Network Protocols and OSI Model

A network protocol is an established set of rules that determine how data is transmitted between different devices in the same network. Essentially, it allows connected devices to communicate with each other, regardless of any differences in their internal processes, structure or design. Network protocols are the reason you can easily communicate with people all over the world, and thus play a critical role in modern digital communications.

List of Network Protocols

There are different network protocols, but they all perform one of three primary actions:

  • Communication
  • Network management
  • Security


Communication protocols allow different network devices to communicate with each other. They are used in both analog and digital communications and can be used for important processes, ranging from transferring files between devices to accessing the internet.

Common types of communication protocols include the following:

  • Automation: These protocols are used to automate different processes in both commercial and personal settings, such as in smart buildings, cloud technology or self-driving vehicles.
  • Instant messaging: Instantaneous, text-based communications on smartphones and computers occur because of a number of different instant messaging network protocols.
  • Routing: Routing protocols permit communication between routers and other network devices. There are also routing protocols specifically for ad hoc networks.
  • Bluetooth: Ever-popular Bluetooth devices — including headsets, smartphones and computers — work due to a variety of different Bluetooth protocols.
  • File transfer: If you have ever moved files from one device to another, either via a physical or digital medium, you’ve used file transfer protocols (FTP).
  • Internet Protocol: Internet Protocol (IP) allows data to be sent between devices via the internet. The internet could not operate as it currently does without IP.

Network Management

Network management protocols define and describe the various procedures needed to effectively operate a computer network. These protocols affect various devices on a single network — including computers, routers and servers — to ensure each one, and the network as a whole, perform optimally.

The functions of network management protocols include the following:

  • Connection: These protocols establish and maintain stable connections between different devices on the same network.
  • Link aggregation: Link aggregation protocols allow you to combine multiple network connections into one link between two devices. This works to increase the strength of the connection and helps sustain the connection should one of the links fail.
  • Troubleshooting: Troubleshooting protocols allow network administrators to identify errors affecting the network, evaluate the quality of the network connection, and determine how administrators can fix any issues.


Security protocols, also called cryptographic protocols, work to ensure that the network and the data sent over it are protected from unauthorized users.

Common functions of security network protocols include the following:

  • Encryption: Encryption protocols protect data and secure areas by requiring users to input a secret key or password in order to access that information.
  • Entity Authentication: Entity authentication protocols create a system that requires different devices or users on a network to verify their identity before accessing secure areas.
  • Transportation: Transportation security protocols protect data while it is transported from one network device to another.

Network Protocol Example

Here are a few examples of the most commonly used network protocols:

  • Hypertext Transfer Protocol (HTTP): This Internet Protocol defines how data is transmitted over the internet and determines how web servers and browsers should respond to commands. This protocol (or its secure counterpart, HTTPS) appears at the beginning of various URLs or web addresses online.
  • Secure Socket Shell (SSH): This protocol provides secure access to a computer, even if it’s on an unsecured network. SSH is particularly useful for network administrators who need to manage different systems remotely.
  • Short Message Service (SMS): This communications protocol was created to send and receive text messages over cellular networks. SMS refers exclusively to text-based messages. Pictures, videos or other media require Multimedia Messaging Service (MMS), an extension of the SMS protocol.

The most popular model used to establish open communication between two systems is the Open Systems Interface (OSI) model proposed by ISO.

OSI Model

OSI stands for Open System Interconnection is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer. OSI consists of seven layers, and each layer performs a particular network function. OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications. OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task. Each layer is self-contained, so that task assigned to each layer can be performed independently.

Functions of the OSI Layers

There are the seven OSI layers. Each layer has different functions. A list of seven layers are given below:

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

Physical (Layer – 1)

OSI Model, Layer 1 conveys the bit stream – electrical impulse, light or radio signal — through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Example: Includes Ethernet, FDDI, B8ZS, V.35, V.24, RJ45..

Functions of a Physical layer:
  • Line Configuration: It defines the way how two or more devices can be connected physically.
  • Data Transmission: It defines the transmission mode whether it is simplex, half-duplex or full-duplex mode between the two devices on the network.
  • Topology: It defines the way how network devices are arranged.
  • Signals: It determines the type of the signal used for transmitting the information.

Data Link (Layer 2)

At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Example: Includes PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay..

Functions of the Data-link layer:
  • Framing: The data link layer translates the physical’s raw bit stream into packets known as Frames. The Data link layer adds the header and trailer to the frame. The header which is added to the frame contains the hardware destination and source address.
  • Physical Addressing: The Data link layer adds a header to the frame that contains a destination address. The frame is transmitted to the destination address mentioned in the header.
  • Flow Control: Flow control is the main functionality of the Data-link layer. It is the technique through which the constant data rate is maintained on both the sides so that no data get corrupted. It ensures that the transmitting station such as a server with higher processing speed does not exceed the receiving station, with lower processing speed.
  • Error Control: Error control is achieved by adding a calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link layer’s trailer which is added to the message frame before it is sent to the physical layer. If any error seems to occur, then the receiver sends the acknowledgment for the re-transmission of the corrupted frames.
  • Access Control: When two or more devices are connected to the same communication channel, then the data link layer protocols are used to determine which device has control over the link at a given time.

