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Computer Network Layering Models: Overview

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Computer Network Layering Models: Overview Computer Network Layering Models: Overview 1. Computer Networking Layering In case of working with complex systems one should consider some special principles: explicit structure allows identification, relationship of complex system’s pieces modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system o e.g., change in gate procedure doesn’t affect rest of system layering is considered harmful 1 2. Computer Networking Reference Models 2.1. DARPA (ARPANnet) Model Single closed network: ARPAnet packet network introduced around 1970s Network of networks created around 1972-74 and supported by DARPA (Defence Advanced Research Projects Agency (RFC 829) Known also under the name TCP/IP five layer model 2 TCP/IP model maps a four-layer conceptual model known as the DARPA model, named after the U.S. government agency that initially developed TCP/IP. Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model. TCP/IP is a suite of protocols used to support data communications between equipment on Local Area Networks (LANs) and between interconnected networks. TCP/IP is also the primary protocol suite used on the global Internet. Network Access Layer: Data Link Layer and Physical Layer are normally grouped together. TCP/IP model uses of existing Data Link and Physical Layer standards. Internet Layer: In the OSI Reference Model the Network Layer isolates the upper layer protocols from the details of the underlying network and manages the connections across the network. The Internet Protocol (IP) is normally described as the TCP/IP Network Layer. Because of the Inter-Networking emphasis of TCP/IP this is commonly referred to as the Internet Layer. Transport Layer. TCP/IP model specifys two Transport Layer protocols. The Transmission Control Protocol (TCP) guarantees that information is received as it was sent. The User Datagram Protocol (UDP) performs no end-to-end reliability checks. Application Layer. Application Layer also includes the OSI Presentation Layer and Session Layer. In this document an application is any process that occurs above the Transport Layer. This includes all of the processes that involve user interaction. The application determines the presentation of the data and controls the session. In TCP/IP the terms socket and port are used to describe the path over which applications communicate. 3 2.2. Open Systems Interconnection (OSI) Model The standard model for networking protocols and distributed applications defined late 1970s is: International Standard Organization's Open Systems Interconnection (ISO/OSI 2012) model. It is known also as 7 (seven) Layer Network Model Layer 1 - Physical. transmitting raw bits over a communication channel. It has to be sure that what one side sends it is the same received by the other side Layer 2 - Data Link. the sender of the layer breaks the input data up into data frames, transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver. Layer 3 - Network. controls the operation of the subnet. It determines how packets are routed from source to destination. Layer 4 - Transport. accepts data from the session layer, split it up into smaller units if needed, pass these to the network layer, and ensure that the pieces all arrive correctly at the other end. Layer 5 - Session. allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides some enhanced services used in some applications. Layer 6 - Presentation concerned with the syntax and semantics of the information transmitted. Layer 7 - Application. contains a variety of protocols that identify and establish the availability of intended communication partners (and the resources required to connect with them), synchronize cooperating applications, and establish agreements on procedures for error recovery and control of data integrity. 4 3. Peer-to-Peer Communication Each layer of the architectural model supports a protocol to communicate with its peer. When a packet is transmitted by a layer, a header consisting of Protocol Control Information (PCI) is added to the data to be sent. OSI terminology: - the packet data (known as the payload) is called a Protocol Data Unit (PDU). - the packet so-formed, called a Service Data Unit (SDU) is passed via a service interface to the layer below. This is sent using the service of the next lower protocol layer. Each layer in a source host system uses its own layer protocol to communicate with its peer layer in the destination host system. In other words, layers talk to each other. Each layer "below" the current layer provides services to the layer above. For example, - the transport protocol communicates end-to-end using the services of the network layer below. - the peer-to-peer communication takes place between the end systems using a communication protocol. - in the case of the data link layer, the communication takes place using the services of the physical layer. The communication takes place with the peer data kink layer protocol in the next directly connected system (either an Intermediate System or an End System). 5 4. Data Encapsulation Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model. Each OSI layer may add its own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.) When an application needs to transmit a message in the network, the data packet is sent to the application layer of the local host. The protocol implemented at this layer, generates a specific header which is added to the data packet. The process continues similarly for each layer crossed by the packet. Additionally, at the data link layer it is also computed a frame checksum which is attached at the end of the packet. The physical layer transmits over the network the received frame without any other modification. The 5 Steps of Data Encapsulation are: 1. The Application, Presentation and Session layers create DATA from users' input. 2. The Transport layer converts the DATA to SEGMENTS 3. The Network layer converts the SEGMENTS to PACKETS (or datagram) 4. The Data Link layer converts the PACKETS to FRAMES 5. The Physical layer converts the FRAMES to BITS. - At the sending computer the information goes from top to bottom while each layer divides the information received from upper layers in to smaller pieces and adds a header. - At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together. 6 .
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