1. Physical Layer

2. Data Communications - Physical Layer2 Physical Layer Essence: Provide the means to transmit bits from sender to receiver involves a lot on how to use (analog) signals for digital information l Theoretical background: signal transmission and Fourier analysis l Transmission media (wires and no wires) l Modulation techniques (the actual encoding), multiplexing, and switching

3. Data Communications - Physical Layer3 Transmission of Information l Well-understood basics l From physics Energy Electromagnetic wave propagation l From mathematics Coding theory Fourier Analysis

4. Data Communications - Physical Layer4 Baud Rate & Bandwidth l Baud Number of changes in the signal in 1 second The number of bits per second is the baud rate times the number of bits per signal. l Bandwidth Maximum rate that the hardware can change a signal Measured in cycles per seconds or Hertz (Hz)

5. Data Communications - Physical Layer5 Digital Transmission System Measures l Propagation delay Time required for signal to travel across media Example: electromagnetic radiation travels through space at the speed of light (C=3x10 8 meters/sec) l Throughput The number of bits per second that can be transmitted Related to underlying hardware bandwidth (maximum times per second the signal can change)

6. Data Communications - Physical Layer6 Propagation and Transmission Time l Example Message length: 500 Bytes Bit transmission Rate: 4 Mb/s Optical fiber length: 2 Km Speed of information in the fiber: 66% C ~ 2*10 8 m/s l Transmission time: (500*8bits) / (4*10 6 b/s) = 10 -3 s = 1 ms l Propagation delay (2000 m) / (2*10 8 m/s) = 0.01 ms

7. Data Communications - Physical Layer7 Maximum Data Rate of a Channel Nyquist Sample Theorem: Relation between digital throughput and bandwidth D = 2 B log 2 K D: maximum data rate, in b/s B: system bandwidth K: possible values of voltages (binary K = 2) Absolute maximum

8. Data Communications - Physical Layer8 RS 232 l פרוטוקול להעברת מידע בשכבה הפיזית. l מורכב מחיבור כבל עם מחבר של 25 פינים. l לרוב מחבר בין המחשב (DTE) למודם (DCE).

9. Data Communications - Physical Layer9 Noise l Undesired signal associated with the transmission l May produce error information l Noise Level: signal-to-noise ratio S/N S: average signal power N: Noise signal power Measured in decibels: 10log 10 S/N

10. Data Communications - Physical Layer10 The Effect of Noise Shannon’s Theorem: Gives capacity in presence of noise. C = B log 2 (1 + S/N) C: effective limit on the channel capacity (b/s) B: hardware bandwidth S/N: signal to noise ratio

11. Data Communications - Physical Layer11 Example : maximal transmission rate on a voice grade line l Signal to noise ratio: 30 dB l Bandwidth B = 3000 Hz l Therefore, the maximum bits per seconds that can be transmitted is C = 3000 log 2 (1+1000) = 29901.69 b/s ~ 30 Kb/s Conclusion: dialup modems have little hope of exceeding 28.8 Kbps Remember: log a x = log b x / log b 2

12. Data Communications - Physical Layer12 The Bottom Line l Shannon’s Theorem means that no amount of clever engineering can overcome the fundamental physical limits of a real transmission system Relates throughput to bandwidth Encourages engineers to use complex encoding l Nyquist’s Theorem means finding a way to encode more bits per cycle improves the data rate Adjusts for noise Specifies limits on real transmission systems

13. Data Communications - Physical Layer13 Transmission media l Copper wires Twisted pairs Coaxial Cables l Glass Fibers l Satellites Geo-synchronous Low orbit l Radio Microwave Infrared Laser beam

14. Data Communications - Physical Layer14 Copper Wires

15. Data Communications - Physical Layer15 Glass Fibers

16. Data Communications - Physical Layer16 Transmission Performance

17. Data Communications - Physical Layer17 ASCII Character Codes

18. Data Communications - Physical Layer18 Modulation Techniques Problem: How can we encode our signals when we can effectively use only a single frequency (or better: small frequency range)? Answer: Apply modulation techniques: l Amplitude Modulation l Frequency Modulation l Phase Modulation

19. Data Communications - Physical Layer19 Modulation A binary signal Amplitude Modulation Frequency Modulation Phase Modulation

20. Data Communications - Physical Layer20 Multiplexing: FDM Problem:Considering that the bandwidth of a channel can be huge, wouldn’t it be possible to divide the channel into sub-channels? Frequency Division Multiplexing: Divide the available bandwidth into channels through frequency filtering, and apply modulation techniques per channel:

21. Data Communications - Physical Layer21 Multiplexing: TDM Time Division Multiplexing:Simply merge/split streams of digital data into a new stream. Data is handled in frames – a fixed series of consecutive bits: