Industrial Ethernet vs PROFINET Architecture, RT Classes, Routing

Overview: Industrial Ethernet vs PROFINET

Industrial Ethernet (IE) is a marketing and technical designation for a class of IEEE 802.3-based networks used in automation. It is not a single protocol. The IE family includes PROFINET, EtherNet/IP, EtherCAT, POWERLINK, SERCOS III, CC-Link IE, and Modbus TCP, each with its own framing, scheduling, and conformance rules. PROFINET is one member of that family, defined by PROFIBUS & PROFINET International (PI) and originally developed in the late 1990s as the Ethernet-based successor to PROFIBUS DP.

The relationship is hierarchical:

• Industrial Ethernet = the physical/datalink category (IEEE 802.3 + automation-specific extensions)
• PROFINET = a specific application-layer protocol stack that runs on top of IE hardware

Confusion arises because PROFINET is the most widely deployed IE protocol in European machinery, and Siemens documentation often uses "Industrial Ethernet" and "PROFINET" interchangeably when referring to a specific S7-1200/S7-1500 port. The port itself is a 100 Mbit/s or 1 Gbit/s Ethernet port; whether traffic on it is PROFINET, S7 communication, TCP/IP, or web diagnostics depends on the configured services.

ISO/OSI Layer Model Mapping

The distinction between IE and PROFINET is clearest when mapped to the OSI layers. Standard office Ethernet uses Layers 1 through 7 with full IP routing (Layer 3) enabled. PROFINET deliberately bypasses IP for its cyclic real-time channel:

The key architectural decision: PROFINET cyclic I/O uses Ethernet Type 0x8892 frames sent directly on Layer 2 with no IP header. This is why a PROFINET controller and device can exchange process data before any IP address is assigned, and why PROFINET RT frames cannot traverse a standard IP router.

The Industrial Ethernet Protocol Family

All of the following are real-time industrial protocols that share the IEEE 802.3 physical and data-link layers but diverge above Layer 2:

Even though all are "Industrial Ethernet," they are not interoperable. A PROFINET controller cannot talk to an EtherNet/IP scanner. The shared substrate stops at the RJ45 jack, the magnetics, and the 802.3 MAC. This is the practical reason for the original question: calling PROFINET "just Industrial Ethernet" is technically incomplete.

PROFINET Architecture and Conformance Classes

PROFINET defines three conformance classes (CC-A, CC-B, CC-C) that determine the features a device must support. These classes are independent of the IE physical layer:

For CC-B and CC-C, PROFINET requires managed Ethernet switches with IEEE 802.1AB LLDP for neighbor detection. PROFINET uses LLDP to auto-discover the physical topology, which is why an unmanaged consumer switch will not allow the controller to fully commission a PROFINET network.

PROFINET Real-Time Communication Classes

PROFINET supports three traffic categories, each with its own QoS treatment:

1. NRT (Non-Real-Time) - Standard TCP/IP/UDP traffic, used for acyclic services, web interface, SNMP, and PROFINET diagnostics. Uses IP and is routable. Frame priority 0.
2. RT (Real-Time) - Cyclic process data exchanged between IO controller and IO devices. Uses Ethernet Type 0x8892, no IP, VLAN priority 6. Cycle times 250 µs to 10 ms are typical; jitter of ≤ 1 ms is achievable on a properly engineered CC-B network.
3. IRT (Isochronous Real-Time) - Reserved bandwidth with hardware-scheduled timeslots. Cycle times down to 31.25 µs and jitter under 1 µs. VLAN priority 7. Requires CC-C and time-aware switches implementing IEEE 1588 precision time protocol synchronization.

The two RT classes are the most important field-level distinction. RT is "fast enough for I/O," IRT is "fast enough for synchronized motion." Mixing RT and IRT on the same cable is allowed; the controller schedules IRT inside a reserved phase and RT in the open phase of each cycle.

