OPC UA modern architecture in SCADA systems - Step-by-Step Execution

Process Flow - OPC UA modern architecture

Client Request

↓

Secure Channel Setup

↓

Session Establishment

↓

Service Request

↓

Server Processes Request

↓

Response Sent

↓

Session Close

↓

Secure Channel Close

This flow shows how a client connects securely to an OPC UA server, establishes a session, sends requests, receives responses, and then closes the session and secure channel.

Execution Sample

SCADA systems

Client -> Server: Open Secure Channel
Client -> Server: Create Session
Client -> Server: Send Read Request
Server -> Client: Send Data Response
Client -> Server: Close Session
Client -> Server: Close Secure Channel

This sequence shows the main steps of communication between an OPC UA client and server in a modern architecture.

Process Table

Step	Action	Details	Result
1	Client sends Open Secure Channel request	Client initiates secure communication	Secure Channel established
2	Client sends Create Session request	Client requests a session for communication	Session created and acknowledged
3	Client sends Read Request	Client asks for data from server	Server processes request
4	Server sends Data Response	Server returns requested data	Client receives data
5	Client sends Close Session request	Client ends the session	Session closed
6	Client sends Close Secure Channel request	Client ends secure communication	Secure Channel closed

💡 Communication ends after secure channel is closed

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
Secure Channel	Not established	Established	Established	Established	Established	Established	Closed
Session	None	None	Created	Created	Created	Closed	Closed
Data Request	None	None	None	Sent	Processed	Processed	Processed
Data Response	None	None	None	None	Received	Received	Received

Key Moments - 3 Insights

Why do we need to open a Secure Channel before creating a Session?

The Secure Channel ensures all communication is encrypted and safe. Without it, the Session cannot be securely established. See execution_table step 1 and 2.

What happens if the Session is not closed properly?

If the Session remains open, resources on the server stay allocated, which can cause issues. Step 5 shows the proper closing of the Session.

Why does the client close the Secure Channel last?

The Secure Channel protects all communication including session close messages. Closing it last ensures all messages are securely sent. See steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the Session created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Action' column for 'Create Session' in the execution_table.

According to variable_tracker, what is the state of the Secure Channel after Step 6?

A Closed

B Established

C Not established

D Created

💡 Hint

Look at the 'Secure Channel' row under 'After Step 6' in variable_tracker.

If the client sends a Read Request before opening the Secure Channel, what would happen?

A Request is processed normally

B Session is created automatically

C Communication fails due to no secure channel

D Data response is sent immediately

💡 Hint

Refer to the concept_flow where Secure Channel setup is the first step before any requests.

Concept Snapshot

OPC UA modern architecture uses a Secure Channel for encrypted communication.
A Session is created inside this channel for client-server interaction.
Clients send service requests (like Read) within the session.
Server processes requests and sends responses.
Sessions and Secure Channels must be properly closed to free resources.

Full Transcript

OPC UA modern architecture starts with the client opening a Secure Channel to ensure encrypted communication. Then, the client creates a Session to interact with the server. Within this session, the client sends service requests such as reading data. The server processes these requests and sends back responses. Finally, the client closes the Session and then the Secure Channel to end communication securely. This flow ensures safe, organized, and efficient data exchange between client and server in industrial systems.

Practice

1. What is the main purpose of the OPC UA modern architecture in industrial systems?

easy

A. To limit data access to only one device

B. To replace all hardware components with software

C. To enable secure and standardized communication between devices

D. To increase the physical size of control systems

Solution

1. Step 1: Understand OPC UA architecture goal

   OPC UA is designed to allow different industrial devices to communicate securely and in a standardized way.

2. Step 2: Evaluate options

   Options A, B, and C do not reflect the purpose of OPC UA. Only To enable secure and standardized communication between devices correctly states the main goal.

3. Final Answer:

   To enable secure and standardized communication between devices -> Option C

4. Quick Check:

   OPC UA purpose = secure, standard communication [OK]

Hint: Focus on communication and security in OPC UA [OK]

Common Mistakes:

• Thinking OPC UA replaces hardware
• Assuming OPC UA limits device access
• Confusing size increase with architecture purpose

2. Which of the following is the correct way to describe an OPC UA client-server interaction?

easy

A. Client sends data to server without request

B. Client requests data and server responds

C. Server requests data from client

D. Server and client operate independently without communication

Solution

1. Step 1: Recall OPC UA client-server model

   In OPC UA, the client initiates requests and the server responds with data or services.

2. Step 2: Analyze options

   Only Client requests data and server responds correctly describes this interaction. Options A and B reverse roles, and D denies communication.

3. Final Answer:

   Client requests data and server responds -> Option B

4. Quick Check:

   Client requests, server responds = true [OK]

Hint: Remember client always asks, server answers [OK]

Common Mistakes:

• Confusing client and server roles
• Thinking server initiates data requests
• Assuming no communication happens

3. Given an OPC UA client subscribing to a server's data node, what output should the client expect when the node's value changes?

medium

A. The client receives a real-time notification with the new value

B. The client must poll the server repeatedly to get updates

C. The server stops sending data until the client requests again

D. The client receives no updates automatically

Solution

1. Step 1: Understand OPC UA subscription model

   When a client subscribes to a data node, the server pushes updates automatically when the value changes.

2. Step 2: Evaluate options

   The client receives a real-time notification with the new value correctly describes this push notification. Options B and D describe polling or no updates, which are incorrect. The server stops sending data until the client requests again is false behavior.

3. Final Answer:

   The client receives a real-time notification with the new value -> Option A

4. Quick Check:

   Subscription = automatic updates [OK]

Hint: Subscription means server pushes updates automatically [OK]

Common Mistakes:

• Confusing subscription with polling
• Thinking client must ask repeatedly
• Assuming no automatic updates

4. You configured an OPC UA client to connect to a server but receive no data. Which is the most likely cause?

medium

A. The server is overloaded with too many clients

B. The server hardware is physically disconnected

C. The client is sending data instead of receiving

D. The client did not subscribe or request data properly

Solution

1. Step 1: Check client-server communication setup

   If the client receives no data, it often means it did not subscribe or request data correctly from the server.

2. Step 2: Analyze other options

   The server hardware is physically disconnected is possible but less common and usually causes connection failure. The client is sending data instead of receiving misunderstands roles. The server is overloaded with too many clients might slow but not block all data.

3. Final Answer:

   The client did not subscribe or request data properly -> Option D

4. Quick Check:

   No data = missing subscription/request [OK]

Hint: Check client subscription first when no data arrives [OK]

Common Mistakes:

• Ignoring subscription setup
• Assuming hardware is always disconnected
• Confusing client and server roles

5. In an OPC UA system, how can you ensure secure communication between multiple clients and servers in a large industrial network?

hard

A. Use OPC UA's built-in encryption and authentication layers

B. Disable security to improve speed

C. Allow anonymous access to all devices

D. Use only physical isolation without software security

Solution

1. Step 1: Identify OPC UA security features

   OPC UA includes built-in layers for encryption and authentication to protect data and verify users.

2. Step 2: Evaluate security options

   Options A, C, and D reduce security and risk data breaches. Only Use OPC UA's built-in encryption and authentication layers uses proper OPC UA security mechanisms.

3. Final Answer:

   Use OPC UA's built-in encryption and authentication layers -> Option A

4. Quick Check:

   Secure communication = encryption + authentication [OK]

Hint: Always enable OPC UA encryption and authentication [OK]

Common Mistakes:

• Disabling security for speed
• Allowing anonymous access
• Relying only on physical isolation