w2 by liuhongmei

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									CPEG 419
Introduction to Networks




    [Week 2]




        University of Delaware CPEG 419   1
Administrative Issues

Homework #1 assigned.
 Due in 2 weeks.




            University of Delaware CPEG 419   2
Transmission Impairments

Types of impairments:
  Attenuation.
  Delay distortion.
  Noise.
  Multi-path Fading (wireless only).




               University of Delaware CPEG 419   3
Attenuation

Weakening of the signal’s power as it
 propagates through medium.
Function of medium type
  Guided medium (wired): logarithmic with
   distance.
  Unguided medium (wireless): more complex
   (function of distance and atmospheric
   conditions).

              University of Delaware CPEG 419   4
Attenuation
Problems and solutions:
  Insufficient signal strength for receiver to
   distinguish between the signal and noise: use
   amplifiers/repeaters to boost/regenerate
   signal.
  Attenuation increases with frequency: special
   amplifiers to amplify high-frequencies
   (equalization).



               University of Delaware CPEG 419   5
Attenuation

 Let Rf be the received signal power at frequency f
 Let Tf be the transmitted signal power at frequency f

The attenuation in dB is:

                                Rf                 
                  A f  10 log                     
                               T                   
                                f                  


                  University of Delaware CPEG 419        6
Delay Distortion

Speed of propagation in guided media
 varies with frequency.
  Different frequency components arrive at
   receiver at different times (more about this
   later).
Solution:
  equalization techniques to equalize distortion
   for different frequencies.
  Use fewer frequencies.

                University of Delaware CPEG 419     7
Noise

Noise: undesired signals inserted
 anywhere in the source/destination path.
Different categories: thermal (white),
 crosstalk, impulse, etc.

                            noise
                                          received signal is an attenuated
              attenuation
transmitter                    +          version of the transmitted signal
                                          plus noise.
                      University of Delaware CPEG 419                 8
Thermal Noise
 Any conductor and electronic device has noise due to
  thermal agitation of electrons
 The thermal noise found in 1Hz is
                       N = k T (W/Hz)
  k = 1.3 e –23 (Boltzmann’s constant)
  T is the temperature in Kelvin
  N is noise power in watts per 1Hz of bandwidth (dBW)
 Total noise is
                          N = k T  B
  B is total bandwidth.

                   University of Delaware CPEG 419       9
Crosstalk
 Wires act as antennas. They broadcast energy when the signal
  switches and receive energy for any other source (e.g., other wires,
  radios, microwave ovens, the big bang, etc.).
 Crosstalk can be reduced by careful shielding and using twisted
  pairs.
 The longer the wires, the more significant the crosstalk.

                                Sf            power found on the wire of interest
Crosstalk gain is C f  10 log        
                               O              power at other wires
                                f     

      Suppose that –10 dBW is transmitted on other wires.
      And the crosstalk gain is 3.
      Then the noise received had power is –7 dBW.
                       University of Delaware CPEG 419                      10
Other noises
Coupling through common impedance (power
 supply noise). This is a major source at the
 transmitter and receiver.
Galvanic Action. Dissimilar metals and moisture
 produce a chemical wet cell (battery).
Triboelectric effect from bends in cable.
Shot Noise. Present in semiconductors.
Contact noise. Due to imperfect contacts.
Popcorn noise. Minor defects in junction in a
 semiconductor, often due to metallic impurities.
                University of Delaware CPEG 419   11
Decibel and Signal-to-
Noise Ratio
Decibel (dB): measures relative strength of 2
 signals.
  Example: S1 and S2 with powers P1 and P2.
     NdB = 10 log10 (P1/P2)


Signal-to-noise ratio (S/N):
  Measures signal quality.
   S/NdB = 10 log10 (signal power/noise power)




                     University of Delaware CPEG 419   12
SNR

   Suppose that we transmit at a very high power, so thermal
   and other noises are small compared to crosstalk.

              Received Signal 
SNR  10 log                    10 log Received Signal  10 log Noise
                  Noise       
     10 log transmitted power  Attenuatio n - 10log transmitted power  Crosstalk
     Attenuatio n - Crosstalk

      This depends on the cable.
      Furthermore, it may not be possible to transmit at such a high
      power that other noises can be neglected.


