Polling vs report-by-exception in SCADA systems - Quick Revision & Key Differences

Recall & Review

beginner

What is polling in SCADA systems?

Polling is a method where the central system regularly asks each device for its status or data at fixed intervals.

beginner

What does report-by-exception mean?

Report-by-exception means devices only send data or alerts when something changes or an event happens, instead of sending data all the time.

intermediate

Which method reduces network traffic: polling or report-by-exception?

Report-by-exception reduces network traffic because data is sent only when needed, not continuously.

intermediate

What is a downside of polling?

Polling can cause unnecessary network load and delays because it asks for data even if nothing has changed.

intermediate

Why might report-by-exception be better for real-time alerts?

Because devices send updates immediately when something changes, report-by-exception allows faster response to important events.

In polling, how does the central system get data?

A By regularly asking devices for data

B Devices send data only when something changes

C Data is sent randomly

D Data is never sent

Polling means the central system asks devices for data at set times.

Which method sends less data over the network?

A Polling

B Report-by-exception

C Both send the same amount

D Neither sends data

Report-by-exception sends data only when needed, reducing network traffic.

What is a disadvantage of report-by-exception?

A It can miss data if no events happen

B It always sends data even if unchanged

C It causes high network load

D It requires constant polling

If no events happen, report-by-exception may not provide regular updates.

Polling is best described as:

A Devices push data when needed

B Data is sent only on errors

C No data exchange

D Central system requests data regularly

Polling means the central system asks for data at intervals.

Which method allows faster reaction to changes?

A Neither

B Polling

C Report-by-exception

D Both are equally fast

Report-by-exception sends updates immediately when changes occur.

Explain the difference between polling and report-by-exception in SCADA systems.

Think about who initiates data sending and when.

What are the advantages and disadvantages of polling compared to report-by-exception?

Consider network load and data timeliness.

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