Physical Layer

NCE 501

Internet protocol stack

• Application: supporting network applications – ftp, smtp, http

• Transport: host-host data transfer– tcp, udp

• Network: routing of datagrams from source to destination– ip, routing protocols

• Data-link: data transfer between neighboring network elements– ppp, ethernet

• Physical: bits “on the wire”

application

transport

network

link

physical

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Layering: Physical Communication

applicationtransportnetworklink

physical

applicationtransportnetworklink

physicalapplicationtransportnetworklink

physical

applicationtransportnetworklink

physical

networklink

physical

data

data

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Protocol layering and data

Each layer takes data from the layer above• Adds header information to create new data unit• Passes new data unit to layer below

applicationtransportnetwork

linkphysical

applicationtransportnetwork

linkphysical

source destination

M

M

M

M

Ht

HtHn

HtHnHl

M

M

M

M

Ht

HtHn

HtHnHl

message

segment

datagram

frame

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Encapsulating Data

Transport

Data Link

Physical

Network

Upper Layer Data

Upper Layer DataTCP Header

DataIP Header

DataLLC Header

0101110101001000010

DataMAC Header

Presentation

Application

Session

Segment

Packet

Bits

Frame

FCS

FCS

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De-encapsulating Data

Upper Layer Data

LLC Hdr + IP + TCP + Upper Layer Data

MAC Header

IP + TCP + Upper Layer Data

LLC Header

TCP+ Upper Layer Data

IP Header

Upper Layer Data

TCP Header

0101110101001000010

Transport

Data Link

Physical

Network

Presentation

Application

Session

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Outline

• What is the Physical Layer?

• What are its functionalities?

• How fast can we send the bits?– Bandwidth, SNR

• Wired and Wireless– Which way to go and why?

• Some key technologies used in Physical Layer, such as OFDM, MIMO

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References etc.

• Fundamentals:– Data Communication and Networking, 4th

edition, Behrouz Forouzan

• More details:– Digital Communication, B.Sklar

• Further Details:– Wireless Communication, Andra Goldsmith– Fundamentals of Wireless Communication, D.

Tse and P. Viswanath

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Physical Layer

• What is the job of physical layer?– Communicate bits reliably from one Data-link layer

to the other.• The bit transfer must take place with minimum

number of errors, to avoid retransmission (packet error etc) in the higher layers.

• It should be able to match the rate at which bits are arriving from higher layers.

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Physical Layer

• To transmit over a physical medium, the bits must be encoded into signals; electrical or optical, depending upon the medium used for transmission.

• Each medium has a certain capacity to carry a number of bits per second. This capacity depends upon bandwidth and Signal to Noise ratio (SNR).

• Capacity determines the number of bits that can be sent per second (transmission rate).

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Physical layer

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Transmission

• The bits can be transmitted by converting them into an electrical signal (for copper wires) or into light signal (fiber).

• It is also possible to transmit the bits over the “air” by converting them into signals of appropriate frequency.

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Physical Layer: Transmitter

Channel Coding

Map bits to Symbols

Convert the symbols levels to a physical

signal

Channel information

Analog Processing, Amplification

Antenna

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Physical Layer

• How many bits per second?

• Can we send as many bits as we want?

• What limits the bit rate?

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Channel Capacity• Channel Capacity is defined as the maximum number of bits

that can be transmitted over a channel. It depends upon the channel bandwidth and SNR condition.

• The channel capacity theorem gives an upper limit on the bps. If higher rates are attempted, the errors will too large. Formally, the channel capacity theorem, also known as Shannon–Hartley theorem or noisy channel coding theorem is defined as follows:

• C=B*log2(1+SNR) bits/s

• C: Channel capacity (bits/second)• B: bandwidth in Hz.• SNR: Signal to Noise ratio

What is Bandwidth?

• Bandwidth is the width of the frequency band used for communication.

• It determines the maximum allowable frequency of the transmitted signal.

• What is the frequency of the transmitted signal?

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Transmission

• Frequency of a transmitted signal is the rate at which the transmitted signal changes.

• More changes means more information: higher bit rate

• Consider a bit sequence being transmitted over a twisted pair using a sequence of pulse of amplitude 1 V and 0 V.

• Duration of each pulse is 1ms.

• What is the data rate?

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Transmission Rate

• Data rate can be increased by reducing the pulse duration to say 1s, resulting in data rate of 1 Mbits/second.

• Downside? The detection device at the receiver has to work faster and better. Can we go faster that that?

• Can we send a pulse of duration 1ns, resulting in bit rate of 1 Gbits/s? Or even higher?

• Is there any limitation on the rate at which we can send pulses over the wires? Be it copper twisted pair, coaxial cable or fiber?

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Rate Limitation

• The rate at which pulses can be sent will depend upon:

– Bandwidth of the medium over which these pulses have traveled.

– the speed of the switching circuitry (Analog to Digital Converters).

• The medium changes nature of the pulses, effectively spreading them out and smoothening the sharp edges.

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Attenuation, Dispersion Effects

Source: Prof. Raj Jain, WUSTL

Inter-symbol interference (ISI)

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Pulse Dispersion

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Channel?

