Source: LinkedIn

OSI Model

OSI stands for Open Systems Interconnection. It has been developed by ISO– International Organization for Standardization, in the year 1984. It is a seven-layer architecture with each layer having specific functionality to perform.

**OSI Layers, Data Units, and Functions:**

Layers	Data Units	Functions
Application Layer	Data	Mail Services, Directory Services, FTAM
Presentation Layer	Data	Encryption/Decryption, Compression
Session Layer	Data	Session Establishment, Synchronization, Dialog Controller
Transport Layer	Segments, Datagram	Segmentation
Network Layer	Packets	Traffic control, Fragmentation, Routing
Data Link Layer	Frames	Flow control, Error control, Access control
Physical Layer	Bits	Bit Synchronization, Bit rate control, Physical Topologies

Layers and their uses:

Physical Layer

Network Topologies:

Mesh Topology: In a mesh topology, every device is connected to another device via a particular channel. If suppose, N number of devices are connected with each other, then a total number of links is required to connect NC .

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Bus Topology: Bus topology is a network type in which every computer and network device is connected to a single cable. If N devices are connected, then the number of cables required is 1 which is known as backbone cable, and N drop lines are required.
Star Topology: In star topology, all the devices are connected to a single hub through a cable. If N devices are connected to each other, then the no. of cables required N.
Ring Topology: In this topology, it forms a ring connecting devices with exactly two neighboring devices.

Transmission Modes:

Simplex Mode : the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit, and the other can only receive.
Half-duplex Mode : each station can both transmit and receive, but not at the same time.
Full-duplex Mode : both stations can transmit and receive simultaneously.

Manchester Encoding: When there is a long sequence of 0s and 1s, there is a problem at the receiving end. The problem is that the synchronization is lost due to a lack of transmissions.

NRZ-level encoding : The polarity of signals changes when the incoming signal changes from ‘1’ to ‘0’ or from ‘0’ to ‘1’. It considers the first bit of data as polarity change.
NRZ-Inverted/ Differential encoding : In this, the transitions at the beginning of the bit interval are equal to 1 and if there is no transition at the beginning of the bit interval are equal to 0.