The OSI (Open Systems Interconnection) protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T from 1977 onwards. The OSI model describes seven 'layers' of interconnection: the physical layer, the data link layer, the network layer, the transport layer, the session layer, the presentation layer, and the application layer. These OSI protocols for those layers are described below.

Physical Layer

This layer deals with the physical plugs and sockets and electrical specification of signals.

This is the medium over which the digital signals are to be transmitted. This can be twisted pair, coaxial cable, optical fiber, wireless, or other media.

Data Link Layer

The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one host to another. It might perform error checking.

Network Layer

• Connectionless Network Service (CLNS)
• Connectionless Network Protocol (CLNP) – ISO 8473
• Connection-Oriented Network Service (CONS)
• Connection-Oriented Network Protocol (CONP) – ISO 8878. This is a minor variant of the previously existing X.25 protocol and is compatible with X.25 networks.
• Network Fast Byte Protocol – ISO 14700

Transport Layer

This is specified by ISO 8072 (service) and ISO 8073 (protocol).

• Transport Protocol Class 0 (TP0)
• Transport Protocol Class 1 (TP1)
• Transport Protocol Class 2 (TP2)
• Transport Protocol Class 3 (TP3)
• Transport Protocol Class 4 (TP4)
• Transport Fast Byte Protocol – ISO 14699

Session Layer

• Session service – ISO 8306 / X.215
• Connection-oriented Session protocol – ISO 8307 / X.225
• Connectionless Session protocol – ISO 9548

Presentation Layer

• Presentation service – ISO 8822 / X.216
• Connection-oriented Presentation protocol – ISO 8823 / X.226
• Connectionless Presentation protocol – ISO 9576

Application Layer

Common-Application Service Elements (CASEs)

• Association Control Service Element (ACSE)
• Remote Operations Service Element (ROSE)
• Reliable Transfer Service Element (RTSE)
• Commitment, Concurrency, and Recovery service element (CCRSE)
• Security Exchange Service Element (SESE)

Application Processes

• Common management information protocol (CMIP) – ISO 9596 / X.700
• Directory services (DS) – X.500
• File transfer, access, and management (FTAM)
• Message handling system (MHS) – X.400)
• Virtual terminal protocol (VTP)
• Remote Database Access (RDA)
• Distributed Transaction Processing (OSI TP)
• Interlibrary Loan Application Protocol (ILAP)
• Document Transfer And Manipulation (DTAM)
• Document Printing Application (DPA)
• Document Filing and Retrieval (DFR)

Routing Protocols

• Intermediate System to Intermediate System (IS-IS) – ISO 10589
• End System to Intermediate System (ES-IS) – ISO 9542
• Interdomain Routing Protocol (IDRP) – ISO 10747