Chapter 8 Data Communication Fundamentals Business Data Communications

Chapter 8: Data Communication Fundamentals Business Data Communications, 4 e

Three Components of Data Communication Data n n Analog: Continuous value data (sound, light, temperature) Digital: Discrete value (text, integers, symbols) Signal n n Analog: Continuously varying electromagnetic wave Digital: Series of voltage pulses (square wave) Transmission n n Analog: Works the same for analog or digital signals Digital: Used only with digital signals

Analog Data-->Signal Options Analog data to analog signal n n n 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 n n Requires a codec (encoder/decoder) Allows use of digital telephony, voice mail

Digital Data-->Signal Options Digital data to analog signal n n n Requires modem (modulator/demodulator) Allows use of PSTN to send data Necessary when analog transmission is used Digital data to digital signal n n n 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 n only transmits analog signals, without regard for data content attenuation overcome with amplifiers signal is not evaluated or regenerated Digital transmission n transmits analog or digital signals uses repeaters rather than amplifiers switching equipment evaluates and regenerates signal

Data, Signal, and Transmission Matrix A Data D D A A D Signal Transmission System

Advantages of Digital Transmission The signal is exact Signals can be checked for errors Noise/interference are easily filtered out A variety of services can be offered over one line Higher bandwidth is possible with data compression

Why Use Analog Transmission? Already in place Significantly less expensive Lower attentuation rates Fully sufficient for transmission of voice signals

Analog Encoding of Digital 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)

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 zeros Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

ASK Illustration 1 0 0 1

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 sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog bandwidth than ASK

FSK Illustration 1 1 0 1

Phase Shift Keying (PSK) Also known as phase modulation (PM) Frequency and amplitude of the carrier signal are kept constant The carrier signal is shifted in phase according to the input data stream Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

PSK Illustration 0 0 1 1

Differential Phase Shift Keying (DPSK) 0 1 1 0

Analog Channel Capacity: BPS vs. Baud=# of signal changes per second BPS=bits per second In early modems only, baud=BPS Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase Increases information-carrying capacity of a channel without increasing bandwidth Increased combinations also leads to increased likelihood of errors

Voice Grade Modems

Cable Modems

DSL Modems

Digital Encoding of Analog Data Primarily used in retransmission devices 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. 8000 samples/sec sufficient for 4000 hz

Converting Samples to Bits Quantizing Similar concept to pixelization Breaks wave into pieces, assigns a value in a particular range 8 -bit range allows for 256 possible sample levels More bits means greater detail, fewer bits means less detail

Codec Coder/Decoder Converts analog signals into a digital form and converts it back to analog signals Where do we find codecs? n n 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 digits Typically, negative=1 and positive=0 Known 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=0 Advantage 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 begins In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses Timing 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 two Modulation rate is greater, so bandwidth requirements are higher Advantages n n Synchronization due to predictable transitions Error detection based on absence of a transition

Manchester Code Transition in the middle of each bit period Transition provides clocking and data Low-to-high=1 , high-to-low=0 Used in Ethernet

Manchester Code

Differential Manchester Midbit transition is only for clocking Transition at beginning of bit period=0 Transition absent at beginning=1 Has added advantage of differential encoding Used in token-ring

Differential Manchester

Digital Encoding Illustration

Asynchronous and Synchronous Transmission Asynchronous Transmission n n 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 n n A block of bits is transmitted in a steady stream without start and stop codes. Synchronization w A separate clock line w To embed the clocking information in the data signal

Asynchronous Transmission

Synchronous Transmission To determine the beginning and end of a block of bits n n Begins with a preamble bit pattern Ends with a postamble bit pattern Frame = preamble + control + data + postamble

Digital Interfaces The point at which one device connects to another Standards define what signals are sent, and how Some standards also define physical connector to be used

Generic Communications Interface Illustration

DTE and 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-232 C (EIA 232 C) EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional, and procedural aspects of the interface Used for connections between DTEs and voice-grade modems, and many other applications

EIA-232 -D new version of RS-232 -C adopted in 1987 improvements in grounding shield, test and loop-back signals the 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 -C provides for 37 -pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps covers functional/procedural portions of R-232 -C n electrical/mechanical specs covered by RS 422 & RS-423

Functional Specifications Specifies the role of the individual circuits Data circuits in both directions allow fullduplex communication Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

Functional specification Table 8. 6 (Page 200)

Procedural Specifications Multiple procedures are specified Simple example: exchange of asynchronous data on private line n n n 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 leads DTE devices usually have male DB 25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications

RS-232 DB-25 Connectors DB-25 Female DB-25 Male

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 DCE Uses NRZ-L encoding n n Voltage < -3 V = binary 1 Voltage > +3 V = binary 0 Rated for <20 Kbps and <15 M n greater distances and rates are theoretically possible, but not necessarily wise

RS-232 Signals (Asynch) Even Parity Odd Parity No Parity