Why PROFINET Omits the IP Layer for Cyclic Traffic

Layer 3 routing is intentionally excluded from PROFINET RT/IRT for three engineering reasons:

1. Predictable latency - An IP router introduces store-and-forward delay (typically 50 – 500 µs per hop) and best-effort queuing. PROFINET RT relies on cut-through switching and a fixed EtherType so the frame can be forwarded in well under 10 µs per hop.
2. No address assignment at startup - PROFINET devices receive their IO device name at controller startup via LLDP and the PROFINET Discovery and Configuration Protocol (DCP). They do not need an IP address to begin cyclic I/O. IP is configured only for acyclic services (web server, parameter assignment via TCP).
3. Determinism isolation from IT traffic - Because the RT stream does not traverse IP, IT services on the same physical cable cannot cause retransmission storms or routing loops in the RT path. Engineering separation is enforced at Layer 2.

PROFINET Frame Structure (RT Frame Example)

A simplified PROFINET RT frame on the wire looks like this (no IP header):

| Dest MAC (6) | Src MAC (6) | VLAN Tag (4, PCP=6) | EtherType 0x8892 (2) | PROFINET RT Payload (36-1440) | FCS (4) |

Key fields in the PROFINET RT payload:

• FrameID - 16-bit identifier assigned by the controller; the IO device filters on this value in hardware.
• CycleCounter - 16-bit incrementing counter; a missed counter indicates a frame loss.
• DataStatus - 1 byte: provider/consumer state, run/stop, validity.
• TransferStatus - 1 byte: bit 0 indicates primary/backup AR.
• I/O data - 1 to 1440 bytes of process image, mapped to slot/subslot configuration.

The corresponding acyclic record-data read uses PROFINET RPC over UDP port 34964 with IP addressing. Acyclic reads are used for parameter assignment, diagnostics, and firmware updates. This is the only context in which a typical PROFINET device cares about its IP configuration.

Network Topology, Device Roles, and Commissioning

A PROFINET network has three device roles:

• IO Controller - The PLC, typically an S7-1500, S7-1200, ET 200SP CPU, or third-party controller. Owns the AR (Application Relationship).
• IO Device - A field device with a PROFINET port, assigned a name and slot configuration by the controller.
• IO Supervisor - A diagnostics station, usually running TIA Portal, that connects via acyclic services to read diagnostic buffers.

Commissioning flow:

1. Assign each IO device a unique PROFINET name (e.g., "valve-1") using the controller's online tools, PRONETA, or a Siemens SIPLUS CMS scanner.
2. Configure the device name in the hardware configuration (GSD file / HSP).
3. Plug the device into the network. The controller sends DCP Identify-Request frames; the device responds with its assigned name and MAC.
4. The controller establishes the AR, performs module/submodule parameterization, and begins cyclic RT exchange.
5. Optional: a supervisor (PG/PC with TIA Portal) connects via acyclic services for live diagnostics.

Comparison: When to Use Which "Industrial Ethernet" Protocol

Selection depends on the existing controller base, the OEM relationship, and the determinism budget:

Coexistence with IT Networks (Vertical Integration)

Most plants run PROFINET on the field level and a separate IP network for SCADA, MES, and ERP. Two integration patterns are common:

1. Shared physical layer with VLAN separation - A single managed switch stack with VLAN 10 = PROFINET (priority tagging for RT), VLAN 20 = IT (TCP/IP). The PROFINET and IT segments are logically isolated at Layer 2. Tools like Siemens SCALANCE XC-200 and XC-300 support PROFINET priority tagging out of the box.
2. Dual physical layer - A dedicated PROFINET ring (typically fiber with MRP redundancy) and a separate IT network. Connections between them are limited to acyclic gateways (e.g., OPC UA server on the PLC, MQTT broker, or NAMUR Open Architecture NOA bridge).

For OPC UA integration, modern Siemens CPUs (S7-1500 firmware V2.5 and later) expose an OPC UA server directly on the PROFINET port without disrupting RT traffic. The OPC UA traffic is NRT and shares bandwidth with PROFINET's acyclic channel. Refer to the SIMATIC S7-1500 OPC UA manual for tuning the acyclic bandwidth budget.

Troubleshooting Common IE / PROFINET Confusion

For an in-depth reference on PROFINET frame timing and the role of LLDP, the official PI document PROFINET Installation Guideline (order number 8.061) is the primary source. Engineers deploying Siemens controllers should also keep the SIMATIC PROFINET Function Manual at hand; it is updated with each TIA Portal release.