Eliminating Access Points with Mesh WLANs
September 30, 2002
Is the cost associated with access points and associated network cabling to high for your budget? Consider deploying a WLAN without access points by combing routing with the ad hoc mode of operation.
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.
Interested in wireless mesh networking? Join us at the 802.11 Planet Conference & Expo, Dec. 3-5 in Santa Clara, CA. One of our workshop sessions will cover Leveraging Multi-Hopping and the Mesh Architecture in 802.11 Networks.