The OSI model

Network Modal:

Reference modal is a virtual modal of network which define how communication should take place. It logically divide the processes into layers which are required for the network communication. This function is known as a layered architecture. A network reference modal helps us to understand the function of communication software which relate to network product development activities. It also provide the basic scheme for creating and implementation of network device.

 There are two popular reference modals:

 OSI modal           TCP/IP modal

Why Reference Modal Needed: 

The goals needed to archive while designing the network modal are following:

• The ability to connect multiple network together in a seamless way
• Network should be able to service loss of subnet hardware with existing communication not being broken
• The architecture should be flexible to meet requirement of various applications

Both of these reference modal OSI & TCP/IP works on the principal of layered architecture. Layering is a process to divide a task into sub-tasks and then solve each sub-task independently. Establishing a well-defined interfaces between layers makes parting easier. Major advantage of layering is to re-use of codes and extensibility.

OSI Reference Modal:

There are multiple user present all over the world using computer network. Hence we need a standard to ensure the world-wide communication system can be delivered and are compatible with each other.

 The International Standard Organization (ISO) presented a  proposal for the standardization of protocols used in computer network is called Open System Interconnection Reference Modal or simply OSI modal. The model was developed in 1984. This modal is called OSI  because it deals with connecting open system ( Systems that are open for communication with other systems).

The Open System Interconnection model is a seven-layer structure that specifies the requirements for communications

between two computers. The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems. This separation into smaller more manageable functions is known as layering. This model allows all network elements to operate together no matter, who created the protocols and what computer vendor supports them. 

The OSI model describes how information or data makes its way from application programs (such as Browser) through a

network medium (such as wire) to another application program located on another network. It ensures greater compatibility and interoperability between various types of network technologies. It is now considered the primary architectural model for inter-computer communications.

Principles on which OSI model was designed:

• A layer should be created where different level of abstraction is needed.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen according to the internationally standardized protocols.
• The number of layers should be large enough that distinct functions should not be put in the same layer and small enough that the architecture does not become very complex.

• The OSI Reference Model is composed of seven layers, each specifying particular network functions.
• The process of breaking up the functions or tasks of networking into layers reduces complexity.
• Each layer provides a service to the layer above it in the protocol specification.
• Each layer communicates with the same layer’s software or hardware on other computers.
• The lower 4 layers (transport, network, data link and physical) are concerned with the flow of data from end to end through
• the network.
• The upper 3 layers of the OSI model (application, presentation and session) are orientated more toward services to the applications.
• Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

With  the OSI model network can be broken up into manageable pieces. The OSI model provide a common language to explain component and their functionality.

The seven OSI modal layers:

The main benefits of the OSI model include the following:

•  Helps users understand the big picture of networking
•  Helps users understand how hardware and software elements function together
•  Makes troubleshooting easier by separating networks into manageable pieces
•  Defines terms that networking professionals can use to compare basic functional relationships on different networks                 
•  Helps users understand new technologies as they are developed
•  Aids in interpreting vendor explanations of product functionality

Layer 1- The Physical Layer:

It is the first or bottom layer of OSI model.  It is concerned with transmitting bits over a communication channel. It is responsible for the actual physical connection between the devices. Physical layer manages the way, how a device should connects to network media. It defines the transmission rate. It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. The Physical Layer of the OSI model is only part of a LAN (Local Area Network).

Components of the physical layer include:

• Cabling system components
• Adapters that connect media to physical interfaces
• Connector design and pin assignments
• Hub, repeater, and patch panel specifications
• Wireless system components
• Parallel SCSI (Small Computer System Interface)
• Network Interface Card (NIC)

Examples :- EIA/TIA-232, RJ45, NRZ.

Layer 2- The Data-Link Layer:

This layer allows a device to access the network to send and receive messages. It is responsible for node-to-node delivery of data. The data link layer provides access to the networking media and physical transmission across the media and this enables the data to locate its intended destination on a network. It receives the data from network layer and creates FRAMES , add physical address to these frames & pas them to physical layer. The data link layer uses the MAC address to define a hardware address in order for multiple stations to share the same medium and still uniquely identify each other.

Functions of Data Link Layer:

• (Framing): Data-link layer divides the bits received from network layer into frames. (Frame contains all the addressing information necessary to travel from source to destination).
• (Physical addressing): After creating frames, data-link layer adds physical address of sender/receiver (MAC address) in the header of each frame.
• (Flow Control): Data-link layer prevents the fast sender from drowning the slow receiver.
• (Error Control): It provides the mechanism of error control in which it detects & retransmits damaged or lost frames.
• (Access Control): When single communication Channel is shared by multiple devices, MAC layer of data-link layer provides help to determine which device has control over the channel.