Network (Layer 3)

Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Example: Includes AppleTalk DDP, IP, IPX..

Functions of Network Layer:
  • Internetworking: An internetworking is the main responsibility of the network layer. It provides a logical connection between different devices.
  • Addressing: A Network layer adds the source and destination address to the header of the frame. Addressing is used to identify the device on the internet.
  • Routing: Routing is the major component of the network layer, and it determines the best optimal path out of the multiple paths from source to the destination.
  • Packetizing: A Network Layer receives the packets from the upper layer and converts them into packets. This process is known as Packetizing. It is achieved by internet protocol (IP).

Transport (Layer 4)

OSI Model, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Example: Includes SPX, TCP, UDP..

Functions of Transport Layer:
  • Service-point addressing: Computers run several programs simultaneously due to this reason, the transmission of data from source to the destination not only from one computer to another computer but also from one process to another process. The transport layer adds the header that contains the address known as a service-point address or port address. The responsibility of the network layer is to transmit the data from one computer to another computer and the responsibility of the transport layer is to transmit the message to the correct process.
  • Segmentation and reassembly: When the transport layer receives the message from the upper layer, it divides the message into multiple segments, and each segment is assigned with a sequence number that uniquely identifies each segment. When the message has arrived at the destination, then the transport layer reassembles the message based on their sequence numbers.
  • Connection control: Transport layer provides two services Connection-oriented service and connectionless service. A connectionless service treats each segment as an individual packet, and they all travel in different routes to reach the destination. A connection-oriented service makes a connection with the transport layer at the destination machine before delivering the packets. In connection-oriented service, all the packets travel in the single route.
  • Flow control: The transport layer also responsible for flow control but it is performed end-to-end rather than across a single link.
  • Error control: The transport layer is also responsible for Error control. Error control is performed end-to-end rather than across the single link. The sender transport layer ensures that message reach at the destination without any error.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Example: Includes NFS, NetBios names, RPC, SQL..

Functions of Session layer:
  • Dialog control: Session layer acts as a dialog controller that creates a dialog between two processes or we can say that it allows the communication between two processes which can be either half-duplex or full-duplex.
  • Synchronization: Session layer adds some checkpoints when transmitting the data in a sequence. If some error occurs in the middle of the transmission of data, then the transmission will take place again from the checkpoint. This process is known as Synchronization and recovery.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Example: Includes Encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI..

Functions of Presentation layer:
  • Translation: The processes in two systems exchange the information in the form of character strings, numbers and so on. Different computers use different encoding methods, the presentation layer handles the interoperability between the different encoding methods. It converts the data from sender-dependent format into a common format and changes the common format into receiver-dependent format at the receiving end.
  • Encryption: Encryption is needed to maintain privacy. Encryption is a process of converting the sender-transmitted information into another form and sends the resulting message over the network.
  • Compression: Data compression is a process of compressing the data, i.e., it reduces the number of bits to be transmitted. Data compression is very important in multimedia such as text, audio, video.

Application (Layer 7)

OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Example: Includes WWW browsers, NFS, SNMP, Telnet, HTTP, FTP..

Functions of Application layer:
  • File transfer, access, and management (FTAM): An application layer allows a user to access the files in a remote computer, to retrieve the files from a computer and to manage the files in a remote computer.
  • Mail services: An application layer provides the facility for email forwarding and storage.
  • Directory services: An application provides the distributed database sources and is used to provide that global information about various objects.

Pros and Cons of the OSI Model

The OSI model has a number of advantages, including:

  • It’s considered a standard model in computer networking.
  • Supports connection less as well as connection-oriented services. Users can leverage connection less services when they need faster data transmissions over the internet and the connection-oriented model when they’re looking for reliability.
  • Has the flexibility to adapt to many protocols.
  • More adaptable and secure than having all services bundled in one layer.

The disadvantages include:

  • Doesn’t define any particular protocol.
  • Session layer, which is used for session management, and the presentation layer, which deals with user interaction aren’t as useful as other layers in the OSI model.
  • Some services are duplicated at various layers, such as the transport and data link layers each have an error control mechanism.
  • Layers can’t work in parallel; each layer has to wait to receive data from the previous layer.

Published by Abdul Samad

Having 17+ years of extensive experience in IT industry, enabled to enhance the team performance and maximize customer satisfaction by strategically managing calls and implementing process improvements. Demonstrated ability to solve problems, meets challenging goals, and expedites delivery. Skilled MSSQL administrator guide team during the crisis situation. Apply Creative thoughts process in re-designing the workflow system to eliminate duplication of effort and increase productivity.

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