Analog vs. Digital Signals and Data

Analog vs. Digital Data and Signals

• Data can be analog (continuous) or digital (discrete).
• Analog signals have infinite values within a range.
• Digital signals have a limited number of values.
• Data communication uses periodic analog signals and nonperiodic digital signals.
• Periodic analog signals require less bandwidth.
• Nonperiodic digital signals can represent variation in data.

Periodic Analog Signals

• Classified as simple (sine wave) or composite.
• A sine wave cannot be decomposed into simpler signals.
• Composite signals are composed of multiple sine waves.
• Sine waves are defined by peak amplitude, frequency, and phase.
• Peak amplitude is the absolute value of the highest intensity, measured in volts.
• Frequency refers to periods in 1 second, expressed in Hertz.
• Period is the time in seconds for a signal to complete one cycle, a full pattern.

Time and Frequency Domains

• Time-domain plots show signal amplitude changes over time.
• Frequency-domain plots show the relationship between amplitude and frequency.
• The frequency domain allows immediate visualization of frequency and peak amplitude values.
• A complete sine wave in the time domain translates to a single spike in the frequency domain.

Composite Signals and Bandwidth

• Single-frequency sine waves are insufficient for data communications
• Composite signals combine multiple simple sine waves.
• Per Fourier analysis, composite signals consist of various frequencies, amplitudes, and phases.
• A periodic composite signal decomposes into signals with discrete frequencies.
• A nonperiodic composite signal decomposes into sine waves with continuous frequencies.
• Bandwidth refers to the difference between the highest and lowest frequencies within a composite signal.

Digital Signals

• In addition to analog signals, Information can be represented via digital signals.
• A digital signal with eight levels needs 3 bits per level.

Bit Rate

• Most digital signals are nonperiodic, rendering frequency and period inappropriate measures.
• Bit rate measures data transmission speed in bits per second (bps).
• Bit interval is the reciprocal of the bit rate.
• Bit length is determined by multiplying propagation speed by the bit interval.

Transmission Methods

• Digital signals are transmitted via baseband or broadband.
• Baseband transmission sends the digital signal directly over a channel without converting it to an analog signal.
• Typically requires a low-pass channel beginning from zero.
• Best with a dedicated medium with wide bandwidth.
• Broadband Transmission
• involves converting the digital signal to analog via modulation for transmission.
• Uses a band-pass channel, which does not start from zero.
• It is necessary when the available channel is a bandpass channel.
• A modem does this in computer data transmission via telephone lines.

Transmission Impairment

• Signals deteriorate during transmission, leading to signal impairment.
• Signal at the beginning is not as same at the end of the medium.
• Three main causes exist (Attenuation, distortion, and noise).
• Attenuation is energy loss during signal transmission.
• Compensated for using amplifiers.
• Decibel (dB) measures relative power loss (attenuation).
• Distortion means that signal changes form or shape
• Each signal component has its own propagation speed through the medium therefore its own delay in arriving at final destination.
• Noise comes in four forms:
• Thermal Noise is random electron motion in the wire.
• Induced noise comes from appliances and motors.
• Crosstalk Noise is from one wire's effect on another.
• Impulse Noise is a high energy signal in a short period of time from many sources.

Signal-to-Noise Ratio (SNR)

• SNR is the radio between signal power and noise power.
• High SNR results in less signal corruption.
• SNR in relation to decibels is described as SNRdb=10log10 SNR

Data Rate Limits

• The speed of data transmission over a channel depends on a few important factors:
• Bandwidth, signal level, and channel quality.
• Nyquist Bit Rate applies to noiseless channels with theoretical calculations
• Shannon Capacity applies to noisy channels

Nyquist Bit Rate Calculation

• Theoretical Maximum bit rate(bps) = 2 x bandwidth x log2 L, where L=signal levels that represent data

Shannon Capacity Calculation

• Shannon's theorem provides the capacity of a system in the presence of noise.
• Capacity(bps) = Bandwidth log2(1 + SNR)
• The Shannon capacity gives us the upper limit.
• The Nyquist formula tells us how many signal levels we need.

Performance Parameters

• Bandwidth:
• capacity of the system in hertz (range of frequencies in Hertz).
• bits per second (speed of bit transmission)
• Throughput: actual data transfer rate
• Latency (delay) : Time it takes a message to arrive from source to destination.
• Latency (delay) = propagation time + transmission time + queuing time + processing time
• Propagation : the time a bit travels from source to destination
• Transmission :is the time required for transmission of a message.
• Depends on the size of the message.
• Queuing time : Time needed for device to hold message; changes with network load.

Transmission Media

• Transmission media are pathways that carry information between sources and destinations.
• Located below the physical layer, controlled by it.

Guided vs. Unguided Media

• Guided media physically connect devices (Twisted pair, coaxial cable, optical fiber).
• Unguided media transmits electromagnetic waves wirelessly (radio, microwave, infrared).

Guided Media

• Guided Media
• Provide a physical conduit from one device to another
• Examples include twisted-pair cable, coaxial cable, and fiber-optic cable
• Twisted and Coaxial Pair Cables
• Use copper conductors to transport electrical signals.
• Optical Fiber
• Transports signals in the form of light.

Twisted-Pair Cable

• Employs one wire for signal transmission and the other as a ground.
• Twisting balances effects and reduces unwanted signals.

Coaxial Cable

• Carries higher frequency signals because it is constructed differently.
• The outer conductor acts as a shield against noise and a second conductor.

Fiber Optic Cable

• Transmits signals using light through glass or plastic.
• Light travels in a straight path until it hits another substance that results a change in direction.
• Fiber optic cables use reflection to guide light.
• Has a glass or plastic core surrounded by a cladding of less dense material.
• Advantages of Fiber Optics:
• Higher bandwidth, less signal loss (attenuation).
• Immunity to electromagnetic interference, and greater resistance to corrosive materials.
• Disadvantages:
• Tricky to install and maintain, unidirectional light propagation, and cost.
• Twisted-pair cables are used in telephone lines for voice and data and in telephone companies.
• Coaxial cable is used in analog (and older digital) telephone networks.
• Optical fiber is used in SONET and some cable TV networks.

Unguided Media - Wireless

• Unguided media transmit electromagnetic waves wirelessly
• Signals are broadcast, available to any device capable of receiving them.

Wireless Propagation Methods

• Ground propagation (below 2 MHz).
• Low-frequency signals follow the curvature of the planet.
• AM radio is an example. Sky propagation (2-30 MHz).
• Higher frequency waves radiate into the ionosphere.
• Example uses include BBC and citizen band. Line-of-sight propagation (above 30 MHz).
• Very high-frequency signals are transmitted in straight lines, requiring directional antennas.
• Satellite TV is an example.

Radio Waves

• Radio waves are suited for multicast communications (radio, television, paging).
• Can penetrate walls.
• Use omnidirectional antennas.

Microwaves

• Utilized for unicast communication; examples include cellular telephone, wireless LANs.
• Cannot penetrate walls.
• Require directional antennas for sight communications.
• Two types of antennas include a parabolic dish an horn.

Omnidirectional vs. Unidirectional Antennas

• Omnidirectional antennas radiate signals in all directions.
• They are easy to install
• They broadcast to a high area
• Susceptible to interference and noise.
• Unidirectional antennas send signals in one direction.
• Directional antennas are more powerful than those that are omni, and there is no interference.
• They are more delicate.

Infrared Waves

• Infrared signals are best for short-range communication in closed areas using line-of-sight propagation.