Physical Layer

Mukesh Chinta

Asst Prof, CSE, VRSEC

Physical Layer

• The physical layer is the first layer of the Open System

Interconnection Model (OSI Model). The physical layer deals

with bit-level transmission between different devices and

supports electrical or mechanical interfaces connecting to the

physical medium for synchronized communication.

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Data & Signals• Data refers to information that conveys some meaning based on some

mutually agreed up rules or conventions between a sender and a receiver and

today it comes in a variety of forms such as text, graphics, audio, video and

animation

• Data can be of two types; analog and digital. Analog data take on continuous

values on some interval. Typical examples of analog data are voice and video.

On the contrary, digital data take on discrete values. Text or character strings

can be considered as examples of digital data.

• Signal is electrical, electronic or optical representation of data, which can be

sent over a communication medium. Stated in mathematical terms, a signal is

merely a function of the data. Analog signals are continuous-valued; digital

signals are discrete-valued.

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Periodic/aperiodic Signals

• Periodic signal exhibits a specific signal pattern that repeats over time,

whereas non-periodic (aperiodic) does not repeat any specific signal pattern.

• A composite signal is a collection of one or more signals having different

frequencies, amplitude and phases. It can be periodic or aperiodic signal.

• A periodic signal is characterized by the following three parameters.

o Amplitude: It is the value of the signal at different instants of time. It is measured

in volts.

o Frequency: It is inverse of the time period, i.e. f=1/T. The unit of frequency is

Hertz (Hz) or cycles per second.

o Phase: It gives a measure of the relative position in time of two signals within a

single period

• Most digital signals are aperiodic and thus, period or frequency is not

appropriate. The bit interval is the time required to send one single bit. The bit

rate is the number of bits send in one second, usually expressed in bits per

second (bps)

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Physical Layer - Standards

• The protocols and operations of the upper OSI layers are performed by

software and are designed by software engineers

• The services and protocols in the TCP/IP suite are defined by the Internet

Engineering Task Force (IETF). Similar to technologies associated with the

Data Link layer, the Physical layer technologies are defined by organizations

such as:

1. The International Organization for Standardization (ISO)

2. The Institute of Electrical and Electronics Engineers (IEEE)

3. The American National Standards Institute (ANSI)

4. The International Telecommunication Union (ITU)

5. The Electronics Industry Alliance/Telecommunications Industry Association

(EIA/TIA)

6. National telecommunications authorities such as the Federal Communication

Commission (FCC) in the USA.

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Physical Layer

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• Describe the role of bits in representing a frame as

it is transported across the local media

Signals on Physical Media

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Physical Media Data Rates

Different physical media support the transfer of bits at different speeds. Data

transfer can be measured in three ways:

• Bandwidth: The capacity of a medium to carry data is described as the raw

data bandwidth of the media. Digital bandwidth measures the amount of

information that can flow from one place to another in a given amount of

time. Bandwidth is typically measured in kilobits per second (kbps) or

megabits per second (Mbps).

• Throughput: Throughput is the measure of the transfer of bits across the

media over a given period of time. Due to a number of factors, throughput

usually does not match the specified bandwidth in Physical layer

implementations such as Ethernet.

• Goodput: A third measurement has been created to measure the transfer of

usable data. Goodput is the measure of usable data transferred over a given

period of time, and is therefore the measure that is of most interest to network

users.8

Data Rates

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The range of frequencies transmitted without being strongly

attenuated is called the bandwidth

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Maximum Data Rate of a Channel

Nyquist theorem: To transmit at a transmission rate of fb Hz requires aminimum bandwidth of Hmin = fb/2 Hz→ This specifies the maximum data rate for the noiseless case as:

Where, V is the number of discrete levels in the signal and H is the

maximum bandwidth.

Shannon’s theorem: If information rate does not exceed channel capacity,there exists a coding technique such that information can be transmitted over anoisy channel, error free.The channel capacity provides the maximum possible data rate for the generalnoisy case as:

Examples

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Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal

with two signal levels. The maximum bit rate can be calculated as

We can calculate the theoretical highest bit rate of a regular telephone line. A

telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is

usually 3162. For this channel the capacity is calculated as

The signal-to-noise ratio is often given in decibels. Assume that SNRdB = 36

and the channel bandwidth is 2 MHz. The theoretical channel capacity can be

calculated as

Transmission Media

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۞Transmission media are actually located below the physical layer and are

directly controlled by the physical layer.

۞A transmissionmedium can be broadly defined as anything that can carryinformation from a source to a destination.۞In data communications, the transmission medium is usually free space,metallic cable, or fiber-optic cable. The information is usually a signal that isthe result of a conversion of data from another form.

Guided Media

• Guided media, which are those that provide a conduit from one

device to another, include twisted-pair cable, coaxial cable, and

fiber-optic cable.

• A signal traveling along any of these media is directed and

contained by the physical limits of the medium.

