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Chap. 3 Data Transmission & Transmission Media

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Data

• Data Entity that convey meaning

• Analog & digital data Analog

various in a smooth way over time analog sensor such as temperature sensor

Digital maintains a constant level then changes to another constant level on/off sensor such as limit sensor, switch

• Periodic & aperiodic data Periodic

pattern repeated over time sensor data or actuating data

Aperiodic pattern not repeated over time failure data, report data

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• Real-time or non-real-time data real-time

time-critical data relatively short data size, frequent appearance time-delay effects system performance

non-real-time data relatively long data size, rare appearance

message

type

characteristic

graphic

filesdata files

numeric control

programs

synchronization

signals

sensor

signals

event

message

allowed delay 1-100s 1-100s 1-100s 1-100ms 20-100ms 0.1-80ms

message length >10kbits 1-10kbits >10kbits 8-64bits <10kbits 8-84bits

frequency of

appearancerarely very rarely very rarely very frequently frequently rarely

classification non-real-time data real-time data

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Signal

• Signals Electric or electromagnetic representations of data, physically propagates along wired or

wireless transmission medium

• Analog signal continuous electric or electromagnetic representations transmitted via wired transmission medium or wireless transmission medium

• Digital signal discrete electric representation transmitted via only wired transmission medium cheaper less susceptible to noise greater attenuation

pulses become rounded and smaller leads to loss of information

digital now preferred choice

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Conversion from data to signal

• analog data to analog signal telephone, analog cellular phone

• digital data to analog signal digital cellular phone analog data -> digital data -> analog signal

• digital data to digital signal LAN

• analog data to digital signal web camera, IP(internet) telephone

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Transmission

• Transmission communication of data by propagation and processing of signals

• Analog transmission analog signal transmitted without regard to content may be analog or digital data attenuated over distance use amplifiers to boost signal also amplifies noise suitable for cheaper voice communication in analog telephone not suitable for data communication in cellular phone

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• Digital transmission concerned with content integrity endangered by noise,

attenuation etc. repeaters used repeater receives signal extracts bit pattern retransmits attenuation is overcome noise is not amplified

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Transmission Impairments

• Signal received may differ from signal transmitted analog - degradation of signal quality digital - bit errors

• Most significant impairments are attenuation noise

• Attenuation signal strength falls off with distance depends on medium received signal strength:

must be enough to be detected must be sufficiently higher than noise

to be received without error so increase strength using amplifiers/repeaters

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• Noise additional signals inserted between transmitter and receiver thermal

due to thermal agitation of electrons uniformly distributed white noise

crosstalk a signal from one line is

picked up by another impulse

irregular pulses or spikes external electromagnetic interference

short duration high amplitude a minor problem for analog signals but a major source of error in digital data

a noise spike could corrupt many bits

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Channel capacity and data rate

• Channel capacity max possible data rate on communication channel channel capacity is a function of

data rate - in bits per second(bps), at which data can be communicated bandwidth - in cycles per second or Hertz noise - average level of noise over the communications path error rate - at which errors occur

limitations due to physical properties

• Nyquist Bandwidth consider noise free channels if rate of signal transmission is 2B then can carry signal with frequencies no greater than B

ie. given bandwidth B, highest signal rate is 2B for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels Nyquist Formula : C = 2B log2M so increase rate by increasing signal levels

at cost of receiver complexity limited by noise & other impairments

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• Shannon Capacity Formula consider relation of data rate, noise & error rate

faster data rate shortens each bit so bursts of noise affects more bits given noise level, higher rates means higher errors

shannon developed formula relating these to signal to noise ratio (in decibels) SNRdB=10 log10 (signal/noise)

capacity C=B log2(1+SNR)

theoretical maximum capacity get lower in practise

• Example spectrum : from 3 to 4 MHz, SNRdB= 24

bandwidth : 1 MHz SNRdB= 24dB = log10 (signal/noise) -> SNR = 251

C = B log2(1+SNR) = 106 log2(1+251) = 8*106

C = 2B log2M

8*106 = 2*106* log2M -> M = 16 => it is necessary to distinguish 16 signal levels

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Traffic and transmission delay

• Bit interval and data rate bit interval : time for transmitting one bit data rate : the number of transmitted bits for 1 sec, which denotes bps

• Traffic the quantity of transmitted data for1 sec if data rate is 1Mbps and 1M bit data are propagated for 1 sec, traffic is 100%

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Transmission Medium

• Transmission medium uuided - wire unguided - wireless characteristics and quality determined by medium and signal for guided, the medium is more important for unguided, the bandwidth produced by the antenna is more important key concerns are data rate and distance

• Guided transmission medium Twisted Pair Coaxial cable Optical fiber

• Unguided transmission medium

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Twisted Pair

• Twisted pair separately insulated twisted together often bundle into cables Usually installed in building during construction

• UTP (unshielded twisted pair) telephone, office network UTP category 3 : twisted length of 7.5-10cm, data rate of 10Mbps UTP category 5 : twisted length of 0.6-0.85cm, data rate of 100Mbps

• STP (shielded twisted pair) use metal braid to reduce electromagnetic interference industrial cable

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• Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range

• Transmission Characteristics Analog

Amplifiers every 5km to 6km Digital

Use either analog or digital signals repeater every 2km or 3km

Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise

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Coaxial Cable

• Coaxial cable outer conductor is braided shield inner conductor is solid metal separated by insulating material covered by padding

• Coaxial Cable Applications most versatile medium television distribution

Ariel to TV Cable TV

long distance telephone transmission can carry 10,000 voice calls simultaneously being replaced by fiber optic

short distance computer systems links local area networks

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• Transmission Characteristics Analog

Amplifiers every few km Closer if higher frequency Up to 500MHz

Digital Repeater every 1km Closer for higher data rates

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Optical Fiber

• Optical fiber class or plastic core laser or light emitting diode specially designed jacket small size and weight

• Benefits greater capacity

data rates of hundreds of Gbps smaller size & weight lower attenuation electromagnetic isolation greater repeater spacing

10s of km at least