- The OSI Model is a theoretical, seven-layered model of how networks work. The Open Systems Interconnect Model (the OSI Model) is a theoretical model of networking that organizes network functions into seven layers (physical layer, data link layer, network layer, transport layer, session layer, presentation layer and application layer) and specifies the communication interfaces between the OSI Model’s layers and between network endpoints utilizing an OSI Model-based protocol suite.
- The OSI Model is a way of thinking about how networks work.
- THEORY: The OSI Model is a theoretical model
- The OSI Model is not a technology.
- The OSI Model is not a protocol.
- The OSI Model is not a program or software.
- The OSI Model sorts network communication functions into seven layers
- Layer functions are not specified and should be transparent to other layers.
- Layer Communications (how layers communicate with each other) ARE specified in the model
- Layer behavior should be invisible other layers
The OSI Model was developed as the ISO designed their own suite of networking protocols (the ISO OSI Protocol Suite). It is used so commonly as a point of reference that you really should learn the OSI Model’s seven layers and learn how the OSI Model explains this layered approach to networking.
The International Standards Organization (ISO) developed a theoretical model of how networks should behave and how they are put together. This model is called the Open Standards Interconnect (OSI) Model. The “ISO OSI Model” was developed because it appeared that IBM was going to patent the design of their SNA networks so that no one else could use IBM’s design model for networking. The ISO OSI model is used throughout the network, internet and telecom industries today to describe various networking issues.
The OSI model is also of use in a learning or training environment where a novice can use it as a point of reference to learn how various technologies interact, where they reside, what functions they perform and how each protocol communicates with other protocols.
The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.
Examples of applications that utilize the network are:
- Instant Message software (AIM, MSN, ICQ, Yahoo)
- Microsoft Windows File Shares
- Web Browsers (Internet Explorer, Firefox, Google Chrome, Safari)
- Network games of Doom, Quake, Unreal (first-person 3D shooter video games)
- IRC (mIRC)
The presentation layer handles the conversion of data between a Standards-based or platform independent formats to a format understood by the local machine. This allows for data to be transported between devices.
The presentation layer performs the following functions:
- Communication with the application layer above.
- Translation of data conforming to cross-platform standards into formats understood by the local machine.
- Communication with the session layer below.
Examples of Presentation Layer Functions:
- Conversion of a Sun .RAS raster graphic to JPG.
- Conversion of ASCII to IBM EBCDIC
- Conversion of .PICT on a MAC to .jpg
- Conversion of .wav to .mp3
The session layer tracks connections, also called sessions. The session layer should keep track of multiple file downloads requested by a particular FTP application, or multiple telnet connections from a single terminal client, or web page retrievals from a web server.
With TCP/IP this functionality is handled by application software addressing a connection to a remote machine and using a different local port number for each connection.
The session layer performs the following functions:
- Communication with the Presentation layer above.
- Organize and manage one or more connections per application, between hosts.
- Communication with the Transport layer below.
Sessions are used to keep track of individual connections to remote servers. Your web browser is an excellent example of the use of sessions.
Your web browser (an application layer object) opens a web page. That page contains text, graphics, Macromedia Flash objects and perhaps a Java applet. The graphics, the Flash object and the Java applet are all stored as separate files on the web server. To access them, a separate download must be started. Your web browser opens a separate session to the web server to download each of the individual files. The session layer keeps track of which packets and data belong to which file and keeps track of where they go (in this case, to your web browser).
If networking software performs reliable data transfer functions, then the detection of errors, and retransmission of data to recover those errors or lost data will occur in software managing this layer. The transport layer may use a variety of techniques such as a Cyclic Redundancy Check, windowing and acknowledgements. If the data is lost or damaged, it is the transport layer’s responsibility to recover from that error.
- Communicate with the Session layer above.
- Reassemble transport Protocol Data Units into data streams
- Reliable protocols operating at this layer will
- Detect errors and lost data
- Recover lost data
- Manage retransmission of data.
- Segmentation of data streams into transport Protocol Data Units.
- Communicate with the Network layer below.
Examples of transport layer protocols include:
- Transmission Control Protocol (Reliable)
- User Datagram Protocol (Unreliable)
The network layer’s job is to figure out the network topology, handle routing and to prepare data for transmission. The network layer is concerned with the following primary functions:
- Communication with the Transport layer above.
- Encapsulation of Transport data into Network layer Protocol Data Units.
- Management of connectivity and routing between hosts or networks.
- Communication with the data link layer below.
Examples of network layer protocols include:
- Internet Protocol
- Internet Control Message Protocol (ICMP or “ping”)
- Internet Gateway Management Protocol (IGMP)
The data link Layer is the second layer of the OSI model. The data link layer performs various functions depending upon the hardware protocol used, but has four primary functions:
- COMMUNICATION WITH NETWORK LAYER
- SEGMENTATION & REASSEMBLY
- BIT ORDERING
- COMMUNICATION WITH PHYSICAL LAYER
- COMMUNICATION with the Network layer above.
- SEGMENTATION of upper layer datagrams (also called packets) into frames in sizes that can be handled by the communications hardware.
- BIT ORDERING. The data link layer organizes the pattern of data bits into frames before transmission. The frame formatting issues such as stop and start bits, bit order, parity and other functions are handled here. Management of big-endian / little-endian issues are also managed at this layer.
- COMMUNICATION with the Physical layer below
This layer provides reliable transit of data across a physical link. The data link layer is concerned with a physical addressing, network topology, physical link management, error notification, ordered delivery of frames, and flow control.
It should be noted that in most modern network interface adaptors, the Physical and Datalink functions are performed by the network interface adaptor.
The OSI Model’s layer 1, the Physical Layer, outlines the characteristics of the physical medium between networked devices, cabling, wiring, fiber strands and the air. Transmission and receipt of data from the physical medium (copper wire, fiber, radio frequencies, barbed wire, string etc.) is managed at this layer.
The Physical Layer receives data from the data link Layer, and transmits it to the wire. The physical layer controls the electrical and mechanical functions related to the transmission and receipt of a communications signal. It also manages the encoding and decoding of data contained within the modulated signal.
Note that for two devices to communicate, they must be connected to the same type of physical medium (wiring). 802.3 Ethernet to 802.3 Ethernet, FDDI to FDDI, serial to serial etc. Two end stations using different protocols can only communicate through a multi-protocol bridge or a router.
The physical layer is responsible for:
- Communication with the data link layer above it.
- Fragmentation of data into frames
- Reassembly of frames into data link Protocol Data Units.
- Transmission to the physical media
- Receiving from the physical media
It should be noted that in most modern network interface adaptors, the physical and data link functions are performed by the adaptor.