1. IS250 Spring 2010 chuang@ischool.berkeley.edu
Physical Layer
IS250
Spring 2010

2. 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)

3. Outline Fundamental concepts Data encoding (Ch. 6, 10)
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)

4. Communication System Transmitter, receiver, medium

5. 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

6. Data and Signal Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
Data

7. Transmission Media
Guided (wired): twisted pair, coaxial cable, optical fiber
Unguided (wireless): RF, microwave (terrestrial & satellite), infra-red

8. Frequencies you may be using today
Radio: kHz (AM); MHz (FM)
TV: 54-88MHz; MHz; MHz
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?

9. 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, , , 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 n

10. Outline Fundamental concepts Data encoding (Ch. 6, 10)
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)

11. Transmission Impairments
Signal received may differ from signal transmitted
Analog transmission: degradation of signal quality
Digital transmission: bit errors
Causes
Attenuation
Noise
Source:

12. 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

13. 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
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

14. Outline Fundamental concepts Data encoding (Ch. 6, 10)
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)

15. Encoding Techniques Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
Data

16. 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

17. 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: …
Step 2: digital data can then be transmitted using digital encoding schemes (previous slide)
Variations: delta PCM, adaptive DPCM

18. 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

19. 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

20. Outline Fundamental concepts Data encoding (Ch. 6, 10)
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)

21. 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

22. 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

23. Example dial-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?

24. 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
Combining Hartley’s Law and Shannon’s Theorem --
Implication: the number of effectively distinguishable messages (K) is determined by the signal-to-noise ratio: K = sqrt(1+S/N)

25. 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

26. 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?
Actual data rate dependent on line quality
56k modems use digital as opposed to analog signals on the telephone lines; else fall back to 33.6kbps
Never caught on since DSL became widely available

27. 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

28. 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)