chapter 15 & 16:. 2 electromagnetic signals analog signal –signal intensity varies in a smooth...
TRANSCRIPT
Chapter 15 & 16:
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Electromagnetic Signals
• Analog Signal – signal intensity varies in a smooth fashion
over time. In other words, there are no breaks or discontinuities in the signal
• Digital Signal – signal intensity maintains a constant level for
some period of time and then changes to another constant level
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Analog and Digital Waveforms
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Periodic Signal Characteristics
• Peak Amplitude (A)– Maximum signal value (strength), measured in volts
• Frequency (f)– Repetition rate– Measured in cycles per second or Hertz (Hz)
• Period (T)– Amount of time it takes for one repetition, T=1/f
• Phase ()– Relative position in time, measured in degrees
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s(t) = (4/) (sin (2ft) + (1/3) sin (2(3f)t))
Frequency Domain Concepts
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Frequency Domain Concepts
• Spectrum of a signal is the range of frequencies that it contains
• Absolute bandwidth of a signal is the width of the spectrum
• Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found
• The greater the bandwidth, the higher the information-carrying capacity of the signal
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Bandwidth
• Width of the spectrum of frequencies that can be transmitted– if spectrum=300 to 3400Hz,
bandwidth=3100Hz
• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion
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Analog Signaling
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Voice/Audio Analog Signals
• Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage
• Human voice has frequency components ranging from 20Hz to 20kHz
• For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz
Voice Signals
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Image/Video: Analog Data to Analog Signals
• Image is scanned in lines; each line is displayed with varying levels of intensity
• Requires approximately 4Mhz of analog bandwidth
• Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal
Digital Signals
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Transmission Media
• Physical path between transmitter and receiver (“channel”)
• Design factors affecting data rate– bandwidth– physical environment– number of receivers– impairments
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Impairments and Capacity
• Impairments exist in all forms of data transmission
• Analog signal impairments result in random modifications that impair signal quality
• Digital signal impairments result in bit errors (1s and 0s transposed)
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Transmission Impairments:Guided Media
• Attenuation– loss of signal strength over distance
• Attenuation Distortion– different losses at different frequencies
• Delay Distortion– different speeds for different frequencies
• Noise– distortions of signal caused by interference
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Transmission Impairments:Unguided (Wireless) Media
• Free-Space Loss– Signals disperse with distance
• Atmospheric Absorption– Water vapor and oxygen contribute to signal loss
• Multipath– Obstacles reflect signal creating multiple copies
• Refraction - Change in signal speed due to atmospheric conditions
• Thermal Noise- White noise, arises from thermal activity of devices
Business Data Communications, 5e 17
Types of Noise
• Thermal (aka “white noise”)– Uniformly distributed, cannot be eliminated
• Intermodulation– When different frequencies collide (creating
“harmonics”)
• Crosstalk– Overlap of signals
• Impulse noise– Irregular spikes, less predictable
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Channel Capacity
• The rate at which data can be transmitted over a given path, under given conditions
• Four concepts– Data rate– Bandwidth– Noise– Error rate
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Data Communication Components
• Data– Analog: Continuous value data (sound, light,
temperature)– Digital: Discrete value (text, integers, symbols)
• Signal– Analog: Continuously varying electromagnetic wave– Digital: Series of voltage pulses (square wave)
• Transmission– Analog: Works the same for analog or digital signals– Digital: Used only with digital signals
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Analog DataSignal Options
• Analog data to analog signal– Inexpensive, easy conversion (e.g., telephone)– Data may be shifted to a different part of the
available spectrum (multiplexing)– Used in traditional analog telephony
• Analog data to digital signal– Requires a codec (encoder/decoder)– Allows use of digital telephony, voice mail
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Digital DataSignal Options
• Digital data to analog signal– Requires modem (modulator/demodulator)– Allows use of PSTN to send data– Necessary when analog transmission is used
• Digital data to digital signal– Requires CSU/DSU (channel service unit/data service
unit)– Less expensive when large amounts of data are
involved– More reliable because no conversion is involved
Analog and Digital Signaling
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Transmission Choices
• Analog transmission– only transmits analog signals, without regard for data
content
– attenuation overcome with amplifiers
– signal is not evaluated or regenerated
• Digital transmission– transmits analog or digital signals
– uses repeaters rather than amplifiers
– switching equipment evaluates and regenerates signal
Analog and Digital Data and Signals
Analog Signal
Digital Signal
Analog Data
Two alternatives:(1) signal occupies the same spectrum as the analog data(2) Analog data are encoded to occupy a different spectrum.
