01. Physical Layer of OSI Model (Layer1)

The 7x Layer model starts from Physical till Application Layer. OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. Physical Layer is the first & lowest layer in the 7Layer OSI Model. It is responsible for ultimate transmission of data between sender & receiver. The physical layer defines the means of transmitting raw bits over the transmission media.  It also defines how the data is carried over the Physical media rather than logical data packets over a physical data link connecting network nodes. The bit stream is grouped into code words or symbols.

Below Figure shows the position of  Physical Layer in the OSI Model:

OSI Model Layers Waqas Karim

Functions/Duties of Physical Layer

  • Real data delivery of bits
  • Physical Medium standards of connectors, cabling, Radio, Wireless, … & their technical specifications like signal levels, impedance, Optical levels, Bandwidth, frequency, …      & their Mechanical specification like length, strength, dimensions, …). For example, Physical layer defines:
    • How many volts should be used to represent a 1 bit and how many for a 0?
    • Whether 1 will present an ON or 0 will present an ON at both sides
    • How many nanoseconds a bit lasts?
    • Whether transmission may proceed simultaneously in both directions?
    • Whether transmission may proceed simultaneously in both directions?
    • How many pins the network connector has and what each pin is used for?
  • Modulation Scheme (PSK, ASK, FSK, QAM, …)
  • Line coding (RZ, NRZ, NRZ–L, NRZ–M, Bipolar, …)
  • Bit synchronization for serial communications
  • L1 Flow Control to match sender/receiver rates
  • Start/Stop Signalling
  • Circuit Switching
  • L1 Multiplexing which is used in SDH, DWDM & OTN
  • Signal Processing including pulse shaping, Equalization, filtering,..
  • Bit-interleaving and other channel coding
  • Physical network topology specifications. It defines which topology of our network would be e.g. bus, ring, mesh or star network)
  • Bit rate Control,
  • Network/Port mode control (Point-to-point, multipoint or point-to-multipoint)
  • Communication type specification (e.g. Serial, parallel communication)
  • Duplex Control (Simplex, half duplex or full duplex transmission mode)
  • Negotiation Control (Auto-negotiation, Manual, …)
  • CSMA-CD Control & specification
  • FEC (Forward Error Correction) for Error Control which is commonly used in SDH & DWDM

 

Physical Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Physical layer are listed in below:

 

 Network Equipment/Components at Physical Layer

Similar to protocols, each layer has associated equipment with it. Some important Equipment that operate at Physical Layer of OSI Model are listed in below:

 

Summary

  • Physical Layer is the 1st & Lowest Layer in OSI 7 Layer Model
  • It performs very important functions like transmission of data, Multiplexing, Line Coding, Modulation, Physical Media standardization, Error Control, Flow Control, …
  • Important Protocols at Physical Layer include WiFi, USB, Bluetooth , RJ45, SDH, DWDM, OTN, MW, RS232, ATM, DSL, …
  • Equipment operating at Physical Layer include Hubs, Repeaters, NIC, Modems, Media Convertors, Ethernet Cables, Optical Fibers, Patch Panels, …

 

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