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During and before the 1980s, there were only a handful of protocols and standards that were around and they belonged to different manufacturers who didn’t have much dialogue with each other. Though eventually computer science and technology continued to be further innovated and become more readily available to companies and the public, it became necessary that a widespread standard would be needed in place to ensure compatibility between all machines. This was especially true about networks, and networking technology. A network is designed to share data and information between machines, a standard that dictates how this information is formatted, transmitted, and received would make it easy for data to be shared openly, even when sending or receiving from networks that are not similar. Requirements for a new standard of implementing open communications led the International Organization for Standardization (ISO) and American Nation Standards Institute (ANSI) to develop a 7 layer network communications model known as Open Systems Interconnect or the OSI. The OSI became a link that allows data to be reliably exchanged and transmitted since guidelines were created to set a standard in how network equipment is manufactured and network OS’s to communicate to each other on a network. The OSI model doesn’t actually perform any tasks or functions but it dictates HOW the work should be performed by other hardware or software between networks so that communication can occur.
The OSI model is made up of these seven layers; the physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. These 7 parts of the model are called a ‘stack’.
Physical Layer The physical layer defines all the mechanical, procedural, functional and electrical specifications for activating, deactivating and maintaining the link between each system . Such characteristics as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors, are defined by physical layer specifications.
Data Link Layer The data link layer provides error-free transfer of data frames from one computer to another over the physical layer. The layers above this layer can assume virtually error-free transmission over the network. The data-link layer provides the following functions. – Establishing and terminating a logical link between two computers identified by their unique network interface card. – Controlling frame flow by instructing the transmitting computer not to transmit frame buffers – Sequentially transmitting and receiving frames – Providing and expecting frame-acknowledgment, and detecting and recovering from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipts – Managing media access to determine when the computer is permitted to use the physical medium – Eliminating frames to create and recognize frame boundaries – Error-checking frames to confirm the integrity of the received frame – Inspecting the destination address of each received frame and determining if the frame should be directed to the layer above
Network Layer The network layer controls the operation of the subnet. It determines which physical path the data takes, based on the network conditions, the priority of service, and other factors. The network layer provides the following functions.
The transport layer makes sure that messages are delivered in the order in which they were sent and that there is no loss or duplication. It removes the concern from the higher layer protocols about data transfer between the higher layer and its peers. The size and complexity of a transport protocol depend on the type of service it can get from the network layer or data link layer. For a reliable network layer, a minimal transport layer is required. Functions of the transport layer include the following.
The session layer establishes a communications session between processes running on different computers and can support message-mode data transfer.
The presentation layer ensures that information sent by the application layer of one system will be readable by the application layer of another system. If necessary, the presentation layer translates between multiple data representation formats by using a common data representation format. The presentation layer concerns itself not only with the format and representation of actual user data but also with data structures used by programs. In addition to actual data format transformation, the presentation layer negotiates data transfer syntax for the application layer.
The application layer is the OSI layer closest to the user. It differs from the other layers because it does not provide services to any other OSI layer, but rather to application processes lying outside the scope of the OSI model. Examples include spreadsheet programs, word-processing programs, banking terminal programs, etc. The application layer identifies and establishes the availability of intended communication partners, synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. Also, the application layer determines whether sufficient resources for the intended communication exist.
Transmission Control Protocol/Internet Protocol was developed in the 60’s as a method that connects large mainframes computers together for the simple purpose of sharing data or information. In the present, most computer operating systems manufactures to incorporate the TCP/IP suit into their software programs allowing for each individual workstation to encompass the ability to transmit, receive, and share information through the largest mainframe available, the Internet. The TCP/IP model is made up of 4 layers, a few layers less than the OSI model. The TCP/IP model consists of from highest to lowest: The Application layer, The Transport layer, The Internet layer and the Link layer or Subnet layer.
Just like the OSI model, the Application layer in the TCP/IP model performs the same sort of function. Only that the Application layer for the TCP/IP corresponds to the Application layer, Presentation layer and Session layer of the 7 layer OSI model.
Transport layers exist in both TCP/IP and OSI model. Even though both models have Transport layers they differ. The TCP/IP model consists of two standard transport protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP uses a reliable data-stream protocol which is connection-oriented and UDP uses an unreliable data-stream protocol which is connectionless oriented.
The Internet layer is a group of protocols and specifications that are used to transport packets from the host across a network, the host specified by a network address (IP address).
The lower level layer of the TCP/IP model, this layer is used by a suite of protocols for the “Internet”. This is used to connect hosts or nodes to a network. This layer is compared to the “Data Link” layer and the “Physical “ layer of the OSI model.
TCP/IP Application Layer VS OSI Application, Presentation, and Session layer.
The similarities in both models are comparable but different at the same time. All though they exist in both, the approach each uses to construct applications is different. In the OSI model the Application layer, Presentation layer and Session layer correspond to the Application layer of the TCP/IP model. They somewhat do the same job but use different protocols, TCP/IP uses FTP, SMTP, TELNET, DNS and SNMP where the OSI model uses: FTAM, VT, MHS, DS, CMIP.
UDP and TCP defined by TCP/IP Transport Layer correspond to many of the requirements of the OSI Transport Layer. Some issues occur over the requirements in the session layer of OSI since sequence numbers and port values can help the Operating System to keep track of active sessions. Most of the TCP and UDP functions and specifications map to the OSI Transport Layer. The TCP/IP and OSI architecture models both employ all connection and connectionless models at the transport layer. The architecture calls the 2 models in TCP/IP simply Connections and Datagrams. The OSI model uses the terms “Connection-mode” and “Connection-oriented” for the connection model and the term “Connectionless-mode” for the connectionless model.
The Network layer of the OSI model is compared to the Internet layer of the TCP/IP model. Both models support “Connectionless” network services, but only the Network layer in the OSI supports connected services. The OSI layer is a “catch-all” for all protocols that assist in network functionality, where the “Internet” layer of the TCP/IP model assist in internetworking using Internet Protocol.
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