OSI protocols
From Wikipedia, the free encyclopedia
 |
The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style. |
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.
OSI protocols are used to implement various networks. The OSI protocols are split into layers for easier organization and understanding. The layers form a hierarchy starting with the application level, then the presentation level, session level, transport level, network level, and the lowest level is the physical level. Each layer receives information from the layer above, processes it and passes it down to the next layer. Each layer adds its own header to the incoming information before it is passed to the level below. Headers generally include address of destination and source, check sums (for error control), type of protocol used in the current layer, and other options such as flow control options and sequence numbers (used to ensure data is sent in order).
Not all layers are mandatory, it depends on which protocols are implemented. The Internet protocol suite is not intended to be compliant with OSI, and efforts to use it as a model for OSI protocols or the OSI model will eventually fail. They have a better chance of working when additional and lesser-known ISO documents are considered, such as the Internal Organization of the Network Layer.
Nevertheless, there have been real-world examples of pure OSI protocol stacks that do not implement all the layers. The Manufacturing Automation Protocol (MAP) user group, focused on real-time control of manufacturing robots of various types, implements layer 1 (physical), a two-sublayer layer 2 (data link) with LLC Type 3 on top of the medium access layer, and then the Layer 7 Manufacturing Message System on top. Layers 3 through 6 are not present. This high-performance stack is intended just for the robots themselves; the robot controller would load files with a full 7-layer stack with FTAM file transfer on top. The Signaling System 7 stack is an OSI derivative, where TCP/IP is not, and compresses and reorganizes the three upper layers.
Contents |
[edit] 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 transmitted. It can be twisted pair, coaxial cable, optical fiber, wireless, or other transmission media.
[edit] 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.
[edit] 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
- Intermediate System to Intermediate System (IS-IS) - later adapted for the TCP/IP model.
This level is in charge of transferring data between systems in a network, using network-layer addresses of machines to keep track of destinations and sources. This layer uses routers and switches to manage its traffic (control flow control, error check, routing etc).
[edit] Transport Layer
The connection-mode and connectionless-mode transport services are specified by ISO 8072; the protocol that provides the connection-mode service is specified by ISO 8073, and the protocol that provides the connectionless-mode service is specified by ISO 8602.
- 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
The transport layer transfers data between destination and source processes. Generally it is either connection-mode, which provides guaranteed delivery (we have to ensure data has no errors) or connectionless-mode, which provides best effort delivery (don't worry about possible errors).
[edit] Session Layer
- Session service – ISO 8306 / X.215
- Connection-oriented Session protocol – ISO 8307 / X.225
- Connectionless Session protocol – ISO 9548
[edit] Presentation Layer
- Presentation service – ISO 8822 / X.216
- Connection-oriented Presentation protocol – ISO 8823 / X.226
- Connectionless Presentation protocol – ISO 9576
This layer defines and encrypts/decrypts data types from the application layer. Protocols such as MIDI, MPEG, and GIF are presentation layer formats shared by different applications.
[edit] Application Layer
[edit] 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)
This keeps track of how each application talks to another application. Destination and source addresses are linked to specific applications.
[edit] 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 (VT) - ISO 9040/9041
- 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)
[edit] 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
