 MHz wide and is
divided into 11 overlapping 22 MHz
wide channels. Since only three of the
channels are non-overlapping, these
are the only useful ones that do not
interfere with one another. Their center frequencies are 2.412, 2.437, and
2.462 GHz. With 22 MHz of bandwidth, it is possible to achieve a data
rate up to 11 Mbps. But, of course,
that data rate also depends upon the
range of transmission, the environment, as well as sources of interference and noise.
The 802.11b standard specifies
the use of direct sequence spread
spectrum (DSSS). This is a technique
that takes the raw serial binary data
and exclusive ORs it with a unique
high speed chipping code. The result
is a high speed serial signal that is
then modulated onto the carrier. The
802.11b standard specifies multiple
modulation methods depending upon
the speed of transmission. If the environment is favorable and the transceivers are within range and noise and
interference are at a minimum, the
maximum data rate can be achieved.
FIGURE 2. A common home network access point or wireless broadband router.
Check out the dual antennas for diversity reception. Courtesy SMC Networks.