ISO-OSI

what is ISO-OSI ?

• the main idea between computer networks is the communication between different systems.
• the purpose of communication is for information sharing

suppose we have two systems (A) and (B) which want to communicate with each other and this two systems are connected to each other through a computer network the distance between (A) and (B) maybe only few meters or they might be across different continents if (A) wants to send some data to (B) then the data has to travel through a network to reach (B) during this transfer the data has to travel through different intermediate nodes to ensure that data reaches from source machine (A) and to the destination machine (B) securely and correctly we need to have some some rules which carry the communication between (A) and (B) this rules called protocols thus for every communication we need to have some protocols which guide the communication. these protocols are grouped together and called as a communication model. the OSI reference model is one such communication model, OSI stands for open system interconnection which means every system participating in this model is open for communication with other systems. OSI reference model defines the functionality of network communication. the OSI model divides the communication into 7 layers.

Some history :

? In the 1977 the international standards organization (ISO) started work on standards to try to ensure compatibility between different manufacture equipment. This is known as the open system interconnection (OSI) model, it consists of definition for the seven layers . Each layer at the transmission end has a direct relationship with the same layers from the bottom.

this is how OSI model looks like

The top 4 layers only implemented by the end systems and the intermediate nodes only implement the bottom three layers, since their task is just to pass on the data along the navigation path.

All the seven layers have a distinct function:

• Physical Layer:

The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. The layer moves the data in the form of electromagnetic or optical signal across the transmission medium. It is responsible for the movement of data from one node to next.

This layer covers all mechanical, electrical, functional and procedural aspect of physical communication. Such characteristics as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

summary : Defines the electrical and mechanical interfacing to the physical medium. Sets up, maintains and disconnects physical links. Include hardware (I/O ports, modems, communication lines, etc.) and software (device driver)

• Data link layer :

Data link layer attempts to provide reliable communication over the physical layer interface. Layer 2 manages the ordering of bits, packets, to and from data segments. The ensuing result is called frames. Data links are responsible for moving frames from one node to another.

A data link layer Create and detect frame boundaries and detect an error and produce an acknowledgement and re-transmit the data. Its responsibility includes, Packet addressing, Media access control, Format the frame used to encapsulate data, Error notification on the Physical layer, Managing of error messaging specific to the delivery of packets.

summary : Establishes error-free paths over physical channel, frames massages, error detection and correction. Manages access to and use of channel. Ensure proper sequence of the transmitted data

• Network layer:

The network layer is responsible for the delivery of individual packets from the source host to destination host. It defines the most optimum path the packet should take from the source to the destination. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node.

Functions generally carried out by OSI are: Traffic direction to the end destination, Packet switching, Packet sequence control, End-to-end error detection, from the data sender to the receiver of data, Congestion control, Network layer flow control and Network layer error control, Gateway services.

summary : 1- Address and routes messages. 2- Sets up communication paths. 3-Flow control

• Transport layer:

Purpose of this layer is to provide a reliable mechanism for the exchange of data between two processes in different computers.This layer transport the data in a sequential manner with no loses.

Transport layer divides large messages into small packets so that easily transmitted. Main functions of transport layer are - guarantee data delivery, flow control, error detection and error recovery.

summary:Provides end-to-end control of a communication session. Allows processes to exchange data reliably

• Session layer:

Session layer provides a mechanism for controlling the dialogue between the two end systems. It defines how to start, control and end conversations (called sessions) between applications.The Session layer utilizes the virtual circuits created by the Transport layer to establish communication sessions.

It establishes, terminates, and monitors communication sessions between applications. Session layer is also responsible for terminating the connection.

summary: Establishes and controls node-system-dependent aspects. Interfaces transport level to logical functions in node operating system.

• Presentation layer:

Presentation layer defines the format of communication data is to exchanged between to system. And this layer is responsible for translation, Protocol conversion, Character set conversion, Interpretation of graphics commands, compression and encryption.

summary: Allow encoded data transmitted via communication path to be presented in suitable format for user manipulator.

• Application layer:

The application layer is responsible for providing services for user. Application layer contains management functions to support distributed applications. Examples of application layer are applications such as file transfer, electronic mail, remote login etc.

summary : Allows a user service to be supported, e.g. resource sharing, file transfers, remote file access, etc.

summarize for all layers :

• Full architecture of OSI model

How data flows through the OSI model

In order for human-readable information to be transferred over a network from one device to another, the data must travel down the seven layers of the OSI model on the sending device and then travel up the seven layers on the receiving end.

For example: Mr. Cooper wants to send Ms. Palmer an email. Mr. Cooper composes his message in an email application on his laptop and then hits ‘send’. His email application will pass his email message over to the application layer, which will pick a protocol (SMTP) and pass the data along to the presentation layer. The presentation layer will then compress the data and then it will hit the session layer, which will initialize the communication session.

The data will then hit the sender’s transportation layer where it will be segmented, then those segments will be broken up into packets at the network layer, which will be broken down even further into frames at the data link layer. The data link layer will then deliver those frames to the physical layer, which will convert the data into a bitstream of 1s and 0s and send it through a physical medium, such as a cable.

Once Ms. Palmer’s computer receives the bit stream through a physical medium (such as her wifi), the data will flow through the same series of layers on her device, but in the opposite order. First the physical layer will convert the bitstream from 1s and 0s into frames that get passed to the data link layer. The data link layer will then reassemble the frames into packets for the network layer. The network layer will then make segments out of the packets for the transport layer, which will reassemble the segments into one piece of data.

The data will then flow into the receiver's session layer, which will pass the data along to the presentation layer and then end the communication session. The presentation layer will then remove the compression and pass the raw data up to the application layer. The application layer will then feed the human-readable data along to Ms. Palmer’s email software, which will allow her to read Mr. Cooper’s email on her laptop screen.

• OSI message passing

• Simplified OSI model

For many industrial protocols the use of the full seven layers of the OSI model is inappropriate as the application may require a high-speed response. hence a simplified OSI model is often preferred for industrial applications where time critical communications is more important than full communications functionality provided by the seven-layer model. Generally, most industrial protocols are written around three layers:

?? The physical layer

?? The data link layer

?? The application layer

• Limitation of Simplified OSI

?? As there is no transport layer, the maximum size of the application messages is limited by the maximum size allowed on the channel

?? As there is no network layer, no routing of messages is possible between different networks

?? As there is no session layer, no full duplex communications are possible

?? As there is no presentation layer, message formats must be the same for all nodes

summarize for the whole article :

Open System Interconnection, OSI is a network model developed by ISO in 1978 where peer-to-peer communications are divided into seven layers. Each layer performs a specific task or tasks and builds upon the preceding layer until the communications are complete. Below are the purposes of each of the seven layers.

The seven layers in the OSI model are usually displayed in reverse order, with layer 7 listed first (at the top) and layer 1 listed last (at the bottom).

1 - Physical layer - responsible for the electrical, mechanical, and timing across the link.

2 - Data link layer (also known as the link layer) - responsible for transmitting data across a link.

3 - Network layer - responsible for routing information through the network and allowing systems to communicate.

4 - Transport layer - responsible for transferring information between endpoints on the network and deals with errors, such as lost or duplicate packets.

5 - Session layer - responsible for managing a session between two applications.

6 - Presentation layer - responsible for the data formatting and display, allowing for compatibility.

7 - Application layer - responsible for user interaction.