Physical Layer of OSI Model (Layer-1)

Physical Layer of OSI Model is the first layer in seven layers for simplification. It is responsible for transmission of data between sending & receiving devices. It also defines the means of transmitting raw bits over the transmission media. Physical layer addresses how the data is carried over the Physical media rather than logical data packets over a physical data link connecting network nodes. Each layer in OSI Model has its own duties & specific functions to perform so that it can support the layers above it. Below figure shows the position of Physical layer in the OSI Model:

Functions & Duties of Physical Layer of OSI Model

Physical layer of OSI model is mainly responsible for standards of medium like connectors, cabling, Radio, Wireless & their technical specifications like signal levels, impedance, Optical levels, Bandwidth and frequency. It also addresses their mechanical specification like length, strength and dimensions. For example, Physical layer defines:

How many amperes of current & volts should be used to represent a 1-bit and how many for 0-bit?
Whether 1 will present an ON or 0 will present an ON at both sides
What is the total duration for a bit of zero or one?
Whether transmission will be carried in both directions or only in one direction (duplex mode)?
How many pins are there in the network connector and which pins are used for data & power supply?

Physical layer also addresses the Modulation Scheme i.e. whether communication will take place using PSK, ASK, FSK or QAM and line coding like RZ, NRZ, NRZ–L, NRZ–M & Bipolar. It addresses Bit synchronization for serial communications and Layer-1 Flow Control to match sender/receiver rates, start/stop Signaling, circuit switching, Layer-1 Multiplexing which is used in SDH, DWDM & OTN. Network Port mode control also falls under the responsibilities of physical layer that whether it is point-to-point, multipoint or point-to-multipoint). Physical Layer also controls the Duplex mode (Simplex, half-duplex or full-duplex transmission mode) and port negotiation (Auto-negotiation, Manual, …).

Protocols at Physical Layer of OSI Model

Important protocols on Physical layer are listed in below:

WiFi
USB
Bluetooth
RJ45
Transmission Protocols like SDH, DWDM, OTN
Microwave for air interface communications
RS232 for Serial Communications
ATM
FDDI
DSL

Network Equipment/Components at Physical Layer

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

Hubs
Repeaters
NIC (Network Interface Cards)
Modems
Media Convertors
Ethernet Cables
Optical Fibers
Patch Panels

Summary

Physical Layer is the first/lowest Layer in OSI seven layer model & it performs many functions while physical media standardization is the most important duty out of that. Important protocols at Physical Layer include WiFi, USB, Bluetooth , RJ45, SDH, DWDM, OTN, Microwave and RS-232, while the relevant equipment at physical layer include Hubs, Repeaters, NIC, Modems, connectors like RJ45, adapters, Ethernet cables like CAT5/CAT6 and Optical Fibers.

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