                             University of Delaware CPEG 419                        13
SNR=13
        0 1 1 0 1 0 1 1 0                                                0 1 1 0 1 0 1 1 0
2                                                                2




0                                                                0



    5    4   3   2   1   0   1   2   3    4   5                      5    4   3   2   1   0   1   2   3   4   5


        0.5 times the bit-rate                                           0.75 times the bit-rate

        0 1 1 0 1 0 1 1 0                                                0 1 1 0 1 0 1 1 0
                                                                 2
2




                                                                 0
0



                                                                     5    4   3   2   1   0   1   2   3   4   5
    5    4   3   2   1   0   1   2   3    4   5

         1 times the bit-rate                                             2 times the bit-rate
                                         University of Delaware CPEG 419                                          14
Multi-path Fading (wireless)
Because of reflections, a signal may take
many paths from transmitter to receiver.
transmitter
                                                 Objects such as
                                                 buildings, people, etc.


                                  receiver

    Signals that take alternative paths will arrive later.

                     University of Delaware CPEG 419                       15
Multi-path reflection or delay
spread (wireless)
                                  line of sight signal                                  late arriving signals
                                                                 1


    f( t ) . .6   ( f( t   D ) . .3   f( t   1.5 . D ) . .1 )

    f( t )                                                      0.5

    f( t     D)                                                                                                 getting
    f( t     1.5 . D )                                           0                                               small
    .5


                                                                0.5
                                                                      2   1     0         1     2      3
                                                                                    t

                                                                              received signal
At 10Mbs, if the difference in paths is 30 meters, then the
alternative signals arrive at exactly the next slot. (Use the fact
that light travels a 300000000 m/s.
                                                            University of Delaware CPEG 419                      16
Channel Capacity 1

Channel Capacity is the rate at which data can
 be transmitted over communication channel.
We saw earlier that to send a binary data at a
 rate R, the channel bandwidth must be greater
 than ½ R.
So, if the bandwidth of the channel is B, it might
 be possible to transmit at a rate of 2B.



                 University of Delaware CPEG 419   17
Channel Capacity 2
For a fixed bandwidth, the data rate can be
 increased by, increasing number of signal levels.
 However, the signal recognition at receiver is
 more complex and more noise-prone.
The data rate becomes
  C = 2B log2V, where V is number voltage levels.
Is it possible to continually increase V to make C
 arbitrarily large?



                 University of Delaware CPEG 419     18
Channel Capacity 3

Noisy channel: Shannon’s Theorem
  Given channel with B (Hz) bandwidth and
   S/N (dB) signal-to-noise ratio, C (bps) is
    C = B log2 (1+S/N)
  Theoretical upper bound since assumes
   white noise (e.g., thermal noise, not impulse
   noise, etc).



                University of Delaware CPEG 419    19
Transmission Media
Chapter 4
Physically connect transmitter and
 receiver carrying signals in the form
 electromagnetic waves.
Types of media:
  Guided: waves guided along solid medium
   such as copper twisted pair, coaxial cable,
   optical fiber.
  Unguided: “wireless” transmission
   (atmosphere, outer space).
               University of Delaware CPEG 419   20
Guided Media: Examples 1
Twisted Pair:
  2 insulated copper wires arranged in regular spiral.
   Typically, several of these pairs are bundled into a
   cable. (What happens if the twist is not regular?
   Reflection?)
  Cheapest and most widely used; limited in distance,
   bandwidth, and data rate.
  Applications: telephone system (home-local
   exchange connection).
  Unshielded and shielded twisted pair.
  What is a differential amplifier?


                  University of Delaware CPEG 419         21
Guided Media: Examples 1

Twisted pair – continued
  Category 3: Unshielded twisted pair (UTP) up to
   16MHz.
  Cat 5: UTP to 100 MHz.
  Table 4.2. Suppose Cat 5 at 200m (the limit of
   100Mbps ethernet is 300m).
     The dB attenuation at 100m is 22.0. So at 200m, the
      attenuation is 44. Suppose we transmit at –80dBW. Then
      the received signal has energy of –124dBW.
     The near-end crosstalk is 32dB per 100m. So the crosstalk
      energy is at –144dBW.
     The SNR is 20dB (neglecting thermal noise).

