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The model divides the functions of a protocol into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behaviour consisting of a series of these layers is known as a 'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or a mixture of both. Typically, only the lower layers are implemented in hardware, with the higher layers being implemented in software.

This OSI model is roughly adhered to in the computing and networking industry. Its main feature is in the junction between layers which dictates the specifications on how one layer interacts with another. This means that a layer written by one manufacturer can operate with a layer from another (assuming that the specification is interpreted correctly.) These specifications are typically known as Request for Comments or "RFC"s in the TCP/IP community. They are ISO standards in the OSI community.

Usually, the implementation of a protocol is layered in a similar way to the protocol design, with the possible exception of a 'fast path' where the most common transaction allowed by the system may be implemented as a single component encompassing aspects of several layers.

This logical separation of layers makes reasoning about the behaviour of protocol stacks much easier, allowing the design of elaborate but highly reliable protocol stacks. Each layer performs services for the next higher layer, and makes requests of the next lower layer. An implementation of several OSI layers is often referred to as a stack (as in TCP/IP stack).

1 Description of Layers

• Physical layer Layer 1. : The physical layer defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. The major functions and services performed by the physical layer are:
  • establishment and termination of a connection to a communications medium.
  • participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
  • modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. This is signals operating over the physical cabling -- copper and fibre optic, for example. SCSI operates at this level.

• Data link layerThe data link layer is level two of the seven-level OSI model. It responds to service requests from the network layer and issues service requests to the physical layer. The data link layer is the layer of the model which ensures that data is transferred c Layer 2. : The Data link layer provides the functional and procedural means to transferDid you mean? decal Population transfer Manhattan Transfer Latin words in English Transfer (movie Electron transfer Fare transfer A technique in propaganda. data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is EthernetEthernet is a packet-based computer networking technology for local area networks (LANs). It defines wiring and signaling for the physical layer, and packet formats and protocols for the media access control (MAC)/ data link layer of the OSI model. Ethern. Other examples of data link protocols are HDLCHigh-Level Data Link Control (HDLC is a bit-oriented synchronous data link Layer 2 protocol developed by the International Organization for Standarization ( ISO). HDLC was imported by the ITU into the X. 25 protocol stack. It was modified by IBM to become and ADCCP for point-to-point or packetA packet (alternatively, datagram the terms are basically synonymous, although in some contexts the two mean subtly different things) is the fundamental unit of information carriage in all modern computer networks. A packet consists of a header which cont-switched networks and LLC and AlohaThe Aloha Protocol is a layer 2 (Layer 2 is the Data Link layer of the OSI model) protocol for LAN networks with broadcast topology. It was used for the first time in the Packet Radio System of the University of Hawaii in 1970. It is a predecessor to the for local area networks. This is the layer at which bridges and switches operate. Connectivity is provided only among locally attached network nodes.

• Network layer Layer 3. : The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions. The router operates at this layer -- sending data throughout the extended network and making the Internet possible, although there are layer 3 (or IP) switches. This is a logical addressing scheme - values are chosen by the network engineer. The addressing scheme is hierarchical.

• Transport layer Layer 4. : The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer. The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the session layer can keep track of the packets and retransmit those that fail.

• Session layer Layer 5. : The Session layer provides the mechanism for managing the dialogue between end- user application processes. It provides for either duplex or half-duplex operation and establishes checkpointing, adjournment, termination, and restart procedures. This layer is responsible for setting up and tearing down TCP/IP sessions.

• Presentation layer Layer 6. : The Presentation layer relieves the Application layer of concern regarding syntactical differences in data representation within the end- user systems. MIME encoding, encryption and similar manipulation of the presentation of data is done at this layer. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

• Application layer Layer 7, the highest layer. : This layer interfaces directly to and performs common application services for the application processes. The common application services provide semantic conversion between associated application processes. Examples of common application services include the virtual file , virtual terminal, and job transfer and manipulation protocols.

The mnemonics "People Design Networks To Send Packets Accurately", "Please Do Not Throw Sausage Pizza Away", and "All People Seem To Need Data Processing" may help you remember the layers.