CYBR 4323 (Privitera) - Chapter 3 - Introduction to Physical Layer
Data rate limits
Depends on: 1. Bandwidth available 2. Level of signals we use 3. Quality of channel
Plotting signals
Horizontal = time Vertical = strength/value of signal Analog is curvy (continuous, flowing) while digital is rectangular (discrete, sudden jumps)
latency
How long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source Latency = propagation time + transmission time + queuing time + processing delay
Fourier analysis to decompose a digital signal
If the digital signal is PERIODIC, which is rare in data communications, the decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies If the digital signal is NONPERIODIC, the decomposed signal still has an infinite bandwidth, but the frequencies are continuous
Period and Frequency
Period: amount of time, in seconds, a signal needs to complete 1 cycle. Frequency: the number of periods in 1 s (Hertz, Hz, cycle per second) - a general measurement of the rate of change of a signal with respect to time - Change in a short span of time means high frequency. Change over a long span of time means low frequency. Period is the inverse of frequency (T = 1/f), and frequency is the inverse of period (f = 1/T)
Physical layers
Physical
Transmission Impairment
Imperfect transmission media leads to signal impairment Three causes of impairment are attenuation, distortion, and noise
Attenuation
Loss of energy As a signal travels through a medium, it loses power (reason for warm/hot wires) Amplifiers are use to amplify the signal
Data types
Analog and Digital
Logical layers
Application, transport, network, or data-link
Periodic analog signals
Simple periodic analog signals - sine wave - cannot be decomposed into simpler signals Composite periodic analog signals - composed of multiple sine waves
If the available channel is a bandpass channel, we cannot send the digital signal directly to the channel; we need to convert the digital signal to an analog signal before transmission.
TRUE
digital signal is a composite analog signal The bandwidth is infinite
TRUE
Transmission of Digital Signals
Two ways: 1. Baseband 2. Broadband A digital signal is a composite analog signal with an infinite bandwidth.
Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if we have
a low-pass channel with an infinite or very wide bandwidth.
Jitter
a problem if different packets of data encounter different delays and the application using the data at the receiver site is time-sensitive If the delay for the first packet is 20 ms, for the second is 45 ms, and for the third is 40 ms, then the real-time application that uses the packets endures jitter
Peak Amplitude
absolute value of its highest intensity, proportional to the energy it carries
Types of signals
analog and digital
Fourier analysis
any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases.
Signal-to-Noise Ratio (SNR)
avg signal power / avg noise power the ratio of what is wanted (signal) to what is not wanted (noise). high SNR = signal is less corrupted by noise low SNR = signal is more corrupted by noise. often described in decibel units
Bandwidth in Bits per Seconds
bandwidth can also refer to the number of bits per second that a channel, a link, or even a network can transmit
In baseband transmission, the required bandwidth is proportional to the _______; if we need to send bits faster, we need more _______.
bit rate bandwidth
nonperiodic composite signal
can be decomposed into a combination of an infinite number of simple sine waves with continuous frequencies, frequencies that have real values
periodic composite signal
can be decomposed into a series of simple sine waves with discrete frequencies, frequencies that have integer values (1, 2, 3, and so on).
Digital signal
can have only a limited number of defined values often as simple as 1 and 0.
Nonperiodic signal
changes without exhibiting a pattern or cycle that repeats over time.
Broadband Transmission (Using Modulation)
changing the digital signal to an analog signal for transmission. Modulation allows us to use a bandpass channel: a channel with a bandwidth that does not start from zero. (more available than a low-pass channel.)
Wavelength
characteristic of a signal traveling through a transmission medium. binds the period or the frequency of a simple sine wave to the propagation speed of the medium depends on both the frequency and the medium property of any type of signal describes the transmission of light in an optical fiber is the distance a simple signal can travel in one period. can be calculated if one is given the propagation speed (the speed of light) and the period of the signal. Wavelength = propogation speed x period normally measured in micrometers (microns) instead of meters
Periodic signal
completes a pattern within a measurable time frame, called a period repeats that pattern over subsequent identical periods completion of one full pattern is called a cycle
A digital signal in time-domain
comprises connected vertical and horizontal line segments. vertical line in the time domain means a frequency of infinity (sudden change) horizontal line in the time domain means a frequency of zero (no change in time)
Phase
describes the position of the waveform relative to time 0. describes the amount of shift along a time axis indicates the status of the first cycle measured in degrees or radians phase shift of 360º corresponds to a shift of a complete period shift of 180° corresponds to a shift of one-half of a period phase shift of 90º corresponds to a shift of one-quarter of a period
Bit Length
distance one bit occupies on the transmission medium Bit length = propagation speed x bit duration
Relationship
explicit relationship between the bandwidth in hertz and bandwidth in bits per second increase in bandwidth in hertz means an increase in bandwidth in bits per second.
fundamental frequency
frequency of the sine wave with frequency f is the same as the frequency of the composite signal The sine wave with frequency 3f has a frequency of 3 times the fundamental frequency; it is called the third harmonic
Analog signal
has infinitely many levels of intensity over a period of time As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path.
