1. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 1 2.4 Transmission Impairment Signals travel through the transmission media, which are not perfect – causes signal impairment. Thus signal at the beginning of the medium is not the same as signal at the end of the medium – what is sent is not what is received. 3 causes of impairment :  Attenuation  Distortion  noise

2. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 2 2.4 Transmission Impairment Attenuation  attenuation means a loss of energy  When signal travels through a medium, it loses some of its energy in overcoming the resistance of medium.  Ex : wire carrying electric signals gets warm, electrical energy in the signal is converted into heat.  To compensate for this loss, amplifiers are used to amplify the signal.

3. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 3 2.4 Transmission Impairment decibel (dB)  is used to show that a signal has lost or gained strength.  decibel (dB) measures the relative strengths of two signals or one signal at two different points.  Negative value of decibel : signal is attenuated  Positive value of decibel : signal is amplified (gained strength)  P 1 and P 2 shows the power of two different signal or power at two different points (in watt !)  Ex : Suppose a signal travels through a transmission medium and its power is reduced to one-half. Calculate the attenuation (loss of power).

4. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 4 2.4 Transmission Impairment decibel (dB)  Ex : A signal travels through an amplifier, and its power is increased 10 times. Calculate the gain of power after the amplification.  Ex 3.28 (pg 82) dBm  Used to measure signal power in milliwatts.  Ex : Convert a power level of 200 mW to dBm  Ex : Calculate the power of a signal if its dBm = -30 dBm

5. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 5 2.4 Transmission Impairment Distortion  Distortion means that the signal changes its form or shape.  Distortion can occur in a composite signal made of different frequencies.  Each signal component has its own propagation speed through a medium and, therefore, its own delay in arriving at the final destination.  The delay causes the signal components to have a different phases from what they have at the sender.  This causes a distortion to the composite signal at the receiver.

6. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 6 2.4 Transmission Impairment Noise  Several type of noise – thermal noise, induced noise, crosstalk and impulse noise.  Thermal noise – random motions of electron in wire which creates an extra signal  Induced noise – from sources such as motors and appliances.  Crosstalk – effect of one wire on the other. One wire acts as a sending antenna, and another wire acts as a receiving antenna.  Impulse noise – a spike (signal with high energy in very short time) comes from power lines, lightning etc.

7. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 7 2.4 Transmission Impairment Signal-to-Noise Ratio (SNR)  the signal-to-noise ratio is define as  SNR shows the ratio what is wanted (signal) to what is unwanted (noise)  High SNR means the signal is less corrupted by noise ; low NSR means the signal is more corrupted by noise.  The SNR is often expressed in dB  Ex : the power of a signal is 10 mW and the power of the noise is 1 uW. What are the values of SNR and SNR dB ?

8. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 8 2.4 Transmission Impairment  Large SNR and low SNR

9. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 9 2.5 Data Rate Limits Data rate (how fast we can send data) depends on 3 factors :  The bandwidth available  The level of signals used  Quality of the channel 2 theoretical formula to calculate the data rate  Nyquist Bit Rate for a noiseless channel  Shannon Capacity for a noisy channel

10. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 10 2.5 Data Rate Limits Nyquist Bit Rate for Noiseless Channel  For a noiseless channel, Nyquist bit rate defines the theoretical maximum bit rate as  Bandwidth : bandwidth of the channel; L is the signal levels used to represent data; bit rate is the bit rate in bits per second (bps).  Increasing the number of a signal may reduce the reliability of the system.  Ex : Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with 2 signal levels. Calculate the maximum bit rate.  Ex : Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with 4 signal levels. Calculate the maximum bit rate.

11. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 11 2.5 Data Rate Limits Shannon Capacity for Noisy Channel  In reality, we cannot have a noiseless channel.  Shannon Capacity is used to determine the theoretical highest data rate for a noisy channel  Bandwidth : bandwidth of the channel; SNR is the signal-to-noise ratio; capacity is the capacity of the channel in bits per second.  In Shannon formula, there is no indication of the signal levels; means that no matter how many levels we have, we cannot achieve a data rate higher than a capacity of the channel  Ex : Consider an extremely noisy channel in which the value of the signal-to- noise ratio is almost zero. Calculate the capacity of the channel.  Ex : Calculate the theoretical highest bit rate of a telephone line given a bandwidth of 3000 Hz and SNR = 3162. How to further increase the capacity ?

12. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 12 2.5 Data Rate Limits  Ex : Assume the signal-to-noise ration is given in SNR dB = 36 dB and the channel bandwidth is 2 MHz. Calculate the theoretical channel capacity. Conclusion from Shannon Capacity and Nyquist Bit Rate  The Shannon Capacity gives the upper limit  The Nyquist bit rate determine how many signals level is need  Ex : We have a channel with a 1 MHz bandwidth. The SNR for this channel is 63. What are the appropriate bit rate and signal level ?

13. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 13 2.6 Performance Some terms used to describe how good is the network Bandwidth  Bandwidth in hertz – is the range of frequencies contained in a composite signal or the the range of frequencies a channel can pass. Ex : bandwidth of a subscriber telephone is 4 kHz.  Bandwidth in Bits per second – refer to the number of bits per second that a channel, a link or network can transmit. Ex : bandwidth of a Fast Ethernet Network is a maximum of 100 Mbps (means that the network can send at a speed of 100 Mbps).  Relationship between bandwidth in hertz and bandwidth in bits per second – an increase in bandwidth in hertz means an increase in bandwidth in bits per second.  Ex : The bandwidth of a subscriber line is 4 kHz for voice or data. The bandwidth of this line for data transmission can be up to 56000 bps using a sophisticated modem to change the digital signal to analog.

14. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 14 2.6 Performance Throughput  is a measure of how fast we can actually send data through a network.  A link may have a bandwidth of B bps, but we can only send T bps through this link with T always less than B.  Ex : A network with bandwidth of 10 Mbps can pass only an average 12000 frames per minute with each frame carrying an average of 10000 bits. What is the throughput of this network ?

15. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 15 2.6 Performance Latency (Delay)  Latency or Delay defines how long it takes for an entire message to completely arrive at the destination from the first time bit is sent out from the source.  4 components related to latency : propagation time, transmission time, queuing time, processing delay. Propagation Time  Measures the time required for a bit to travel from the source to the destination.  Ex : What is the propagation time if the distance between the 2 points is 12000 km ? Assume the speed to be 2.4 x 10 8 m/s in cable.

16. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 16 2.6 Performance Transmission Time  The time between the first bit leaving the sender and the last bit arriving at the receiver.  Ex : What are the propagation time and the transmission time for a 2.5-kbyte message (an email) if the bandwidth of the network is 1 Gbps ? Assume that the distance between the sender and the receiver is 12000 km and light travels at 2.4 x 10 8 m/s.  Ex : What are the propagation time and the transmission time for 5 Mbyte message (an image) if the bandwidth of the network is 1 Mbps ? Assume the distance between the sender and the receiver is 12000 km and light travels at 2.4 x 10 8 m/s.

17. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 17 2.6 Performance Queuing Time  The time needed for each intermediate or end device to hold the message before it can be processed.  Changes with the load imposed on the network.  Heavy traffic causes the queuing time to increase.  An intermediate device, such as router, queues the arrived message and processes them one by one.

18. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 18 2.6 Performance Bandwidth-Delay Product  Bandwidth-delay defines the number of bits that can fill the link.  Case 1  Assume we have a link with bandwidth of 1 bps and the delay of the link is 5s.  Bandwidth x delay product = 5 bits is the maximum number of bits that can fill the link at any time on the link.

19. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 19 2.6 Performance Bandwidth-Delay Product  Case 2  Assume we have a link with bandwidth of 5 bps and the delay of the link is 5s.  Bandwidth x delay = 25 bits is the maximum number of bits that can fill the link at any time on the link.  Bandwidth-delay product is important to determine the maximum number of bit per packet to be sent through the channel.

20. Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 20 2.6 Performance Jitter  Jitter is a problem if different packets of data encounter different delays and the application using the data at the receiver site is time-sensitive ( e.g. audio and video data).  If the delay of the 1st packet is 20 ms, for the 2nd is 45 ms and for the 3rd is 40 ms, then the real-time application that uses the data endures jitter.