Open System Interconnection

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Bell Njaidi 1 OSI (Open System Interconnection) model Assignment

OSI : 

OSI The Open System Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design.

OSI Layers : 

OSI Layers

OSI model : 

OSI model In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

Description of OSI layers : 

Description of OSI layers A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. OSI Seven Layer Model from top to bottom, are the Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers.

Layer 1: Physical Layer : 

Layer 1: Physical Layer The Physical Layer defines the electrical and physical specifications for devices. Defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBAs used in storage area networks) and more.

functions and services of the layer:1 : 

functions and services of the layer:1 Establishment and termination of a connection to a communications medium. Participation in the process whereby the communication resources are effectively shared among multiple users. E.g., contention resolution and flow control.

functions and services of the layer:1 Cont:d : 

functions and services of the layer:1 Cont:d Modulation or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link. Ethernet incorporates both this layer and the Data Link Layer

Layer 2: Data Link Layer : 

Layer 2: Data Link Layer The Data Link Layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical Layer. The Data Link Layer is concerned with local delivery of frames between devices on the same LAN.

functions and services of Layer:2 : 

functions and services of Layer:2 Data link layer error control is not used in LAN protocols such as Ethernet, but in modems and wireless networks. WAN Protocol architecture provides the functional and procedural means to transfer data. provide the means to detect and possibly correct errors that may occur in the Physical Layer.

functions and services of Layer:2 Cont:d : 

functions and services of Layer:2 Cont:d IEEE 802 LAN architecture This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sub layer. Data packet queuing or scheduling whereby controlling the rate of transmission.

Layer 3: The network layer : 

Layer 3: The network layer This layer provide switching and routing technologies. Creating logical paths, known as virtual circuits, for transmitting data from node to node. This is a logical addressing scheme- values are chosen by the network engineer. Careful analysis of the Network Layer indicated that it could have at least Three sub layers.

Layer 3: Cont;d : 

Layer 3: Cont;d Subnet work Access - considers protocols that deal with the interface to networks, such as X.25. (IPv4 and IPv6 carry interface addresses rather than node addresses, hence are in this class). Subnet work Dependent Convergence- brings the level of a transit network up to the level of networks on either side. Subnet work Independent Convergence -handles transfer across multiple networks such as CLNP, or IPv7 ISO 8473.

functions and services of the Layer:3 : 

functions and services of the Layer:3 Routing and forwarding are functions of this layer. Congestion control and packet sequencing. Addressing, internetworking and error handling. maintains the quality of services requested by the Transport Layer.

Layer 4: Transport Layer : 

Layer 4: Transport Layer This layer provides transparent transfer of data between end systems, or hosts(users). actual OSI protocols, are in five classes of connection-mode transport protocols ranging from class 0 ( also known as TP0) to class 4 (also known as TP4). TP0 provides the least features and contains no error recovery, hence used on error-free connections network layers . TP4 designed for less reliable networks, similar to the Internet.

Layer 4: Cont:d : 

Layer 4: Cont:d Other examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Though not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer.

Layer 4: Cont’d : 

Layer 4: Cont’d All OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. both of which TCP is incapable.

functions and services of the Layer:4 : 

functions and services of the Layer:4 Controls the reliability of a given link through:- flow control. segmentation/desegmentation. and error control. Keep track of the segments and retransmit those that fail (state and connection oriented protocols).

functions and services Cont;d : 

functions and services Cont;d Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office. Deals with the dispatch and classification of mail and parcels sent. Only manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only.

Layer 5: Session Layer : 

Layer 5: Session Layer This layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures.

functions and services of the Layer:5 : 

functions and services of the Layer:5 The OSI model made this layer responsible for graceful close of sessions. Property of the TCP. Check pointing and recovery, which is not usually used in the Internet Protocol Suite. Commonly implemented explicitly in application environments that use remote procedure calls. It deals with session and connection coordination.

Layer 6: Presentation Layer : 

Layer 6: Presentation Layer This layer provides independence from differences in data representation. higher-layer entities can use different syntax and semantics, as long as the presentation service understands both and the mapping between them.

Layer 6: Cont;d : 

Layer 6: Cont;d e.g., ( encryption) by translating from application to network format, and vice versa. The presentation service data units are then encapsulated into Session Protocol data units, and moved down the stack.

functions and services of the Layer:6 : 

functions and services of the Layer:6 Transforms data into the form that the application layer can accept. Providing freedom from compatibility problems. Formats and encrypts data to be sent across a network.

functions and services of the Layer:6 Cont;d : 

functions and services of the Layer:6 Cont;d Use basic encoding rules of Abstract Syntax Notation One (ASN.1) hence sometimes called the syntax layer. ASN.1 convert an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Layer 7: Application Layer : 

Layer 7: Application Layer OSI layer closest to the end user. OSI application layer and the user interact directly with the software application. software applications implement a communicating component. software applications fall outside the scope of the OSI model

functions and services of the Layer:7 : 

functions and services of the Layer:7 Identifies communication partners. By determining the identity and availability of communication partners for an application with data to transmit. determines resource availability. By deciding whether sufficient network or the requested communication exist.

functions and services of the Layer:7Cont;d : 

functions and services of the Layer:7Cont;d synchronizes communication. Manages cooperation between all communication applications.

examples of application layer implementations : 

examples of application layer implementations Hypertext Transfer Protocol (HTTP) File Transfer Protocol (FTP) Simple Mail Transfer Protocol (SMTP) and X.400 Mail...

OSI model Layers Diagram : 

OSI model Layers Diagram