3. physical layer - dcuee224/2012-2013/notes/03_physical_layer_print.pdf · there are two...

EE210 Data Communications and Computer Networks 1

3. Physical Layer3. Physical Layer

The physical layer is responsible for transmitting raw bits over a communication channel.

Design issues deal with mechanical, electrical, and timing interfaces, and the physical transmission mediumThere are a wide variety of physical media over which data may be transmitted.

E.g. wire, cable, fibre, radio, satellite.The media are rated according to their transmission speed (bit rate) and likelihood of errors (bit error rate)For each medium there are fundamental limit on transmission speed

No medium gives error free transmission (although some are better than others)


OverviewOverview

Fundamentals of signal transmissionBit signalingBit synchronisationMedia types


3.1 Fundamentals of Signal Transmission

Fourier theory says that all signals are composed of a possibly infinite number of sinusoidsTherefore all signals can be considered to be composed of sinusoids.We refer to the difference between the maximum and minimum frequency of a signal as the bandwidth of that signal

Bandwidth, B

f2f1


Fundamental Limits of Transmission MediaFundamental Limits of Transmission Media

All channels allow only a limited set of frequencies to be passed. This limit is named bandwidth of that channel.

Actually, cut-off is not usually sharp, so usually bandwidth refers to frequency at which half the power gets through.

All channels are subject to background noise (random perturbations of the line voltage and current). Sources of noise include:

crosstalk: a signal on one line is picked up by adjacent lines as a small noise signal

Near-End Crosstalk (NEXT) caused when a strong transmitter output signal interferes with a much weaker incoming receiver signal.

impulse noise: caused by external activity or equipment which generates electrical impulses on the line which cause large signal distortion for their duration.thermal noise (white noise): caused by the thermal agitation of electrons associated with each atom in the device or transmission line material. It consists of random frequency components of continuously varying amplitude.


Fundamental Limits of Transmission Media:Fundamental Limits of Transmission Media:

Shannon Channel CapacityShannon Channel Capacity

The maximum bit rate of a channel depends on the bandwidth of the channel (W) and the Signal to Noise Ratio (SNR)

Shannon Channel Capacity C=W log2(1+SNR) bits/secSNR=Avg. Signal Power/Avg. Noise Power

Note that this is a theoretical limit and is rarely achieved in practice.


Additional limitations of transmission media

Signal Attenuation is the phenomenon whereby the Amplitude of a signal decreases as it propagates along a transmission line.

Attenuation is a function of distance and frequency of signalRepeaters are used to increase the power of the signal at appropriate intervals

Signal Distortion involves the Shape of the signal becoming altered as it propagates along the line.

One cause of distortion is the different attenuation rates for different frequency components of the signal.

This can be addressed using an equalizerDelay distortion is caused by different frequency components of the signal propagating at slightly different speeds.

If the frequency components of one bit are delayed sufficiently they will overlap with the component of the next bit resulting in Inter Symbol Interference (ISI).


3.2 Converting Bits to Signals

There are two fundamentally different ways to produce digital signalsDC or lowpass where bits are represented using square waves. Common encoding schemes include:

Non-return-to-zero (NRZ)BipolarManchester

Bandpass or modulated where bits are represented using fixed frequency sinusoids. This is used when the channel does not pass low frequency signals. Common modulation techniques include:

Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PSK)


3.2.1 Encoding schemes: Polar Schemes3.2.1 Encoding schemes: Polar SchemesPolar encoding schemes rely on the voltage level to make a determination of whether a binary 1 or a 0 was sent.Common Schemes:

Unipolar NRZ: binary 1: +A volts, binary 0: 0 volts.Polar NRZ: binary 1: +A/2 volts, binary 0: –A/2 volts.

Half the power requirement of unipolar NRZBipolar: Consecutive binary 1s: +/- A/2 volts, binary 0: 0 volts

Produces a frequency spectrum with less low frequency components.

Drawbacks to polar encoding schemes are:Long strings of either 1s or 0s can cause loss of timing informationSystematic errors in polarity can cause all 1s to be read as 0s and vice versa


Encoding schemes: Transition OrientatedEncoding schemes: Transition Orientated

Manchester encoding: binary 1: transition from A/2 to –A/2 in middle of bit time interval, binary 0: transition from –A/2 to +A/2 in middle of bit time interval.Differential Manchester encoding: There is a transition at the centre of each bit, but there is only a transition at the start of a 0 bit.NRZ inverted (NZRI): starting at a fixed signal level, binary 1: denoted by a transition and binary 0: by no transition.

The maximum frequency of a NZRI signal is half that of Bipolar and Manchester encoded signals so it only requires half the transmission bandwidthUse NRZI in Wide Area Networks (WANs) while the other methods are generally used in LANs.


Example of Encoding Schemes: Representing 101011100Example of Encoding Schemes: Representing 1010111001 0 1 0 1 1 1 0 0

UnipolarNRZ

Polar NRZ

NRZ Inverted

Bipolar

Manchester

Differential Manchester


3.2.2 Modulation Schemes3.2.2 Modulation SchemesA single frequency signal, known as the Carrier, is selected to lie within acceptable range of frequencies. Amplitude, Frequency or Phase is then varied, or keyed, in accordance with the data signal to be transmitted.

Amplitude shift keying (ASK or AM) is rarely used because of attenuation problems.Frequency shift keying (FSK or FM), is used with lower bit rate modems. Relatively simple demodulation circuitry.

The Signalling Rate or Baud Rate is the number of times per second the amplitude, frequency or phase of the transmitted signal changes.

amplitude

frequency

phase


Phase Modulation Schemes

Phase shift keying (PSK): Uses two fixed signals with a 180 phase difference to each other to indicate a 0 or a 1 respectively.

