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SLO # 5: �Network Communication and Protocols��Sub topic: 5.2 OSI Model ��Adapted by: Younus Bashir

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Differentiate among seven layers of Open System Interconnection (OSI) model;�Cognitive level: Understanding �

Student Learning outcome # 5.2.1

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OSI MODEL

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  • The International Standards Organization (ISO) based in Geneva, developed standards for international and national data communications.
  • In the early 1970s, ISO developed a standard model of a data communication system.
  • The Open System Interconnection (OSI) model describes how data is sent and received over a network.
  • This model breaks down data transmission over a series of seven layers. Each layer has a responsibility to perform specific tasks.

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  • In OSI model, control is passed from one layer to the net, starting at the application layer in one station and proceeding to the bottom layer (the physical layer), over the physical link to the next station.

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Layer 7- Application Layer

[Software layer]

[Provides services to end user]

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  • Application provides services to end-user.
  • It interacts with operating system or application software whenever user wants to send files, read messages or perform other network related activities.
  • This is an application like Firefox, Outlook, or Internet Explorer.

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Layer 6- Presentation Layer

[Software layer]

[Converts data into standard format]

  • Presentation layer is also called the Translation layer.
  • The data from the application layer is extracted here and manipulated as per the required format to transmit over the network and can be understand by other layers.
  • At the receiving end it also formats the information so that it looks the way the user can understand. (See image)
  • Functions of presentation layers are:

Translation, encryption/ decryption, compression.

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Layer 5- Session Layer

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[Software layer]

[Session establishment, maintenance and termination]

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  • Session layer enables two applications or two pieces of the same application to communicate across the network.
  • It performs security, name recognition, logging and other similar functions.
  • It also establishes, maintains and ends communication with the receiving computer.

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Layer 4- Transport Layer

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[Heart of Operating system]

[Session establishment, maintenance and termination]

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  • Transport layer establishes connections between two computers on the network. 
  • It is responsible for the End to End delivery of the complete message.
  • Transport layer also provides the acknowledgment of the successful and right format data transmission.
  • It re-transmits the data if an error is found.

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Layer 3- Network Layer

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[Hardware Layer]

[Selects physical path-way]

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  • Network layer decides which physical path-way the data should take to reach the destination.
  • It also takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available.
  • The sender & receiver’s IP address are placed in the header by network layer.

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Layer 2- Data Link Layer

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[Hardware Layer]

[Defines format of data on the network]

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  • Data link layer defines the format of data on the network. This layer converts the data into packets and checks them before putting them on the path-way.
  • When a packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address.

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Layer 1- Physical Layer

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[Hardware Layer]

[Defines cables and signaling]

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  • Physical layer defines cables and signaling. It provides hardware means such as cables and connectors for sending and receiving data.

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Identify protocols and devices used on every layer of OSI model; �Cognitive level: Understanding �

Student Learning outcome # 5.2.1

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Protocol

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A protocol is a set of rules and conventions that govern how computers exchange information over a network medium.

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Layers of OSI Model

Protocol Name

Function

Device Used

Physical Layer

X.25 & IEEE 802

Provides hardware such as cables and connectors for sending and receiving data.

Cables and connectors

e.g fiber, wireless.

Data link Layer

X.25 & IEEE 802

Places data packets on to the pathway for transmission.

Switches and NICs

Network Layer

Internet protocol

Controls routing and forwarding of data between the source and destination.

Router

Transport Layer

TCP, UDP

Transfers data between source and destination and is responsible for error recovery and flow control.

Router

Session Layer

NetBIOS, WinSock

Starts and stops communication sessions between applications.

Router and gateway

Presentation Layer

Windows OS, SSL, SSH, JPEG, MP3, ASCII & ANSI

Converts data into a format that can be carried by the lower layer or converts data into that application layer can understand

Gateway

Application Layer

HTTP, FTP, SMTP

Provides interaction between the end user and software

Gateway

Protocols and devices used at different layers

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It’s easy to remember the sequence of OSI Model 7 Layers using this simple sentence: ��“All people seem to need data processing.”�All = Application Layer�People= Presentation Layer�Seem = Session Layer�To = Transport Layer�Need = Network Layer�Data = Data Link Layer�Processing = Physical Layer�

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Past Paper Review Questions:��

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Past Paper Review Questions:��

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Past Paper Review Questions:��

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Resources

Computer Science grade 11�National Book Foundation