Three ways to go wireless on the LAN
Components of a successful wireless LAN implementation involve more than just
compliant radio cards and access points. To complete a system, you also need
application connectivity software with which end-user devices can communicate with
application software or databases located on a centralized server.
The common forms of connectivity software that implementers use to connect wireless
clients are terminal emulation, direct database connectivity, and wireless middleware.
Let's take a look at each of these alternatives.
Terminal emulation software runs on an end-user device, which then operates as a
terminal and communicates directly with application software running on a host-based
system. For example, VT220 terminal emulation communicates with applications running on
a Unix host and 5250 terminal emulation works with AS/400-based systems.
Terminal emulation software's advantage is its low initial cost. Keep in mind,
though, that wireless systems using terminal emulation may not be able to maintain
continuous connections with legacy applications, which have timeouts set for the more
reliable wired networks. (Timeouts automatically disconnect a communications session if
they don't sense activity within a given time period.) As a result, corporate MIS folks
may spend a lot of time responding to end-user complaints of dropped connections and
the associated issues of incomplete data transactions. Thus, implementing terminal
emulation can have a significant deleterious effect on long-term support costs.
Direct database connectivity encompasses application software running on a client
that interfaces over TCP/IP directly with a database located on a server. With this
configuration, the software on the end-user device provides all application
functionality. This provides flexibility when developing applications, because the
programmer has complete control over what functions are implemented and is not
constrained by the legacy applications on the host. Direct database connections are
often the best approach if you need a lot of flexibility in writing the application
A problem, however, is that the direct database approach relies on TCP/IP, which is
not well suited for traversing a wireless network. TCP/IP uses a significant amount of
bandwidth overhead when re-establishing connections after a break, and supports the
transmission of packets with relatively large headers.
Direct database connectivity and terminal emulation can work for both wired and
wireless networks. Most often, however, the optimum connectivity software for wireless
LANs is designed specifically for that medium. Wireless middleware software provides
intermediate communications between end-user devices and the application software
located on a server. The middleware, which generally runs on a dedicated platform
attached to the wired LAN, processes the packets that pass between the LAN and the
wireless access point. It provides efficient and reliable communications over the
wireless network, while maintaining appropriate connections to application software and
databases on the server via the more reliable wired LAN.
The following are features to look for in middleware products:
- Optimization techniques: Many middleware products include
data compression at the transport layer to help minimize the number of bits sent over
the wireless link. Some implementations of middleware use header compression, where
mechanisms replace traditional packet headers with a much shorter bit sequence before
- Intelligent restarts: With wireless networks, a transmission
may be unexpectedly cut at midstream. Intelligent restart is a recovery mechanism that
detects the premature end of a transmission. When the connection is reestablished, the
middleware resumes transmission from the break point instead of at the beginning.
- Data bundling: Some middleware is capable of combining smaller
data packets into a single large packet for transmission over the wireless network,
which can help lower transmission service costs of WANs. Since most wireless data
services charge users by the packet, data bundling results in a lower aggregate cost.
- Store-and-forward messaging: Middleware queues traffic to
ensure delivery to users who become disconnected from the network. Once the destination
station comes back online, the middleware sends the stored packets.
- Screen scraping and reshaping: The development environment of
some middleware products allows developers to use visual tools to "scrape"
and "reshape" portions of existing application screens to more effectively fit data on
the smaller display of some non-PC wireless devices.
- Support for
HREF="http://www.computer.org/internet/v2n1/perkins.htm">Mobile IP: Some
middleware products offer home and foreign agent functions to support the use of Mobile
IP protocols, which seamlessly handles changes in a client's point of attachment to the
Internet. This enables wireless users to roam across networks based on different
- Operational support mechanisms: Some middleware products offer
utilities and tools to monitor the performance of wireless appliances, enabling you to
better troubleshoot problems.
I recommend using wireless middleware when implementing large, complex projects.
Though you'll spend $10,000 to $25,000 per site for the hardware and software, you'll
easily recover the initial expense with money saved in supporting the system.
Some popular wireless middleware products include:
Next time we'll take a look at the emerging IEEE 802.11a high-speed wireless LAN