                    University of Delaware CPEG 419               22
Examples 2
 Coaxial Cable
   Hollow outer cylinder conductor surrounding inner wire conductor;
    dielectric (non-conducting) material in the middle.
   Less capacitance than twisted pair, so less loss at high frequencies.
    Also, Coaxial has more uniform impedance.
   Applications: cable TV, long-distance telephone system, LANs.
   Repeaters are required every few kilometers at 500MHz.
   +’s: Higher data rates and frequencies, better interference and
    crosstalk immunity.
   -’s: Attenuation at high frequency (up to 2 GHz is OK) and thermal
    noise.



                      University of Delaware CPEG 419               23
Examples 3

Optical Fiber
  Thin, flexible cable that conducts optical waves.
  Applications: long-distance telecommunications,
   LANs (repeaters every 40km at 370THz!).
  +’s: greater capacity, smaller and lighter, lower
   attenuation, better isolation,
  -’s: Not currently installed in subscriber loop. Easier
   to make use to current cables than install fiber.



                   University of Delaware CPEG 419           24
               Examples 3 – types of fiber
                 Step-index multimode                                                                lower index of refraction
                                                                                                           shorter path
                                                                                                           longer path
                     absorbed                        total internal                            higher index of refraction
                                                       reflection

  Since the signal can take many different paths, the arrival the received signal is smeared.
                                 Input Signal                                                               Output Signal
         1.5                                                                      40


           1
f( t )
                                                                             g( t ) 20
         0.5


           0
               0.6   0.4   0.2   0   0.2       0.4   0.6     0.8   1   1.2          0
                                           t                                             0.6   0.4   0.2     0   0.2       0.4   0.6   0.8        1   1.2
                                                                                                                       t
                                                           University of Delaware CPEG 419                                                   25
Examples 3 – types of fiber

Single mode



  If the fiber core is on the order of a wavelength,
  then only one mode can pass.

  Wavelengths are 850nm, 1300nm and 1550nm (visible
  spectrum is 400-700nm). 1550nm is the best for highest and
  long distances.

                     University of Delaware CPEG 419           26
Wavelength-division
multiplexing (WDM)

Wavelength-division multiplexing
  Multiple colors are transmitted.
  Each color corresponds to a different
   channel.
  In 1997, Bell Labs had 100 colors each at
   10Gbps (1Tbps).
  Commercial products have 80 colors at
   10Gbps.

               University of Delaware CPEG 419   27
Wireless Transmission

Omni-directional – the signal is
 transmitted uniformly in all directions.
Directional – the signal is transmitted only
 in one direction. This is only possible for
 high frequency signals.




               University of Delaware CPEG 419   28
Terrestrial Microwave

Parabolic dish on a tower or top of a building.
Directional.
Line of sight.
With antennas 100m high, they can be 82 km
 (50 miles).
Use 2 – 40 GHz.
2 GHz: bandwidth 7MHz, data rate 12 Mbps
11 GHz: bandwidth 220MHz, data rate 274 Mbps

                University of Delaware CPEG 419   29
Satellite Microwave

1 – 10 GHz (Above 10 GHz, the atmosphere
 attenuates the signal, and below 1 GHz there is
 too much noise).
Typically, 5.925 to 6.425 GHz for earth to
 satellite and 4.2 to 4.7 GHz for satellite to earth.
 (Why different frequencies?)
A stationary satellite must be 35,784 km (22000
 miles) above the earth.
The round-trip delay is about ½ a second.

                 University of Delaware CPEG 419    30
Other

Cell phones – Omni-directional. GSM-900 uses
 900MHz, GSM-1800 and GSM-1900 (PCS).
 Typical data rate seems to be around 40kbps.
 But the protocol is specified to 171kbps.
802.11 wireless LANs
  Omni-directional
  802.11b 2.4 GHz up to 11Mbps
  802.11a 5 GHz up to 54Mbps
Infrared – Line of sight, short distances.
                 University of Delaware CPEG 419   31
Types of Connections
 Long-haul – about 1500km (1000 miles) undersea, between major cites,
  etc. High capacity: 20000-60000 voice channels. Twisted pair, coaxial, fiber
  and microwave are used here. Microwave and fiber are still being installed.
 Metropolitan trunks – 12km (7.5 miles) 100,000 voice channels. Link long-
  haul to city and within a city. Large area of growth. Mostly twisted pair and
  fiber are used here.
 Rural exchange trunks – 40-160km link towns. Twisted pair, fiber and
  microwave are used here.
 Subscriber loop – run from a central exchange to a subscriber. This
  connection uses twisted pair, and will likely stay that way for a long time.
  Cable uses coaxial and is a type of subscriber loop (it goes from central
  office to homes). But a large number of people share the same cable.
 Local area networks (LAN) – typically under 300m. Sizes range from a
  single floor, a whole building, or an entire campus. While some use fiber,
  most use twisted pair as twisted pair is already installed in most buildings.
  Wireless (802.11) is also being used for LAN.

