Chapter 3 Data and Signals

1. Chapter 3 Data and Signals

2. Position of the physical layer & Services

3. Chapters Chapter 3 Signals Chapter 4 Digital Transmission Chapter 5 Analog Transmission Chapter 6 Multiplexing Chapter 7 Transmission Media Chapter 8 Circuit Switching and Telephone Network Chapter 9 High Speed Digital Access

4. Chapter 3 Signals

5. Note To be transmitted, data must be transformed to electromagnetic signals.

6. A Transmission System Transmitter Receiver Communication channel Transmission: Communication of data by propagation and processing of signals through a communication channel Signal: Electromagnetic energy that moves through the transmission medium Transmitter • Converts information into signal suitable for transmission • Injects energy into communications medium or channel • Telephone converts voice into electric current • Modem converts bits into electric current Receiver • Receives energy from medium • Converts received signal into form suitable for delivery to user • Telephone converts current into voice • Modem converts electric current into bits

7. Analog Data: Continuous value data (sound, light, temperature) Digital Data: Discrete value (text, integers, symbols) Analog Signals: Continuously varying electromagnetic wave have an infinite number of values over a period of time Digital Signals: Series of voltage pulses (square wave) have only a limited number of values over a period of time. Maintains a constant level then changes to another constant level Signals are classified into: Periodic Signals: Consists of repeated patterns over identical period of times (cycles) Aperiodic Signals: change without a pattern that repeats over time 3.1 Analog and Digital

8. Figure 3.1 Comparison of analog and digital signals How signals are represented on a graph

9. Periodic analog and digital signals

10. Note: In data communication, we commonly use periodic analog signals and aperiodic digital signals.

11. 3.2 Analog Signals Sine Wave Phase Examples of Sine Waves Time and Frequency Domains Composite Signals Bandwidth

12. Figure 3.2A sine wave • Sine wave is the most fundamental form of a periodic analog signal (a simple signal ) • Three main characteristics that describe Simple Sine Wave: • Amplitude • Frequency • Phase

13. Simple Signal Characteristics: 1- Amplitude Peak amplitude is the absolute value of the highest/lowest signal value

14. Simple Signal Characteristics: 2- Frequency Period = The time needed to complete a full pattern (cycle) Frequency: Number of completed (patterns) cycles per second

15. Note Frequency is the rate of change with respect to time. Change in a short span of time means high frequency.Change over a long span of time means low frequency.

16. Note If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite.

17. Note Frequency and period are the inverse of each other.

18. Example 3.3 The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows: 1 ms = 10-3 second

19. Example 3.5 The period of a signal is 100 ms. What is its frequency in kilohertz? Solution First we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz).

20. Simple Signal Characteristics: 3- Phase • Phase: the position of the signal relative to time zero (where the signal starts with respect to time zero) • measured in degrees or radians (1 degree is radian ) • It describes the amount of the shift happened to the first cycle (its offset with respect to time zero)

21. Note: Phase describes the position of the waveform relative to time zero.

22. Mathematical description of sine wave • S instantaneous Amplitude • A peak Amplitude • F frequency • Phase

23. Figure 3.6Sine wave examples

24. Figure 3.6Sine wave examples (continued)

25. Figure 3.6Sine wave examples (continued)

26. The Medium bandwidth Medium Bandwidth meaning depends on the type of the transmitted signal through the medium: • Analog bandwidth is associated with Analog signals. • it is the difference between the highest and the lowest (the range of ) frequencies that the medium can pass safely (expressed in Hz) • Range of frequencies in a signal • High bandwidth Medium Less error in the transmitted data • Needs Expensive equipments • Low bandwidth Medium more errors • cheaper • Digital Bandwidth is associated with Digital signals • It is the maximum bit rate that a medium can pass expressed in bits per second (bps)

27. Example 4 A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all integral frequencies of the same amplitude. Solution B = fh- fl 20 = 60 - fl fl = 60 - 20 = 40 Hz

