Purdue Model

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Overview

The Purdue model is derived from the Purdue Enterprise Reference Architecture (PERA) model, which is a widely used conceptual model that describes the internal connections and dependencies of important components in ICS networks. The Purdue model is also known as the Industrial Automation and Control System reference model.

The Purdue model consists of three zones: the manufacturing zone (OT) and enterprise zone (IT), separated by a demilitarized zone (DMZ), which is used to restrict direct communication between the OT and IT systems. The intention behind adding this extra layer is to confine the network or system compromises within this layer and provide uninterrupted production. The three zones are further divided into several operational levels.

Purdue Model-1759409371112.webp

Zones and Levels

Enterprise Zone (IT Systems)

The enterprise security zone is a part of IT, in which supply-chain management and scheduling are performed using business systems such as SAP and ERP. It also locates the data centers, users, and cloud access. The enterprise zone consists of two levels.

Level 5 (Enterprise Network)

This is a corporate level network where business operations such as B2B (business-to-business) and B2C (business-to-customer) services are performed. Internet connectivity and management can be handled at this level. The enterprise network systems also accumulate data from all the subsystems located at the individual plants to report the inventory and overall production status.

Level 4 (Business Logistics Systems)

All the IT systems supporting the production process in the plant lie at this level. Managing schedules, planning, and other logistics of the manufacturing operations are performed here. Level 4 systems include application servers, file servers, database servers, supervising systems, email clients, etc.

Protocols Used in Level 5 and Level 4:

IDMZ == Level 3.5

Manufacturing Zone (OT Systems)

All the devices, networks, control, and monitoring systems reside in this zone. The manufacturing zone consists of four levels.

Level 3 (Operational Systems/Site Operations)

In this level, the production management, individual plant monitoring, and control functions are defined. Production workflows and output of the desired product are ensured at this level. Production management includes plant performance management systems, production scheduling, batch management, quality assurance, data historians, manufacturing execution/operation management systems (MES/MOMS), laboratories, and process optimization. Production details from lower levels are collected here and can then be transferred to higher levels or can be instructed by higher-level systems.

Protocols Used in Level 3:

Level 2 (Control Systems/Area Supervisory Controls)

Supervising, monitoring, and controlling the physical process is carried out at this level. The control systems can be DCSs, SCADA software, Human–Machine Interfaces (HMIs), real-time software, and other supervisory control systems such as engineering works and PLC line control.

Protocols Used in Level 2:

Level 1 (Basic Controls/Intelligent Devices)

Analysis and alteration of the physical process can be done at this level.

Analysis and alteration of the physical process can be done at this level. The operations in basic control include "start motors," "open valves," "move actuators," etc. Level 1 systems include analyzers, process sensors, and other instrumentation systems such as Intelligent Electronic Devices (IEDs), PLCs, RTUs, Proportional Integral Derivative (PID) controllers, Equipment Under Control (EUC), and Variable Frequency Drives (VFDs). PLCs was used in level 2 with a supervisory functionality, but it is used as a control function in level 1.

Level 0 (Physical Process)

In this level, the actual physical process is defined, and the product is manufactured. Higher levels control and monitor operations at this level; therefore, this layer is also referred to as Equipment Under Control (EUC). Level 0 systems include devices, Sensors (e.g., speed, temperature, pressure), Actuator, or other industrial equipment used to carry out the manufacturing or industrial operations. A minor error in any of the devices at this level can affect overall operations.

Protocols Used in Level 1 and Level 0:

Industrial Demilitarized Zone (IDMZ)

The demilitarized zone is a barrier between the manufacturing zone (OT systems) and enterprise zone (IT systems) that enables a secure network connection between the two systems. The zone is created to inspect overall architecture. If any errors or intrusions compromise the working systems, the IDMZ holds the error and allows production to be continued without interruption. IDMZ systems include Microsoft domain controllers, database replication servers, and proxy servers.

graph TB
    subgraph Enterprise["ENTERPRISE ZONE (IT Systems)"]
        L5["Level 5: Enterprise Network
Corporate operations, B2B/B2C services
Internet connectivity, inventory reporting"]
        L4["Level 4: Business Logistics Systems
Application servers, file servers, database servers
Production scheduling and planning"]
    end
    
    subgraph DMZ["INDUSTRIAL DMZ"]
        IDMZ["Security Barrier
Domain controllers, database replication
Proxy servers"]
    end
    
    subgraph Manufacturing["MANUFACTURING ZONE (OT Systems)"]
        L3["Level 3: Operational Systems
Production management, MES/MOMS
Data historians, quality assurance"]
        L2["Level 2: Control Systems
SCADA, DCS, HMI
Supervisory control, PLCs"]
        L1["Level 1: Basic Controls
PLCs, RTUs, IEDs
Sensors, actuators, analyzers"]
        L0["Level 0: Physical Process
Manufacturing equipment
Sensors, actuators, industrial devices"]
    end
    
    L5 --> L4
    L4 --> IDMZ
    IDMZ --> L3
    L3 --> L2
    L2 --> L1
    L1 --> L0
    
    style Enterprise fill:#e3f2fd,stroke:#1976d2,stroke-width:3px
    style DMZ fill:#fff3e0,stroke:#f57c00,stroke-width:3px
    style Manufacturing fill:#f3e5f5,stroke:#7b1fa2,stroke-width:3px
    style IDMZ fill:#ffe0b2,stroke:#e65100,stroke-width:2px
    style L5 fill:#bbdefb,stroke:#1976d2,stroke-width:2px
    style L4 fill:#bbdefb,stroke:#1976d2,stroke-width:2px
    style L3 fill:#e1bee7,stroke:#7b1fa2,stroke-width:2px
    style L2 fill:#e1bee7,stroke:#7b1fa2,stroke-width:2px
    style L1 fill:#e1bee7,stroke:#7b1fa2,stroke-width:2px
    style L0 fill:#e1bee7,stroke:#7b1fa2,stroke-width:2px

![Figure 18.82: OT technologies and protocols over the Purdue model

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