chapter 8: data communication fundamentals business data communications, 4e
TRANSCRIPT
Chapter 8:Data CommunicationFundamentals
Business Data Communications, 4e
Three Components of Data Communication
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 Data-->Signal Options
Analog data to analog signal Inexpensive, easy conversion (eg
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 Data-->Signal 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
Data
Signal
TransmissionSystem
A
DD
DA
A
Data, Signal, and Transmission Matrix
Advantages of Digital Transmission
The signal is exactSignals can be checked for errorsNoise/interference are easily filtered outA variety of services can be offered over one lineHigher bandwidth is possible with data compression
Why Use Analog Transmission?
Already in placeSignificantly less expensiveLower attentuation ratesFully sufficient for transmission of voice signals
Analog Encoding of Digital Data
Data encoding and decoding technique to represent data using the properties of analog wavesModulation: the conversion of digital signals to analog formDemodulation: the conversion of analog data signals back to digital form
Modem
An acronym for modulator-demodulatorUses a constant-frequency signal known as a carrier signalConverts a series of binary voltage pulses into an analog signal by modulating the carrier signalThe 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)
Amplitude Shift Keying (ASK)
In radio transmission, known as amplitude modulation (AM)The amplitude (or height) of the sine wave varies to transmit the ones and zerosMajor disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude
1 0 0 1
ASK Illustration
Frequency Shift Keying (FSK)
In radio transmission, known as frequency modulation (FM)Frequency of the carrier wave varies in accordance with the signal to be sentSignal transmitted at constant amplitudeMore resistant to noise than ASKLess attractive because it requires more analog bandwidth than ASK
1 1 0 1
FSK Illustration
Phase Shift Keying (PSK)
Also known as phase modulation (PM)Frequency and amplitude of the carrier signal are kept constantThe carrier signal is shifted in phase according to the input data streamEach phase can have a constant value, or value can be based on whether or not phase changes (differential keying)
0 0 1 1
PSK Illustration
0 1 1
Differential Phase Shift Keying (DPSK)
0
Analog Channel Capacity: BPS vs. Baud
Baud=# of signal changes per secondBPS=bits per secondIn early modems only, baud=BPSEach signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phaseIncreases information-carrying capacity of a channel without increasing bandwidthIncreased combinations also leads to increased likelihood of errors
Voice Grade Modems
Cable Modems
DSL Modems
Digital Encoding of Analog Data
Primarily used in retransmission devicesThe 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.8000 samples/sec sufficient for 4000hz
Converting Samples to Bits
QuantizingSimilar concept to pixelizationBreaks wave into pieces, assigns a value in a particular range8-bit range allows for 256 possible sample levelsMore bits means greater detail, fewer bits means less detail
Codec
Coder/DecoderConverts analog signals into a digital form and converts it back to analog signalsWhere do we find codecs? Sound cards Scanners Voice mail Video capture/conferencing
Digital Encoding of Digital Data
Most common, easiest method is different voltage levels for the two binary digitsTypically, negative=1 and positive=0Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level
NRZ-L
Differential NRZ
Differential version is NRZI (NRZ, invert on ones)Change=1, no change=0Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level
NRZI
Problems With NRZ
Difficult to determine where one bit ends and the next beginsIn NRZ-L, long strings of ones and zeroes would appear as constant voltage pulsesTiming is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted
Biphase Alternatives to NRZ
Require at least one transition per bit time, and may even have twoModulation rate is greater, so bandwidth requirements are higherAdvantages Synchronization due to predictable
transitions Error detection based on absence of a
transition
Manchester Code
Transition in the middle of each bit periodTransition provides clocking and dataLow-to-high=1 , high-to-low=0Used in Ethernet
Manchester Code
Differential Manchester
Midbit transition is only for clockingTransition at beginning of bit period=0Transition absent at beginning=1Has added advantage of differential encodingUsed in token-ring
Differential Manchester
Digital Encoding Illustration
Asynchronous and Synchronous Transmission
Asynchronous Transmission Data are transmitted one character at a time. Timing (synchronization) is maintained within
each character, by the use of start elements and stop elements.
Synchronous Transmission A block of bits is transmitted in a steady
stream without start and stop codes. Synchronization
A separate clock line To embed the clocking information in the data signal
Asynchronous Transmission
Synchronous Transmission
To determine the beginning and end of a block of bits Begins with a preamblepreamble bit pattern Ends with a postamblepostamble bit pattern
Frame =
preamble + control + data + postamble
Digital Interfaces
The point at which one device connects to anotherStandards define what signals are sent, and howSome standards also define physical connector to be used
Generic CommunicationsInterface Illustration
DTE and DCE
DTE DTE
host com puter term inal
in terface in terface
m odem m odem
DCE
Four Characteristics of Interfaces
Mechanical: The actual physical connection of the DTE to the DCE.Electrical: Voltage levels and timing of voltage changes.Functional: The functions that are performed.Procedural: The sequence of events for transmitting data.
RS-232C (EIA 232C)
EIA’s “Recommended Standard” (RS)Specifies mechanical, electrical, functional, and procedural aspects of the interfaceUsed for connections between DTEs and voice-grade modems, and many other applications
EIA-232-D
new version of RS-232-C adopted in 1987improvements in grounding shield, test and loop-back signalsthe prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace
RS-449
EIA standard improving on capabilities of RS-232-Cprovides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bpscovers functional/procedural portions of R-232-C electrical/mechanical specs covered by
RS-422 & RS-423
Functional Specifications
Specifies the role of the individual circuitsData circuits in both directions allow full-duplex communicationTiming signals allow for synchronous transmission (although asynchronous transmission is more common)
Table 8.6(Page 200)
Functionalspecification
Procedural Specifications
Multiple procedures are specifiedSimple example: exchange of asynchronous data on private line Provides means of attachment
between computer and modem Specifies method of transmitting
asynchronous data between devices Specifies method of cooperation for
exchange of data between devices
Mechanical Specifications
25-pin connector with a specific arrangement of leadsDTE devices usually have male DB25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications
DB-25 Female
DB-25 Male
RS-232 DB-25 Connectors
RS-232 DB-25 Pinouts
RS-232 DB-9 Connectors
Limited RS-232
RS-422 DIN-8
Found on Macs
DIN-8 Male DIN-8 Female
Electrical Specifications
Specifies signaling between DTE and DCEUses NRZ-L encoding Voltage < -3V = binary 1 Voltage > +3V = binary 0
Rated for <20Kbps and <15M greater distances and rates are
theoretically possible, but not necessarily wise
RS-232 Signals (Asynch)
Even Parity
Odd Parity
No Parity