OSI Reference Model

OSI Reference Model

(Open System Interconnection)

This model was established in 1970 by ISO. It is data communication model.
It is logical and conceptual model.
This model has 7 layers:
1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data link Layer
7. Physical Layer

1) Application Layer

Deals with the communication of software on different machines.
For example: HTTP, FTP, SMTP, Telnet and DNS servers, all operating systems, web browsers, communication software (messengers, Skype etc).

2) Presentation Layer

Three activities are taking place at this layer:
1) Encryption: The process of converting the plain text in to cipher text for data confidentiality is called encryption. i.e. DES, 3DES and AES
2) Translation: Coverts protocol from one form to other. Like IPX to IP and vise versa.
3) Compression: It simply works with the compression of data like win zip.

3) Session Layer

A time period in which two machines communicate is called a session.
This layer deals with:

• Session establishment: To establish a session before data communication.
• Session management: To manage the session throughout the communication.
• Session termination: To manually terminate the session or there is a network down.

4) Transport Layer

• It uses protocols to transfer data from one machine to another machine.
• Two protocols work on this layer, TCP/IP and UDP.

TCP/IP (Transmission control protocol/Internet protocol)

• It provides error recovery.
• It relies on IP for end-to-end delivery of data, including routing issues.

Functions of TCP/IP
1. Multiplexing using port number

• TCP identifies applications by their port numbers.
• Multiplexing relies on the use of a concept called a socket
• Socket consist of three things

1. IP address
2. Transport protocol (TCP or UDP)
3. Port number

• Multiplexing helps in running multiple applications on a machine. PC keeps every application separate and transfers data accordingly.
• Every application and protocol has port number.
• Port number identifies data that which application this data belong to.
• Like:

Telnet 23
SMTP 25
HTTP 80
HTTPS 443
DNS 53
Range of Port number is 1 – 65536.
1 – 1024 are already assigned.

2. Error recovery

• TCP numbers data bytes using the sequence and acknowledgement fields in the tcp header
• TCP sends data in sequence
• Sender also sends re-transmission timer.

3. Flow control using Windowing

• TCP implements flow control by taking advantage of sequence and acknowledgement fields in the TCP header.
• Window size is the number of packets sent before getting acknowledgement.
• It starts with smaller size of window than gradually increases until some error occurs.

UDP (user data gram protocol)

• It is connection less protocol.
• It also works at transport layer.
• It does not do any error recovery thus it takes less bandwidth.
• It perform
• Data transfer
• Data segmentation
• Multiplexing using port numbers

5) Network Layer

• It deals with the function of path selection and logical addressing
• The protocols use are:
• IP (internet protocol) in TCP/IP
• IPX in Novell
• DDR in AppleTalk
• Every network and host has IP address.
• Router works on this layer

IP define three different network classes:

• Class A 1-126
• Class B 128-191
• Class C 192-223

6) Data Link Layer

• This layer is divided into two sub-layers
• LLC Sub-layer: it create link between network and data link layer. For example ARP.
• Mac Sub-layer: it deals with error detection, addressing and orderly delivery of frames.
• Addressing: In many networks more than two devices are attached to the same physical network.
• Data link layer defines addresses to make sure that correct device listen and receive data.
• In Ethernet data link layer uses Media Access Control (MAC) address.
• It is 48-bit long address.
• It also performs error detection.
• It put a field of FCS (frame check sequence) in a frame. This field holds the value of CRC (cyclical redundancy check) algorithm.
• This CRC value is calculated by mathematical formula applied on data in the frame.
• Same value in generated on destination, if values are same than there is no error.
• It does not perform error recovery only error detection

7) Physical layer

• It is pure hardware layer of OSI model.
• On this layer frame is converted into bits.
• Hubs, repeaters and all cables work on this layer.

Data on different layers

• Data on Transport layer is called a segment.
• Data on Network layer is called a packet.
• Data on Data link layer is called a frame.
• Data on Physical layer is converted into bits.