Wireless Networking

Dealing with cabling problems is one of a network manager's primary jobs. Troubleshooting cables for breaks and bad connections and overseeing the relocation of computers are not only time-consuming tasks , but frustrating as well.

Installing new cable can be an expensive proposition, costing hundreds or even thousands of dollars per network connection. The cable itself is not that expensive unshielded twisted-pair wire, for example, costs only a few cents per inch. But paying union technicians to pull new cable is costly, especially in old buildings that may contain asbestos or other hazardous materials.

As a result, some network managers turn to wireless networking, rather than cable, when they expand existing LANs or build new ones. Wireless LANs give network designers a level of flexibility unavailable to wire-based systems. Wireless networkswhich send and receive data through a transmission and reception device attached directly to the network adapter cardmake moving PCs connected to wireless networks a much simpler process than with wire-based systems.

Network managers can merely move the PC, without adding cable or testing its associated connections. This capability makes wireless networks a tempting alternative to cable-based ones.

Going WirelessPros And Cons

Wireless LANs offer network managers a host of advantages over traditional hard-wired network technologies. In addition to simplifying the move of networked PCs, they can also be less expensive to buy and buildover the lifetime of the LANthan cabled LANs. The most obvious cost savings come from not needing to remove and install and test the cable. Their initial installation costs, however, are higher, since you must purchase the wireless transmission and reception electronics. Some wireless connections won't require building permits , as can be the case with large-scale cable installations. This feature can save additional time and money.

There are tradeoffs, however. By nature of the medium, wireless networks transmit at slower data rates than wire-based networks. They also impose limitations on the number of connected nodes and how far apart those nodes can be placed from each other.

Some wireless networksinfrared onesrequire line-of-sight communications. This limitation restricts their usefulness in many situations, such as in so-called hard-walled offices and multi-story buildings.

Manufacturers use three basic technologies to carry data over their wireless networking products: infrared light signals, narrow- band Radio Frequency (RF) signals, and spread-spectrum RF signals. Here's a look at the basics of how each operates and what their operational characteristics mean to network managers and users.

Making Light Of Data

Infrared networking products transport data via light waves that are invisible to humans . Infrared light, falling in the 1,000 gigahertz (GHz) and higher range, shares all the properties of visible light: It can be reflected off, but cannot penetrate , solid objects such as walls.

The primary advantage of infrared technology is its great bandwidth, allowing it to carry hundreds of megabits of data per second. In addition, because no government body regulates use of light frequencies, infrared data transmission is unlicensed.

Infrared networks share the technology used with the remote-control units that come with home electronics equipment, such as TVs and stereos. Receivers for wireless networks, like the channel changer and TV, must be visible to the transmitter, either directly or via reflection.

Infrared-based networks can be implemented with mirrors that focus the light signal to an extremely tight beam. Because focusing delivers essentially all of the transmitted signal to the receiver, it permits high-speed communication that can equal or surpass that of 16Mbit/sec Token Ring networks. Mirror-based systems are well suited for point-to-point applications, such as building-to-building connections where transceivers are seldom moved. They are less satisfactory for in-building networks, however: The receiver must have a direct, unobscured view of the sending unit, and any movement in either unit can break the connection.

To work around this line-of-sight requirement, some infrared networks spread, or "flood" light around an area, which allows a single transmitter to reach multiple receivers, while reducing the effects of the transmitter or receiver being moved. A more diffused signal, however, reduces data rates and shortens the distance over which the signal can be reliably sent.

Another disadvantage of infrared networks is their susceptibility to interference from other light sources, including the sun and some lighting fixtures. Focused systems, because they produce a stronger light beam, offer greater immunity to light interference than unfocused ones.

Up In The Air

Radio frequency transmission, while not limited to line-of-sight environments, offers its own set of thorny technical issues, including available bandwidth, signal reflections and interference, and Federal Communications Commission (FCC) regulations.

Manufacturers have taken two tacks in developing RF network products. Narrowband transmission requires licensing by the FCC because it needs a "clear" communications channelone that is uninterrupted by other narrow-band transmitters. Spread-spectrum transmission uses special unregulated frequencies that require no FCC licensing.

Narrow-band networking products transmit data directly on a center frequency, much like a radio broadcast, so the transmitter and receiver must be tuned to the same bandwidth. Like the signals from radio and TV stations , narrow-band signals are subject to interference from signal reflections. This interference is caused when signals reflected off walls and other objects arrive at an antenna at different intervals.

Such "ghosts" make data communication unreliable. Unlike the human eye, communications equipment is not sophisticated or intelligent enough to discern the difference between reflections and the "real" transmission.

Vendors of narrow-band wireless products must thus be able to guarantee their customers a clear channel. A clear channel can only be ensured by carefully allocating each available frequency band to make sure that two nearby networks do not share the same frequency.

Spreading Data Around

Spread-spectrum transmission distributes, or " spreads ," a radio signal over a broad frequency range. To do this, spread-spectrum networks use what is called a predetermined pseudo-random sequence to transmit data. This pseudo-random sequence is actually a predetermined digital signal pattern that places data on a combination of frequencies from across the entire spread-spectrum band.

A receiving device must thus know the specific signal pattern used by the transmitting device to decode data. This technique makes spread-spectrum LANs secure and reliable. In fact, the spread-spectrum technology was developed by the U.S. military during World War II for secure voice communications.

One of spread-spectrum's main benefits is that it allows multiple networks to share a single frequency as long as different pseudo-random sequences transfer data. In these situations, the signals from one network are interpreted by another as random noise and are ignored. Another advantage of spread-spectrum networks: They don't require line-of-sight communications, making them suitable for hard-wall offices as well as open office environments.

Wireless Topology Issues

Like their cable-based counterparts, wireless networks must provide clearly defined methods of accessing the transmission channel. Vendors use two basic techniques for granting this access: One, called a peer-level system, lets every node on the network communicate directly with every other node. The second uses a dedicated server architecture, with all network devices communicating through a central control station.

Peer-level wireless networks typically cost less to build than those with central control units, primarily because they don't require the central hardware. But because there is no central point of control in peer-level systems, managing the network, including providing security, collecting network statistics, and performing network management and diagnostics, can be difficult.

Because they do feature a dedicated server environment, controller-based wireless networks permit centralized management, security, and maintenance capabilities. The central control unit also provides access to other services, including local and wide area networks.

The Wireless Marketplace

A number of vendors, including NCR, Motorola, Windata, and Photonics, market wireless networking products. These offer a variety of features and benefits, including varying degrees of performance, transmission characteristics, and network operating system support. Additionally, the IEEE has formed a committee, 802.11, to study and standardize wireless networks.

Many of the new pen-based, hand-held PCs include wireless networking capabilities as an option. These products can give end users even greater flexibility in their networking choices.

This tutorial, number 40, written by Jim Carr, was originally published in the November 1991 issue of LAN Magazine/Network Magazine.