The OSI and TCP/IP Reference Models
Halzen Ashton SmithThe OSI and TCP/IP Reference ModelsThe OSI and TCP/IP Reference ModelsProtocols are required to enable network users to communicate. These protocols are structured into layered network architectures.The two most popular existing models in network communications today are:the OSI (Open System Interface) modelthe TCP/IP (Transmission Control Protocol/Internet Protocol) modelOSI REFERENCE MODELThere are seven numbered layers. The purpose off each of the seven layers in the model is to carry out part of the communication process in a well-defined way. Achieved in a hierarchical fashion; i.e. the higher layers request the services of the lower layers to carry out their functions. Data travels across the network and is passed from layer to layer until the lowest layer is reached. Actual physical communication takes place at the lowest layer and is passed up through the layers to the highest layer and from there to the user.OSI Model: LayersLayer 1: The Physical LayerLayer 2: The Datalink LayerLayer 3: The Network LayerLayer 4: The Transport LayerLayer 5: The Session LayerLayer 6: The Presentation LayerLayer 7: The Application LayerOSI Layer 1: Physical LayerThe physical layer is mainly concerned with the transmission of data over a communications link, i.e. a telephone cable or LAN cable. The function of the Physical Layer is to provide ''mechanical, electrical, functional, and procedural means to activate a physical connection for bit transmission'' (ISO/IEC7498: 1984). Basically, this means that the typical role of the physical layer is to transform bits in a computer system into electromagnetic (or equivalent) signals for a particular transmission medium (wire, fiber, ether, etc.). Repeaters operate at this level.OSI Layer 2 : The Data Link LayerThis layer attempts to make the physical layer reliable for the transmission of data. It provides synchronization and error detection control, which allows the upper layers to assume error-free transmission over the link. Data received from the Network Layer is formatted into frames and sent to the Physical Layer for transmission. Bridges and switches operate at this level and use the machine's MAC address to filter traffic.OSI Layer 3 : The Network LayerThe network layer relieves the higher layers of the routing, switching and signaling functions required to establish the necessary physical network links.It accepts data from layer 4 and selects the best route to transfer it between end systems. The computer communicates with the network at this layer to specify the destination address and to request network facilities such as priority. The IP protocol is used at this level for packet forwarding routing. Routers are used at this level. They use packet addresses to choose the optimum route across the network.OSI Layer 4 : The Transport LayerThe transport layer ensures reliable, cost effective, end to end transfer of data. Like the data link layer, the transport layer has error control functions. Whereas the data link layer was concerned with the traffic across physical links, the transport layer is concerned with traffic between the layers. This layer provides a data transfer service, which shields the upper layers from any concern with the detailed way in which data is transferred.OSI Layer 5 : The Session LayerThe Session Layer permits two parties to hold ongoing communications called a session across a network. It provides dialogue control, which is the setting up and monitoring of connections. If the lower four layers of the model are unreliable, the session layer will attempt to correct the faults and maintain the connection with the higher layers becoming aware of any problem. Thus the applications on either end of the session can exchange data or send packets to another for as long as the session lasts. The Session layer handles session setup, data or message exchanges, and teardown when the session ends. It also monitors session identification so only designated parties can participate and security services to control access to session information.OSI Layer 6 : The Presentation LayerThe presentation layer defines the format and representation of the data exchanged between users. Video displays and printers may use different data formats. Without this layer these different formats would require individual treatment. Data Encryption and compression, and code and format conversions are some of the facilities offered by this layer. For outgoing messages, it converts data into a generic format that can survive the rigors of network transmission; for incoming messages, it converts data from its generic networked representation into a format that will make sense to the receiving application.OSI Layer 7 : The Application LayerThe Application Layer is the top layer of the reference model. It provides a set of interfaces to allow equipment and application programs to exchange information in the OSI environment. It is the source of all data to be transferred including services such as networked file transfer, message handling, and database query processing. Gateways work at this level.OSI Reference Model
TCP/IP REFERENCE MODELThe TCP/IP reference model, is based on a suite of protocols in which each protocol solves a particular network communications problem. The TCP/IP model can be used in a heterogeneous environment that has equipment from many different vendors.There are four layers on the TCP/IP reference model.Layer 1: The Network LayerLayer 2: The Internet LayerLayer 3: The Transport LayerLayer 4: The Application Layer13TCP/IP Layer 1: Network LayerThe network access layer is responsible for exchanging data between a host and the network and for delivering data between two devices on the same network. Node physical addresses are used to accomplish delivery on the local network. Functions performed at this level include encapsulation of IP datagrams (i.e. the packet format defined by Internet Protocol.) into the frames transmitted by the network, and the mapping of IP addresses into the physical addresses used by the network. The TCP/IP Network Access Layer can encompass the function of all three lower layers of the OSI reference model (Network Layer, Data Link Layer, and Physical Layer.)TCP/IP Layer 2 : Internet LayerThe internet layer is responsible for sending source packets from any network on the internetwork and have them arrive at their destination regardless of the path they took. The Internet Protocol (IP) is used in this layer and it provides the packet delivery service on which the TCP/IP is based. The IP protocol implements a system of logical host addresses called IP addresses. The IP addresses are used by the internet and higher layers to identify devices and to perform internetwork routing.TCP/IP Layer 3: Transport LayerThe transport layer is responsible for the reliability, flow control, and error correction of data being sent across the network.Its main protocol is called the transmission control protocol (TCP). TCP provides reliable data delivery service with end-to-end error detection and correction.User Datagram Protocol (UDP) is another protocol used which provides slow-overhead, connectionless datagram delivery service.UDP is unreliable but enhances network throughput when error correction is not required at the host-to-host-layer. Both protocols deliver data between the Application Layer and the Internet Layer. The users may choose either best suited to their means.TCP/IP Layer 4 : Application LayerThe application layer is responsible for handling high-level protocols, issues of representation, encoding and dialog control. This layer is broadly equivalent to the application, presentation and session layers of the OSI model. It gives an application access to the communication environment. Examples of protocols found at this layer are TelnetFTP (File Transfer Protocol)SNMP (Simple Network Management Protocol)HTTP (Hyper Text Transfer Protocol)SMTP (Simple Mail Transfer Protocol)TCP/IP Reference Model
TCP/IP Reference Model
TCP/IP Reference Model
TCP/IP Reference Model
