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Physical Layer. IS250 Spring 2010 [email protected] Summary. Physical layer is concerned with the communication of data encoded as signals transmitted over a medium Fundamental techniques: encoding, modulation, multiplexing

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Summary
Summary

  • Physical layer is concerned with the communication of dataencoded as signals transmitted over a medium

    • Fundamental techniques: encoding, modulation, multiplexing

  • Channel capacity influenced by hardware bandwidth, encoding scheme, transmission impairments (noise and attenuation)


Outline
Outline

  • Fundamental concepts

    • Data, signal, transmission (Ch. 5)

    • Transmission media (Ch. 7)

    • Multiplexing (Ch. 11)

    • Transmission impairments (Ch. 8.2)

  • Data encoding (Ch. 6, 10)

  • Channel capacity (Ch. 7)


Communication system
Communication System

  • Transmitter, receiver, medium

http://i.ehow.com/images/GlobalPhoto/Articles/4996474/illustration-main_Full.jpg


Communication system1
Communication System

  • Transmitter, receiver, medium

  • Data, Signal, Transmission

    • Data: entities that convey meaning (can be digital or analog)

    • Signals: electric or electromagnetic representations of data (can be digital or analog)

    • Transmission: communication of data by propagation and processing of signals


Data and signal
Data and Signal

  • Digital data, digital signal

  • Analog data, digital signal

  • Digital data, analog signal

  • Analog data, analog signal

Data


Transmission media
Transmission Media

  • Guided (wired): twisted pair, coaxial cable, optical fiber

  • Unguided (wireless): RF, microwave (terrestrial & satellite), infra-red


Frequencies you may be using today
Frequencies you may be using today

  • Radio: 535-1605kHz (AM); 88-108MHz (FM)

  • TV: 54-88MHz; 174-216MHz; 470-806MHz

  • Cell phones: 850, 900, 1800, 1900MHz

  • Cordless phones: 900MHz, 2.4GHz, 5.8GHz

  • Wi-Fi: 2.4GHz (802.11b/g); 5GHz (802.11a)

  • Q: how do radio/tv stations and receivers, cell phones and towers, etc., share the airwaves?

  • Q: how are 500 channels of TV programming sent over the cable?


Multiplexing
Multiplexing

  • Combining multiple data streams into a single signal

    • Allows resource sharing (e.g., of a communication channel)

  • Many different forms of multiplexing

    • Time division multiplexing (TDM)

      • GSM, SONET

    • Frequency division multiplexing (FDM)

      • Applications: Broadcast radio/TV, DSL

      • Wave division multiplexing (WDM) for fiber optic communication

      • Orthogonal FDM (OFDM) used in DSL, 802.11, 802.16, etc.

    • Spread spectrum

      • Flavors: Frequency hopping (FHSS), direct sequence (DSSS)

      • Transmitter & receiver coordinates via pseudo-random number generator

      • Basis for CDMA (code-division multiple access) technologies

    • Spatial multiplexing

      • e.g., wireless MIMO antennae used in 802.11n


Outline1
Outline

  • Fundamental concepts

    • Data, signal, transmission (Ch. 5)

    • Transmission media (Ch. 7)

    • Multiplexing (Ch. 11)

    • Transmission impairments (Ch. 8.2)

  • Data encoding (Ch. 6, 10)

  • Channel capacity (Ch. 7)


Transmission impairments
Transmission Impairments

  • Signal received may differ from signal transmitted

    • Analog transmission: degradation of signal quality

    • Digital transmission: bit errors

  • Causes

    • Attenuation

    • Noise

Source: http://www.telebyteusa.com/primer/fig9.gif


Attenuation and noise
Attenuation and Noise

  • Attenuation

    • Signal strength falls off with distance

    • Received signal strength:

      • must be enough to be detected

      • must be sufficiently higher than noise to be received without error

    • Attenuation is an increasing function of frequency

  • Noise: additional signals inserted between transmitter and receiver

    • Thermal: thermal agitation of electrons (also called “white noise”)

    • Intermodulation: signals that are the sum and difference of original frequencies sharing a medium

    • Crosstalk: signal from one line is picked up by another

    • Impulse: irregular pulses or spikes that are high in amplitude and short in duration, e.g., external electromagnetic interference


Analog v digital transmission
Analog v. Digital Transmission

  • Digital transmission better than analog transmission in supporting long distance communication. Why?

