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The PDU is passed to layer N−1, where it is known as the service data unit (SDU). At layer N−1 the SDU is concatenated with a header, a footer, or both, producing a layer N−1 PDU. It is then passed to layer N−2. The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device.
This article lists protocols, categorized by the nearest layer in the Open Systems Interconnection model.This list is not exclusive to only the OSI protocol family.Many of these protocols are originally based on the Internet Protocol Suite (TCP/IP) and other models and they often do not fit neatly into OSI layers.
OSI model Layer Protocol data unit (PDU) Function [3] Host layers 7 Application: Data: High-level protocols such as for resource sharing or remote file access, e.g. HTTP. 6 Presentation: Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption: 5 Session
Protocol data units of the OSI model are: [1] The Layer 4: transport layer PDU is the segment or the datagram. The Layer 3: network layer PDU is the packet. The Layer 2: data link layer PDU is the frame. The Layer 1: physical layer PDU is the bit or, more generally, symbol.
There is also a form of CMIS that is developed to operate directly on top of the LLC sublayer. It is called the LAN/MAN Management Protocol (LMMP), formerly it was the Common Management Information Services and Protocol over IEEE 802 Logical Link Control (CMOL). This protocol does away with the need for the OSI stack as is the case with CMIP.
X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976. [1] [2]
The OSI model defines the application layer as only the interface responsible for communicating with host-based and user-facing applications. [10] OSI then explicitly distinguishes the functionality of two additional layers, the session layer and presentation layer , as separate levels below the application layer and above the transport layer.
OSIsoft, LLC is a manufacturer of application software for real-time data management, called the PI System. Founded in 1980, OSIsoft was privately held and headquartered in San Leandro, California .