• Twisted-pair and coaxial cable use metallic (copper) conductors

that accept and transport signals in the form of electric current.

• Optical fiber is a cable that accepts and transports signals in the

form of light.

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Magnetic Media

• One of the most common ways to transport data from one computer to

another is to write them onto magnetic tape or removable media (e.g.,

recordable DVDs), physically transport the tape or disks to the destination

machine, and read them back in again.

• It is often more cost effective, especially for applications in which high

bandwidth or cost per bit transported is the key factor.

• Although the bandwidth characteristics of magnetic tape are excellent, the

delay characteristics are poor. Transmission time is measured in minutes or

hours, not milliseconds.

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Twisted Pair

Twisted-pair is one of the oldest and still most common transmission media

of cabling that is used for telephone communications and most modern

Ethernet networks. Twisted pairs can be used for transmitting either analog

or digital signals.

The wires are twisted together in a helical form, just like a DNA molecule.

Twisting is done because two parallel wires constitute a fine antenna. When

the wires are twisted, the waves from different twists cancel out, so the wire

radiates less effectively..

There are two basic types, shielded twisted-pair (STP) and unshielded

twisted-pair (UTP).15

UTP

The most common twisted-pair cable used in communications is referred to as

unshielded twisted-pair (UTP).

This cable consists of 4 twisted pairs of metal wires (that means there are 8 wires in

the cable). Each pair is twisted with a different number of twists per inch to help

eliminate interference from adjacent pairs and other electrical devices. Each twisted

pair consists of two metal conductors that are insulated separately with their own

coloured plastic insulation.

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Categories of UTP

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Category rating based on number of twists per inch andthe material used

STP

• This cable has a metal foil or braided-mesh covering that covers each pair of

insulated conductors. The metal foil is used to prevent infiltration of

electromagnetic noise. This shield also helps to eliminate crosstalk.

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STP is suited for environments with electrical interference and also provides

better performance at higher data rates. But the extra shielding makes the STP

cables quite bulky and more expensive that UTP cables

Co-axial Cable

• A coaxial cable consists of a stiff copper wire as the

core, surrounded by an insulating material. The insulator

is encased by a cylindrical conductor, often as a closely-

woven braided mesh. The outer conductor is covered in

a protective plastic sheath.

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Types of Co-axial Cable

• Although coaxial cabling is difficult to install, it is highly resistant to signal

interference. It can support greater cable lengths between network devices

and greater bandwidth than twisted-pair cable. Coaxial cables are capable of

transmitting data at a fast rate of 10Mbps.

• Thicknet and Thinnet are two varieties of coaxial cable, but rarely used.

Ethernet can run approx 100mts (328 feet) with UTP, while coaxial cable

increases this distance to 500mts (1640 feet). The RG numbering system used

with coaxial cables refers to cables approved by U.S. Department of Defense

(DoD).

• t

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Connectors

• To connect coaxial cable to devices, it is necessary to use coaxial

connectors. The most common type of connector is the Bayone-Neill-

Concelman, or BNC, connectors. BNC connectors are sometimes

referred to as bayonet mount, as they can be easily twisted on or off.

There are three types: the BNC connector, the BNC T connector, the

BNC terminator

• Coaxial cable applications include analog and digital telephone

networks, cable TV networks, Ethernet LANs and short range

connections.

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Optical Fiber

• Fiber-optic cable or optical fiber consists of thin glass fibers that can

carry information in the form of visible light. The typical optical fiber

consists of a very narrow strand of glass or plastic called the core.

• Around the core is a concentric layer of less dense glass or plastic called

the cladding, whose refractive index is less than that of the core. The

outer most layer of the cable is known as the jacket, which shields the

cladding and the core from moisture, crushing and abrasion.

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Light Propagation

• Optical fibers transmit a beam of light by means of total internal

reflection. When a light beam from a source enters the core, the core

refracts the light and guides the light along its path. The cladding

reflects the light back into the core and prevents it from escaping

through the medium.

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Single mode Vs Multimode

• Fiber optic cables support two modes of propagating light, which are

multimode and single mode. In multimode, many beams from a light

source traverse along multiple paths and at multiple angles. In single

mode, the beams propagate almost horizontally.

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Laser Connectors

• LED or LASER (Light Amplification by Stimulated Emission of Radiation)

acts as the source converting electric pulse to light pulses and photodiode acts

as receiver doing viceversa. Fiber optic cables uses 3 types of connectors,

which are :

• SC (Subscriber Connector)- used to connect cable TV

• ST (Straight Tip)- to connect network devices

• MT-RJ (Mechanical Transfer-Registered Jack)- for network applications.

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A Comparison

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Physical Layer Devices

Cables

Connectors

Repeaters

Passive Hub

Simple Active Hub

Transmitters

Multiplexers

Receivers

Transceivers

Couplers