The traditional architecture for wireless LANs includes access points, which
forms a backbone enabling users to roam throughout a facility and interface
with resources on the wired network. It’s a proven, workable method, and that’s
why the use of access points is the most common method used today to fully cover
a facility. You can sometimes even go without access points if you want to have
client devices communicate directly, but that severely decreases the range of
the wireless LAN.
But there is another way.
"Infrastructure" Relatively Expensive
In an "infrastructure mode" of operation, the radio-based network
interface card (NIC) in user devices automatically finds and associates with
the access point having the strongest beacon signal. A user’s radio NIC must
be associated with an access point before the user can send data over the network.
Data en route from one user to another, whether located on the wireless or wired
side of the network, must go through the access point.
To support infrastructure mode, companies must install Ethernet switches and
cabling to interconnect the access points to the wired backbone of the network.
This enables the roaming protocols to work. As a client device moves from one
access point to another, it will re-associate with the next access point. The
previous access point must then move any of the user’s buffered packets to the
new access point via the Ethernet network.
The costs
of the wireless LAN backbone, which consists of access points, Ethernet switches
and cabling, constitutes a relatively large percentage of the total cost of
the wireless LAN. Access points for enterprise applications range in cost, up
to over a thousand dollars each depending on features. Along with installation
services, the cost of backbone hardware is a major consideration for companies
deploying wireless LANs.
Ad Hoc Simplifies, but Shortens Ranges
802.11 offers peer-to-peer communications, called ad
hoc mode, between user devices without the need for access points. The advantage
of ad hoc mode is the cost savings of not requiring hardware and installation
services for the wireless LAN backbone. Users simply plug in a radio NIC, switch
to ad hoc mode, and start communicating with other wireless users directly.
The problem, however, is that ad hoc alone doesn’t provide very good range.
Because ad hoc directly communicates to the destination user, range limitations
of the radio become a major issue. For example, a typical 802.11b radio NIC
indoors has a range of approximately 200 feet. If the user you’re trying to
interface with is located on the opposite end of the building, perhaps one or
two floors beneath you, then you won’t be able to connect. This discourages
many companies from deploying ad hoc mode on a widespread scale.
Routing Reduces Range Issues
There’s a relatively new application of ad hoc mode that goes beyond the usual
802.11 functionality that involves the addition of routing mechanisms in clients
to form a wireless mesh network. With the addition of routing software, devices
implementing ad hoc mode can forwarding packets closer to the intended destination.
This configuration makes it possible to extend the range of all ad hoc users
as packets jump from user to user to get to the destination node, assuming the
density of users is high enough to enable hopping from user-to-user.
A problem with this approach is that there are not many products currently
on the market that offer wireless routing combined with end user devices. MeshNetworks
is the only company known that currently sells a system enabling every node
(i.e., each radio NIC) on the network to become a router and extend the range
for ad hoc users. MeshNetworks’ system enables end user devices to build and
maintain their own routing tables. Packets hop around congestion obstacles that
offer attenuation, making this type of network very efficient. If there are
plenty of users, you can even turn down the transmit power of each user’s radio
NIC to make better use of the spectrum.
The problem with ad hoc routing, though, is that most users generally need
a wired connection to the Internet and corporate resources. If none of the ad
hoc users interface with the wired network, then users needing access to wired-based
services will not be happy. In addition, the concentration of users is critical.
If there are not enough users uniformly distributed throughout the facility,
then there will be somewhat spotty coverage.
Despite the pitfalls of ad hoc mode, the idea of not purchasing and installing
access points should cause an IT manager to think twice before deploying a wireless
LAN. If there are enough users, then ad hoc mode combined with routing can provide
good performance with significant cost savings.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
Join Jim for discussions as he answers questions in the 802.11 Planet Forums.
