The OSI model was conceptualized to standardize the characteristics of communication of computing systems without any regard to their underlying technology and internal structure. It aims to provide inter-operability between diverse systems of communication using standard protocols.
What is OSI Model
OSI (Open systems interconnect) is a reference model that depicts data communication over a network. It was created based on a proposal from International Organization for Standardization (ISO). Its official title is ISO OSI Reference Model because it relates to or describes systems that are available for communication with other devices. It also defines standards for:
- Communication between devices.
- Means to inform devices about when to transmit/ not transmit the data.
- Methods that ensure accurate data flow rate to devices.
- Methods that ensure the data is received by the intended recipient.
- Arrangement and connection of physical transmission media.
It divides the computer network architecture into seven (7) layers of logical progression. Lower layers perform functions like routing of binary data and handling electrical signals. Issues such as network request and response, data representation and network protocols are handles by the higher levels of the model.
Fig. 1 – Seven Layers of OSI model
Why was OSI Model Developed
This model was developed with the following purposes in mind:
- To create a platform that is common for hardware manufacturers and software developers. This promotes the design of networking products that correspond with each other over the network.
- To assist network administrators by breaking down large data chunks into smaller segments. These are easier to comprehend, supervise and troubleshoot.
Characteristics of Seven Layers in OSI Model
Layer wise characteristics of OSI model are as follows :
1. Physical Layer in OSI Model
Physical layer of the OSI model has the following characteristics:
- Deals with transmitting raw bits of data over a physical medium.
- It is hardware specific and is responsible for actual physical connection between a network medium and your computer.
- Data is handled in the form of 1s and 0s. These are represented by light pulses, electric voltages or radio frequencies.
- All devices functioning at this layer handle signaling.
- Ethernet cables, Token ring networks , pin-outs, cable types, connectors are some of the components included in this layer.
Fig. 2 – Physical Layer in OSI Model
2. Data Link Layer in OSI Model
Data link layer of OSI model has the following characteristics :
- Enables data movement over a link from one device to another
- Media access control.
- Packet addressing.
- Formatting the frame that is used to encapsulate data.
- Error notification on physical layer.
- It orders bits and packets to and from data segments. This ensuing result is called as frames. They contain data that are set in an orderly manner.
- Ensure error-free communication between two devices by correct transmission of frames.
Data Link Layer is further divided into 2 components. They are termed as Media Access Control (MAC) and Logical Link Control (LLC) respectively. This division is mainly attributed to the complex structure of data link layer.
Fig. 3 – Data Link Layer in OSI Model
3. Network Layer in OSI Model
Network layer of OSI model has the following characteristics:
- Responsible for establishing paths that are used for the transfer of data packets between network devices.
- Traffic direction.
- Addressing; Service and logical network addresses.
- Packet switching.
- Controlling packet sequence.
- Complete error detection; from sender to receiver.
- Congestion control.
- Gateway services.
Fig. 4 – Network Layer in OSI Model
4. Transport Layer in OSI Model
Transport layer in OSI-model has the following characteristics:
- Responsible for message delivery between the networked hosts. Messages are fragmented and reassembled by this layer. It also controls the reliability of any given link.
- Guaranteed delivery of data.
- Name resolution.
- Flow control.
- Error detection and recovery.
TCP and UDP are the common transport protocols used at this layer.
Fig. 5 – Transport Layer in OSI Model
5. Session Layer in OSI Model
Session layer of the OSI-model has the following characteristics:
- Establish, monitor and terminate the communication session between applications.
- Name lookup, security functions and data synchronization.
- Placement of header information in a packet.
- Determines whether the messages that are exchanged during a session are full or half duplex.
Fig. 6 – Session Layer in OSI Model
6. Presentation Layer in OSI Model
Presentation layer of the OSI model has the following characteristics:
- This layer defines the syntax used by hosts in the network to communicate.
- Data compression.
- Encryption and decryption of data.
- Data translation.
- Interpreting graphics commands.
- Protocol conversion.
- Gateway services.
Fig. 7 – Presentation Layer in OSI-Model
7. Application Layer in OSI Model
Application layer of OSI model has the following characteristics:
- It works as an interface between the software and the network protocol on the computer. It provides services that are necessary to support the applications.
- This layer provides interface for FTP applications, email and telnet.
Some common protocols in this layer include:
- File Transfer Protocol (FTP).
- Simple Mail Transfer Protocol (SMTP).
- Internet Message Access Protocol (IMAP).
- Hypertext Transfer Protocol (HTTP).
- Post Office Protocol (POP).
Fig. 8 – Application Layer in OSI Model
Why Use OSI Model
- It is truly a generic model. Changes in one layer does not affect any other layers in the model.
- Functions of each layer are described clearly. This in turn supports industry standardization.
- Standardization permits multiple vendor development.
- Software design, development and maintenance is easier because the model breaks the communication processes into smaller segments.
- Different types of software and network hardware can easily communicate with each other.
- It is adaptable and more secure.
Limitations of OSI Model
- It is sometimes considered to be too generic and is often criticized for not defining any particular protocol.
- New protocols may sometimes find it difficult to fit into this model. It is because OSI was created before any of these newer protocols.
- Service duplication can be seen in data link and transport layers.
- There is a great deal of inter-dependency between the layers and hence work cannot be completed in parallel.
- It is regarded as slow since it requires an agreement between 3 parties: service provider and users.
- Today, most LANs are very powerful and have lower error rates. Hence, data integrity feature provided by OSI is no longer needed.