Components of a successful wireless LAN implementation involve more than just IEEE 802.11- 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 software.
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 transmission.
- 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 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 domains.
- 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 standard.