OSI Model

OSI stands for Open System Interconnection. An open system is a model that allows any two systems to communicate even if their architectures are different.

OSI is a super model for communication between different computers. It is complete model that covers all aspects of network communications. It was developed by International Standards Organization (ISO) in 1983.

OSI model consists of seven layers. Each layer performs a specific function in network communication. OSI model is called Reference Model because it presents an ideal rather than reality. No communication protocol follows this model by 100%.

 

Layers of OSI Model

OSI model consists of seven Iayers that are as follows:

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

 

1. Physical layer

Physical layer is the bottom layer of OSI model. It transmits stream of bits and defines the data is transmitted over the network and what control signals are used. Its main function is to control how a stream of bits is sent and received over the physical medium. Physical layer must decide the following:

1. Characteristics of Media: The physical layer defines the characteristics and type of transmission medium.
2. Representation of Bits: The bib are encoded into electrical signals for transmission. The physical layer defines the type of encoding.
3. Data Rate: Physical layer defines the number of bits that will be sent in each second.
4. Synchronization: The clocks of sender and receiver are also synchronized.
5. Line Configuration: The physical layer defines the attachment of communication devices with medium.
6. Transmission Mode: The physical layer defines the direction of transmission between two devices: simplex, half—duplex or full-duplex

Common protocols used at this level are IEEE 802, IEEE 802.2, FDDI.

 

2. Data Link Layer

The Data Link Layer is responsible for the reliability of the physical link established at layer 1. Data link layer must decide the following:

1. Framing: The data link layer divides the stream of bits into manageable data units called frames.
2. Flow Control: If the rate of sending data is more than the rate of receiving data, data link layer imposes a flow control mechanism to prevent this situation.
3. Error Control: The data link layer detects and retransmits damaged or lost frames. It also prevents the duplication of frames.
4. Access Control: If two or more devices are connected to the same link, data link layer determines which device has control over the link at a given time.

Data Link Layer is divided in two sub-layers:

i. LLC

LLC stands for Logical Link Control. It is the upper sub-layer. It ensures reliability of physical connection. The standard protocol IEEE 802.2 is the most commonly used standard.

Point-to-Point Protocol (PPP) is an important standard at this OSl level. It is used to communicate across point-to-point links. It is important protocol for wide area networking.

 

ii. MAC

MAC stands for Media Access Control. It specifies how workstations cooperatively share the transmission medium. The IEEE 802.3 standard specifies a medium-access method known as “carrier sense multiple access with collision detection (CSMA/CD)!”

 

3. Network Layer

The network layer is responsible for establishing, maintaining, and terminating network connections. It manages the delivery at data from source to destination.

Network layer determine logical path between sender and the receiver. There may be many network between two computers: This layer manages to send data from source computer to the destination computer.

Common protocols that operate at this level are IP, lPX, and X.25.

 

4. Transport Layer

The transport layer controls the flow of data. It ensures that messages are delivered error free. It divides large messages into small packets for efficient transmission. These packets are reassembled, checked for errors and acknowledged at receiving side. If there are errors in transmission, the data is retransmitted.

Common protocols that operate at transport layer are TCP, UDP, SPX, and NetBEUl.

 

5. Session Layer

The session layer establishes, manages, and terminates user connections. A session is an exchange of messages between computers. It synchronizes user tasks.

Synchronization involves the use of checkpoints in data stream. If a failure occurs, only madam from the last checkpoint is mummified. Suppose we want to send 1000 pages of data. Checkpoint can be used after each 100 pages. If there is an error at page 320, the pages from 301 will be retransmitted. Page from 1 to 300 will not be retransmitted.

Winsock and NetBIOS are visually shown as functioning at the session layer.

 

6. Presentation Layer

The presentation layer performs data reformatting, data compression and encryption.

Data Reformatting: When two computers exchange data, the data is changed to bit streams before it is transmitted. Two computers may use different encoding techniques. The presentation layer at sending computer changes data according to the sender's format. The presentation layer at receiving according to the computer changes data receiver’s format.

Encryption: The presentation layer encrypts the data before transmission. It means that the sender transforms the original information to another form and sends the resulting message over the network. The receiver again transforms the message back to its original form. It is called decryption.

Compression: Data compression reduces the number of bits to be transmitted. The presentation layer compresses a large amount of data into small size.

Common protocols that operate at the presentation layer include SMB, NCP, & NFS.

 

7. Application Layer

The application layer is the top-most layer of OSI model. It provides services directly to user applications. It enables the user to access the network. It provides user interfaces and support for services such as email, remote file access and transfer, shared database management, and other types of distributed information services.

File Transfer: It allows a user to access, retrieve and manage files in a remote computer.

Mail Services: It provides the basis for email forwarding and storage facilities.

Directory Services: It provides distributes database sources and access for global information about various objects and services.

• Data Communications and Networks