OSI REFERANCE MODEL

The OSI reference model, as shown under, is developed by International Standards Organization (ISO) as a step towards international standardization of the protocols used in the networking.

·        The model is called OSI (Open System Interconnection) because it deals with connecting open systems that is, systems that are open for communication with other systems.

·        OSI Defines: - How network device contact each other, if they have different language, How they communicate each other?

·        OSI is not a physical model; rather it is a set of guidelines that application developer can use to create application that run on network. It also provides framework for creating and implementing networking standards, devices, internetworking scheme.

·        O.S.I Has 7 Layer, divided into 2 groups.

·     The Layer 4-3-2-1 layer defines how data is transmitted end-to-end.

·     The Layer 7-6-5 defines how the application within one station will communicate with each other. Responsible for application communication between hosts.

·        Layers are as under:

o   A layer should be created where a different level of abstraction is needed.

o   Each layer should perform a well defined function.

o   The function of each layer should be chosen with an eye towards defining internationally standardized protocols.

o   The layer boundaries should be chosen to minimize the information flow across the interfaces.

o   The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity.

(1) Physical Layer:

·        The physical layer of the OSI model establishes the physical characteristics of the network such as the type of cable, connectors, and the length of the cable.

·        This layer also defines the electrical characteristics of the signals used to transmit the data. The electrical characteristics include signal voltage swing and duration of voltages.

·        The physical layer transmits the binary data (bits) as electrical or optical signals depending on medium.

·        This layer is concern with the following:

o  Physical characteristics of interface and medium: This layer defines the characteristics of the interface between the devices and the transmission medium.

o  Representation of bits: Data of this layer consists of a stream of bits without any interpretation. They must be encoded into signals (electronics or optical), which is defined by the physical medium.

o  Data rates: The number of bits sent each second, is also defined b the physical layer.

o  Line configuration: They physical layer is concerned with the connection of device to the medium. It can be Point-to-Point or Multipoint Configuration.

o  Transmission Mode: The physical layer also defined the direction of transmission between two devices (Simplex, Half-Duplex or Full-Duplex).

(2) Data Link Layer:

·        The data link layer defines how the signal will be placed.

·        In this layer, the data frames are broken down into individual bits that can be translated into electronic signals and sent over network. On the receiving end the bits are reassembled into frames for processing by upper levels.

·        If an acknowledgement is expected and not received, the frame will be resent.

·        Error and corrupt data detection and correction is also performed at the data link layer

·        The data link layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-to-node delivery.

• Function: Framing, Combine Packets into Bytes And bytes into Frame

o  Framing: the data link layer divides the stream of bits received from the network layer into manageable data units called frames.

o  Physical Addressing: If frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the physical address of the sender and receiver of the frame.

o  Flow control: If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer imposes the flow control mechanism to prevent overwhelming the receiver.

o  Error control: The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It uses the mechanism to prevent duplication of frames. Error control is normally achieved through a trailer added to the end of the frame.

o  Access control: When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

(3) Network Layer:

—  The network layer is primarily concerned with addressing and routing.

—  Logical addresses or IP addresses are translated into physical addresses or machine addresses for transmission at the network layer.

—  On the receiving side, the translation process is reversed.

—  The network layer also determines the route from the source to the destination computer to deliver packets.

—  Routes are determined based on packet addresses and network conditions. Traffic control measures are also implemented at the network layer.

—  If 2 systems are connected in the same network, there is no need for the network layer.

—  Function: Routing, Provides Logical addressing which router used for path determination

o  Logical Addressing: The physical addressing implemented by the data link layer handles the addressing problem locally. If packet passes the network boundary, it needs another addressing system to help distinguish the source and destination system.

o  Routing: When independent networks or links are connected together to create internetwork, the connecting device (router or gateway) route the packets to their final destination. This is one more mechanism provided by this layer.

(4) Transport Layer:

— The transport layer is responsible for the control of flow and ensuring that messages are delivered error free source -to-destination (end-to-end).

— On the originating side, messages are packaged for efficient transmission and assigned a tracking number.

— On the receiving side, the packets are reassembled, checked for errors, and acknowledged.

— The transport layer performs error handling by ensuring that all data is received in the proper sequence without errors. If there are errors the data is retransmitted.

·     This layer ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level.

·     For security purpose, this layer may create a connection between the two end ports, a logical path between source and destination.

Function: End-to-End Connection, Perform error correction before retransmit

o Service -point addressing: Computer runs several programs at the same time. So this layer is responsible for delivering data to specific process (port) of computer.

It contains service-point address (port address) and therefore, network layer gets each packet to the correct computer on correct network.

Segmentation and Reassembly: A message is divided into transmittable segments, each segment containing sequence number, which enables layer to reassembling the message correctly upon arriving at the destination and can identify those packets which are lost during the transmission.

o Connection control: The transport layer can be either connection or connection oriented.

 A connection-less transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine.

Whereas connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packet.

o Error control: This layer is more concern with end -to-end control rather than single link control of data link layer. The receiving transport layer makes sure that the entire message arrives without error. Error correction is usually achieved through retransmission.

(5) Session Layer:

·        The session layer is responsible for establishing, managing, and terminating a connection called session.

·        A session is an exchange of messages between computers. Logon, name recognition and security functions occur while establishing a session.

·        Managing the session involves synchronization of user tasks and messages.

·        Synchronization involves the use of checkpoints in the data stream. In the event of a failure, only the data from the last checkpoint has to be resent.

Function: Keeps different application data separate, Dialog control, Synchronization

o  Dialog control: The session layer allows two systems to enter into a dialog. It allows the communication between two processes to take place either in half-duplex or full duplex.

o  Synchronization: This layer allows a process to add check points into a stream of data.

(6) Presentation Layer:

·        This layer is concern with the syntax and semantics of the information exchanged between 2 systems. The presentation layer is concerned with translating, interpreting, and converting the data from various formats.

·        Data is compressed for transmission and uncompressed on receipt in this layer. Encryption techniques are also implemented at the presentation layer. The redirector operates at the presentation layer by redirecting I/O operations across networks.

·        Function: Present Data, Handles Processing Such as Encryption, Compression  and Translation Services

o  Translation: Information should be changed to bit streams before being transmitted. Because different computers us e different encoding system, the presentation layer is responsible for interoperability between these different encoding methods.

o  Encryption: To carry sensitive information, a system must be able to assure privacy. Transformation is carried out at this layer.

o  Compression: Data compression reduces the number of bits to be transmitted. It is highly required when transmitting data of text, audio and video.

(7) Application Layer:

·        This layer provides the operational system with direct access to the network services.

·        It also provides an interface so that Application (Web Browser) that are running on the local machine can access the network services. So It enables user or software to access the network.

·        It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

o  Network Virtual Terminal: A network virtual terminal is a software version of a physical terminal and allows a user to log on to a remote host.

o  File Transfer and Access Management: This application allows a user to access files in a remote computer.

o  E-Mail Service: This application provides the basis for email forwarding and storage.

o  Directory Service: This application provides distributed database source and access for global information about various objects and services.

Summary of Layers

·        Physical Layer: To transmit bits over a medium.

·        Data Link Layer: To organize bits into frames; to provide node-to-node delivery.

·        Network Layer: To move packets from source to destination; to provide internetworking.

·        Transport Layer: To provide reliable end-to-end message delivery and error recovery.

·        Session Layer: To establish, manage and terminate sessions.

·        Presentation Layer: To translate, encrypt, and compress data.

·        Application Layer: To allow access to network resources.