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Summary
Physical layer is concerned with the communication of data encoded 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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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)
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Communication System
Transmitter, receiver, medium
http://i.ehow.com/images/GlobalPhoto/Articles/4996474/illustration-main_Full.jpg
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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
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Data and Signal Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
Data
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Transmission Media Guided (wired): twisted pair, coaxial cable, optical fiber Unguided (wireless): RF, microwave (terrestrial & satellite),
infra-red
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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?
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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
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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)
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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
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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
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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
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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)
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Encoding Techniques Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
Data
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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
0v
3.2v
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2. Analog Data, Digital Signal
Step 1: convert analog data into digital data via sampling and 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
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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)
data
carrier
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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
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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)
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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
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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
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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?
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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
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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
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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?
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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
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Summary
Physical layer is concerned with the communication of data encoded as signals transmitted over a medium- Fundamental techniques: encoding, modulation, multiplexing
Channel capacity influenced by hardware bandwidth, encoding scheme, transmission impairments (noise and attenuation)
