Data Communication Fundamentals: Business Data Communications, 5e

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 Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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Data, Signal, and Transmission Matrix

A Data

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Transmission System

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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 Communications, 5e

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Why Use Analog Transmission? • • • •

Already in place Significantly less expensive Lower attenuation rates Fully sufficient for transmission of voice signals

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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 Communications, 5e

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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 Communications, 5e

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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)

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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 Business Data Communications, 5e

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ASK Illustration

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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 Business Data Communications, 5e

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FSK Illustration

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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) Business Data Communications, 5e

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PSK Illustration

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Differential Phase Shift Keying (DPSK)

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Voice Grade Modems

QuickTimeª and a TIFF (LZW) decompressor are needed to see this picture.

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Cable Modems • Permits Internet access over cable television networks.  • ISP is at or linked by high­speed 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, time­division multiplexing  technique (see Chapter 17) • Splitter is used to direct TV signals to a TV and the data  channel to a cable modem Business Data Communications, 5e

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Cable Modem Layout

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Asymmetric Digital Subscriber Line (ADSL) • • • •

New modem technology for high­speed digital transmission over  ordinary telephone wire.  Telephone central office can provide support for a number of ISPs,  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 4­kHz voice signal. 

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The data and voice signals are combined on the twisted pair line  using frequency­division­multiplexing techniques (Chapter 17)

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DSL Modem Layout

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Digital Encoding of Analog Data • Evolution of telecommunications networks to digital  transmission and switching requires voice data in digital  form • Best­known 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. • Good­quality voice transmission can be achieved with a  data rate of 8 kbps • Some videoconference products support data rates as low  as 64 kbps Business Data Communications, 5e 24

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 Business Data Communications, 5e

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Codec • Coder/Decoder • Converts analog signals into a digital form and converts it back to analog signals • Where do we find codecs? – – – –

Sound cards Scanners Voice mail Video capture/conferencing Business Data Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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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

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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

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Digital Encoding Illustration

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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

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Analog Encoding of Analog Information • Voice­generated sound wave can be represented  by an electromagnetic signal with the same  frequency components, and transmitted on a  voice­grade 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 – Analog­to­analog modulation permits frequency­ division multiplexing (Chapter 17) Business Data Communications, 5e

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Asynchronous and Synchronous Transmission • For receiver to sample incoming bits  properly, it must know arrival time and  duration of each bit that it receives

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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 Business Data Communications, 5e

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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 Communications, 5e

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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 Communications, 5e

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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%)

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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 Communications, 5e

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