Business data communications 4e chapter 8

Chapter 8: Data Communication Fundamentals 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, 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 ✘ Data encoding decoding technique to of and Digital Data 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 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 RS-232C (EIA 232C) ✘ 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 million bps ✘ covers functional/procedural portions of R-232-C ✘ 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) Procedural Specifications ✘ Multiple procedures are specified ✘ Simple 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 leads ✘ DTE devices usually have male DB25 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 ✘ Voltage < -3V = binary ✘ Voltage > +3V = binary ✘ Rated for Signal Options ✘ Digital data to analog signal ✘