Functionality and Characteristics of OSI Model

Functions of each layer in the OSI model:

1) Physical Layer

• It specifies the transmission media between two connecting devices.
• In addition, it specifies the information rate(number of bits sent each second) within the defined media.
• It defines the topology of the network. The topology might be Bus, Ring, Star, Mesh, Tree, or Hybrid.
• It defines a data transmission mode. It could be Simplex, Half-Duplex, or Duplex.
• It defines the sort of data encoding utilized in the transmission.
• It describes the line configuration of the network. It could be point-to-point or multiport.

2) Data – Link Layer

• Allows media access using framing: It allows the upper layers to get into the media using framing since it performs physical addressing of the data.
• Controls data: It executes flow, error, and access control of the data. It controls the information rate of the transmission to regulate the information flow. It uses the header information or checksum bits to regulate the error. Most importantly, it performs access control of the information utilizing the MAC address.

3) Network Layer

• Logical Addressing: Every computer has a unique IP(Internet Protocol) address. The layer attaches the foundation and destination IP address to the data so that it could be transmitted even in different networks. Internet Protocol Version 4(IPv4) and Internet Protocol Version 6(IPv6) addressing comes under the network layer for logical addressing.
• Routing: Routing is a process through which the data packets can travel from one node to a different in some type of computer network. In this layer, the routing decisions are mainly predicated on IP addresses or logical addressing.
• Path Determination: Path determination is the method of selecting a path from various available paths based on the routing information. Path determination is done by the network layer for finding the absolute most optimum path for data transmission.

4) Transport Layer

• Segmentation: Dividing the data received into multiple data segments may be termed segmentation. The layer performs the assembly as well as reassembly of data at the sender’s and receiver’s sides respectively. Each segment has the source and destination ‘port’ and ‘sequence’ numbers. The port number helps to direct each data segment to the proper application, as the sequence number keeps them in a correct sequence when the segmented data is received at the receiver’s side.
• Flow Control: This layer controls the flow of the data during transmission. It is mainly done to avoid any data loss and enhance data transmission efficiency.
• Error Control: The transport layer checks for any type of errors in the data using the checksum bits which can be within the data header. It can also request for retransmission of some data if it’s not received at the receiver’s end.
• Connection Control: This layer also maintains the connection suitably involving the devices. For connection-oriented transmission, TCP(Transmission Control Protocol) is used. TCP is fairly slow but is reliable. It can be used for long-distance transmissions. For connectionless transmission, UDP(User Datagram Protocol) is utilized. UDP is fast however, not reliable. It is mainly used for short-distance transmissions.

5) Session Layer

• Authentication: Authentication is a procedure for verifying the user. The session layer may ask the devices to enter valid login credentials, to be able to maintain a safe data connection.
• Authorization: Authorization is the process of determining the user’s authority to gain access to the data. The layer determines whether the device has permission to gain access to those data elements or not.
• Synchronization: The session layer integrates the sender and receiver. It adds various checkpoints with the information to synchronize data at the sender’s and receiver’s sides. In case of any crash or transfer failure, the information transmission could be resumed from the final checkpoint. There is no need to retransfer the entire data.

6) Presentation Layer

• Data Translation: Data translation refers to transforming data from another. The presentation layer transforms the high-level user language data to the very same low-level machine-level language, and vice versa. These are some of the standards used by this layer for translation are ASCII, EBCDIC, etc.
• Data Encryption and Decryption: Data encryption is the method of converting an ordinary text into cypher text for security. Encryption is applied to the info at the sender’s side. Data decryption is the method of converting a ciphertext into plain text. It is applied to the info at the receiver’s side. The presentation layer uses the SSL(Secure Socket Layer) for message encryption and decryption.
• Data Compression: Data compression is the method of reducing how many bits in the data. It can either be lossy or lossless. Lossless compression is certainly caused by a preference for many important data items.

7) Application Layer

• File Transfer: The Application layer mainly facilitates the file transfer between two networking devices with the help of FTP(File Transfer Protocol).
• Web Surfing: Web surfing is only possible in this layer. Some protocols such as HTTP(Hypertext Transfer Protocol), HTTPS(Hypertext Transfer Protocol Secure), etc. do allow web surfing.
• Emails: Electronic mails may be sent from one device to another on the network only through the application layer. Some protocols are SMTP(Simple Mail Transfer Protocol), etc. are used for sending emails on the network.
• Network Virtual Terminal: This layer facilitates the remote host login in the network with the help of protocols such as Telnet, etc. It can be called the program version of the physical terminal in the network.

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Functionality and Characteristics of OSI Model

Characteristics of OSI Model

1) Physical Layer

• Handles transmitting raw bits of data over a physical medium.
• It’s hardware-specific and is in charge of the actual physical connection between a network medium and your computer.
• Data is in the form of 1s and 0s. They are represented by light pulses, electric voltages, or radio frequencies.
• All devices functioning only at that layer handle signalling.
• Ethernet cables, Token ring networks, pin-outs, cable types, and connectors are a number of the components included in this layer.

2) Data – Link Layer

• It enables data transfer through a link from one device to another. 
• Media access control.
• Packet addressing.
• It formats the frame that is used to encapsulate data.
• Error notification on the physical layer.
• It orders bits and packets between the data segments. This ensuing result is known as frames. They contain data that can be set in an orderly manner.
• It also ensures error-free communication between two devices.

3) Network Layer

• Accountable for establishing paths that are used for the transfer of data packets between network devices.
• Traffic direction.
• Addressing; Service and logical network addresses.
• Routing.
• Packet switching.
• Controlling packet sequence.

4) Transport Layer

• This layer is responsible for data delivery involving the networked hosts. Data is fragmented and reassembled by this layer. In addition, it controls the reliability of any given link.
• Guaranteed delivery of data.
• Name resolution.
• Flow control.
• Error detection and recovery.

5) Session Layer

• It Establishes, 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 which can be exchanged during a period are full or half-duplex.

6) Presentation Layer

• This layer defines the syntax utilized by hosts in the network to communicate.
• Data compression.
• Encryption and decryption of data.
• Data translation.
• Interpreting graphics commands.
• Protocol conversion.
• Gateway services.

7) Application Layer

• It performs as an operating system the program and the network protocol on the computer. It offers services that are necessary to guide the applications.
• This layer provides an interface for FTP applications, email, and telnet.

OSI model protocols

The Protocols of the OSI model in each layer are defined below:

OSI model vs TCP/IP model

Advantages of OSI Model over TCP/IP

• Protocols may be replaced by new protocols when technology changes.
• Provide support for connection-oriented services in addition to connectionless services.
• It is just a standard model in computer networking.
• Supports connectionless and connection-oriented services.
• Offers flexibility to conform to various forms of protocols

Disadvantages/limitations of OSI Model

• You can use it only as a reference model.
• Doesn’t define any specific protocol.
• In the network layer model, some services are duplicated in several layers like the transport and data link layers
• Layers can’t work in parallel as each layer need to wait to acquire data from the prior layer.

Uses of OSI model

The uses of the 7 layers of the OSI model can be defined as:

• Troubleshooting: The model allows IT professionals to logically break down complex network challenges into manageable tasks, thus providing checkpoints. If a consumer is having network performance problems, you can check the grade of the connection at Layer 1, Layer 2, Layer 3, etc. to narrow down the performance issue.
• Communication: While the majority of IT professionals are knowledgeable about the TCP/IP internet model, there are other communication networks in the wild. Getting the OSI model allows IT professionals to keep in touch with each other inconsistent and easily recognizable terms to cut back on miscommunications.

What is OSI Model and 7 Layers of the OSI Model Explained

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