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Lecture 2 data communication physical layer
- 1. Data Communication Lecture 2 PhysicalLayer: Signals & Digital Transmission
- 2. Physical Layer Topicsto Cover Signals Digital Transmission Analog Transmission Multiplexing Transmission Media
- 3. Analog & DigitalData Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values.
- 4. To be transmitted,data must be transformed to electromagnetic signals. Note
- 5. Signals can beanalog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values. Note
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- 9. Cont’ Three parametersto describe a sine wave 1. Peak amplitude 2. Frequency and time period 3. Phase
- 10. The bandwidth ofa composite signal is the difference between the highest and the lowest frequencies contained in that signal. Note
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- 13. Digital Signals Inaddition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.
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- 15. Bit Rate &Bit Interval (contd.)
- 16. Bit Interval andBit Rate Example Example A digital signal has a bit rate of 2000 bps. What is the duration of each bit (bit interval) Solution Solution The bit interval is the inverse of the bit rate. Bit interval = 1/ 2000 s = 0.000500 s = 0.000500 x 106 s = 500 s
- 17. The bit rateand the bandwidth are The bit rate and the bandwidth are proportional to each other. proportional to each other. Note
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- 19. Analog versus digitalsignals
- 20. Low Pass &Band Pass
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- 22. Data Rate Limits A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors: 1. The bandwidth available 2. The level of the signals we use 3. The quality of the channel (the level of noise)
- 23. Noiseless Channel: NyquistBit Rate Defines theoretical maximum bit rate for Noiseless Channel: Bit Rate=2 X Bandwidth X log2L Such that L is he signal level
- 24. Example Example Consider a noiselesschannel with a bandwidth of 3000 Hz transmitting a signal with two signal levels. The maximum bit rate can be calculated as Bit Bit Rate = 2 Rate = 2 3000 3000 log log2 2 2 = 6000 bps 2 = 6000 bps
- 25. Increasing the levelsof a signal may reduce the reliability of the system. Note
- 26. Noisy Channel: ShannonCapacity Defines theoretical maximum bit rate for Noisy Channel: it represents the maximum rate at which information can be reliably transmitted over a communication channel Capacity=Bandwidth X log2(1+SNR) SNR stands for "Signal-to-Noise Ratio." It is a measure used to quantify the ratio of the strength of a signal. A higher SNR indicates a stronger signal relative to the noise, which generally results in better signal quality and more reliable communication.
- 27. Example Example Consider an extremelynoisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity is calculated as C = B log C = B log2 2 (1 + SNR) = B log (1 + SNR) = B log2 2 (1 + 0) (1 + 0) = B log = B log2 2 (1) = B (1) = B 0 = 0 0 = 0
- 28. Example Example We can calculatethe theoretical highest bit rate of a regular telephone line. A telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is usually 3162. For this channel the capacity is calculated as C = B log C = B log2 2 (1 + SNR) = 3000 log (1 + SNR) = 3000 log2 2 (1 + 3162) (1 + 3162) = 3000 log = 3000 log2 2 (3163) (3163) C = 3000 C = 3000 11.62 = 34,860 bps 11.62 = 34,860 bps
- 29. Example Example We have achannel with a 1 MHz bandwidth. The SNR for this channel is 63; what is the appropriate bit rate and signal level? Solution Solution C = B log C = B log2 2 (1 + SNR) = 10 (1 + SNR) = 106 6 log log2 2 (1 + 63) = 10 (1 + 63) = 106 6 log log2 2 (64) = 6 Mbps (64) = 6 Mbps Then we use the Nyquist formula to find the number of signal levels. Bit Rate=2 X Bandwidth X log2L 6 6 Mbps = 2 Mbps = 2 1 MHz 1 MHz log log2 2 L L L = 8 L = 8 First, we use the Shannon formula to find our upper First, we use the Shannon formula to find our upper limit. limit.
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- 31. Transmission Impairments Signalstravel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation attenuation, , distortion distortion, , and noise noise. .
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- 33. Decibel Used tosignal gained or lost strength dB = 10 log P2/P1 (Power) dB Voltage = 20 log (V1/V2) dB Current = 20 log (I1/I2) The dB is not an absolute quantity, it is always a RATIO of two quantities. The unit can be used to express power gain (P2>P1), or power loss (P2<P1) -- in the latter case the result will be a negative number.
- 34. Example Suppose asignal travels through transmission medium and its power is reduced to one half…. Calculate attenuation loss?
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- 36. Performance One importantissue in networking is the performance of the network—how good is it?
- 37. Performance Bandwidth Throughput Latency (Delay) Bandwidth-Delay Product
- 38. Throughput Throughput is ameasure of the actual data transfer rate in a communication system or network, representing the amount of data successfully transmitted from the source to the destination over a given period of time. Throughput is often expressed in bits per second (bps), kilobits per second (Kbps), megabits per second (Mbps), or gigabits per second (Gbps), depending on the scale of the communication system.
- 39. Latency Latency =propagation time + transmission time + queuing time + processing time
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- 41. In computernetworks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay is equal to d / s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c
- 42. In data communications,bandwidth-delay product refers to the product of a data link's capacity (in bits per second) and its end-to- end delay (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maximum amount of data on the network circuit at any given time, i.e. data that has been transmitted but not yet received. Sometimes it is calculated as the data link's capacity multiplied by its round trip time
- 43. Physical Layer Topicsto Cover Signals Digital Transmission Analog Transmission Multiplexing Transmission Media
- 44. According to theNyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal. Note
Editor's Notes
- #27 Faint ضعيفه
- #33 The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities The decibel (dB) is a very commonly used and often misunderstood unit of measurement. The dB is a logarithmic unit expressing the RATIO of two powers. It is defined as: Number of dB = 10 log (P2/P1). The dB is not an absolute quantity, it is always a RATIO of two quantities. The unit can be used to express power gain (P2>P1), or power loss (P2<P1) -- in the latter case the result will be a negative number. The decibel actually comes from a logarithmic unit of measurement called a "Bel", named after Alexander Graham Bell. One Bel is defined as a power ratio of ten (or ten times the power). It was originally used to measure acoustic power (sound) ratios in telephony. The Bel is a rather large unit, so the decibel (dB), which is 1/10 of a bel, is more commonly used. Although the dB is defined with respect to power, it has become common practice to also use it to express voltage or current ratios, in which case it is defined as: dB Voltage = 20 log (V1/V2), or dB Current = 20 log (I1/I2).
- #40 In computer networks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay is equal to d / s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c