two-way paging.
The actual, raw data rate for paging technology is quite slow ( 2,400 bps) and the latency for one-way and two-way paging may vary from 20 seconds to several minutes. However, this is acceptable for some DA applications, such as capacitor switching.
Licensed radio can be used with a variety of methods and configurations in the Very High Frequency (VHF) and Ultra High Frequency (UHF) spectrums. Each frequency spectrum (band) has its strengths and weaknesses. The VHF band offers non-line-of-sight propagation characteristics and is less affected by atmospheric noise and physical obstructions (buildings, trees, etc.) than higher frequencies. Its weakness is that it provides lower available bandwidth ( 9. 6 kbps or less). However, this is not a major problem for most DA systems. (See Figure 1, previous page.)
Licensed radio is well suited for implementing automatic sectionalizing, if suitable licenses can be obtained by the utility. WiMAX, a new licensed radio technology operating from 2-11Ghz and 10-66GHz, promises to provide WiFi-like service over areas ranging in the tens of kilometers.
Spread spectrum radio technology has been used since World War II because it
was largely immune to enemy interference and jamming. These qualities are still desirable for networks that require high reliability, security and availability. Advances over the last several years have made it a dependable and proven communications technology.
Spread-spectrum radios operate in the 900 MHz, 2. 4 GHz, and 5. 8 GHz unlicensed frequency ranges. All users within wireless range of each other share the entire band, with each using only a small portion of frequency band in a random fashion. The chance of more than one user being on the same channel at the same time and causing interference is very low, even in densely populated areas.
The bandwidth offered by spread-spectrum product lines is generally around 100 to 250 kbps—more than enough for DA applications. Communication signals can be obstructed by foliage (especially pine needles) on a direct path from the source. Some spread-spectrum products offer the flexibility for mesh-networking (peer-to-peer architecture), which can be used to communicate via alternate routing around a partially obstructed direct path (see Figure 2).
Recently, unlicensed radio has been the most popular choice for handling DA communications due to its combination of low cost, high overall reli-
ability and availability, advanced security features, and excellent data throughput capabilities. In addition, eliminating the application process required for licensed frequencies reduces the startup time for the communication system. Coverage can be a problem in some terrains, but most utilities have been able to mitigate this problem through the use of repeaters and mesh networks.
Large infrastructure investments by cellular carriers and the economies of scale provided by millions of subscribers have made cellular technology an attractive option for DA. One drawback for utilities is the possible lack of certain service options and even basic coverage in some rural markets. To mitigate this, it may be possible to connect to a digital cellular system over greater than normal distances by using radios with a higher power rating and better antenna than a typical handset.
A modem or module is needed to use cellular data services—these generally cost around $200, which is a reasonable price for a DA node. Ongoing monthly charges can be inexpensive, too. Digital cellular data service can be obtained in some areas for as low as $7 to $11 per month, making digital cellular one of the most economical approaches for DA communications, although it can mean “locking in” to one particular vendor.
Satellite communication is an excellent “gap filler” technology for hard-to-reach areas. New service offerings have the following characteristics:
• Bandwidth of 30 to 100 kbps;
• Latency of 2 to 8 seconds (longer latency for some service offerings);
• Availability from most locations in the U.S.;
• Two-way capability; and,
• Price of less than $150 per month.
Satellite technology is not recommended for widespread use in DA applications. This is primarily due to the high cost of
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