                          University of Delaware CPEG 419                    32
University of Delaware CPEG 419   33
Data Encoding

Transforming original signal just before
 transmission.
Both analog and digital data can be
 encoded into either analog or digital
 signals.




              University of Delaware CPEG 419   34
Digital/Analog Encoding
Encoding:
      g(t)                                                         g(t)

      (D/A) Encoder           Digital Medium                Decoder
Source                                                              Destination
    Source System                                       Destination System

Modulation:
       g(t)                                                        g(t)
      (D/A) Modulator       Analog Medium                Demodulator
Source                                                              Destination
    Source System                                       Destination System
                      University of Delaware CPEG 419                     35
Encoding Considerations

Digital signaling can use modern digital
 transmission infrastructure.
Some media like fiber and unguided
 media only carry analog signals.
Analog-to-analog conversion used to shift
 signal to use another portion of spectrum
 for better channel utilization (frequency
 division mux’ing).
              University of Delaware CPEG 419   36
Digital Transmission
Terminology

Data element: bit.
Signaling element: encoding of data
 element for transmission.
Unipolar signaling: signaling elements
 have same polarization (all + or all -).
Polar signaling: different polarization for
 different elements.

               University of Delaware CPEG 419   37
More Terminology

Data rate: rate in bps at which data is
 transmitted; for data rate of R, bit
 duration (time to emit 1 bit) is 1/R sec.
Modulation rate = baud rate (rate at
 which signal levels change).




               University of Delaware CPEG 419   38
Digital Transmission:
Receiver-Side Issues

Clocking: determining the beginning and
 end of each bit.
  Transmitting long sequences of 0’s or 1’s can
   cause synchronization problems.
Signal level: determining whether the
 signal represents the high (logic 1) or low
 (logic 0) levels.
  S/N ratio is a factor.

                University of Delaware CPEG 419   39
Comparing Digital
Encoding Techniques

Signal spectrum: high frequency means
 high bandwidth required for transmission.
Clocking: transmitted signal should be
 self-clocking.
Error detection: built in the encoding
 scheme.
Noise immunity: low bit error rate.

              University of Delaware CPEG 419   40
Digital-to-Digital Encoding
Techniques

Nonreturn to Zero (NRZ)
Multilevel Binary
Biphase
Scrambling




             University of Delaware CPEG 419   41
NRZ Techniques

Use of 2 different voltage levels.
NRZ-L: positive voltage represents one
 binary value; negative voltage, the other.
NRZI (Nonreturn to zero, invert on ones):
 transition (low-to-high or high-to-low)
 represents “1”; no transition, “0”.
NRZI is an example of differential
 encoding: decoding based on comparing
 polarity of adjacent signal elements.
               University of Delaware CPEG 419   42
Multilevel Binary
Use more than 2 signal levels.
Bipolar-AMI: “0”: no signal; “1”: positive
 and negative pulse; consecutive “1”s
 alternate in polarity: avoid synchronization
 loss.
Pseudoternary: opposite representation.
Long sequence of 0’s or 1’s still a problem
 for bipolar-AMI and pseudoternary
 respectively.
               University of Delaware CPEG 419   43
Biphase
Manchester: transition in the middle of bit
 period.
  Carries data and provides clocking.
  Low-to-high: “1”.
  High-to-low: “0”.
Differential Manchester:
  Mid-bit transition only provides clocking.
  “0”: transition in the beginning of bit interval.
  “1”: no transition.
                University of Delaware CPEG 419   44
Scrambling
Avoid long sequences of 0’s or 1’s.
Bipolar with 8-zeros substitution (B8ZS)
  Inserts transitions when transmitting 8
   consecutive “0”s.
High-density bipolar-3 zeros (HDB3)
  Inserts pulses when transmitting 4
   consecutive “0”s.
Receiver must recognize insertions and
 re-generate original signal.
               University of Delaware CPEG 419   45
Digital-to-Analog Encoding

Transmission of digital data using analog
 signaling.
Example: data transmission of a PTN.
PTN: voice signals ranging from 300Hz to
 3400 Hz.
Modems: convert digital data to analog
 signals and back.
Techniques: ASK, FSK, and PSK.
              University of Delaware CPEG 419   46
Amplitude-Shift Keying

2 binary values represented by 2
 amplitudes.
Typically, “0” represented by absence of
 carrier and “1” by presence of carrier.
Prone to errors caused by amplitude
 changes.



