The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T. The standardization process began in 1977. While the seven-layer OSI model is often used as a reference for teaching and documentation, the protocols originally conceived for the model did not gain popularity, and only X.400, X.500, and IS-IS have achieved lasting impact. The goal of an open-standard protocol suite instead has been met by the Internet protocol suite, maintained by the Internet Engineering Task Force (IETF). Overview The OSI protocol stack is structured into seven conceptual layers. The layers form a hierarchy of functionality starting with the physical hardware components to the user interfaces at the software application level. Each layer receives information from the layer above, processes it and passes it down to the next layer. Each layer adds encapsulation information (header) to the incoming information before it is passed to the lower layer. Headers generally include address of source and destination, error control information, protocol identification and protocol parameters such as flow control options and sequence numbers. Layer 1: physical layer This layer deals with the physical plugs and sockets and electrical specification of signals only. This is the medium over which the digital signals are transmitted. It can be twisted pair, coaxial cable, optical fiber, wireless, or other transmission media. Layer 2: data link layer The data link layer packages raw bits from the physical layer into frames (logical, structured packets for data). It is specified in ITU-T Rec. X.212 [ISO/IEC 8886], ITU-T Rec. X.222 and others. This layer is responsible for transferring frames from one host to another. It might perform error checking. This layer further consists of two sublayers: MAC and LLC. Layer 3: network layer Connectionless Network Service (CLNS) – ITU-T Rec. X.213 [ISO/IEC 8348]. SCCP is based on X.213. Connectionless Network Protocol (CLNP) – ITU-T Rec. X.233 [ISO/IEC 8473-1]. Connection-Oriented Network Service (CONS) – ITU-T Rec. X.213 [ISO/IEC 8348]. Connection-Oriented Network Protocol (X.25) – ITU-T Rec. X.233 [ISO/IEC 8878]. This is the use of the X.25 protocol to provide the CONS. Network Fast Byte Protocol – ISO/IEC 14700 End System to Intermediate System Routing Exchange Protocol (ES-IS) - ISO/IEC 9452 (reprinted in RFC 995). Intermediate System to Intermediate System Intra-domain Routing Protocol (IS-IS) - ISO/IEC 10589 (reprinted in RFC 1142), later adapted for the TCP/IP model. End System Routing Information Exchange Protocol for