Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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).

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 ?

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

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 = How to further increase the capacity ?

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 ?

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 bps using a sophisticated modem to change the digital signal to analog.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 frames per minute with each frame carrying an average of bits. What is the throughput of this network ?

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 km ? Assume the speed to be 2.4 x 10 8 m/s in cable.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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 ) if the bandwidth of the network is 1 Gbps ? Assume that the distance between the sender and the receiver is 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 km and light travels at 2.4 x 10 8 m/s.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.

Chapter 2 : Data Communications BENG 4522 Data Communications & Computer Networks 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.