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Paper presented by Vasant Dave at 31st Convention of Indian Water Works Association (February, 1999)

The Third Eye of the Water Supply Manager --
Radio Telemetry

1.0 Communication related issues

1.2 A water supply system consists of pumps, electric motors, switchgear, valves, and instrumentation. They require close co-ordination to yield maximum benefits. They are controlled by monitoring rainfall, water-levels, pH, turbidity, pipeline pressure and flow. Since sites are normally inaccessible, you require sophisticated automation, which is possible by computers. But they are not effective in telemetry applications unless supported by reliable communication. The issue before the Water Supply Manager is to select an appropriate communication system.

2.0 What is Telemetry ?

2.2 For longer distances, we convert DC voltage or current to audio tones and send them over wire. This is called modulation, and the reverse (i.e. converting the varying signal to data) is called demodulation. A device to perform it is called a modem.

2.3 An analog signal is a continuously varying wave. If we measure its height at specific points in time, we obtain a series of voltages with numeric values. These values can be represented in binary form and transmitted as a series of bits. A bit is a binary digit, either 0 or 1, whose combination in form of a code represents information in digital communication¹.

3.0 Types of Communication System

Other selection criteria are the initial, recurring and maintenance costs.

3.2 We do not get disturbed by dead telephones during rains any more. We also accept faulty connections and cross-talk. Water Supply Managers have often asked: "What's the use of a facility if it can't be relied upon in emergency ?" They have sought alternate medium for telemetry. Optic fibre technology with data speeds upto 400 Mega bits per second is overdesigned for 4.8 Kbps required by water supply telemetry.

3.3 Very Small Aperture Terminals (V-SATs) offer speeds upto 2 Mbps. A captive hub station costs exorbitantly high. In India, V-SAT service-providers offer shared-hub facilities at initial cost comparable to radio communication. The system consists of a central hub and several remote stations. Data from a remote terminal (e.g. at a lake) is transmitted to the satellite which reflects it to the hub. It returns the message to the satellite which then directs it to another remote terminal (e.g. at the Central Control Station). One terminal cannot communicate to another bypassing the hub. Thus a message travels four times between the earth and the satellite before delivery. That results in a time-lag which many of us might have experienced while conversing over phone with the North Eastern states.

3.4 Light signals or radio waves travel at 300,000 km per second in vacuum. Suppose a signal travels from a remote site to Central Control Station 500 km away in the city. Its propagation delay can roughly be calculated as:

500 km / 300,000 km per sec. = 0.0017 seconds (1.7 milliseconds)

When the signal goes via satellite it has to travel four times 36,000 km. The propagation delay is:

4 x 36,000 km / 300,000 km per sec. = 0.48 seconds (480 milliseconds)

Thus V-SAT takes 280 time longer than radio. The delay doubles for a reply. We see that transit time seriously affects the total response time. Since half a second is added to each transmission in either direction, the transmission time adds significantly to the total response time in telemetry application¹.

3.5 During peak hours, the satellite-transponder might be fully used by all customers of the service provider adding to delay till the line gets free. If response time and availability are not critical, V-SAT could help. Brihanmumbai Municipal Corporation (BMC) proposes to utilise V-SAT in their III Water Supply Project.

4.0 Radio Telemetry -- the pros & the cons

4.2 Radio waves travel in space on a horizontal plane. The earth's curvature confines them to 40 km. Normally the span between every other point along a water pipeline is less. Each intervening point acts as a repeater. It revives the signal for re-emission. It can also drop a signal or insert a new one. A barrier such as a hill can block the line-of-sight. A repeater on its top would help but incur cost. Sometimes radio link is impossible. A tall building near BMC's Golanji Hill Reservoir shadows it from radio rays.

4.3 Antenna is a vital part of the wireless system. Tower design considers obstacles in the line-of-sight, wind velocity during cyclones, and the kind of soil. A waterlogged site like BMC's Bhandup Intermediate Pumping Station would require pile foundation. That would incur extra cost.

4.4 Traction and high-tension lines, transformers, and heavy electric machines generate electromagnetic impulses. They 'stab' a radio signal. Radio modems that sense impulses and repeat lost signals can resolve the problem.

4.5 We can erect towers only if the Special Advisory Committee on Frequency Allocation (SACFA) approves. That was doubted for BMC's Pali Hill Reservoir, which is near Sahar and Santa Cruz Airports.

