Question

Research the OSI reference model and provide the steps for how data travels from an Application to the recipient using a real-life example.

Answer 1

Layer Number	Layer Name	Description
7	Application	Provide user interface to send and receive the data
6	Presentation	Encrypt, format and compress the data for transmission
5	Session	Initiate and terminate session with remote system
4	Transport	Break data stream in smaller segments and provide reliable and unreliable data delivery
3	Network	Provide logical addressing
2	Data Link	Prepare data for transmission
1	Physical	Move data between devices

==> The Application Layer

This is the last and the topmost layer of the OSI model. This layer provides an interface between application programs running in system and network. If any application needs to access any resource that is available in remote system, it interacts with this layer. Then this layer encompasses the protocols and services that the application will employ to access that resources.

There are two types of application programs; network-aware and network-unaware. An application program is considered as network-aware when it can make any sort of network request. If an application program can’t make any kind of network request, it is considered as network-unaware program.

==> The Presentation Layer

The sixth layer of OSI model is the Presentation layer. Applications running in local system may or may not understand the format that is used to transmit the data across the network. The presentation layer works as the translator in OSI model. When receiving data from application layer, it converts that data in such a format that can be sent over the network. When receiving data from session layer, it reconverts that data in such a format that the application which will use the incoming data can understand.

==> The Session Layer

The session layer is the fifth layer of OSI model. It is responsible for setting up, managing, and dismantling sessions between presentation layer entities and providing dialogs between computers.

When an application makes a network request, this layer checks whether the requested resource is available in local system or in remote system. If requested resource is available in remote system, it tests whether a network connection to access that resource is available or not. If network connection is not available, it sends an error message back to the application informing that connection is not available.

If network connection is available, it establishes a session with remote system. For each individual request, it uses a separate session. This allows multiple applications to send or receive data simultaneously. When data transmission is completed, it terminates the session.

==> The Transport Layer

The transport layer is the fourth layer of OSI model. It provides following functionality: -

Segmentation

On sending computer, it breaks data stream into smaller pieces before transmission. Each piece is known as segment and the process of breaking data into smaller pieces is known as segmentation. On receiving computer, it joins all segments back in data stream. So the upper layers receive data in the format in which it was sent.

Data transportation

This layer establishes a logical connection between sending system and receiving system and uses that connection to provide end-to-end data transportation. For data transportation, it mainly uses two protocols; TCP and UDP.

Multiplexing

Through the use of port numbers, this layer also provides connection multiplexing. Connection multiplexing allows multiple applications to send and receive data simultaneously

==> The Network Layer

The third layer of OSI model is the Network Layer. This layer takes data segment from transport layer and adds logical address to it. A logical address has two components; network partition and host partition. Network partition is used to group networking components together while host partition is used to uniquely identity a system on a network. Logical address is known as IP address. Once logical address and other related information are added in segment, it becomes packet.

This layer decides whether the packet is intended for local system or remote system. It also specifies the standards and protocols which are used to move the data packets across the networks.

To move data packet between two different networks, a device known as router is used. Router uses logical address to take routing decision. Routing is a process of forwarding data packet to its destination.

==> The Data Link Layer

The Data Link Layer is the second layer of OSI model. This layer defines how networking components access the media and what transmission methods they use. This layer has two sub-layers; MAC and LLC.

MAC (Media Access Control)

This sub layer defines how the data packets are placed in media. It also provides physical addressing. Physical address is known as MAC address. Unlike logical addresses which need to be configured, physical addresses are pre-configured in NIC. MAC address is used to uniquely identify a host in local network.

LLC (Logical Link Control)

This sub layer identifies the network layer protocol. On sending computer, it encapsulates the information of the Network Layer protocol in LLC header from which the Data Link layer receives the data packet. On receiving computer, it checks the LLC header to get the information about the network layer protocol. This way a data packet is always delivered to the same network layer protocol from which it was sent.

==> The Physical Layer

The Physical Layer is the first layer of OSI model. This layer specifies the standards for devices, media and technologies which are used in moving the data across the network such as:-

1. · Type of cable used in connecting the devices
2. · Patterns of pin used in both sides of cable
3. · Type of interface card used in networking device
4. · Type of connector used to connect the cable with network interface
5. · Encoding of digital signals received from the Data Link layer based on the attached

This was exapmle of osi model