Components of the Data-link layer include:

• Network interface cards
• Ethernet and Token Ring switches
• Bridges

Examples :- Ethernet, Frame Relay, FDDI.

Layer 3- The Network Layer:

It is responsible for the source to destination delivery of a packet across multiple networks. It defines logical addressing so that any endpoint can be identified. The network layer ensures that the packet gets its point of origin to its final destination. Network layer defines how routing works and how routes are learned so that the packets can be delivered this function is known as routing. Routers operate at Layer 3. The networking industry has evolved to the point that it requires a common layer 3 addressing system. The Internet Protocol (IP) addresses make networks easier to both set up and connect with one another. The Internet uses IP addressing to provide connectivity to millions of networks around the world.  The network layer also defines how to fragment a packet into smaller packets to accommodate different media.

Functions of Network Layer:

• (Internetworking): It provides Internetworking.
• (Logical Addressing): When packet is sent outside the network, network layer adds Logical (network) address of the sender & receiver to each packet.
• Network addresses  (IP address) are assigned to local devices by network administrator and assigned dynamically by special server called DHCP (Dynamic Host Configuration Protocol).
• (Routing): When independent network are connected to create internetwork several routes are available to send the data from sander to destination. These network are interconnected by routers & gateways that route the packet to final destination.

Examples :- IP, IPX, AppleTalk.

Layer 4- The Transport Layer:

The transport layer offers end-to-end communication between end devices through a network. Depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications. It is responsible for process-to-process delivery of the entire message. Transport layer looks after the delivery of entire message considering all its packets and make sure that all packets are in order.

The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system. At the receiver side, transport provides services to application layer and takes services form network layer. At the source side, transport layer receives message from upper layer into packets and reassembles these packets again into message at the destination.

The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Transport Layer provides two types of services:

• Connection Oriented Transmission: In this type of transmission the receiving devices sends an acknowledge back to the source after a packet or group of packet is received. It is slower transmission method.

• Connectionless Transmission: In this type of transmission the receiving devices does not sends an acknowledge back to the source. It is faster transmission method.

Functions of Transport Layer:

• Segmentation of message into packet & reassembly of packets into message.
• Computers run several processes, transport layer header include a port address with each process.
• Flow control facility prevents the source form sending data packets faster than the destination can handle.
• Transport layer ensures that the entire message arrives at the receiving without error.
• Application identification
• Client-side entity identification
• Confirmation that the entire message arrived intact
• Segmentation of data for network transport
• Control of data flow to prevent memory overruns
• Establishment and maintenance of both ends of virtual circuits
• Transmission-error detection
• Realignment of segmented data in the correct order on the receiving side
• Multiplexing or sharing of multiple sessions over a single physical link

Layer 5- The Session Layer:

Session layer is the fifth layer of OSI Model. It has the responsibility of beginning, maintaining and ending the communication between two devices, called session. This includes the control and management of multiple bi-directional messages using dialogue control. It also provides for orderly communication between devices by regulating the flow of data. The session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. It also synchronizes dialogue between two hosts' presentation layers and

manages their data exchange. The session layer offers provisions for efficient data transfer.

Functions of session layer:

• Establishing, Maintaining and ending a session
• Dialog Control & Dialog separation
• Virtual connection between application entities
• Synchronization of data flow & Creation of dialog units
• Connection parameter negotiations
• Partitioning of services into functional groups
• Acknowledgements of data received during a session
• Retransmission of data if it is not received by a device

Examples :- SQL, ASP(AppleTalk Session Protocol)

Layer 6- The Presentation Layer:

The presentation layer is responsible for how an application formats the data to be sent out onto the network. The presentation layer basically allows an application to read (or understand) the message. The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data formats by using a common format. It was designed for data encryption, decryption and compression.

Functions of Presentation Layer:

• Encryption and decryption of a message for security
• Compression and expansion of a message so that it travels efficiently
• Graphics formatting
• Content translation
• System-specific translation

Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

Layer 7- The Application Layer:

The application layer is the OSI layer that is closest to the user. The application layer provides an interface for the end user operating a device connected to a network. It provides supports for services such as e-mail, file transfer, access to the world wide web. This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail)  that is, this application layer is the data the user views while using these applications. The application layer establishes the availability of intended communication partners, synchronizes and establishes agreement on procedures for error recovery and control of data integrity.  It differs from the other layers in that it does not provide services to any other OSI layer, but rather only to applications outside the OSI model.

Functions of Application Layer:

• Accessing the World Wide Web
• Mail Services
• Ability to print on a network
• File transfer & Access
• Remote log-in

You can download these notes, use this link-

https://drive.google.com/open?id=0B6Ov-Oe4YuYUMktNbUxvVzJOSm8