Data Comm for Business, Inc.
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Connecting Remote LANs
Wireless WAN Links Reduce Telephone Cost to Zero!
Overview: Radio modem links connect remote LANs or connect remote terminals to a host computer. Use unlicensed spread-spectrum radios in the 900 Mhz, 2.4 GHz, and 24 GHz bands or licensed microwave units in the 23 GHz band. Available speeds vary from 38.4Kbps to full duplex ethernet at 10 Mbps. 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 was able to connect remote LANs using routers with radios, and another used radio modems to connect terminals and multiplexers to a remote host computer.
The most common method used to connect remote office LANs is a dedicated phone line, 56 Kbps DSUs (or slower modems), and routers. By replacing the DSUs and phone line with two radio modems running 64Kbps, you can achieve equivalent performance without the phone line. The initial cost of radios is usually returned in less than a year since DSUs aren't needed and phone line costs are usually several hundred dollars per month. Figure One details one installation that provides low cost 64Kbps connectivity between two LANs at an installed cost of about
$3,500.00 for the radio modems. When using radio modems, you don't have to purchase DSUs (typically $1,000 per pair or more). When compared to leased telephone lines that normally cost $200.00 or more per month, a one year payback is common. Routers are needed when connecting LANs with either telephone lines or radios.
Figure One. Typical Wireless System.
By using more expensive microwave radios, the performance of this system can increase to either T-1 speeds of 1.54 Mbps or full ethernet at 10 Mbps. As when using telephone circuits that support higher speeds, replacing the routers with higher speed models is required. Equivalent telephone circuits at T-1 speeds often cost over $1,000 per month. Although telephone DDS lines normally top out at T-1 speed, microwave links can run at full ethernet speeds... effectively extending your ethernet LAN over the wide area.
Router application on wireless systems is identical to that of wired systems. The same considerations used with DDS, analog leased lines, frame relay, or ISDN circuits come into play. Consider the system throughput limitations from limited bandwidth and select an appropriate router. The exception to this rule would be the full ethernet speed 10Mbps links available on the 23/24GHz systems and the RL2 product. The RL2 and 23/24Ghz products are MAC layer ethernet bridges. Although bridging is adequate and these systems don't actually require a router, routing may be preferred for performance reasons. For those systems, use an ethernet wire speed router.
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.
* LOS: Line of Sight
|Model|| Max. Range|| Max imum Speed|| Frequency|| Licensed || Antenna
| 875 ||3 Miles LOS* ||10Mbps Plus T1 ||23 GHz|| Yes ||9 Dish
| 950 ||10 Miles LOS*|| 10 Mbps AND T1 or 20 Mbps || 23 GHz ||Yes|| 2 Dish
| 950 ||Over 10 Miles LOS*|| 10 Mbps OR T1 or Multiple (4) T1|| 23 GHz|| Yes ||4 Dish
| RL2 ||3 Miles LOS*|| T-1 (1.5 Mbps) ||2.4 Ghz|| NO ||Yagi
| 890 ||1000 Ft. LOS*|| 10 Mbps ||24GHz ||No|| 9 Dish
| 990 ||2000 Ft. LOS*|| 10 Mbps ||24GHz ||No|| 9 Dish
| BL-121 ||Over 4 Miles LOS**||T-1 at 4 Miles, down to 256Kbps at 20 miles ||2.4GHz ||No|| 24" Mesh Dish|
** Requires a router at each end of the wireless link
Figure Two. Typical System Characteristics.
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 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 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 reduce this probability. We generally recommend 900 MHz for our rural customers and 2.4 GHz for those in urban or built-up areas. For short hops in urban areas, 900 MHz is probably a safe bet. With cost savings between 900 MHz and 2.4 GHz at about $1,000 per link, many customers prefer to go with 900 MHz for their price-consious applications.
Since systems in the spread-spectrum license-free bands require no license preparation and little systems engineering, the installation cost is less than 23 GHz systems. The radios are also much less expensive than the higher performance 23 GHz radios. New in 1997 is the 24 GHz unlicensed band. We carry two units that operate at full ethernet speed in this frequency with ranges of 1000' to 2000'.
Licensed radios in the 23 GHz band 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. These radios require precise "aiming" and a true line of sight between the antennae. Systems range from the low end using a nine inch dish with a range of several hundred feet to three miles up to large systems with four foot diameter dishes and ranges of 20 miles or more. Most of the systems we've installed use either the nine inch or two foot diamater dishes and ranges of one to 10 miles. Obstructions are skirted by using repeater systems or higher mounting locations. The 23 GHz systems require systems engineering.
The Decision Process
Here are a few questions that help determine is wireless is the preferred choice for any particular installation.
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.
- What is the distance involved?
- If it's much greater than 10-20 miles, wireless is less cost effective because repeaters are required.
- What is the required bandwidth?
- Low cost radio modems are available for up to 64 Kbps. Units operating at 2.4 GHz operate up to T-1 speeds, and full T-1 (or multiple T-1) units are available at in the 23 GHz licensed band. If you need full ethernet connectivity (10Mbps ethernet), then you definitely need microwave units at 23 or 24 GHz.
- 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.
Data Comm for Business, Inc. is a manufacturer and distributor of a diverse
line of data communications and networking products. Their experts can help
you connect remote sites with phone (1-800-4-dcbnet), email
(firstname.lastname@example.org), or personal (807 Pioneer, Champaign, Il. 61820) assistance.
Check out their Web page at http://www.dcbnet.com .
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