• Channel is the medium of transmission.• Wired:

– Twisted Pair: • Level 1: Bandwidth 400 kHz• Cat 6: Ethernet upto 250 MHz

– Coaxial Cable: Bandwidth > 400 MHz– Optical Fiber

• Wireless: Varying frequency bands– Cellular– Satellite– WLAN/WPAN/WBAN etc etc

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Bandwidth

• The number of transitions that can be sustained over a medium depends upon its bandwidth.

• For a channel of bandwidth 5000 Hz, 10000 such changes can take place. In other words, smallest pulse duration is 0.1 ms. If pulses of durations 1 s are transmitted, there will be considerable distortion and transmitted bit sequences cannot be recovered at the receiver.

• This pulse rate is referred to as the symbol rate or ‘baud rate’. For a channel of bandwidth B, maximum allowable symbol rate is 2B. However, in practice, only B symbols may be transmitted per second.

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Noise

• In common use, the word noise means any unwanted sound. In computing it can be considered unwanted information; that is, data that is not the desired signal, but is simply produced as an unwanted by-product of other activities.

• Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects. Thermal and shot noise are unavoidable, while other types depend mostly on manufacturing quality and semiconductor defects.

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Noise Sources

• Naturally occurring noise– Thermal radiation: generated through the random thermal

motion of electrons in a conduction medium. Thermal noise is system inherent.

– Absorption noise: when energy is absorbed by a body the same energy is reradiated as noise as shown by the theory of black body emission.

– Shot- and 1/f noise

• Man-made noise sources– Quantization noise– Oscillator phase noise– Interference treated as noise

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Signal to Noise Ratio• Signal-to-noise ratio (often abbreviated SNR or S/N) is a measure

used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise.

• SNR is normally stated in dB or dBm. Both definitions are as follows:

• dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one mW. It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form

N

s

P

PSNR 10log10

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Bit Rate• It is the bandwidth and the SNR that determines how many bits can

be transmitted through a particular medium. Higher the bandwidth, more the number of transitions we can have (narrower pulses). Higher the SNR, more the number of levels that each pulse can have.

• Ultimately, the user is interested in the total number of bits per second (bps) that can be transmitted with specific reliability. The reliability is measured in terms of Bit Error Rate (BER). Most applications have a specific BER requirement; e.g. 10-4 or 10-6.

• It is possible to trade bps for lower BER. If BER requirement is stringent (10-8), it is possible to achieve that by reducing the overall bit rate (bps) and vice versa.

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Channel Capacity• Channel Capacity is defined as the maximum number of bits that

can be transmitted over a channel. It depends upon the channel bandwidth and SNR condition.

• The channel capacity theorem gives an upper limit on the bps. If higher rates are attempted, the errors will too large. Formally, the channel capacity theorem, also known as Shannon–Hartley theorem or noisy channel coding theorem is defined as follows:

• C=B*log2(1+SNR) bits/s

• C: Channel capacity (bits/second)• B: bandwidth in Hz.• SNR: Signal to Noise ratio

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Channel Capacity

• Consider a channel of bandwidth 20 MHz. What is the capacity of the channel is the SNR is 20 dB (and 0 dB)?

• If SNR decreases from 20 to 0 dB, is there any way to maintain the same bit rate?

• C = Blog2(1+snr)• Case 1: SNR=20 dB, snr = 100

– Capacity C = 20*log2(101) =133Mbps• Case 2: SNR = 0 dB, snr =1

– Capacity=20*log2(2)=20 Mbps

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Noisy Channel Coding Theorem• Shannon's theorem shows how to compute a channel

capacity from a statistical description of a channel, and establishes that given a noisy channel with capacity C and information transmitted at a line rate R, then if– R < C

• there exists a coding technique which allows the probability of error at the receiver to be made arbitrarily small. This means that theoretically, it is possible to transmit information nearly without error up to nearly a limit of C bits per second.

• The converse is also important. If– R > C

• the probability of error at the receiver increases without bound as the rate is increased. So no useful information can be transmitted beyond the channel capacity.

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Spectral Efficiency

• Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth in a specific communication system. It is a measure of how efficiently a limited frequency spectrum is utilized by the physical layer protocol, and sometimes by the media access control.

• The link spectral efficiency of a digital communication system is measured in bit/s/Hz.

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Bits/s/Hz

System Bandwidth (Hz) Bit Rate Bits/s/Hz

Dial-up Modem 4000 Hz 56 kbps 14

DSL 1.5 MHz 1.5- 12 Mbps 1-8

DVB-H 8 MHz 5.5 to 11 MBps 0.68 to 1.4

Fixed WiMAX 20 MHz 96 Mbps 4.8

2G, GSM 20 kHz 10.4 kbps 0.52

802.11b/g 20 MHz 54Mbps 2.7

802.11n 20 MHz 144.4Mbps 7.22

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Spectral Efficiency

• It can be seen that different techniques have different bit rates, even when the allocated bandwidth is the same.

• What makes the dial-up modem so much more spectrally efficient than say WiFi?

• It is a combination of bandwidth, SNR, nature of the channel that determines the overall bit rate.

• Wired channels are much more spectrally efficient due to reasonably time-invariant nature of the channel and overall higher SNR.

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