1.1 The OSI Model

Why the OSI Model Exists

Before the OSI model, every vendor built proprietary networking stacks. An IBM mainframe could not talk to a DEC minicomputer. A Siemens PLC could not share a cable with a Honeywell controller. Each vendor owned the full stack from the physical wire to the application.

The International Organization for Standardization (ISO) published the Open Systems Interconnection model (OSI model) in 1984 as ISO/IEC 7498-1 to solve this. The model divides network communication into seven layers. Each layer has one responsibility. Each layer communicates only with the layers directly above and below it. Any vendor implementing a layer correctly can interoperate with any other vendor’s implementation of the adjacent layers.

No real protocol implements OSI exactly. TCP/IP collapses layers 5, 6, and 7 into one. PROFINET RT skips layers 3 and 4 entirely. The model’s value is as a shared language for describing and troubleshooting network behavior. To use that language, you need to know what each layer does.

The Seven Layers

Layer	Name	PDU	Key Protocols	Device
7	Application	Data	HTTP, SNMP, Modbus TCP, OPC UA	—
6	Presentation	Data	TLS, encoding	—
5	Session	Data	RPC, NetBIOS	—
4	Transport	Segment / Datagram	TCP, UDP	—
3	Network	Packet	IPv4, IPv6, ICMP	Router
2	Data Link	Frame	Ethernet, 802.1Q	Switch
1	Physical	Bit	—	NIC, cable, SFP

Key terms:

• PDU (Protocol Data Unit) — the name for the unit of data at each layer: bit (L1), frame (L2), packet (L3), segment or datagram (L4), data (L5 to L7)
• Encapsulation — each layer wraps the data from the layer above with its own header before passing it down
• Decapsulation — each layer strips its header when receiving data from the layer below

Knowing the layers is only half the picture. The other half is understanding how data actually moves through them.

How Data Flows Through the Layers

Data flows down the stack on the sender and up the stack on the receiver. Each layer adds a header when sending and strips it when receiving.

This encapsulation model explains a key design decision in industrial protocols: why some of them bypass the upper layers entirely.

Why Industrial Protocols Bypass the Upper Layers

PROFINET RT, MRP, and IEC 61850 GOOSE all operate at Layer 2. They use EtherType values directly and skip IP and TCP entirely.

The reason is latency. TCP retransmission introduces variable and unbounded delay. A retransmitted segment might arrive 200 ms late. For a PLC with a 4 ms cycle time, that is catastrophic. By staying at Layer 2, these protocols eliminate the retransmission mechanism and deliver frames with deterministic, sub-millisecond latency.

The trade-off is that Layer 2 frames cannot be routed. PROFINET RT traffic cannot cross a router. It stays within a single VLAN or Ethernet segment. This constraint shapes every industrial network design decision covered in the rest of this book.

The OSI model is most useful not as a design guide but as a diagnostic tool.

OSI as a Troubleshooting Framework

When a device is unreachable, start at Layer 1 and work up. Each layer depends on the one below it. A Layer 3 routing problem is impossible if the Layer 1 link is down.

The flowchart above is the mental model. The script below automates it. Run it against any OT device to get a Layer 1 to 4 status report in seconds, without logging into each switch manually.

import subprocess, socket

def check_ping(host: str) -> tuple[bool, str]:
    r = subprocess.run(["ping", "-c", "2", "-W", "1", host],
                       capture_output=True, text=True)
    if r.returncode == 0:
        for line in r.stdout.splitlines():
            if "rtt" in line or "round-trip" in line:
                return True, line.strip()
    return False, "no response"

def check_port(host: str, port: int) -> tuple[bool, str]:
    try:
        with socket.create_connection((host, port), timeout=2):
            return True, f"port {port} open"
    except OSError as e:
        return False, str(e)

devices = [
    ("192.168.10.1",  [22, 80]),
    ("192.168.10.11", [502]),
    ("192.168.10.12", [502, 44818]),
]

for host, ports in devices:
    ping_ok, ping_detail = check_ping(host)
    print(f"{host:20s}  {'L3 OK' if ping_ok else 'L3 FAIL'}  {ping_detail}")
    for port in ports:
        ok, detail = check_port(host, port)
        print(f"  {'L4 OK' if ok else 'L4 FAIL'}  {detail}")

A device that fails at L3 but has a working link has an IP configuration or routing problem. A device that passes L3 but fails at L4 on port 502 has a Modbus service that is not running or a firewall blocking it.

Key Takeaways

OSI is a troubleshooting language

Use it to communicate precisely. “The problem is at Layer 2” means the link is up but frames are not forwarding correctly. Start at Layer 1 and work up.

PROFINET RT and MRP live at Layer 2

They bypass IP for determinism. They cannot cross a router. Keep all devices in the same VLAN.

What Comes Next

The OSI model gives you the vocabulary. The next section covers Ethernet — the Layer 1 and 2 technology that every industrial network runs on. Understanding the Ethernet frame format and MAC addressing is the foundation for everything that follows, including VLANs, MRP, and PROFINET.

References

• ISO/IEC 7498-1:1994 — Information technology — Open Systems Interconnection — Basic Reference Model
• Tanenbaum, A. S., and Wetherall, D. J. (2011). Computer Networks (5th ed.). Pearson.
• RFC 1122 — Requirements for Internet Hosts — Communication Layers (IETF, 1989)