WLAN Application Connectivity Options

By Jim Geier

February 20, 2003

Because of impairments of wireless LANs such as RF interference and occasional loss of connectivity, some applications don't fare very well. Learn what options you have in order to provide reliable connections for applications over wireless networks.

Common applications, such as Web surfing and e-mail, perform very well over wireless LANs. All it takes is a browser and e-mail software on the client device. Users may lose a wireless connection from time-to-time, but the protocols in use for these relatively simple applications are fairly resilient under most conditions.

Beyond these simple applications, however, you will likely find the need to incorporate connectivity software that provides an interface between a user's client device and the end system containing an application or database. Applications could be warehouse management software running on an IBM AS/400, a modeling application located on a UNIX box, or a time management system resident on an old mainframe system. The databases would be part of a client server system where part or all of the application software resides on the client device and interfaces with a database such as Oracle or Sybase.

In these cases, you need application connectivity software in addition to traditional WLAN components (i.e., wireless radio NICs and access points) to enable communications between the client device and the application software or databases located on a centralized server. The primary options you have available beyond using Web browser technology are terminal emulation, direct database connectivity, and wireless middleware. Let's take a closer look at each one of these.

Terminal Emulation

Terminal emulation software runs on an end-user device and lets the client operate as a traditional terminal, communicating directly with application software running on a host-based system. The terminal merely presents screens to the use and accepts input rendered by the applications software. For example, VT220 terminal emulation communicates with applications running on a UNIX host, 5250 terminal emulation works with AS/400-based systems, and 3270 terminal emulation interfaces with IBM mainframes.

The advantage of using terminal emulation is its low initial cost. The wireless systems using terminal emulation, however, may not be able to maintain continuous connections with legacy applications, which have timeouts set for more reliable wired networks. Timeouts will automatically disconnect a session if they don't sense activity within a given time period. As a result, the corporate IT group 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 disastrous effect on long-term support costs.

Direct Database Connectivity

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 enables 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.

Wireless Middleware

Wireless middleware software, offered by vendors such as Wavelink and Connect, 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 wide area networks. Since some 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, such as PDAs and bar code scanners.

  • End system support: Wireless middleware interfaces with a variety of end system applications and databases. If you have multiple types of applications and data bases that clients need access to, then wireless middleware can act as a concentrator. For example, a user can use the middleware connection to interface with applications on an AS/400 and UNIX box simultaneously without needing to be concerned about running the correct terminal emulation software.

  • Security controls: In addition to application management, middleware products often offer access control and encryption mechanisms that counter the issues of 802.11 wired equivalent privacy (WEP) . Some middleware is beginning to include the ability to identify and counteract the presence of rogue access points and denial of service attacks.

  • Operational support mechanisms: Some offer utilities and tools to monitor the performance of wireless appliances, enabling you to better troubleshoot problems.

I strongly recommend using wireless middleware when implementing large, complex projects. Though you could spend tens of thousands of dollars per site for the hardware and software, you'll easily recover the initial expense with money saved in supporting the system.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs.

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