1 Chap. 3 Data Transmission & Transmission Media

2 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

3 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 files data files numeric control programs synchronization signals sensor signals event message allowed delay1-100s 1-100ms20-100ms0.1-80ms message length>10kbits1-10kbits>10kbits8-64bits<10kbits8-84bits frequency of appearance rarelyvery rarely very frequentlyfrequentlyrarely classificationnon-real-time datareal-time data

4 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

5 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

6 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

7 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

8 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

9 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

10 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 log 2 M  so increase rate by increasing signal levels  at cost of receiver complexity  limited by noise & other impairments

11 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)  SNR dB =10 log 10 (signal/noise)  capacity C=B log 2 (1+SNR)  theoretical maximum capacity  get lower in practise Example  spectrum : from 3 to 4 MHz, SNR dB = 24  bandwidth : 1 MHz  SNR dB = 24dB = log 10 (signal/noise) -> SNR = 251  C = B log 2 (1+SNR) = 10 6 log 2 (1+251) = 8*10 6  C = 2B log 2 M  8*10 6 = 2*10 6 * log 2 M -> M = 16 => it is necessary to distinguish 16 signal levels

12 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%

13 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

14 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 cm, data rate of 10Mbps  UTP category 5 : twisted length of cm, data rate of 100Mbps STP (shielded twisted pair)  use metal braid to reduce electromagnetic interference  industrial cable

15 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

16 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

17 Transmission Characteristics  Analog  Amplifiers every few km  Closer if higher frequency  Up to 500MHz  Digital  Repeater every 1km  Closer for higher data rates

18 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