  • Analog signal transmitted without regard to content

    • Signal is subject to attenuation and noise

    • Amplifiers can be used to boost signal strength, but noise is also amplified

  • Digital transmission involves processing of content

    • Signal is subject to attenuation and noise

    • Repeaters can be used to boost signal strength

      • Repeater receives signal, extracts bit pattern, retransmits clean signal without noise

    • Attenuation is overcome, and noise is not amplified


Outline2
Outline

  • Fundamental concepts

    • Data, signal, transmission (Ch. 5)

    • Transmission media (Ch. 7)

    • Multiplexing (Ch. 11)

    • Transmission impairments (Ch. 8.2)

  • Data encoding (Ch. 6, 10)

  • Channel capacity (Ch. 7)


Encoding techniques
Encoding Techniques

  • Digital data, digital signal

  • Analog data, digital signal

  • Digital data, analog signal

  • Analog data, analog signal

Data


1 digital data digital signal

3.2v

0v

1. Digital Data, Digital Signal

  • Digital signal as discrete, discontinuous voltage pulses

    • Binary data encoded into signal elements

    • Bit duration (function of data rate), voltage levels have to be specified

  • Example 1: RS-232

  • Example 2: USB

    • USB uses NRZI (non-return-to-zero inverted) encoding

      • Presence of transition encodes a “1”

      • Absence of transition encodes a “0”

    • Data rates: 1.5Mbps, 12Mbps, 480Mbps


2 analog data digital signal
2. Analog Data, Digital Signal

  • Step 1: convert analog data into digital data via samplingand quantization (e.g., pulse code modulation)

    • Example: 4-bit PCM

      • Analog data input (in red)

      • 16 quantized levels can be represented using 4 bits

      • Therefore each sample converted into 4 binary bits

      • Digital data output: 1001101111001101111011101111…

  • Step 2: digital data can then be transmitted using digital encoding schemes (previous slide)

  • Variations: delta PCM, adaptive DPCM


3 analog data analog signals
3. Analog Data, Analog Signals

  • Example: broadcast radio, TV

  • Carrier signal modulated by analog data

  • Types of analog modulation

    • Amplitude modulation (AM)

    • Frequency modulation (FM)

    • Phase modulation (PM)

  • Why modulate analog signals?

    • Higher frequency can give more efficient transmission

    • Permits frequency division multiplexing by using different carrier frequencies for different channels (see slide on multiplexing)

carrier

data


4 digital data analog signal
4. Digital Data, Analog Signal

  • Example: using a modem (modulator-demodulator) to send data over analog public telephone system

  • Digital Modulation very similar to Analog Modulation:

    • ASK (amplitude shift keying): values represented by different amplitudes of carrier

      • Usually, one amplitude is zero, i.e., detect presence or absence of carrier

    • FSK (frequency shift keying): values represented by different frequencies (near carrier)

    • PSK (phase shift keying): phase of carrier signal shifted to represent data

  • Can be combined: e.g., QAM (quadrature amplitude modulation) is combination of ASK and PSK


Outline3
Outline

  • Fundamental concepts

    • Data, signal, transmission (Ch. 5)

    • Transmission media (Ch. 7)

    • Multiplexing (Ch. 11)

    • Transmission impairments (Ch. 8.2)

  • Data encoding (Ch. 6, 10)

  • Channel capacity (Ch. 7)


Channel capacity
Channel Capacity

  • Hardware cannot change signal states (e.g., voltage levels) instantaneously  transmission systems have limited bandwidth

  • Bandwidth (B): maximum rate that the hardware can change a signal (measured in Hertz, or cycles per second)

  • Data rate (D): rate at which data can be communicated (measured in bits per second)

  • Channel capacity (C): maximum data rate, which is determined by hardware bandwidth


Channel capacity1
Channel Capacity

  • Nyquist (1928): D < 2B

    • the number of independent pulses that could be put through a telegraph channel per unit time is limited to twice the bandwidth of the channel

  • Hartley (1928): D < 2B log2(K)

    • where K is the number of distinct messages that can be sent

    • Nyquist result is special case of K=2


Example dial up modem w qam comer 10
Exampledial-up modem w QAM (Comer 10)

  • B = 2400Hz

  • V.32 modem:

    • K = 32

    • D < 2*2400*log232 = 24000bps

  • V.32bis modem:

    • K = 128

    • D < 2*2400*log2128 = 33600bps

  • But these modems can only support data rates of 9600bps and 14400bps, respectively. Why?


Shannon s theorem 1948
Shannon’s Theorem (1948)

  • Channel capacity in the presence of noise:

    C = B log2(1+S/N)

    Where

    • C is effective channel capacity

    • B is hardware bandwidth

    • S/N is the Signal-to-Noise Ratio


Decibels db
Decibels (dB)

  • Engineers like to express signal-to-noise ratio in decibels (dB) using the following quantity:

    10log10(S/N)

  • Example: a signal-to-noise ratio of 100 is expressed as 20dB

  • Example: a signal-to-noise ratio of 30dB is the same as 10^(30/10) or 1000


Application
Application

  • Conventional telephone system

    • Engineered for voice

    • Bandwidth is 3000Hz

    • SNR ~= 30dB

    • Effective capacity is:

      3000log2(1+1000) ~= 30000bps

    • Conclusion (Comer, p.130): dial-up modems have little hope of exceeding 28.8Kbps

    • Q: So what about those 56k modems?


Implications
Implications

  • Nyquist/Hartley: encoding more bits per cycle will improve data rate

  • Shannon: no amount of clever engineering can overcome the fundamental physical limits of a real transmission system


Summary1
Summary

  • Physical layer is concerned with the communication of dataencoded as signals transmitted over a medium

    • Fundamental techniques: encoding, modulation, multiplexing

  • Channel capacity influenced by hardware bandwidth, encoding scheme, transmission impairments (noise and attenuation)


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