If a signal changes instantaneously, its frequency is _______.
infinite
Digital data
information that has discrete states (think digital clock - suddenly changes numbers) data take on discrete values and can be converted to a digital signal or modulated into an analog signal for transmission across a medium
Analog data
information that is continuous (think analog clock - continuously ticks forward Data take on continuous values and can be captured by a microphone and converted to an analog signal or sampled and converted to a digital signal
Throughput
measure of how fast we can actually send data through a network Bandwidth is different from throughput: - 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 - bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data
Decibel
measures the relative strengths of two signals or one signal at two different points to show that a signal has lost or gained strength dB is negative is signal is attenuated, positive if amplified
Propagation Time
measures the time required for a bit to travel from the source to the destination distance / propagation speed propagation speed depends on medium and frequency
Digital Signal
more than two levels we can send more than 1 bit for each level Most digital signals are nonperiodic
The bandwidth-delay product defines the
number of bits that can fill the link.
In data communications, we commonly use
periodic analog signals and nonperiodic digital signals.
Bandwidth
range of frequencies contained in a composite signal the difference between the highest and the lowest frequencies contained in that signal normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is 5000 − 1000, or 4000 bandwidth of the nonperiodic signals has the same range, but the frequencies are continuous
Increasing the levels of a signal may ______ the reliability of the system
reduce
Baseband Transmission
sending a digital signal over a channel without changing the digital signal to an analog signal requires a low-pass channel: channel with a bandwidth that starts from zero
Frequency-domain plot
show the relationship between amplitude and frequency concerned with only the peak value and the frequency. Changes of amplitude during one period are not shown frequency domain is easy to plot and conveys the information that one can find in a time domain plot A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
TIme-domain plot
shows changes in signal amplitude with respect to time Phase is not explicitly shown on a time-domain plot
Composite Signals
signal made of many simple sine waves.
Sine wave
the most fundamental form of a periodic analog signal its change over the course of a cycle is smooth and consistent, a continuous, rolling flow. represented by three parameters: the peak amplitude, the frequency, and the phase.
Bandwidth-Delay Product
the product of bandwidth and delay is the number of bits that can fill the link. important if we need to send data in bursts and wait for the acknowledgment of each burst before sending the next one
Bandwidth in Hertz
the range of frequencies contained in a composite signal or the range of frequencies a channel can pass
Distortion
the signal changes its form or shape can occur in a composite signal made of different frequencies Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration
Queuing Time
the time needed for each intermediate or end device to hold the message before it can be processed not a fixed factor - changes with the load imposed on the network heavy traffic on the network, the queuing time increases
Noiseless Channel: Nyquist Bit Rate
theoretical maximum bit rate BitRate = 2 x bandwidth x log2L - bandwidth is the bandwidth of the channel - L is the number of signal levels used to represent data, - BitRate is the bit rate in bits per second When we increase the number of signal levels, we impose a burden on the receiver.
Noise
thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal Thermal noise: random motion of electrons in a wire creates an extra signal not originally sent Induced noise: from sources such as motors and appliances; they act as a sending antenna, and the transmission medium acts as the receiving antenna Crosstalk: effect of one wire on the other; One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise: a spike (a signal with high energy in a very short time) that comes from power lines, lightning, etc.
propagation speed
time between the first bit leaving the sender and the last bit arriving at the receiver message size / bandwidth
The Shannon capacity gives us the ________ the Nyquist formula tells us _______ we need.
upper limit how many signal levels
Bit Rate
used to describe digital signals the number of bits sent in 1s, bits per second (bps).
Noisy Channel: Shannon Capacity
we cannot have a noiseless channel 1944 - Claude Shannon determine the theoretical highest data rate for a noisy channel: Capacity = bandwidth x log2(1 + SNR) - bandwidth is the bandwidth of the channel - SNR is the signal-tonoise ratio - capacity is the capacity of the channel in bits per second no indication of the signal level, which means that no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel.
If a signal does not change at all, its frequency is _____.
zero
Bandwidth in two contexts
❑The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass. ❑ The second, bandwidth in bits per second, refers to the speed of bit transmission in a channel or link.