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This chapter explores the characteristics and differences between analog and digital signals. Analog signals vary smoothly over time, while digital signals maintain constant levels before changing. Key concepts such as peak amplitude, frequency, and bandwidth are discussed, alongside their implications for signal quality and data transmission capacity. Impairments from both analog and digital transmission are examined, including attenuation, distortion, and noise. Additionally, it covers various media for transmission and the conversion processes for data signals to ensure efficient communication.
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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
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
s(t) = (4/) (sin (2ft) + (1/3) sin (2(3f)t)) Frequency Domain Concepts
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
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
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
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
Transmission Media • Physical path between transmitter and receiver (“channel”) • Design factors affecting data rate • bandwidth • physical environment • number of receivers • impairments
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)
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
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
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 Business Data Communications, 5e
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
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
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
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
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
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
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
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
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
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
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.
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
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
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
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
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
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
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%)
