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Designing the Ultimate RTU— Communication Options

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Designing the Ultimate RTU— Communication Options Wireless Communications Technical Paper (continued from page 3) from either other radios in the area or from conditions) but real world RTU operation Designing the Ultimate RTU— Communication Options outside sources in adjacent bands such would likely be in the 1 to 20 Megabits per as cell phone operating frequencies. If second depending on quality of the RF link. interference exists, a frequency hopping By Mike Maes, Larry Terwisscha radio will keep re-transmitting data packets In addition to licensed and unlicensed radios, Data-Linc Group Bellevue, Washington USA on different sub-bands until the data gets which involve private networks and user Abstract- A broad range of communication options are available for SCADA systems through. The result can be a reduction in control over system reliability, there are two to transfer data from Remote Telemetry Units (RTUs) to a central location for system data throughput, but rarely a complete failure other choices. One is cellular and the other monitoring, process control and data logging. The selection of an appropriate of the system. Direct sequence will usually is satellite communication. With cellular communication method is dependent on many factors including distance between the provide higher absolute data throughput, but data packet services, data is collected and RTUs and the central location, the existence or absence of wires or fi ber optic cable can be overwhelmed by noise in which case typically stored on an Internet web site. for communication, data speed required, topography and economics. data throughput can drop to zero. Unless Initial cost may be low, but operating costs there is a requirement for very high data are usually based on a per packet charge. Telephone Line, Dedicated Wire & Fiber Optic Methods rates, frequency hopping radio modems Reliability of service is dependant on cell (operating the 900 MHz band) are usually the phone coverage and the service providers When data rate requirements are low (1200 to amplify the signal. With private lines (dedi- best choice for the typical RTU. Frequency operations. The major advantage of satellite 19,200 bits per second) and either private or cated wires), there is no monthly fee, and hopping radios are available with data rates radio communications is that it is rarely Telco wires are available, several types of wire private line modems are also always on-line from 9600 bits per second to over 1 Megabits limited by topography and is universally modems will fi t the purpose. With Telco lines and have relatively short connection delays per second and have either RS-232 serial or available even in very remote locations. (which will have ongoing monthly charges of 50 ms. Private line modems have typical Ethernet interfaces. 2.4GHz direct sequence Satellite communications is generally the associated them), dial-up modems, which practical ranges of 4 to 25 miles depending on are now available with data rates up to 54 most costly and most RTU applications avoid usually involve the lowest monthly cost, can modulation type and data rate. Megabits per second (theoretical, noise using this method when possible. be used if connection time delays of typically free and short range of a few hundred feet 30 to 60 seconds can be tolerated in the appli- Direct Ethernet wire connection can be made cation. Otherwise, leased line modems, which over modest distances of 300 feet on CAT5 Wireless Ethernet Application Wireless Ethernet Application are always on line and therefore have lower wire. Fiber optic modems are available with Using FHSS Using IEEE 802.11g/b data delays, are an option. If the Telco leased either serial (RS-232) or Ethernet if cable is lines are loaded, communication distance in place or can be economically installed. is unlimited as the telephone company will Fiber optic cables provide the highest speeds (Continued on page 2) Wired & Fiber Communication Comparison Diagram 3 A typical equipment arrangement Diagram 4 An equipment arrangement for for a multi-drop wireless SCADA system a multi-drop high speed wireless system using frequency hopping modems. which includes laptop computer wireless integration utilizing. Table 1 The principle characteristics of each of the options. 4 PN 152-09902-001 1 • 3535 Factoria Blvd SE, Suite 100 • Bellevue, WA 98006 USA • Phone (425) 882-2206 • FAX (425) 867-0865 • www.data -linc.com • 3535 Factoria Blvd SE, Suite 100 • Bellevue, WA 98006 USA • Phone (425) 882-2206 • FAX (425) 867-0865 • www.data-linc.com Telephone Line, Dedicated Dial-up Telephone Line Application Wireless Communications Wire & Fiber Optic Methods Wireless communication methods offer higher data rates, typically 115.2 Kilobits considerable advantages over hardwired or per second to many Megabits per second. (continued from page 1) telephone based methods and are gaining In the Americas and a few other countries, popularity in industry. Wireless technologies they operate in the 902–928 MHz or 2.400- and offer long distances, but are costly to are versatile, most provide high data rate 2.483 GHz band. In most of the rest of the install and diffi cult to repair if damaged. and generally are easy to deploy. There world, only the 2.4GHz band is permitted. are many varieties of wireless technologies, The 900 MHz band is preferable for RTUs as One additional option is DSL/Cable where both private (licensed and unlicensed) and it is somewhat less dependant on true line-of- such service is available. This option has subscriber based. sight and the range is over twice that of 2.4 high speed, unlimited distance (where GHz radios at maximum effective transmit service is provided), relatively low initial Licensed wireless modems require FCC power of 4 watts in North America. cost but rather high monthly expense. operating licenses. They typically operate Since communications is provided by a in the 200MHz to 400MHz frequency bands Unlicensed wireless modems utilize one of third party, data communication interrup- which enables them to operate at long two types spread spectrum technology. The tion can occur without notice. distances without line-of-sight. Their two objective of spread spectrum technology is Diagram 1. A typical dial-up communication system primary disadvantages are the requirement to allow many wireless modems to operate where the control room computer dials each RTU to to apply for and maintain an operating simultaneously without interference. One establish a temporary point-to-point session. Leased Line Telephone Line Application license and their low data throughput of method is called direct sequence and the typically 9600 bits per second or less. In other frequency hopping. It is beyond certain locations, the unlicensed bands are the scope of this paper to discuss the two completely fi lled and no new licenses are types of spread spectrum except for a few available. important characteristics. Unlicensed wireless modems have become Frequency hopping spread spectrum is very widely used because they do not generally considered to be the most robust require an operating license and have much with higher immunity to interfering noise (Continued on page 4) Wireless Communications Comparison Diagram 2. A point to multi-point leased line or dedicated copper system where each RTU is polled by the master computer. Communications are continuous in this system. The diagram also shows how a wireless Ethernet link may be used to access the system. Table 2 The principle characteristics of each wireless communication method 2 3 Field Proven Performance ~ Industrial Grade Support • 3535 Factoria Blvd SE , Suite 100 • Bellevue, WA 98006 USA • Phone (425) 882-2206 • FAX (425) 867-0865 • www.data-linc.com .
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