Complex demodulation circuitry needed to recover the reference phase.

Differential PSK (DPSK): Uses a phase shift of 90 relative to the current signal to indicate a binary 0, and a 270 phase shift for binary 1.

This has simpler demodulation equipment.


Example of Example of Phase Modulation: Representing 10110011: Representing 10110011

1 0 1 1 0 0 1 1

Carrier

PSK

DPSK


Multilevel Modulation Techniques

So far we have had Bit rate = Baud Rate, where each signal element corresponded to just one bit of data, but more bits per signal element can be encoded.

Common Techniques include: Quadrature phase shift keying (QPSK) or 4 PSK which allows 4 phase changesQuadrature Amplitude Modulation (QAM) which changes phase and amplitude

The more bits per signal element the higher the throughput is, but the more complex the scheme is.


Quadrature Amplitude Modulation1 0 1 1 0 0 1 1

Bits Amplitude Phase0 0 1V 0°0 1 1V 180°1 0 2V 0°1 1 2V 180°

Amplitude changePhase change


3.3 Transmission Modes

For various reasons, data streams are often considered to be composed of various elements:

Bits – 0 or 1Characters – eight bit sequencesBlocks or frames – potentially variable numbers of bits

It is necessary to determine the start and end of these elements. The technique for doing this is synchronization.There are two transmission modes:

asynchronous and synchronous.We will examine bit synchronization for both synchronous and asynchronous transmissions


3.3.1 Asynchronous Transmission

Used primarily when the data to be transmitted is generated at random intervals. E.g.: a user typing at a keyboard communicating with a computer.

Generally used in applications where the data to be transferred consists of characters, each character being encoded using 7 or 8 binary bits, common coding schemesbeing ASCII and EBCDIC.

As data is transferred randomly there may be long intervals during which no data signal is present on the line. The receiver must be able to resynchronise at the start of each new character received.


Bit Synchronisation For Asynchronous Transmission

Each received bit is sampled as near to its centre as possible, to ensure that the correct value is read. To do this the receiver clock runs at N times the transmitted bit rate, N=16 is typical, ensuring that the received bit will always be sampled close to its centre.

A start bit is required to start clock count.

Start 0 1 0

8 clockperiods

16 clockperiods

16 clockperiods

16 clockperiods

bit stream

clock


Disadvantage of Asynchronous Transmission

The use of the start and stop bits for each byte transferred means the method is inefficient in its use of transmission capacity.

The bit synchronisation method becomes less reliable as the bit rate increases.

So we look at some of the synchronous transmission schemes.


3.3.2 Synchronous Transmission

Used for large blocks of data at higher bit rates. A frame of data is transmitted as a contiguous bit stream with no delay between each 8-bit element.

The receiver clock operates in synchronism with the received signal.There are two methods of achieving this:

Embedding the clock information into the transmitted signal and having the receiver extract it.

Requires either return to zero or transition orientated scheme.The receiver keeps a local clock, which is kept synchronized with the received signal using a Digital Phase Lock Loop (DPLL)

May be used for NRZ schemesRequires sufficient transitions to keep synchronization

• Uses bit stuffing (more later)


3.4 Media Types

There are a number of transmission media types used for Data Communication. The choice of medium depends on:

Distance to be covered

Desired Bite Rate (in bits per second, bps)

Cost Considerations

Media are often categorised as:Guided

Wireless

Satellite


3.4.1 Guided Media: Two-Wire Open Lines

Signal WireReference Wire

Used mainly for directly connecting devicesover small distances (< 50 m) and at moderate bit rates (< 19.2 bps)

Signals get distorted due to:Crosstalk between the two signalsSusceptibility to Noise Signals from other (external) electrical sources


Guided Media: Twisted Pair Lines

The proximity of signal and reference lines means noise signals are picked up by both wires

Shielded twisted pairs offer better noise immunityskin effect, which increases attenuation as the bit rate of the transmitted signal increases.

Offer higher bit rates1.5 Mbps up to 5 km51 Mbps less than 300 m

E.g. high speed data to home (ADSL) and LANs


Guided media: Coaxial Cable

This is made up of an inner conductor surrounded by an insulator and that surrounded again by the outer conductor

Use of the dielectric material and outer conductor effectively isolate the core conductor from external noise interference.

Coaxial cables can be used at rates from over 10Mbps, over distances of several hundred meters.

E.g. Cable TV and Cable Modem


Guided Media: Optical Fibre

Optic fibre does not use electrical signals to transmit the data, rather it uses light. It transmits these signals through thin glass fibres.

Light beams are also immune to electromagnetic interference and crosstalk

Currently up to 40 Gbps with repeaters every 50 km. Theoretically, up to 50 Tbps = 50,000 Gbps.

E.g. access and backbone networks, very fast LANs


3.4.2 Wireless: Terrestrial Microwave/Radio

Provides omnidirectional or unidirectional signalling depending on transmitter and antenna.

Suffer from factors such as bad weather conditions and obstruction by man-made objects.

Use microwaves for large distances, radio waves for shorter distances. Limited by the curvature of the Earth.

E.g. AM and FM radio, TV, Cellular telephony, wireless LANs


3.4.3 Satellite

The first satellites were launched in the 1970’s and now the most common are the Geostationary ones at 36,000 km above the Earth.Satellites are using direct line of sight between the transmitters and receivers.Data transmitted using electromagnetic (radio) waves propagatingthrough the atmosphere.Typically many signals will be multiplexed onto a single satellite channel utilising a high bit rate.Applications:

Areas with little wired infrastructureMobile communicationBroadcast communicationRapid deployment (military)

3. physical layer - dcuee224/2012-2013/notes/03_physical_layer_print.pdf · there are two...

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