The Open Systems Interconnection (OSI) model is a conceptual framework for understanding how computer networks operate. It was developed by the International Organization for Standardization (ISO) in 1984, and it consists of seven layers, each with a specific function in the communication process. In this article, we will discuss the OSI model in detail and explore each layer's functions.
Physical Layer (Layer 1):
The physical layer is the lowest layer of the OSI reference model. It is responsible for the actual physical connection between devices, including the transmission and reception of raw bit streams over a physical medium. The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
The functions of the physical layer are as follows:
a) Bit synchronization: The physical layer provides synchronization of the bits by providing a clock. This clock controls both sender and receiver, thus providing synchronization at the bit level.
b) Bit rate control: The physical layer defines the transmission rate, i.e., the number of bits sent per second.
c) Physical topologies: The physical layer specifies the way in which the different devices/nodes are arranged in a network, i.e., bus, star, or mesh topology.
d) Transmission mode: The physical layer also defines the way in which the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex, and full-duplex.
Examples of physical layer devices include hubs, repeaters, modems, and cables. The physical layer, along with the data link layer and the network layer, is also known as the lower layers or hardware layers.
Data Link Layer (DLL) (Layer 2):
The data link layer is responsible for the node-to-node delivery of the message. It ensures error-free data transfer from one node to another over the physical layer. When a packet arrives in a network, the data link layer transmits it to the host using its MAC address.
The data link layer is divided into two sub-layers: the Logical Link Control (LLC) and the Media Access Control (MAC). The packet received from the network layer is further divided into frames, depending on the frame size of the Network Interface Card (NIC). The DLL encapsulates the sender and receiver's MAC address in the header. The receiver's MAC address is obtained by placing an Address Resolution Protocol (ARP) request onto the wire, asking "Who has that IP address?" The destination host replies with its MAC address.
The functions of the data link layer are as follows:
a) Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.
b) Physical addressing: After creating frames, the data link layer adds physical addresses (MAC address) of the sender and/or receiver in the header of each frame.
c) Error control: The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.
d) Flow control: The data rate must be constant on both sides; otherwise, the data may get corrupted. Thus, flow control coordinates the amount of data that can be sent before receiving acknowledgment.
e) Access control: When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time.
A packet in the data link layer is referred to as a frame. The DLL is handled by the NIC (Network Interface Card) and device drivers of host machines. Examples of data link layer devices include switches and bridges, while routers are examples of network layer devices. Let's move on to the next layer.
Network Layer (Layer 3):
The network layer is responsible for the creation and management of logical paths, or "routes," for data to travel between devices on different networks. It provides the means to transfer data across multiple interconnected networks, such as the internet, and ensures that the data is delivered to the correct destination. The network layer is also responsible for addressing, routing, and traffic control.
One of the primary functions of the network layer is addressing, which is used to uniquely identify devices on a network. Internet Protocol (IP) is the most widely used protocol at the network layer and it provides a hierarchical addressing scheme, using IP addresses, to ensure that each device on the network has a unique identity. IP addresses are used to route packets of data between devices on different networks, using a variety of routing protocols, such as Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP).
Another important function of the network layer is routing, which determines the path that data takes through a network in order to reach its destination. Routing protocols are used to exchange information about the state of the network, such as available paths and network congestion, in order to make intelligent decisions about how to route data. This ensures that data is delivered quickly and efficiently, while also avoiding congestion and network failures.
Traffic control is also a key responsibility of the network layer. It manages the flow of data across the network by implementing congestion control mechanisms, such as packet queuing and flow control, to prevent the network from becoming overwhelmed. By managing the flow of data, the network layer can ensure that data is delivered reliably and with minimal delay.
Transport Layer (Layer 4):
The transport layer is responsible for providing reliable, end-to-end communication between applications running on different devices. It takes data from the application layer and breaks it up into smaller units, called segments, which can be transmitted over the network. The transport layer also provides flow control, error recovery, and congestion control mechanisms to ensure that data is delivered reliably and efficiently.
One of the primary functions of the transport layer is segmentation, which involves breaking up larger packets of data into smaller segments that can be transmitted over the network. This allows for more efficient use of network resources and can help to avoid congestion. The transport layer also adds a header to each segment, which includes information such as source and destination ports, sequence numbers, and acknowledgment numbers.
Flow control is another important function of the transport layer. It manages the rate at which data is transmitted between devices in order to prevent the receiving device from becoming overwhelmed. The transport layer achieves this by using mechanisms such as sliding windows, which allow for a variable number of segments to be transmitted before waiting for an acknowledgment.
Error recovery is also a key responsibility of the transport layer. It ensures that data is delivered reliably, even in the presence of errors or packet loss. This is accomplished through the use of techniques such as retransmission, where lost or damaged packets are retransmitted until they are successfully received.
Congestion control is another important function of the transport layer. It manages the flow of data across the network in order to prevent congestion and network failures. Congestion control mechanisms such as window-based flow control, slow start, and congestion avoidance, are used to ensure that data is transmitted at a rate that is optimal for the network, without causing congestion or network failures.
Session Layer (Layer 5):
The session layer establishes, maintains, and terminates communication sessions between applications on different hosts. It allows two applications to establish a session and exchange data in either simplex, half-duplex or full-duplex mode.
The functions of the session layer are:
Session establishment, maintenance, and termination: This layer allows the establishment of a session between applications on different hosts, maintains the session while it is being used, and then terminates the session when it is no longer needed.
Session synchronization: The session layer allows for synchronization between the communicating applications. This ensures that the data is exchanged in the correct order.
Checkpointing and recovery: The session layer enables the checkpointing of data during a session. This allows for the recovery of data that may have been lost or corrupted during transmission.
Session layer is implemented in the operating system and/or application.
Presentation Layer (Layer 6):
The presentation layer is responsible for data representation, encryption, and compression. It transforms the data received from the application layer into a form that is suitable for transmission and presentation to the user.
The functions of the presentation layer are:
Data encryption and decryption: The presentation layer is responsible for encrypting and decrypting data to provide security and privacy during transmission.
Data compression: The presentation layer can compress data to reduce the amount of data that needs to be transmitted.
Data conversion: The presentation layer can translate between different data formats to ensure that the data can be presented in a form that is meaningful to the user. The presentation layer is implemented in the operating system and/or application.
Application Layer (Layer 7):
The application layer is responsible for providing services to the user. It is the layer closest to the user and interacts directly with the user's application. It provides access to network services, such as email, file transfer, and remote login.
The functions of the application layer are:
Application services: The application layer provides access to network services, such as email, file transfer, and remote login.
User interface services: The application layer provides a user interface for the user to interact with the network services.
The application layer is implemented in the operating system and/or application.
In conclusion, the OSI reference model provides a standard for how different devices communicate over a network. The different layers work together to ensure that data is transmitted reliably and efficiently. Understanding the different layers of the OSI model is essential for network administrators, engineers, and technicians.
