Unit 3

Unit 3

Physical Layer

OSI Model

•The Open Systems Interconnection (OSI) model is areference framework for understanding datacommunications between any two networked systems.

•It divides the communications processes into sevenlayers.

•Each layer both performs specific functions to supportthe layers above it and offers services to the layersbelow it.

•The three lowest layers focus on passing traffic throughthe network to an end system.

•The top four layers come into play in the end system tocomplete the process.

OSI Model

OSI Layers

An Exchange Using the OSI Model

Physical Layer

Functions of physical Layer

•Physical characteristics of interfaces and media: it defines thecharacteristics of the interfaces between the devices and thetransmission medium. It also defines the type of transmissionmedium.

•Representation of bits: this layer consists of a stream of bits(sequence of 0s and 1s) without any interpretation. To betransmitted, bits must be encoded in to signals- electrical or optical.This layer also defines the type of encoding (how 0s and 1s arechanged in to signals).

•Data rate: the transmission rate – the number of bits sent eachsecond-is also defined by the physical layer.

•Synchronization of bits: the sender and receiver must besynchronized at the bit level. In other words, the sender and thereceiver clocks must be synchronized.

•Line configuration: this layer is also concerned withthe connection of devices to the medium. In apoint-to-point configuration, two devices areconnected together through a dedicated link. In amultipoint configuration, a link is shared betweenseveral devices.

•Physical topology: the physical topology defineshow devices are connected to make a network.Device can be connected using following topologies:

•Mesh topology

•Star topology

•Ring topology, and

•Bus topology

•Transmission mode: the layer also defines thedirection of transmission between twodevices. It can be:

•Simplex

•Half duplex, and

•Full duplex

Physical Layer Design Isuue

Physical layer design issues

•The major task of the physical layer is to provideservices for the data link layer.

•Following are the services provided or duties ofthe physical layer or the design issues of thephysical layer

Duties ofphysical layer

Transformationfrom bit to signal

Bit rate control

Bitsynchronization

Multiplexing

Circuit switching

Transformation from bit to signal:

•The data link layer consists of 0’s and 1’s in the bitform. This bit stream cannot travel as it is on thetransmission medium.

•So the physical layer converts the bit stream into asignal which is suitable for the transmissionmedium.

Bit rate control:

•The transmission medium decides the highest valueof bit rate and the physical layer acts as a bit ratecontroller.

•The design of the physical layer hardware andsoftware will determine the data rate.

•Multiplexing:

Physical layer can use different techniques ofmultiplexing, in order to improve the channelefficiency.

•Switching:

There are three switching methods, namelycircuit switching, message switching andpacket switching. Out of which circuitswitching is the function of physical layer.

Physical layer specifications

Physical layer specification

•Installation of cables or other physical mediums forthe physical layer needs the awareness of all thespecification that affects the process.

•For example one needs to know about the Ethernetstandards published by IEEE 802.3 which specifiesthe wiring configuration, the Media Access Control(MAC) and collision detection mechanism.

•They also specify the maximum length of thecable segment, the distance betweenworkstations and number of repeatersallowed to be used.

•One should also know about the otherstandards such as ANSI/EIA/TIA 568,commercial building telecommunicationcabling standards, local building codes etc.

Physical Layer Signalling

Physical layer signaling:

•The digital data coming from a digitalcomputer or some other source cannot be putdirectly on the network medium, because theformat of this signal is not suitable for itsdirect transmission. This takes place at thephysical layer and it is called as physical layersignaling.

•This data is converted into a suitable format orline code and then transmitted over acommunication channel.

•The various formats used are called as linecodes.

•The formats or line codes used commonly innetworking are:

–Non-return to zero (NRZ) and return to zero (RZ)unipolar formats.

–NRZ and RZ polar format

–Non-return to zero bipolar formats

–Manchester format

–Differential Manchester code

–Write others also

Network Devices Used At Physical Layer

•All types of hubs

•Terminators

•Couplers

•Connectors

•Repeaters

•Multiplexers

•Receivers

•Transceivers

•Serial Interfacing Devices

•RS 232

•RS 449

•RS 530

•RS 422 & RS 423