Business data communications 5e by stallings chapter 16

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 –

– Analog: Continuously varying electromagnetic wave

– Digital: Series of voltage pulses (square wave)

• Transmission

– Analog: Works the same for analog or 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

Transmission System

A

D A

A

Data, Signal, and Transmission Matrix

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 attenuation rates

• Fully sufficient for transmission of voice

signals

• Modulation: the conversion of digital

signals to analog form

• Demodulation: the conversion of analog

data signals back to digital form

• An acronym for modulator-demodulator

• Uses a constant-frequency signal known as

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

1 0 0 1

ASK Illustration

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

1 1 0 1

FSK Illustration

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)

0 0 1 1

PSK Illustration

0 1 1

Differential Phase Shift Keying

(DPSK)

0

Voice Grade Modems

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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 (see Chapter 17)

• Splitter is used to direct TV signals to a TV and the data

Cable Modem Layout

Asymmetric Digital Subscriber Line (ADSL)

• New modem technology for high-speed 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 4-kHz voice signal

• The data and voice signals are combined on the twisted pair line

using frequency-division-multiplexing techniques (Chapter 17)

DSL Modem Layout

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

Converting Samples to Bits

• Quantizing

• Similar concept to pixelization

• Breaks wave into pieces, assigns a value in

• 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

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

• 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

– 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

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

Digital Encoding Illustration

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

Asynchronous and Synchronous

Transmission

• For receiver to sample incoming bits

properly, it must know arrival time and

duration of each bit that it receives

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

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

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

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%)

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, bit divisor leaves 32-bit remainder

33-• For a CRC of length N, errors undetected

• Overhead is low (1-3%)