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ISO-TSAP Class 0
ISO Transport Service Access Point (TSAP) on TCP port 102 is primarily used in OSI protocols, notably for communication between industrial control systems and automation devices such as PLCs and SCADA systems. It facilitates the transport layer's connection-oriented communication, serving as a gateway to multiple higher-level protocols and services conforming to ISO standards..
Port 102 is assigned to ISO-TSAP (Transport Service Access Point) Class 0, which represents the standard interface point for connection-oriented transport in the OSI (Open Systems Interconnection) model. Specifically, TSAP enables Session Layer services by offering reliable transport provided by ISO Transport Protocol Class 0 over TCP. Though OSI protocols have largely been supplanted by TCP/IP standards, port 102 remains essential where OSI stacks persist — notably in secure, real-time communication environments.
One of the primary contemporary use cases for port 102 is facilitating communication via protocols such as Siemens' S7 (used for PLCs) and other industrial automation protocols. These protocols often leverage port 102 to manage command and data transfer sessions between programming devices, control servers, engineering workstations, and embedded controllers. The connection-oriented nature of Class 0 services ensures reliable data delivery which is critical in industrial control system (ICS) environments.
Despite declining general use of OSI protocols, port 102 has a niche but vital role within certain verticals, especially where long-lived, legacy OT (Operational Technology) systems are deployed. SCADA and Distributed Control Systems (DCS) often maintain compatibility with ISO-TSAP to ensure interoperability, deterministic behavior, and robust communication in tightly controlled industrial networks.