Comment

1 September 2005

Remote radio or telephone?

A comparison and case study on control room data.

By Curt Wendt

At some point during projects involving remote terminal units (RTUs), you need to make a decision on how to send the field data to the control room. Often this becomes a choice between radio and telephone technologies. Both technologies have advantages and disadvantages. There is no one-size-fits-all solution. In general, frame relay, analog leased lines, cellular digital packet data (CDPD) (where available), and radio are competitive with each other. Radio and phone technologies can deliver similar data rates. However, as radio data rates increase, the effective range of the system decreases. What is effective at 1 mile may not work at all from 10 miles away. Digital data service (DDS) is more expensive but is the only digital phone technology that provides a dedicated line. Code division multiple access (CDMA) and general packet radio service (GPRS) are the least expensive technologies. As this technology matures and becomes more widely available, it may be a good match for supervisory control and data acquisition systems (SCADA) telemetry systems.

Local phone companies can quote phone-based system installation costs for a complete system, making initial capitol costs easy to estimate. Alternately, users purchase their own data to phone interface equipment. The monthly telephone fee pays for most maintenance of telephone-owned equipment. The local support for phone systems varies widely. During local emergencies, the phone system may overload.

Radio systems generally have a higher initial capitol cost. You'd need a radio survey in most cases and maybe a license. The hardware needed includes radio transceivers, towers, radio frequency (RF) cable, and antennas. Although there are no monthly fees, you need specially trained staff or an outside contractor to supply system maintenance.

Radio systems can suffer from adjacent channel interference, noise, and lightning damage. In some locations, the sight of a tall radio tower is unacceptable to local residents.

Communication options are available to work with SCADA. In a SCADA system, you'd need a data transmission over an extended distance to many sites. A common challenge involves putting communications in place to connect far-flung sites back to a central location. These systems need to work over extended distances (usually miles) and carry a limited amount of information on a cyclic basis with little or no time delays. Data rates of 9.6K and 19.2K bits per second (bps) are typical for SCADA radio systems.

In most cases, phone or radio are the main choices to get data from point A to point B. Radio and telephone technologies have progressed to a point where they are easy to install, easy to use, and affordable. Various types of leased lines and cell phone technology are available. Dialup connections using plain old telephone service (POTS) generally don't meet the performance requirements of SCADA projects. Also, many phone companies are phasing out analog-leased lines in favor of digital solutions. In some areas without phone service, radio may be the only realistic option.

With the introduction of data-designated radio bands and license-free equipment, radio has become more appealing. Radio data stability is enhanced by power and lower data rates. For SCADA radio systems, using the most powerful transmitters allowed and minimizing the data sent so you don't have high data rates is generally a successful strategy. You need to identify radio system constraints early. These include not only technical constraints but site-specific issues. Few residents appreciate a 90-ft pole in their back yard.

Phone technology

A telephone company leased line provides a dedicated analog connection between either two sites or multiple sites available at all times. Leased lines are available in 4-wire circuits and in point-to-point and point-to-multipoint configurations. In certain areas, users have reported difficulty in getting prompt repair of these lines. In some service areas, these lines are no longer available. Leased line modems interface serial ports to the leased line. They are available up to 56K bps. You need bridging, a service the phone company generally provides, to connect multiple sites to the same host. A monthly service charge includes a mileage rate from the phone company central office (CO) to the user sites. Bridging is an additional monthly charge.

Digital data service

DDS provides a dedicated digital link between two or more sites. The connection is permanent and available at all times. It offers the flexibility of point-to-point or point-to-multipoint configurations. Information transmits end-to-end in a digital format, which improves the error rate. DDS provides fixed transmission speed choices of 2.4K, 4.8K, 9.6K, 19.2K, and 56K bps. You need bridging to connect multiple sites to the same host. Channel service unit/data service unit (CSU/DSU) equipment at each site interfaces the phone equipment to a network or data line. You can buy CSU/DSUs from the phone company or independent retailers. The phone company provides bridging to connect multiple sites to the same host.

The amount of bandwidth you order (2.4K to 56K bps) and the distance of sites from a central office determines the monthly charges. Purchasing only as much bandwidth as you need can control costs. Servicing phone company equipment is included in the monthly charge. Again, bridging is an additional monthly charge.

Frame relay

Frame relay is a digital-wide area networking technology shared connection; all users share a common cloud of connections. It uses an efficient packet-switching technology. Frame relay doesn't provide error checking, and it is protocol independent. The frame relay equipment, not the network, encapsulates the data. Frame relay is entirely digital, which reduces the chance of error and offers excellent transmission rates. It operates from 56K to 1.544M bps. Frame relay sends information in packets called frames through a shared frame relay network. Each frame contains all the information necessary to route it to the correct destination. So in effect, each endpoint can communicate with many destinations over one access link to the network. Frame relay offers a committed information rate (CIR), the minimum speed the data will transmit. You'll see frame routers or frame relay assembler/disassemblers (FRADs) in use at each site; they typically connect to an Ethernet router or bridge. You don't need separate routing equipment for multiple sites. The host site usually comes with a greater bandwidth connection than the end sites. CIR and bandwidth determine charges; pricing is not distance or mileage sensitive. Connections to multiple sites do not affect the monthly rate; neither does the amount of data transferred.

