Network Models

 Network Protocol

Network protocols are a set of rules outlining how connected devices communicate across a network to exchange information easily and safely. Protocols serve as a common language for devices to enable communication irrespective of differences in software, hardware, or internal processes.

The rules governing a network include guidelines that regulate access methods, allowed physical topologies, cabling types, and data transfer speed. The entire set of communication processes is divided into several layers to describe and analyse data communication networks.

 Each layer performs a specific task and provides services to an adjacent layer. A protocol is a controlled sequence of messages that is exchanged between two or more systems to accomplish a given task. Protocol specifications define this sequence together with the format or layout of the messages that are exchanged.

It uses two popular models: the OSI reference model and the TCP/IP model.

Layering

Layering is a networking concept that involves dividing a complex system into smaller, manageable layers. Each layer performs a specific function, making it easier to develop, test and modify the network. The benefits of layering in networking include:

- Modular design: With a layered design, each layer can be managed and maintained independently of the others, simplifying network management.

- Interoperability: Different vendors can create products compatible with each other, as long as they follow the specifications for each layer.

- Simplified troubleshooting: Layering isolates problems to a specific layer, making it easier to identify and fix issues without disrupting the entire network.

- Standardization: Layering facilitates the development of standards for network communication, ensuring compatibility between different systems and devices.

- Scalability: A layered design allows a network to be easily scaled up or down as needed, by adding or removing layers or components.

- Flexibility: Layering provides a flexible framework for adding new features and functionality to a network, without affecting the existing layers or applications.

OSI Reference Model

The open systems interconnection (OSI) model is a conceptual model created by the International Organization for Standardization that enables diverse communication systems to communicate using standard protocols. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.

The OSI Model can be seen as a universal language for computer networking. It is based on the concept of splitting up a communication system into seven abstract layers, each one stacked upon the last.

7. The application layer

The Application Layer: content requested and returned in the required format. This is the only layer that directly interacts with data from the user. Software applications like web browsers and email clients rely on the application layer to initiate communications. However, it should be made clear that client software applications are not part of the application layer; rather the application layer is responsible for the protocols and data manipulation that the software relies on to present meaningful data to the user.

Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer Protocol is one of the protocols that enables email communications).

6. The presentation layer

The Presentation Layer: encryption, compression, translation

This layer is primarily responsible for preparing data so that it can be used by the application layer; in other words, layer 6 makes the data presentable for applications to consume. The presentation layer is responsible for translation, encryption, and compression of data.

Two communicating devices communicating may be using different encoding methods, so layer 6 is responsible for translating incoming data into a syntax that the application layer of the receiving device can understand.

If the devices are communicating over an encrypted connection, layer 6 is responsible for adding the encryption on the sender’s end as well as decoding the encryption on the receiver's end so that it can present the application layer with unencrypted, readable data.

Finally, the presentation layer is also responsible for compressing data it receives from the application layer before delivering it to layer 5. This helps improve the speed and efficiency of communication by minimizing the amount of data that will be transferred.

5. The session layer

The Session Layer: session of communication

This is the layer responsible for opening and closing communication between the two devices. The time between when the communication is opened and closed is known as the session. The session layer ensures that the session stays open long enough to transfer all the data being exchanged, and then promptly closes the session in order to avoid wasting resources.

The session layer also synchronizes data transfer with checkpoints. For example, if a 100-megabyte file is being transferred, the session layer could set a checkpoint every 5 megabytes. In the case of a disconnect or a crash, after 52 megabytes have been transferred, the session could be resumed from the last checkpoint, meaning only 50 more megabytes of data need to be transferred. Without the checkpoints, the entire transfer would have to begin again from scratch.

4. The transport layer

The Transport Layer: segment, transport, reassembly

Layer 4 is responsible for end-to-end communication between the two devices. This includes taking data from the session layer and breaking it up into chunks called segments before sending it to layer 3. The transport layer on the receiving device is responsible for reassembling the segments into data the session layer can consume.

The transport layer is also responsible for flow control and error control. Flow control determines an optimal speed of transmission to ensure that a sender with a fast connection does not overwhelm a receiver with a slow connection. The transport layer performs error control on the receiving end by ensuring that the data received is complete, and requesting a retransmission if it isn’t.

Transport layer protocols include the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

3. The network layer

The Network Layer: packet creation, transport, packet assembly

The network layer is responsible for facilitating data transfer between two different networks. If the two devices communicating are on the same network, then the network layer is unnecessary. The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender’s device, and reassembles these packets on the receiving device. The network layer also finds the best physical path for the data to reach its destination; this is known as routing.

Network layer protocols include IP, the Internet Control Message Protocol (ICMP), the Internet Group Message Protocol (IGMP), and the IPsec suite.

2. The data link layer

The Data Link Layer: frame creation, frames sent between networks

The data link layer is very similar to the network layer, except the data link layer facilitates data transfer between two devices on the same network. The data link layer takes packets from the network layer and breaks them into smaller pieces called frames. Like the network layer, the data link layer is also responsible for flow control and error control in intra-network communication (The transport layer only does flow control and error control for inter-network communications).

1. The physical layer

The Physical Layer: sending cable, bitstream, receiving cable

This layer includes the physical equipment involved in the data transfer, such as the cables and switches. This is also the layer where the data gets converted into a bit stream, which is a string of 1s and 0s. The physical layer of both devices must also agree on a signal convention so that the 1s can be distinguished from the 0s on both devices.

TCP/IP Model

1. Host to Network Layer

Network Interface Layer is this layer of the four-layer TCP/IP model. This layer is also called a network access layer. It helps you to define details of how data should be sent using the network.

It also includes how bits should optically be signalled by hardware devices that directly interface with a network medium, like coaxial, optical, coaxial, fibre, or twisted-pair cables.

A network layer is a combination of the data line as defined in the article on the OSI reference model. This layer defines how the data should be sent physically through the network. This layer is responsible for the transmission of the data between two devices on the same network.

2. Internet Layer

An internet layer is a second layer of TCP/IP layers of the TCP/IP model. It is also known as a network layer. The main work of this layer is to send the packets from any network, and any computer till they reach the destination irrespective of the route they take.

The Internet layer offers the functional and procedural method for transferring variable-length data sequences from one node to another with the help of various networks.

3 . Transport Layer

The transport layer builds on the network layer in order to provide data transport from a process on a source system machine to a process on a destination system. It is hosted using single or multiple networks and also maintains the quality of service functions.

It determines how much data should be sent where and at what rate. This layer builds on the messages that are received from the application layer. It helps ensure that data units are delivered error-free and in sequence.

The transport layer helps you to control the reliability of a link through flow control, error control, and segmentation or de-segmentation.

The transport layer also offers an acknowledgement of the successful data transmission and sends the next data in case no errors occur. TCP is the best-known example of the transport layer.

4 . Application Layer

The application layer interacts with an application program, which is the highest level of the OSI model. The application layer is the OSI layer, which is closest to the end-user. It means the OSI application layer allows users to interact with other software applications.

The application layer interacts with software applications to implement a communicating component. The interpretation of data by the application program is always outside the scope of the OSI model.

Differences between OSI & TCP/IP