Analog data are encoded using a codec to produce a digital bit stream.
Digital Data
Digital data are encoded using a modem to produce analog signal.
Two alternatives:(1) signal consists of two voltage levels to represent two binary values(2) digital data are encoded to produce a digital signal with desired properties.
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Analog and Digital Treatment of Signals
Analog Transmission
Digital Transmission
Analog Signal
Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data.
Assumes that the analog signal represents digital data. Signal is propagated through repeaters; at each repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal.
Digital Signal
Not used. Digital signal represents a stream of 1s and 0s which may represent digital data or may be an encoding of analog data. Signal is propagated though repeaters; at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.
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Advantages of Digital Transmission
• Cost – large scale and very large scale integration has caused continuing drop in cost
• Data Integrity – effect of noise and other impairments is reduced
• Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division
• Security & Privacy – Encryption possible• Integration – All signals (Voice. Video, image,
data) treated the same
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Analog Encoding of Digital Data
• Data encoding and decoding technique to represent data using the properties of analog waves
• Modulation: the conversion of digital signals to analog form
• Demodulation: the conversion of analog data signals back to digital form
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Modem
• An acronym for modulator-demodulator• Uses a constant-frequency signal known as
a carrier signal• Converts a series of binary voltage pulses
into an analog signal by modulating the carrier signal
• The receiving modem translates the analog signal back into digital data
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Methods of Modulation
• Amplitude modulation (AM) or amplitude shift keying (ASK)
• Frequency modulation (FM) or frequency shift keying (FSK)
• Phase modulation or phase shift keying (PSK)
Voice Grade Modems
• Designed for digital transmission over ordinary phone lines
• Uses 4-kHz bandwidth
• Adheres to ITU-T standards
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Cable Modems
• Permits Internet access over cable television networks.
• ISP is at or linked by high-speed line to central cable office
• Cables used for television delivery can also be used to deliver data between subscriber and central location
• Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique
• Splitter is used to direct TV signals to a TV and the data channel to a cable modem
Cable Modems
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Asymmetric DigitalSubscriber Line (ADSL)
• New modem technology for high-speed digital transmission over ordinary telephone wire.
• At central office, a combined data/voice signal is transmitted over a subscriber line
• At subscriber’s site, twisted pair is split and routed to both a PC and a telephone– At the PC, an ADSL modem demodulates the data signal for the PC.
– At the telephone, a microfilter passes the 4-kHz voice signal.
• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques.
ADSL Modem Application
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Digital Encoding of Analog Data
• Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form
• Best-known technique for voice digitization is pulse-code modulation (PCM)
• The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.
• Good-quality voice transmission can be achieved with a data rate of 8 kbps
• Some videoconference products support data rates as low as 64 kbps
Pulse-Code Modulation Example
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Analog Encoding of Analog Information
• Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line.
• Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band– A higher frequency may be needed for effective
transmission– Analog-to-analog modulation permits frequency-
division multiplexing
Analog Sine-Wave Signals
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Asynchronous Transmission
• Avoids timing problem by not sending long, uninterrupted streams of bits
• Data transmitted one character at a time, where each character is 5 to 8 bits in length.
• Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.
• Simple and cheap but requires an overhead of 2 to 3 bits per character
Asynchronous Transmission
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Synchronous Transmission• Block of bits transmitted in a steady stream without
start and stop codes. • Clocks of transmitter and receiver must somehow be
synchronized– Provide a separate clock line between transmitter and
receiver; works well over short distances, – Embed the clocking information in the data signal.
• Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern
• The data plus preamble, postamble, and control information are called a frame
Synchronous Transmission
• More efficient than asynchronous transmission
• Preamble, postamble and control information are typically < 100 bits
• Introduces the need for error checking
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Error Control Process
• All transmission media have potential for introduction of errors
• All data link layer protocols must provide method for controlling errors
• Error control process has two components– Error detection: redundancy introduced so that the
occurrence of an error will be detected
– Error correction: receiver and transmitter cooperate to retransmit frames that were in error
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Error Detection: Parity Bits
• Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit character=12.5%)
• Noise impulses are often long enough to destroy more than one bit
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Error Detection: Cyclic Redundancy Check (CRC)
• Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2-N
• Overhead is low (1-3%)
Error Detection Process
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