Polling vs report-by-exception in SCADA systems - Hands-On Comparison

Polling vs Report-by-Exception in SCADA Systems

📖 Scenario: You are working with a SCADA (Supervisory Control and Data Acquisition) system that monitors temperature sensors in a factory. The system can collect data in two ways: polling (asking sensors for data regularly) or report-by-exception (sensors send data only when temperature changes significantly).

🎯 Goal: Build a simple program that simulates both polling and report-by-exception methods to collect temperature data from sensors. You will create data, configure thresholds, apply the main logic for each method, and display the collected data.

📋 What You'll Learn

Create a dictionary called sensors with sensor IDs as keys and their current temperatures as values.

Create a variable called threshold to set the temperature change limit for report-by-exception.

Write a function called polling_collect that returns all sensor data.

Write a function called report_by_exception_collect that returns only sensors with temperature changes greater than threshold compared to previous readings.

Print the results of both collection methods.

💡 Why This Matters

🌍 Real World

SCADA systems monitor industrial equipment and processes. Efficient data collection helps reduce network load and speeds up response to important changes.

💼 Career

Understanding polling and report-by-exception is key for roles in industrial automation, system monitoring, and network optimization.

1

Create initial sensor data

Create a dictionary called sensors with these exact entries: 'sensor1': 22.5, 'sensor2': 23.0, 'sensor3': 21.8.

SCADA systems

# Create the sensors dictionary with exact temperature values
# Your code here

Hint

Use curly braces to create a dictionary with keys as sensor IDs and values as temperatures.

2

Set temperature change threshold

Create a variable called threshold and set it to 0.5 to represent the minimum temperature change for report-by-exception.

SCADA systems

sensors = {'sensor1': 22.5, 'sensor2': 23.0, 'sensor3': 21.8}
# Set the threshold variable to 0.5
# Your code here

Hint

Just assign the number 0.5 to the variable named threshold.

3

Implement polling and report-by-exception logic

Write two functions: polling_collect that returns the full sensors dictionary, and report_by_exception_collect that takes previous_readings dictionary and returns a dictionary of sensors where the absolute temperature difference is greater than threshold.

SCADA systems

sensors = {'sensor1': 22.5, 'sensor2': 23.0, 'sensor3': 21.8}
threshold = 0.5

# Define polling_collect function
# Define report_by_exception_collect function
# Your code here

Hint

Use a for loop to compare current and previous temperatures. Return all sensors for polling.

4

Display collected data

Create a variable previous_readings with {'sensor1': 22.0, 'sensor2': 23.0, 'sensor3': 21.0}. Then print the results of polling_collect() and report_by_exception_collect(previous_readings).

SCADA systems

sensors = {'sensor1': 22.5, 'sensor2': 23.0, 'sensor3': 21.8}
threshold = 0.5

def polling_collect():
    return sensors

def report_by_exception_collect(previous_readings):
    changed = {}
    for sensor, temp in sensors.items():
        if abs(temp - previous_readings.get(sensor, temp)) > threshold:
            changed[sensor] = temp
    return changed

# Create previous_readings dictionary
# Print polling_collect() result
# Print report_by_exception_collect(previous_readings) result
# Your code here

Hint

Use print statements to show the full sensor data and the sensors with significant changes.

Practice

1. What is the main difference between polling and report-by-exception in SCADA systems?

easy

A. Polling sends data only on changes; report-by-exception requests data regularly.

B. Polling and report-by-exception both send data at fixed intervals.

C. Polling requests data regularly; report-by-exception sends data only on changes.

D. Polling uses less network bandwidth than report-by-exception.

Solution

1. Step 1: Understand polling behavior

   Polling means the system asks for data at fixed times, regardless of changes.

2. Step 2: Understand report-by-exception behavior

   Report-by-exception sends data only when there is a change or event to report.