28. 3.3 Digital Signals 1 = Positive voltage > 0 0 = Zero voltage Most Digital Signals are Aperiodic Frequency is not appropriate

29. Figure 3.16 Two digital signals: one with two signal levels and the other with four signal levels

30. Figure 3.17Bit rate and bit interval Bit Interval (digital signal period): The time required to send (represent) one single bit (time units) Bit Rate (digital bandwidth) : Number of bits sent per second (bps)

31. Example 6 A digital signal has a bit rate of 2000 bps. What is the duration of each bit (bit interval) Solution The bit interval is the inverse of the bit rate. Bit interval = 1/ 2000 s = 0.000500 s = 0.000500 x 106ms = 500 ms

32. Figure 3.12Signal corruption • No transmission medium can pass all the signal frequencies safely • A medium may • Pass some frequencies • Block others • Weaken others

33. 3.4 Transmission Impairments Impairments: Factors that make the received signal different from the transmitted one

34. Attenuation • Attenuation: Loss of energy due to resisting the medium (Signal strength falls off with distance) • Increases with signal frequency • Ex. A wire carrying electrical signal becomes warm after some time • Amplifiers (analog signals) and repeaters (digital signals) are used to handle attenuation • Attenuation affects analog signals

35. Distortion • Distortion : Signal changes in shape • Distortion will cause different bits to overlap • Usually occurs to composite signal due to different propagation delays of its components

36. Noise • Thermal Noise due to random motion of electrons in a wire which will create an extra signal • Induced Noise: caused by motors and electrical equipments. • crosstalk noise: Two wires beside each others (hearing another conversation in the background while talking with the phone) • Impulse noise: irregular pulses or noise spikes of short duration and high amplitude • caused by power lines or lightning • Very critical in case of digital signals (primary source of error in digital data communication)

37. Propagation time= time required for a bit to move between two nodes. Propagation time = distance of the link / Propagation speed Propagation speed : Speed of light in the medium. Depends on the medium Light travels at 3x108 m/sVacuum (free space), lower in the air and much lower in a cable (2/3 in vacuum) 3-6 PERFORMANCE Other Physical layer definitions:

38. PropagationTime

39. Transmission time = The time it takes the sender to transmit (put) all the bits of a frame into the link Transmission time = Length of frame (message) in bits / link data rate (bandwidth) (bps) Queuing Time = Time needed for each intermediate device to hold the message before it can be processed. Latency (delay): Total message delivery time = Transmission time + Propagation time + processing time+ queuing time Other Physical layer definitions – Transmission Time

40. Example 3.46 What are the propagation time and the transmission time for a 2.5-kbyte message (an e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s. Solution We can calculate the propagation and transmission time as shown on the next slide:

41. Example 3.46 (continued) Note that in this case, because the message is short and the bandwidth is high, the dominant factor is the propagation time, not the transmission time. The transmission time can be ignored.

42. Example 3.47 What are the propagation time and the transmission time for a 5-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s. Solution We can calculate the propagation and transmission times as shown on the next slide.

43. Example 3.47 (continued) Note that in this case, because the message is very long and the bandwidth is not very high, the dominant factor is the transmission time, not the propagation time. The propagation time can be ignored.

44. Figure 3.33 Concept of bandwidth-delay product bandwidth-delay product = Length of the link in bits

45. Example 3.48 We can think about the link between two points as a pipe. The cross section of the pipe represents the bandwidth, and the length of the pipe represents the delay. We can say the volume of the pipe defines the bandwidth-delay product

46. Note The bandwidth-delay product defines the number of bits that can fill the link.

47. Figure 3.31 Filling the link with bits for case 1

48. Figure 3.32 Filling the link with bits in case 2 5 bps 25 bits

49. Wavelength: the distance a simple signal can travel in one period (or the distance occupied by one cycle) Depends on both the signal frequency and the medium Wavelength = Propagation speed x Period = Propagation speed / frequency Other Physical layer definitions - Wavelength