Connecting Remote Terminals
Wireless WAN Links Reduce Telephone Cost to Zero!


Overview: Radio modem links connect remote terminals to a host computer without phone company charges. Use unlicensed spread-spectrum radios in the 900 Mhz, 2.4 GHz band or 5.8 Ghz band. Available speeds vary from 38.4Kbps to multiple T-1s and ethernet speeds. This communications technology can save hundreds of dollars per month by completely eliminating telephone company charges. In this application note, we show how one customer used radio modems to connect terminals and multiplexers to a remote host computer.

The most common method used to connect remote office terminals with a host computer is a dedicated phone line, 56 Kbps DSUs (or slower modems), and multiplexers. By replacing the DSUs and phone line with two radio modems running 64Kbps, you can achieve equivalent performance without the phone line. The installed cost of radios is usually returned in less than a year since DSUs aren't needed and phone line costs are normally several hundred dollars per month. Figure One details one installation that provides low cost 64Kbps connectivity between two locations at an installed cost of about $3,500.00 for the radio modems. If using radio modems, you don't have to purchase DSUs. When compared to leased telephone lines that normally cost $200.00 or more per month, a one year payback is easily achieved.

Figure One. Typical Wireless System.

By using more expensive radios, the performance of this system can increase to either T-1 speeds of 1.544 Mbps (24 56Kbps channels) or full ethernet speeds. Most multiplexer installations run at 56 Kbps or slower and medium speed radio links are ideal for their application.

Multiplexers

Multiplexers allow multiple terminals (or PCs emulating terminals) to communicate over the same line. Using DCB multiplexers in a wireless systems is almost identical to wired systems. The same considerations used with DDS, analog leased lines, frame relay, or ISDN circuits come into play. Consider the system throughput requirements and select appropriate radio equipment. All DCB SR and SPL series multiplexers work through appropriate radio links. Where supported by the radios, multi-drop systems are even feasible. DCB multi-drop multiplexer systems support as many as 64 terminals using multiple radio locations.... all on the same license-free spread spectrum system.

Distance Limititations

The maximum distance covered by these sytems varies from several hundred feet to over 20 miles. Most systems require paths termed "line of sight". This is a relative term; as in the case of many short hops, you can't actually see one site from the other. In other cases (especially the high speed microwave systems), the antennae must be within view of each other... no obstructions are allowed. Distances can be extended greatly and obstructions skirted by using repeaters. DCB's Customer Support staff can help design a system that works for your location. The chart shown in Figure Two shows typical ranges for different system configurations.


Frequencies


The most economical systems use a license-free band in the 900MHz range. All license-free systems use "spread-spectrum" techniques that were once exclusive military communications methods. This method reduces frequency congestion and minimizes interference. Some spread-spectrum systems also operate in a license-free portion of the 2.4 Ghz or 5.8 Ghz band. This frequency is much more "like" microwaves. It has a very narrow beamwidth and typically has shorter ranges. While the 900 Mhz radios often work adequately without a true line of sight path, 2.4 Ghz and 5.8 Ghz systems almost always require direct line of sight paths. Another factor to consider is frequency congestion. With a proliferation of devices operating on the 900 Mhz band and no license requirements, interference between unrelated systems will become more of a problem in the future. The characteristics of higher frequency bands such as 2.4 Ghz include popular WiFi wireless LAN and cordless phone frequencies. We generally recommend 900 MHz for our rural customers and 2.4 or 5.8 GHz for those in urban or built-up areas. For short hops in urban areas, 900 MHz is probably a safe bet. For reliable operation, a site survey should be performed to determine the radio environment. Other radio frequency emitters can be characterized and the proper equipment selected only after enough knowledge of the operating environment is gained.


Licensed radios offer the maximum in reliability and protection. The frequencies are government licensed and must be co-ordinated with other users in the same frequency band. Licensing can take several months and adds to the expense of the system. Licensed radios are more expensive due to the licensing and engineering requirements.

The Decision Process

Here are a few questions that help determine is wireless is the preferred choice for any particular installation.
What is the distance involved?
If it's much greater than 10-20 miles, wireless is less cost effective because repeaters are required.
Is multi-drop appropriate?
Multi-drop capable radios can sometimes be used to minimize system cost.
Is there line-of-sight between locations?
If not, you may have to install a repeater, increasing system cost.
What is the required bandwidth?
Low cost radio modems operate at 64 Kbps or ethernet speeds.
How much does a telephone circuit cost?
If the path is across town and you can get 128 Kbps ISDN for $25 per month, you probably should go with telephone lines instead of a repeater system of radio modems at 64Kbps. BUT, that is rarely the case. And, when computing return on investment (ROI) or payback time, be sure to include the DSUs and telco installlation charges in the telephone circuit capital requirements. No DSU is required for radio circuits. Although it's anyone's guess, try to estimate future telephone charge increases.
Obtain current pricing on the radio equipment.
As with computers products, prices are decreasing and features are increasing.
Don't forget the cost of installation.
All communications equipment must be installed and maintained.
The bottom line is perpetual cost savings without performance compromises. Select the system that best meets those requirements and enjoy the cost savings month after month.


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