OSI Model (Tutorial-6)

What is the OSI model ?

• Open Systems Interconnection model is fundamental to all communications between network devices. 
• Developed in 1974 by ISO after the American Department of Defence began using the TCP/IP suite of protocols. 
• Finally adopted in 1977. It is now the theoretical model for how communication takes place between network devices.

What are the seven layers ?

Layers OSI Model

• Layer 7 
• Layer 6 
• Layer 5 
• Layer 4 
• Layer 3 
• Layer 2 
• Layer 1

Layers

• In the sense of purpose and responsibility, each layer is separate and independent 
• Each has its own function, but also provides a service to those layers above and below itself 
• The model should be considered an aid to understanding the nature of communication on the network and useful in sorting out troubles that might occur on a network 
• By providing, it allows both software engineers and hardware manufacturers ensure their products work together.

The Layers at Work

• When communicating, each OSI layer talks with the same layer in the other device 
• E.g. the Application Layer of Device A communicates with the Application Layer of Device B, by passing the data through the other layers 
• The Application Layer of each device is not concerned with how the other layers are functioning, but it does rely on them to do their job

How to Remembering the Layers?

Application                 All

Presentation               People

Session                      Seem

Transport                   To
Network                    Need

Data Link                   Data

Physical                      Processing

How does data flow?

• When data is sent from the application on the source computer the following happens 
• Data in the form of a packet moves down through the layers 
• When it reaches the Physical Layer it is ready to be sent along the cable 
• At the Physical Layer the bits may be analogue or digital, in the form of electrical, light or radio waves
• The data is transmitted to the destination device 
• It travels up through the layers of the OSI model, reaching the user. 
• As data moves down through the layers it is encapsulated – ie additional information is added as headers or trailers 
• The data in the packet does not change 
• See the following diagram

Encapsulation

Application Layer

• ‘Closest’ layer to the user 
• Works with the applications you use to communicate over the network 
• E.g.. Services include SMTP, HTTP and FTP 
• Clicking on a link on a web page issues a command for the browser to retrieve the relevant information from the Internet 
• In this example your computer is the source, and the host of the web site information is the destination 
• The application completes your request and delivers the information to your computer

Application Services

• File Services 
• Electronic-mail Services 
• Network-printing Services 
• Application Services 
• Database Services

Presentation Layer

This layer has three fundamental functions 

Data Presentation 

• Enables receiving device to understand the information sent from the source 
• Converts data from native format (abstract syntax) to a common format (transfer syntax), e.g. ASCII

Data Compression

• By reducing the volume of data, transfers can take place in less time 
• Packets are examined and such things as spaces in text removed 
• The destination device returns the data to its original format before passing to the Application Layer 

Data Encryption

• Allows data to be converted to a form which hides its meaning, apart from those you wish to see it 
• Not all data is encrypted on its journey across the network 
• In order for decryption to occur at the destination device a ‘key’ is required

Session Layer

• Primarily responsible for handling the session between devices (beginning, maintaining and finishing) 
• Enforces order in the communication between devices 
• Regulates the flow of data 
• It takes responsibility for the following ….

Session Services

• Establishing a Connection 
• Ending the Connection 
• Handshaking – SYN and ACK packets 
• ‘Keep alive messages’ 
• Session must be terminated (otherwise one device will be still transmitting without any device actually listening) 
• Dialogue Control (simplex, half-duplex, full-duplex) 
• Dialogue Separation – checkpoints within the transmission which allow the detection of lost packets, and subsequent retransmission

Transport Layer

• Ensures reliable transport of packets from source to destination 
• Also manages the speed of transmission – flow control 
• There are two types of transmission (Connection-Oriented Transmissions and Connectionless Transmissions)

Connection-Oriented Transmissions

• Also known as ‘ Reliable Transport Method’ – uses acknowledgement (ack) packets on successful receipt of data. 
• Extra packets slows down communication 

Features are

• Reliability 
• Slower Communication 
• Packets are re-transmitted if unrecognisable or not received 
• Once all the data is received successfully , the packet is re-assembled and the Transport Layer passes it to the Session Layer

Connectionless Transmissions

• In this mode the transmitting device does not require acknowledgements from the receiver, and continues to transmit on the assumption that the data was received 

Features are:

• Little or No Reliability 
• Faster Transmission 
• Packets are not Re-transmitted

Flow Control

• Establishes the maximum speed at which both sender and receiver can communicate at 
• Transport Layer determines largest packet size which can be sent 
• Packets are numbered – to allow re-assembly in the correct order

Network Layer

• Responsible for the correct addressing and delivery of packets of data 
• These are known as datagrams 
• Uses the network address ( this is a logical address – and does not depend upon any hardware in the device, or the device’s physical location) 

The Network Layer does the following:

• Adds the address to the packet (encapsulation) 
• Maps the network address to the devices physical address 
• Determines the best path for the packet (routing) 
• Ensures that the packet is in the correct format for the destination

How does it work ?

• Encapsulation at the Transport Layer involves adding the address of the sender to the datagram 
• The destination address is now added. Both addresses are logical. 
• Both addresses are necessary for packets to move between end systems. 
• If a packet must move to another network, a routing protocol is required 
• If different packet lengths are used on the different networks, the Network Layer formats the data accordingly 
• The primary piece of hardware which works on this layer is the router.

Data Link Layer

Has two sub layers of its own: 

• Logical Link Control (LLC) 
• Media Access Control (MAC) 
• LLC acts between protocols such as Internet Protocol (IP) and the MAC method. 
• MAC is responsible for the connection to the physical media (eg cable)

MAC

• Each NIC has a unique number hard coded in to the card – its physical address 
• The first 6 digits denote the manufacturer, the next six are unique) – type “winipcfg” on your PC 
• When the MAC address is added to the packet it is now known as a frame 
• It now has all the information required to travel from the source to the destination

Physical Layer

• The lowest, bottom, layer – responsible for the physical connection between devices 
• The NIC converts the data (bits) in to transmission signals. 
• Transmissions may be analogue or digital 
• Responsible for the rate of transmission 
• Includes all components such as the type of connector (RJ-45, Token Ring, BNC, SC connector) 
• Devices at this level include NICs, repeaters, hubs and concentrators.

OSI versus TCP/IP Model

• OSI model is an important concept 
• Protocol most in use on modern networks is TCP/IP 
• TCP/IP does not map its layers precisely to OSI model 
• OSI = 7 layers, TCP/IP = 4 layers (sometimes a 5th physical layer is referred to)

TCP/IP Model

• Application or Process Layer – concerned with how data at both ends is handled. 
• Transport Layer – manages flow of data 
• Internet Layer – consists of several protocols, primary protocol is IP (providing hierarchical addressing scheme 
• Data Link (or Network Interface) Layer – manages transmission of data within the network 
• Physical Layer – not really defined, TCP/IP leaves the physical connection to manage itself

Conclusion

• OSI open system interconnect model gives a layered approach to the network model and describes how the thing goes one by one in a particular order . 
• Each layer a perform a unit of work on each packet called packet data unit 
• Data Encapsulation Technique of wrapping the information on data unit at each layer . 
• TCP/IP Model contains four layers in comparison to OSI model it is also called DoD model .