4.6 However, similar issues are unlikely to arise in all cities. Radio Telemetry offers several distinct advantages viz., it operates over a wide range of conditions, responds fast, communication is possible with those sites which cannot be connected by telephone lines, its relocation is easy, and it enables mobile communication. Wireless is considered to be most suitable and cost-effective for water supply telemetry. Tamil Nadu Water & Drainage Board selected it for Second Chennai Water Supply Project, and Bangalore Water Supply & Sewerage Board for Cauvery Water Supply Project.

5.0 How Radio Telemetry assists the Water Supply Manager

5.1 Let us assume that we have to do certain functions at sites A, B and C, as indicated above.

5.2 At Site A, a meter measures the water level. If the reading is an analogue signal, we need to digitize it. We then feed it into a modem that converts it to audio tones. The radio transmitter adds electric energy. The antenna turns it into magnetic energy and sends it in space.

5.3 The antenna at Control Station C picks it up and reconverts to electric energy. The radio receiver amplifies it. The modem interprets so that a computer can sense it. The PC compares the input with a 'desirable water level' in memory. If it varies, the PC buzzes and shows it on the monitor. Suppose a pump at Site B is to be switched on. The operator enters a command on the keyboard. That goes to the modem, which sends it to the radio. The antenna transmits it in space.

5.4 The antenna at Site B receives the signal. It travels via radio, modem and converter to a relay. That triggers a contactor, which starts the pump.

6.0 Role of VHF/UHF Radiomodem

6.2 Most radio telemetry systems use VHF or UHF. Very High Frequency ranges from 30 MHz to 300 MHz. Ultra High Frequency covers up to 3000 MHz. Permission to use spot frequencies in these bands should be obtained from WPC (Wireless Planning & Co-ordination wing of the Department of Telecom). That prevents interference from other users.

6.3 Rugged radios and modems of reputed manufacturers are readily available. Telephone-line modems are inexpensive and often used in radio circuits. However radio signals suffer from fading, noise and static interference which those modems cannot handle. Now special radiomodems have been developed. Awareness about their features might assist the Water Supply Manager to determine their value vis-a-vis cost.

6.4 Depending upon the complexities involved in a system, either 'dumb' or 'intelligent' modems can be selected. Dumb ones only modulate and demodulate. Intelligent ones perform several more functions. That is possible by using of Application Specific Integrated Circuits in their design. ASICs possess memory and can be programmed by the user.

6.5 We usually send data to radio modem by RS-232C interface. Some modems also have a second connector. It enables six bits of digital output and input and eases alarm and control functions. The circuits can fully tie in with the network. The output bits operate controls in SCADA (System Control And Data Acquisition). The input bits read status and cause a relative change at a similar unit elsewhere in the network. The user can select the set-up by software.

6.6 Instead of sending data as it comes, some radio modems arrange it in batches. They are packed, addressed and transmitted. The receiving radio modem checks and acknowledges (ACK) correct packets. The sender repeats any packet that is not 'ACK'ed. This trait ensures reliable data transmission amid electromagnetic noise.

6.7 Error detection and correction in data communication is often achieved by the radiomodem through Cyclic Redundancy Checking (CRC) which ensures less than 10 error bits per trillion (10/10¹²). Such accuracy virtually extends the operator's arm to closely control remote equipment.

6.8 Most Programmable Logic Controllers (PLCs) in a SCADA system operate by polling from the master station. There are transparently operating radiomodems which simply provide the means to communicate between the master and the remotes. On the other hand certain radiomodems have in-built capabilities to execute polling. They can straightaway be used to build-up a star network with Control Station at its center.

A polling network works like this:

• The Central Control Room sends a signal meaning 'Do you want to send me any data ?' to one remote station.
• The remote replies by sending a packet of data, or by a signal meaning 'No, thank you.'
• The CCS then contacts the next remote and repeats the process.
• After contacting all the remotes, the CCS starts all over again.

6.9 Carrier Sense Multiple Access (CSMA) responds faster than polling. It is a mesh network wherein one remote can directly contact any other. They all keep listening on air. Only the one that is called wakes up to respond. Suppose two remotes happen to transmit together. Then both fall silent for a random period as if saying 'pahle aap'. The one that resumes first gets through. The one which follows, gets an 'engaged' tone. So it awaits awhile before retrying.

6.10 These features raise the price of the radiomodem but often reduce the overall cost of the Radio Telemetry system.

Conclusion

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