3. Final Answer:

   Polling requests data regularly; report-by-exception sends data only on changes. -> Option C

4. Quick Check:

   Polling = regular requests, Report-by-exception = event-driven [OK]

Hint: Polling = ask often; report-by-exception = send only on change [OK]

Common Mistakes:

• Confusing which method sends data regularly
• Thinking both methods behave the same
• Assuming report-by-exception always uses more bandwidth

2. Which of the following is the correct syntax to configure polling interval in a SCADA system configuration file?

easy

A. polling-interval = 5 seconds

B. polling_interval = '5s' # Poll every 5 seconds

C. pollingInterval: 5sec

D. polling interval = 5s

Solution

1. Step 1: Identify correct syntax format

   Configuration files often use key = 'value' format with quotes for time strings.

2. Step 2: Check each option for syntax correctness

   polling_interval = '5s' # Poll every 5 seconds uses correct key, equals sign, and quotes with valid time unit '5s'. Others have syntax errors or invalid formats.

3. Final Answer:

   polling_interval = '5s' # Poll every 5 seconds -> Option B

4. Quick Check:

   Correct config syntax uses key = 'value' [OK]

Hint: Config keys use underscores and quotes for values [OK]

Common Mistakes:

• Using spaces instead of underscores in keys
• Missing quotes around time values
• Using colons instead of equals sign

3. Given this SCADA polling setup code snippet:

polling_interval = 10  # seconds
last_value = 100
current_value = 100
if current_value != last_value:
    send_report()
else:
    wait(polling_interval)

What will happen when this code runs?

medium

A. The code causes an error due to missing function definitions.

B. send_report() is never called because values are equal.

C. send_report() is called every 10 seconds regardless of value changes.

D. send_report() is called only if current_value changes from last_value.

Solution

1. Step 1: Analyze the condition

   The code compares current_value and last_value; if different, it calls send_report().

2. Step 2: Check values given

   Both values are 100, so condition is false; else branch waits for polling_interval.

3. Final Answer:

   send_report() is called only if current_value changes from last_value. -> Option D

4. Quick Check:

   Report sent only on value change [OK]

Hint: send_report() triggers only on value difference [OK]

Common Mistakes:

• Assuming send_report() runs every time
• Ignoring the if condition logic
• Thinking wait() causes an error

4. You have a SCADA system using report-by-exception, but it never sends updates even when values change. What is the most likely cause?

medium

A. Exception reporting is disabled or misconfigured.

B. The system is polling too frequently.

C. Network bandwidth is too high.

D. Polling interval is set too low.

Solution

1. Step 1: Understand report-by-exception behavior

   It sends data only when exceptions or changes occur and reporting is enabled.

2. Step 2: Identify cause of no updates

   If no updates are sent despite changes, reporting is likely disabled or misconfigured.

3. Final Answer:

   Exception reporting is disabled or misconfigured. -> Option A

4. Quick Check:

   Disabled reporting stops updates [OK]

Hint: Check if exception reporting is enabled first [OK]

Common Mistakes:

• Confusing polling interval with report-by-exception
• Assuming high bandwidth stops updates
• Thinking frequent polling affects exception reporting

5. A SCADA system monitors temperature sensors. To reduce network load but ensure critical alerts are sent immediately, which approach is best?

hard

A. Use report-by-exception for normal data and polling for critical alerts.

B. Use report-by-exception only, ignoring polling.

C. Use polling only with a very short interval.

D. Use polling with a long interval and no exception reporting.

Solution

1. Step 1: Analyze network load and alert needs

   Polling often increases network load; report-by-exception reduces it by sending only changes.

2. Step 2: Combine methods for best result

   Using report-by-exception for normal data saves bandwidth, while polling ensures critical alerts are checked regularly.

3. Final Answer:

   Use report-by-exception for normal data and polling for critical alerts. -> Option A

4. Quick Check:

   Combine polling and exception reporting for efficiency [OK]

Hint: Mix polling and exception reporting for best balance [OK]

Common Mistakes:

• Using polling only causes high network load
• Ignoring critical alert needs
• Relying solely on report-by-exception may miss urgent data