IE 20303

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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

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Analog DataSignal Options

• Analog data to analog signal– Inexpensive, easy conversion (e.g., 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

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Digital DataSignal 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

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Analog and Digital Signaling

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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

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Analog and Digital Data and Signals

Analog Signal

Digital Signal

Analog Data Two alternatives:(1) signal occupies the same spectrum as the analog data(2) Analog data are encoded to occupy a different spectrum.

Analog data are encoded using a codec to produce a digital bit stream.

Digital Data Digital data are encoded using a modem to produce analog signal.

Two alternatives:(1) signal consists of two voltage levels to represent two binary values(2) digital data are encoded to produce a digital signal with desired properties.

Analog and Digital Treatment of Signals

Analog Transmission

Digital Transmission

Analog Signal Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data.

Assumes that the analog signal represents digital data. Signal is propagated through repeaters; at each repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal.

Digital Signal Not used. Digital signal represents a stream of 1s and 0s which may represent digital data or may be an encoding of analog data. Signal is propagated though repeaters; at each repeater, stream of 1s and 0s is recovered from inbound signal and used to generate a new digital outbound signal.

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Advantages of Digital Transmission

• Cost – large scale and very large scale integration has caused continuing drop in cost

• Data Integrity – effect of noise and other impairments is reduced

• Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division

• Security & Privacy – Encryption possible• Integration – All signals (Voice. Video, image,

data) treated the same

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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

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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

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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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Modulation of Analog Signals for Digital Data

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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

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1 0 0 1

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

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1 1 0 1

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)

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0 0 1 1

PSK Illustration

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Voice Grade Modems

• Designed for digital transmission over ordinary phone lines

• Uses 4-kHz bandwidth• Adheres to ITU-T standards

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Cable Modems

• Permits Internet access over cable television networks. • ISP is at or linked by high-speed 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, time-division multiplexing technique• Splitter is used to direct TV signals to a TV and the data

channel to a cable modem

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Cable Modems

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Asymmetric DigitalSubscriber Line (ADSL)

• New modem technology for high-speed digital transmission over ordinary telephone wire.

• 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 4-kHz voice signal.

• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques.

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ADSL Modem Application

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Digital Encoding of Analog Data

• Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form

• Best-known 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.

• Good-quality voice transmission can be achieved with a data rate of 8 kbps

• Some videoconference products support data rates as low as 64 kbps

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Pulse-Code Modulation Example

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Digital Encodingof 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

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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

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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

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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; maximum modulation rate is twice NRZ

• Advantages– Synchronization due to predictable transitions– Error detection based on absence of a

transition

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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 and other LANs

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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 Signal Encoding Schemes

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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

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Analog Sine-Wave Signals

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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

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Asynchronous Transmission

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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

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Synchronous Transmission

• More efficient than asynchronous transmission

• Preamble, postamble and control information are typically < 100 bits

• Introduces the need for error checking

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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: redundancy introduced so that the

occurrence of an error will be detected– Error correction: receiver and transmitter cooperate

to retransmit frames that were in error

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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%)• Noise impulses are often long enough to

destroy more than one bit

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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, 33-bit divisor leaves 32-bit remainder

• For a CRC of length N, errors undetected are 2-

N

• Overhead is low (1-3%)

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Error Detection Process