Chapter 16: Data Communication Fundamentals Business Data Communications, 5e 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 Business Data Commun ications, 5e 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 Business Data Commun ications, 5e 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 Business Data Commun ications, 5e 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 Business Data Commun ications, 5e Data, Signal, and Transmission Matrix A Data D D A A D Transmission System Signal Business Data Commun ications, 5e 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 Business Data Commun ications, 5e Why Use Analog Transmission? • • • • Already in place Significantly less expensive Lower attenuation rates Fully sufficient for transmission of voice signals Business Data Commun ications, 5e 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 Business Data Commun ications, 5e 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 Business Data Commun ications, 5e 10 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 Business Data Commun ications, 5e 27 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 Business Data Commun ications, 5e 28 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 Business Data Commun ications, 5e 29 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 – Synchronization due to predictable transitions – Error detection based on absence of a transition Business Data Commun ications, 5e 30 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 Business Data Commun ications, 5e 31 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 Business Data Commun ications, 5e 32 Digital Encoding Illustration Business Data Commun ications, 5e 33 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 Business Data Commun ications, 5e 34 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 (Chapter 17) Business Data Commun ications, 5e 35 Asynchronous and Synchronous Transmission • For receiver to sample incoming bits properly, it must know arrival time and duration of each bit that it receives Business Data Commun ications, 5e 36 Asynchronous Transmission • Avoids timing problem by not sending long, uninterrupted streams of bits • Data transmitted one character at a time, where each character is to 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 to bits per character Business Data Commun ications, 5e 37 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 Business Data Commun ications, 5e 38 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 – Error correction Business Data Commun ications, 5e 39 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%) Business Data Commun ications, 5e 40 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, 33bit divisor leaves 32-bit remainder • For a CRC of length N, errors undetected are 2-N • Overhead is low (1-3%) Business Data Commun ications, 5e 41 ... and regenerates signal Business Data Commun ications, 5e Data, Signal, and Transmission Matrix A Data D D A A D Transmission System Signal Business Data Commun ications, 5e Advantages of Digital... to direct TV signals to a TV and the data channel to a cable modem Business Data Commun ications, 5e 20 Cable Modem Layout Business Data Commun ications, 5e 21 Asymmetric Digital Subscriber Line... The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques (Chapter 17) Business Data Commun ications, 5e 22 DSL Modem Layout Business Data