OSI Model

The Open Systems Interconnection (OSI) Model is a seven-layer conceptual framework for describing how data is communicated between devices. It is a teaching and design reference — not a product specification. Modern IACS networks usually run TCP/IP on Ethernet; OSI still gives engineers a shared vocabulary for where media, switches, routers, sessions and applications fit.

The seven layers (overview)

Layer	Name	Purpose	Examples
7	Application	Network services to software; how application data is exchanged	CIP, OPC UA, MQTT, Modbus TCP, HTTP, FTP, SMTP
6	Presentation	Formatting, encoding, compression, encryption	ASCII, Unicode, JSON, XML, SSL/TLS
5	Session	Establish, manage and tear down conversations	Session control; OPC Classic / DCOM (session-ish)
4	Transport	End-to-end delivery, ports, reliability or speed	TCP, UDP
3	Network	Logical addressing and routing between networks	IP, ICMP, routers, L3 switches
2	Data Link	Frames, MAC addressing, local delivery	Ethernet, MAC, VLANs, L2 switches
1	Physical	Bits on the wire/air — electrical, optical, mechanical	Copper, fibre, RS-232/485, IEEE 802.3 / 802.11

Layer 1 — Physical

Layer 1 defines how bits travel: media, connectors, voltages, signalling rates and wireless PHY. Standards examples include EIA/TIA RS-232 and RS-485 for serial, and IEEE 802.3 (Ethernet) / 802.11 (wireless) for LAN PHYs.

Physical layout ties to network topologies (star, bus, ring, mesh, hybrid). In OT practice, a large share of “network” troubleshooting is Layer 1: cabling, connectors, power, noise and media mismatch.

Layer 2 — Data link

The data link layer frames bits for the local segment: MAC addressing, media access control (MAC sublayer) and logical link control (LLC). Ethernet is the dominant LAN technology in modern plants.

Layer 2 switches forward frames by MAC within a LAN. VLANs create separate Layer 2 broadcast domains on shared switches. See Switches and VLANs for managed vs unmanaged switches, trunks and the “VLAN ≠ security alone” caution.

Layer 3 — Network

Layer 3 provides logical addressing and routing between networks — primarily IP, with ICMP for diagnostics and error reporting. Routers and Layer 3 switches forward packets hop by hop. Addressing, ARP, subnet masks/CIDR and default gateways are covered in TCP/IP Fundamentals.

Layer 4 — Transport

Transport makes two hosts appear connected end-to-end: TCP (reliable, connection-oriented) vs UDP (fast, connectionless), plus port numbers that identify services. Detail: TCP/IP Fundamentals.

Layers 5–7 — Session, presentation, application

Session (5) establishes, manages and ends conversations. Presentation (6) handles representation — character sets, encoding, compression and encryption (for example SSL/TLS). Application (7) is where end-user and industrial protocols live: FTP, SMTP, Telnet, HTTP on the IT side; Modbus TCP, CIP, OPC UA, MQTT and similar on the OT side.

Distinguish application software (HMI, MES) from application-layer protocols that carry their data. Many industrial stacks effectively collapse Layers 5–7 into one “application” conversation. An exception often cited in training is OPC Classic / DCOM, which behaves more like an explicit session layer above the network stack — see OPC.

Protocol gateways

A gateway in the OSI sense connects dissimilar systems — for example translating between a DCS highway and a PLC network. Full seven-layer gateways can be slower and more expensive than simple bridges or routers; use them when protocols truly differ. Related protocol pages: OPC, Modbus, CIP.

Encapsulation

Sending hosts pass data down the stack; each layer adds headers (and sometimes trailers). The receiver reverses the process (decapsulation).

Application Data
        ↓
Layer 7 - Application
        ↓
Layer 6 - Presentation
        ↓
Layer 5 - Session
        ↓
Layer 4 - TCP / UDP
        ↓
Layer 3 - IP
        ↓
Layer 2 - Ethernet
        ↓
Layer 1 - Physical Medium

Problems with the OSI model (IACS caveats)

Functional specs only — OSI describes roles, not a mandatory product implementation; real stacks are TCP/IP-centric.
Seven layers are somewhat arbitrary — useful pedagogy, not physics.
Awkward Layer 2 split — MAC vs LLC is historically messy relative to Ethernet practice.
Layers 5–7 often collapse in OT — industrial protocols rarely expose clean session/presentation/application boundaries.
Still invaluable for teaching, design reviews and structured troubleshooting (“which layer failed?”).

Why the model still matters

OSI gives vendors and engineers a common language for interoperability and for isolating faults layer by layer — even when products do not implement every layer as a separate module.

Next recommended reading

The OSI Model is the framework. Next, learn how modern networks implement it with the TCP/IP suite — addressing, ARP, routing, TCP/UDP and ports.

→ TCP/IP Fundamentals

Summary diagram