Databus Issue: 2006 2 05/17/2006
Creating a Wireless Point-to-Point Network
Burt Lonergan Founders of Sun Microwave
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For many years, the terms metropolitan area network and wide area network were differentiated by the size of the area each encompassed. The metropolitan area network was typically within an area no greater than 25 miles; the wide area network covered everything greater than 25 miles. With the advances and changes in technology over the past 15 years, these two terms have become extremely confusing. Today, a metropolitan area network essentially exists in an urban area, while a wide area network is found in a rural environment. For the purposes of this discussion, both structures will be collectively referred to as a metropolitan area network (“MAN”).
To create a MAN, one has three technologies to consider – fiber, copper (leased line) and wireless. The least costly with the lowest bandwidth is copper; the most costly is fiber with the greatest bandwidth. In between these two media is wireless or microwave. In some cases, wireless might be the only solution since the other two solutions are not available. Another significant differentiator is whether one wants to eliminate recurring expenses or wants to use capital monies. Copper and fiber represent a small initial investment but a substantial, recurring monthly expense; furthermore, you never own it. The wireless approach symbolizes an up-front expense but a minor recurring cost (a maintenance and/or support contract, if desired). A final factor is control of the MAN. With leased lines or fiber, a third party controls the network and the availability. With wireless, the customer administers the MAN. All of these technologies are viable. Normally, one will find two or three of these media used in a successfully developed and deployed metropolitan area network.
If one decides to explore a wireless solution, one needs to realize that this technology normally requires line-of-sight – the ability to see from one point to the other. To ascertain this condition, one should undertake a sight survey/engineering study. This endeavor ought to be taken on by a firm competent in wireless technology and experienced with this type of task.
After doing its preliminary work – determination of the approximate coordinates, a vertical profile of the path and an aerial photograph review, the contractor will come on-site to ascertain visually that clear line-of-sight (“LOS”) exists and to verify the groundwork already completed. Should LOS not exist, how can it be obtained? Do towers need to be erected? Does a repeater site need to be established? Are any of these approaches economically feasible? If LOS can be established, the next step is to undertake a spectrum analysis. Using sampling equipment, one surveys the surrounding areas looking for other transmitting equipment that could potential impact the proposed link. With these two steps completed, the customer then needs to start making a few decisions regarding this potential MAN:
1. If the path distance is 4,000 meters or less, laser (as referred to as free space optics or optical wireless bridge) could be used for the transmission media. Like all wireless technology, this equipment has its Achilles heel – fog. If the location for the system is in an area that can be impacted for long periods of time by fog and one cannot usually see from point A to point B, laser is not a viable approach. If fog is a potential problem, then microwave radio is the only workable approach.
2. The microwave radio system, like a laser, has its potential problem. As previously mention, the laser link can be impacted by fog. The microwave hop can be affected by rain. In both cases, an availability calculation should be accomplished; this is a theoretical analysis based upon statistical history in a specific regional area. Typically, a system is stated in terms of 9’s availability. The table below outlines the options:
Availability Annual Outage (Minutes)
99.999 (Five 9’s) 5.3
99.99 (Four 9’s) 53.0
99.9 (Three 9’s) 526.0
99 (Two 9’s) 5,260.0
The selection criterion is the importance of the data to be carried over the wireless equipment. For a critical application, one should have five 9’s; for a continuously used but not mission critical function, one can use four 9’s; for an occasionally employed link, two or three 9’s might be acceptable
3. If radio is to be implemented, does one go with licensed or unlicensed equipment? Of the two, licensed is usually considered a safer approach. The transmit and receive frequencies are coordinated and controlled by the Federal Communication Commission, thereby, providing some protection. Unlicensed systems usually offer faster deployment and more rapid redeployment. The potential problem is interference – two radios stepping on each other. The unwritten rule is “he who gets/uses the frequencies first, shall prevail.” However, too often organizations do not follow this gentleman’s agreement and just step on each other. This situation results in degraded and/or interrupted performance for all parties.
4. For the laser solution, the offered bandwidth is from 100 Mbps to 2.5 Gbps. For licensed radios the capacity ranges from 50 Mbps to 622 Mbps while the unlicensed units can provide 10 Mbps to 1.25 Gbps of capability. Consequently, the customer must evaluate the current and future transmission speed desired for each link or hop.
To this point, only traditional line-of-sight equipment has been addressed. Recently, though, new technology has been introduced offering non-line-of-sight capabilities. These units can operate across a path that is partially or totally obstructed. Determining the feasibility of these radios is a little more difficult. First, the path must be modeled to determine if the equipment can theoretically provide an acceptable bandwidth. Then the path should be tested to ascertain if the connection is possible.
One significant advantage that wireless technology has over leased lines or fiber is the ability to develop literally fault-tolerant solutions. There are three approaches that can be deployed. The first is called “dual path.” In this case, two wireless solutions are implemented across a single path. Both systems are constantly active. If one fails, the other just picks up the added burden until the problem has been corrected. The second approach is a “protected system” or “hot standby.” This scenario has a back-up system always poised to take over if the primary system fails. It continues to function until the down equipment is brought back on line. The third method is “route diversity.” This solution is the completion of a ring. Assume that one has a link from Point A to Point B and from Point A to Point C. By installing a link from Point B to Point C, a ring would be completed. If a radio failed, traffic would follow the other path.
Once the survey work has been completed and a detailed report prepared, the customer should look at acquisition of the link(s). The procurement process can go to bid or to CMAS. When one looks at the time to prepare, administer, negotiate and award a bid, the state provided vehicle (CMAS) will certainly be less expensive. Next, and foremost, it cannot be over emphasized that the hardware and services should be acquired from a single vendor. Awarding the equipment to one company and the installation to another assures problems. This situation can also develop if the firm that completed the site survey is not the company implementing the MAN. To avoid finger pointing and to have a single source for problem resolution, give the contract to one company.
Another important factor when selecting a vendor is to ensure that the company has a valid contractor’s license. State law requires that firm performing these services be licensed by the Contractors State License Board. This is a major issue and will be addressed thoroughly in a future article.
Today, wireless technology has matured to the point that it is a viable vehicle for providing extremely reliable networks with major amounts of bandwidth. With the continuing growth for voice and video applications, microwave has developed to the point that it can meet the challenges facing any organization looking to develop a Metropolitan Area Network.
Burt Lonergan is one of the founders of Sun Microwave dba Sun Wireless. The company, located in San Diego, focuses on providing turnkey wireless solutions for both the public and private sectors. The firm has been in business for more than five years and does business throughout California. Burt can be reached at burt@sunmcirowave.com or (858) 560-0400. Visit www.sunmicrowave.com.
