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.