Physical Layer

Introduction

• The physical layer is the lowest layer/Layer 1 of the OSI model.

Features

• The data units in this layer are called Signals(analog or digital) of Bits.
• The main network devices used/found in the Physical layer are a Repeater and Hub i.e., this layer does not work with devices having any software rather it is pure hardware.
• This layer studies about transmitting medium i.e., wired(cables and connectors) or wireless.
• This layer also works on and defines transmission modes i.e., duplex, half duplex, simplex, etc.
• This layer also defines and uses multiplexing techniques to send and receives signals over/from a transmitting medium in the LAN area to improve the efficiency of the transmitting medium.
• This layer also studies about the topologies used in the network.
• An example of the Physical layer is RS-232.

Functions

• This layer receives the unstructured raw bit stream (called data bits) from the data link layer as a frame of bits and then converts it into respective or required analog and digital signals and finally transmits over/from a physical medium such as cables/wireless. The signal may be discrete(digital) or electrical or optical(analog).
• This layer describes the electrical/optical, mechanical, and functional hardware interfaces or structures of the physical medium.
• This layer carries the data in the form of signals obtained from the above higher layers such as the data link layer, network layer, etc.
• This layer supports the Data encoding mechanism i.e., it modifies the simple digital signal pattern/bits (1s and 0s) of a PC to better accommodate the characteristics of the physical medium available either analog or digital. Thus, this layer determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) mode.
• This layer transmits bits as electrical or optical signals which is suitable for the available physical medium, and also determines what physical medium options can be used. It also determines how many volts/db (decibels) should be used to represent a given signal state, using a given physical medium.
• This layer defines the mechanical and electrical definition of the network medium (cable) and network hardware.
• This layer defines how data is inserted onto the cable and retrieved from it.
• The physical layer is responsible for passing data bits onto and receiving them from the connecting medium.
• This layer gives the data-link layer (layer 2) its ability to transport a stream of serial data bits between two communicating systems.
• This layer has no understanding of the meaning of the bits but deals with the electrical and mechanical characteristics of the signals and signaling methods.