Cellular technology

CDPD uses the same frequencies as analog cellular (around 800 MHz) during quiet time and offers Internet protocol (IP) messaging at 19.2K bps. Coverage, shared with voice, is intermittent and isn't available in all areas. It may also become unavailable during local disasters, such as flash floods or hurricanes. An older technology, CDPD has proven economical historically, but it's now phasing out, and newer technologies are replacing it nationwide. Cellular allows users to easily add more units to a remote location, as long as service is available. Each site requires CDPD radio modems. A simple whip or stub antenna is generally all you need, though a directional antenna can improve reception in fringe areas. Some CDPD modems are now upgradeable to GPRS or CDMA. Data use determines monthly costs, though some plans permit unlimited use. Plans typically include a certain amount of data. Billing includes data sent in excess of that at a fixed rate per kilobyte. Hardware costs are inexpensive.

General packet radio service

GPRS is a data service you can deploy on either a global system for mobile communications (GSM) or time division multiple access (TDMA) systems. GPRS facilitates instant connections whereby you can send or receive information immediately. High immediacy is an important feature for time-critical applications where it would be unacceptable to wait even 30 extra seconds.

With GPRS, the information splits into separate but related packets before transmitting and reassembling at the receiving end. The Internet itself is another example of a packet data network. Packet switching means GPRS radio resources see use only when users are actually sending or receiving data. Each site requires terminals supporting GPRS. Coverage for this new technology is limited but should expand over the next few years. Data use determines monthly costs.

CDMA2000 1X technology supports voice and data services over a standard (1X) CDMA channel. It provides wireless Internet services at peak data rates of up to 153K bps. Each site requires terminals supporting CDMA. This as well has limited coverage, but it should expand over the next few years. And again, data use determines monthly costs.

Radio technology

The FCC has allocated special dedicated RF channels for point-to-multipoint systems such as SCADA. These channels, known as multiple address systems (MAS), are in the 900 MHz frequency band. MAS systems are configured in a point-to-multipoint arrangement only. You can transmit for two-way (interrogate/response) communications between a master station and its remote sites. This is a data-only frequency band using 5-watt transmitters. Data transmission speeds are available up to 9.6K bps and increase SCADA system response to alarms. FCC licenses grant exclusive use of a designated frequency within a 70-mile radius that reduces interference. Because of recent FCC frequency re-farming, a new set of frequencies became available in the 938/952 MHz band. You'll need a license to operate.

Spread spectrum radios in the 900 MHz band are popular because of their ease of use and license-free status. They operate in a data-only band and have communications speeds up to 19.2K bps. To meet FCC rules, the radios use frequency-hopping or direct sequence spread-spectrum techniques. Since they operate on non-exclusive frequencies, you should expect some interference. Therefore, radios manage the effects of interference by avoiding frequency bands, changing hopping sequences, and sending small data packets with auto retries. Spread spectrum radios have a shorter effective range than licensed systems, primarily because they operate using only 1 watt. If you need repeaters to extend coverage, they are easy to implement.

Spread spectrum radios are also manufactured for the 2.4 GHz band. Due to the higher frequency, you should expect more signal attenuation from trees and the like. They are widely available from a number of manufacturers. Using directional antennas can reduce interference. You'll need a radio frequency study to ensure system operation and properly locate the master radio. This technology requires no monthly fees and no licensing fees.

Surveys

You might need a software survey, field survey, and license application to determine the configuration of the system and whether you need additional repeater sites. An engineering firm normally provides these services. The owner implements and maintains the radio infrastructure including antenna towers and repeater sites.

Software surveys are a rough determination that a radio path may exist. Also, you must consider community buy-in for required pole heights. Draw line-of-sight paths between master radio towers and end sites on the plots to verify the possible presence of a communication link. Radio studies factor in many variables impacting the system, including site locations, terrain, antenna heights, antenna gains, transmission line losses, transmitter power, receiver sensitivity, fade margin, and vegetation (land use). While path studies provide valuable assistance in system planning, they are not infallible. It is difficult to consider the effects of manmade obstructions or foliage without performing an actual on-the-air test.

You should conduct a field survey early, preferably during the initial design of the system. Installation of radios and antennas with known gains are temporary at each master tower. Radio technicians at each remote location receive the signal using a test pole and rotate a directional test antenna to maximize the intensity of the signal and ensure reception is through the main antenna lobe. Data adjustment is mathematical, allowing for any differences between the field test hardware and the proposed operational system hardware. A fade margin (20-30 dB) adds to the sensitivity of the radio to produce a minimum received signal strength indication target.

You might need repeaters to extend the coverage area. A repeater system simply retransmits received radio signals to extend the overall area it can serve. If the radio survey shows repeater sites are necessary for the system to work, choose them carefully to reduce the number of paths studied. Evaluating multiple radio paths can become very expensive and time-consuming.

Radio license

When a prospective user applies for a new radio frequency, the FCC considers the proposed use and whether the frequency is already in use by a nearby user to avoid interference. Since frequency congestion is a significant problem in metropolitan areas, the FCC has opened new frequencies between channels and is encouraging narrower bandwidths to permit more users to operate in a given area.

Poles, towers, antennas

Poles and towers come from different materials, including galvanized steel, fiberglass, aluminum, 316 stainless steel, and concrete. Some camouflaged pole designs include fake trees, light poles, church signs, and large flagpoles. A structural engineer should certify structural soundness of all pole and tower systems and their ability to handle maximum wind loads. Water tanks offer an elevated location, but cell companies often buy space on these at a premium. You could also co-locate on cell towers (on municipal property), especially if you've reserved space during contract negotiations. A SCADA radio system typically uses two types of antennas: omni directional antennas (master sites) and directional Yagi antennas (at the end sites).

Behind the Byline

Curt Wendt is I&C group leader at CDM in Maitland, Fla.