TEL  415E    Data  Communica2ons  

İstanbul  Teknik  Üniversitesi  Elektrik  Elektronik  Fakültesi  

Elektronik  Haberleşme  Mühendisliği  Fall  2012  

WEEK   Course  Outline   DATE  

1   Introduc2on   24/9  

2   Protocol  Architecture,  TCP/IP,  and  Internet-­‐Based  Applica2ons   1/10  

3   Data  Transmission   8/10  

4   Transmission  Media  (+  Project  Proposals)   15/10  

5   Signal  Encoding  Techniques   22/10  

6   -­‐  (Cumhuriyet  Bayramı)   29/10  

7   Midterm  Exam   5/11  

8   Digital  Data  Communica2ons  Techniques     12/11  

9   Data  Link  Control  Protocols   19/11  

10   Mul2plexing  (+  Project  Progress  Reports)   26/11  

11   Circuit  Switching  and  Packet  Switching   3/12  

12   Midterm  Exam   10/12  

13   Rou2ng  in  Switched  Data  Networks     17/12  

14   Project  Workshop    (+  Project  Final  Reports)   24/12  

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Data  and  Computer  Communica2ons  

Ninth  Edi2on  by  William  Stallings  

Chapter  3  –  Data  Transmission    

Data  and  Computer  Communica2ons,  Ninth  Edi2on  by  William  Stallings,  (c)  Pearson  

Educa2on  -­‐  Pren2ce  Hall,  2011  

Data  Transmission              What  we've  got  here  is  failure  to  communicate.                                Paul  Newman  in  Cool  Hand  Luke  

 

Data  Transmission  

 The  successful  transmission  of  data  depends  on  two  factors:  

 •  quality  of  the  signal  being  transmiced  

   •  characteris2cs  of  the  transmission  medium  

Transmission  Terminology  

Data  transmission  occurs  between  transmi<er  and  receiver  over  some  transmission  medium.  

Communica@on  is  in  the  form  of  electromagne@c  

waves.  

Guided  media    

twisted  pair,  coaxial  cable,  op2cal  fiber  

Unguided  media  (wireless)  

air,  vacuum,  seawater  

Transmission  Terminology  

Transmission  Terminology  

•  Simplex    

– signals  transmiced  in  one  direc2on  •  eg.  Television    

•  Half  duplex  – both  sta2ons  transmit,  but  only  one  at  a  2me  

•  eg.  police  radio    

•  Full  duplex  – simultaneous  transmissions  

•  eg.  telephone  

Frequency,  Spectrum  and  Bandwidth  

       

Ø analog  signal  •  signal  intensity  varies  smoothly  with  no  breaks  

Ø digital  signal  •  signal  intensity  maintains  a  constant  level  and  then  abruptly  changes  to  another  level  

Ø periodic  signal  •  signal  pacern  repeats  over  2me  

Ø aperiodic  signal  •  pacern  not  repeated  over  2me  

Time  Domain  Concepts  

Analog  and  Digital  Signals  

Periodic  Signals  

Sine  Wave  

•  peak  amplitude  (A)  –  maximum  strength  of  signal  –  typically  measured  in  volts  

•  frequency  (f)  –  rate  at  which  the  signal  repeats  –  Hertz  (Hz)  or  cycles  per  second  –  period  (T)  is  the  amount  of  2me  for  one  repe22on  –  T  =  1/f  

•  phase  (φ)  –  rela2ve  posi2on  in  2me  within  a  single  period  of  signal  

(periodic continuous signal)

Varying  Sine  Waves  s(t)  =  A  sin(2πi  +Φ)  

Wavelength  (λ)  the  wavelength  of  a  signal  is  the  distance  occupied  by  a  single  

cycle  

can  also  be  stated  as  the  distance  between  two  points  of  corresponding  phase  of  two  consecu@ve  cycles  

assuming  signal  velocity  v,  then  the  wavelength  is  

related  to  the  period    as  λ  =  vT  

or  equivalently  λf  =  v  

especially  when  v=c  •  c  =  3*108  ms-­‐1  (speed  of  light  in  free  space)  

Frequency  Domain  Concepts  

•  signals  are  made  up  of  many  frequencies  •  components  are  sine  waves  •  Fourier  analysis  can  show  that  any  signal  is  made  up  of  components  at  various  frequencies,  in  which  each  component  is  a  sinusoid  

•  can  plot  frequency  domain  func2ons  

Addi2on  of    Frequency  Components  

(T=1/f)  

Frequency  Domain  

Representa2ons    

•  frequency  domain  func2on  of  Fig  3.4c  

•  frequency  domain  func2on  of  single  square  pulse  

Spectrum  &  Bandwidth  

Signal  with  dc  Component  

Data  Rate  and  Bandwidth  

any  transmission  system  has  a  limited  band  of  frequencies  

this  limits  the  data  rate  that  can  be  carried  on  

the  transmission  medium  

square  waves  have  infinite  components  and  hence  an  infinite  

bandwidth  

most  energy  in  first  few  

components  

limi@ng  bandwidth  creates  

distor@ons  

There is a direct relationship between data rate and bandwidth.

