Logistical Logic of Wireless Systems

Jun 1, 2000 12:00 PM, By Joseph R. Knisley

The use of wireless local area networks (LANs) is growing among businesses seeking an alternative to traditional hard-wired solutions.

Imagine an office in which employees do not have a fixed desk or work area. Instead, they are part of a team that meets in a conference room or at some other location. They carry around laptops, which almost always need a convenient connection to the hard-wired local area network (LAN) in the building. But dragging around connection cables and constantly looking for a data port can be annoying. Is there a simple solution to avoid these headaches? Installing a wireless LAN that works in harmony with your existing hard-wired LAN system is the answer.

Logistics is the driving force spurring the use of wireless LANs. Temporary or remote workers can benefit by using wireless systems anywhere an area is set up, used briefly, and then dismantled. And in most cases, the wireless system is an extension of, but not a replacement for, the wired network, which usually operates at a higher transmission speed than the wireless segment.

Adding to the benefits of this technology is the reduction in cost of wireless system equipment. One of the reasons prices have come down, and perhaps the most significant contributor to the expected growth in wireless use, is the ratification of an IEEE standard. Standard 802.11, Wireless LAN Media Access Control Physical Specification, defines a 2-megabit-per-second (Mbit/sec) transmission speed, with 1-Mbit/sec in harsh environments. Recently, the IEEE updated the standard with a 10-Mbit/sec specification.

Understanding wireless. A radio frequency (RF) wireless LAN within a building has two basic components: the access point and an interface. Connected to the wired LAN, the access point includes the radio transmitter/receiver (transceiver) and an omnidirectional antenna for propagating the signal. The interface is the wireless LAN adapter, which includes the transceiver and antenna. A small card recessed within a desktop PC, notebook, or laptop serves as a transceiver.

A second method of propagation (for low power radio signals) uses a radiating cable, which serves as an antenna. You terminate a 15/8-in. diameter radiating cable, also called "leaking coax," at the far end by a 30W, 50-ohm resistor.

You can also use a fixed wireless link when you don't want to tie two or more buildings together with a LAN using a leased telephone line. In this case, rather than using an antenna that radiates in all directions, use a directional antenna. One manufacturer offers a product that complies with IEEE Std. 802.11 and provides LAN connectivity between buildings as far as six miles apart.

By using a layout of directional antennas, another product connects a number of buildings that can be on a campus, across the street, or across town. You can connect as many as 200 network interface cards on each network segment.

The system supports IEEE Std. 802.3 (standard for an Ethernet) and Ethernet II (TCP/IP) LANs, and transmits data at up to 10 Mbit/sec through even the worst weather. This is because the unit uses direct sequence spread spectrum (DSSS) technology, and thus is unaffected by environmental conditions. At the same time, it is secure to the interception of data.

Let's look at a few applications where wireless is useful: An industrial manufacturer, producing heavy iron and brass products, sets up a wireless communications system in a warehouse to allow forklift operators to gather and input warehouse inventory data. Extending the reach of the facility's wired LAN with a wireless branch brings increased inventory-tracking accuracy, improved productivity, tighter control, and valuable time and money savings.

The system allows 23 warehouse personnel, using industrial-grade touch screen computers, to record inventory, receiving, moving, and shipping. Operating in the 2.4 GHz to 2.4835 GHz unlicensed band, the wireless LAN connects to the warehouse's hard-wired LAN at one of five strategically placed access points (antennas), installed at the level of the warehouse ceiling.

While the forklift operator enters most data into the touch screen computer, warehouse workers can collect barcode information through handheld scanners attached to the computer. The warehouse LAN, which uses a 10Base-T Ethernet network, transmits the collected data to a computer (server) at the corporate headquarters every five minutes, via a leased dedicated telephone line.

Another manufacturing company uses a radiating cable as the antenna system in a 300,000-sq-ft warehouse. In the past, it was difficult to contact managers working on the production floor: Voice paging would add distracting noise to the space, and it was difficult to hear the phones ringing. The solution called for a wireless distributed communications system that could serve both voice and data communications.

The new system integrates the landline phone system and a wireless data system on a combined radio frequency system backbone using a 1.2 GHz personal communications system (PCS) from a local PCS carrier. Supported on ceiling beams and fed by a coaxial cable, a run of radiating cable extends down the entire length of the factory, on either side. With this layout, an employee's handheld PCS phone is never more than 50 ft from the antenna.

These alternatives to traditional hard-wired solutions are making their way into the industry. Interoperability, added range, aggressive pricing, flexible packaging, and improved performance are all key to speeding up future applications of wireless LANs.

Sidebar: Wireless Distributed Communications

Wireless distributed communications for in-building use emerged in the mid-1900s. These systems simply provided wireless communications coverage inside buildings where radio frequency (RF) from an outside source could not penetrate. At first, these systems were primarily made up of private trunking radio services for maintenance, fire, etc. After that, the early wired data systems for LANs and other slow speed wireless data devices came on the scene.