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Analog  and  Digital  Data  Transmission  •  data    

– en22es  that  convey  informa2on  

•  signals  – electric  or  electromagne2c  representa2ons  of  data    

Ø signaling  – physically  propagates  along  a  medium  

•  transmission  – communica2on  of  data  by  propaga2on  and  processing  of  signals  

Acous2c  Spectrum  (Analog)  

Digital  Data  

Examples:  

Text  

Character  strings  

IRA:  International Reference Alphabet (7 Bits => 128 Patterns)  

Advantages  &  Disadvantages    of  Digital  Signals  

Audio  Signals  •  frequency  range  of  typical  speech  is  100Hz-­‐7kHz  •  easily  converted  into  electromagne2c  signals  •  varying  volume  converted  to  varying  voltage  •  can  limit  frequency  range  for  voice  channel  to  300-­‐3400Hz  

Video  Signals  

•  to  produce  a  video  signal  a  TV  camera  is  used  

•  USA  standard  is  483  lines  per  frame,  at  a  rate  of  30  complete  frames  per  second  –  actual  standard  is  525  lines  but  42  lost  during  ver2cal  retrace  

•  horizontal  scanning  frequency  is  525  lines  x  30  scans  =  15750  lines  per  second  

•  max  frequency  if  line  alternates  black  and  white  

•  max  frequency  of  4.2MHz  

Conversion  of  PC  Input  to  Digital  Signal  

-­‐  

Analog  Signals  

Digital  Signals  

Analog  and  Digital  

Transmission  

Transmission  Impairments  

•  signal  received  may  differ  from  signal  transmiced  causing:  – analog  -­‐  degrada2on  of  signal  quality  – digital  -­‐  bit  errors  

•  most  significant  impairments  are  – acenua2on  and  acenua2on  distor2on  – delay  distor2on  – noise  

ATTENUATION  

Received  signal  strength  must  be:  • strong  enough  to  be  detected  • sufficiently  higher  than  noise  to  be  received  without  error  

Strength  can  be  increased  using  amplifiers  or  repeaters.  

Equalize  acenua2on  across  

the  band  of  frequencies  used  by  using  loading  coils  or  amplifiers.  

Ø     signal  strength  falls  off  with  distance  over  any  transmission  medium    

Ø     varies  with  frequency  

Acenua2on  Distor2on  

Delay  Distor2on    •  occurs  because  propaga2on  velocity  of  a  signal  through  a  guided  medium  varies  with  frequency  

•  various  frequency  components  arrive  at  different  2mes  resul2ng  in  phase  shiis  between  the  frequencies  

•  par2cularly  cri2cal  for  digital  data  since  parts  of  one  bit  spill  over  into  others  causing  intersymbol  interference  

Noise  unwanted  signals  inserted  between  transmicer  and  receiver  

is  the  major  limi2ng  factor  in  communica2ons  system  performance  

No  =  kTB  

Categories  of  Noise  

 

Intermodula@on  noise  

• produced  by  nonlineari2es  in  the  transmicer,  receiver,  and/or  intervening  transmission  medium  

• effect  is  to  produce  signals  at  a  frequency  that  is  the  sum  or  difference  of  the  two  original  frequencies  

Categories  of  Noise  Crosstalk:  

–  a  signal  from  one  line  is  picked  up  by  another  

–  can  occur  by  electrical  coupling  between  nearby  twisted  pairs  or  when  microwave  antennas  pick  up  unwanted  signals  Impulse  Noise:  

–  caused  by  external  electromagne2c  interferences  

–  noncon2nuous,  consis2ng  of  irregular  pulses  or  spikes  

–  short  dura2on  and  high  amplitude  –  minor  annoyance  for  analog  signals  

but  a  major  source  of  error  in  digital  data  

Channel  Capacity  

Maximum  rate  at  which  data  can  be  transmi<ed  over  a  given  communica@ons  channel  under  given  condi@ons  

data  rate      

 in  bits  per  second  

bandwidth    

 in  cycles  per  second  or  Hertz  

noise    

 average  noise  level  over  path  

error  rate    

 rate  of  corrupted  

bits  

limita@ons  due  to  physical  proper@es  

main  constraint  

on  achieving  efficiency  is  noise  

Nyquist  Bandwidth  In  the  case  of  a  channel  that  is  noise  free:  •  if  rate  of  signal  transmission  is  2B  then  can  carry  signal  with  frequencies  no  greater  than  B    –  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  

–  C=  2Blog2M  (Hartley  Law)  

•  data  rate  can  be  increased  by  increasing  signals  –  however  this  increases  burden  on  receiver  –  noise  &  other  impairments  limit  the  value  of  M  

Shannon  Capacity  Formula  •  considering  the  rela2on  of  data  rate,  noise  and  error  rate:  –  faster  data  rate  shortens  each  bit  so  bursts  of  noise  corrupts  more  bits  

–  given  noise  level,  higher  rates  mean  higher  errors  •  Shannon  developed  formula  rela2ng  these  to  signal  to  noise  ra2o  (in  decibels)  

•  SNRdB=10  log10  (signal/noise)  •  capacity  C  =  B  log2(1+SNR)  

–  theore2cal  maximum capacity  –  get  much  lower  rates  in  prac2ce  

•  SNR  =  Eb/No  *  Rb/B  

Summary  •  transmission  concepts  and  terminology  

– guided/unguided  media  

•  frequency,  spectrum  and  bandwidth  •  analog  vs.  digital  signals  •  data  rate  and  bandwidth  rela2onship  •  transmission  impairments  

– acenua2on/delay  distor2on/noise  •  channel  capacity  

– Nyquist/Shannon  

QUESTIONS  ?    REMINDER:  PROJECT  PROPOSALS  ARE  DUE  OCTOBER  10  

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