              University of Delaware CPEG 419   47
Frequency-Shift Keying

2 binary values represented by 2
 frequencies.
        s(t )  A cos(2f1t )," "
                               1
         s(t )  A cos(2f 2t ),"0"
Frequencies f1 and f2 are offset from
 carrier frequency by same amount in
 opposite directions.
Less error prone than ASK.
              University of Delaware CPEG 419   48
Phase-Shift Keying

Phase of carrier is shifted to represent
 data.
Example: 2-phase system.
     s(t )  A cos(2f ct  ),"1"
     s(t )  A cos(2f ct ),"0"
Phase shift of 90o can represent more
 bits: aka, quadrature PSK.
               University of Delaware CPEG 419   49
Analog-to-Digital Encoding

Analog data transmitted as digital signal,
 or digitization.
Codec: device used to encode and decode
 analog data into digital signal, and back.
2 main techniques:
  Pulse code modulation (PCM).
  Delta modulation (DM).


              University of Delaware CPEG 419   50
Pulse Code Modulation 1

Based on Nyquist (or sampling) theorem:
 if f(t) sampled at rate > 2*signal’s highest
 frequency, then samples contain all the
 original signal’s information.
Example: if voice data is limited to
 4000Hz, 8000 samples/sec are sufficient
 to reconstruct original signal.


               University of Delaware CPEG 419   51
PCM 2

Analog signal -> PAM -> PCM.
  PAM: pulse amplitude modulation; samples
   of original analog signal.
  PCM: quantization of PAM pulses; amplitude
   of PAM pulses approximated by n-bit integer;
   each pulse carries n bits.




               University of Delaware CPEG 419   52
Delta Modulation (DM)

Analog signal approximated by staircase
 function moving up or down by 1
 quantization level every sampling interval.
Bit stream produced based on derivative
 of analog signal (and not its amplitude):
 “1” if staircase goes up, “0” otherwise.
Parameters: sampling rate and step size.

              University of Delaware CPEG 419   53
Analog-to-Analog Encoding
Combines input signal m(t) and carrier at
 fc producing s(t) centered at fc.
Why modulate analog data?
  Shift signal’s frequency for effective
   transmission.
  Allows channel multiplexing: frequency-
   division multiplexing.
Modulation techniques: AM, FM, and PM.

               University of Delaware CPEG 419   54
Amplitude Modulation (AM)

Carrier serves as envelope to signal being
 modulated.

   S AM (t )  [1  m(t )] cos(2f ct )
Signal m(t) is being modulated by carrier
 cos(2p fct).
Modulation index: ratio between
 amplitude of input signal to carrier.
                University of Delaware CPEG 419   55
Angle Modulation

FM and PM are special cases of angle
 modulation.
FM: carrier’s amplitude kept constant
 while its frequency is varied according to
 message signal.
PM: carrier’s phase varies linearly with
 modulating signal m(t).

               University of Delaware CPEG 419   56
Spread Spectrum 1

Used to transmit analog or digital data
 using analog signaling.
Spread information signal over wider
 spectrum to make jamming and
 eavesdropping more difficult.
Popular in wireless communications



              University of Delaware CPEG 419   57
Spread Spectrum 2

2 schemes:
  Frequency hopping: signal broadcast over
   random sequence of frequencies, hoping from
   one frequency to the next rapidly; receiver
   must do the same.
  Direct Sequence: each bit in original signal
   represented by series of bits in the
   transmitted signal.


               University of Delaware CPEG 419   58
Transmission Modes
Assuming serial transmission, ie, one
 signaling element sent at a time.
Also assuming that 1 signaling element
 represents 1 bit.
Source and receiver must be in sync.
2 schemes:
  asynchronous and
  synchronous transmission.

              University of Delaware CPEG 419   59
Asynchronous Xmission 1

Avoid synchronization problem by
 including sync information explicitly.
Character consists of a fixed number of
 bits, depending on the code used.
Synchronization happens for every
 character: start (“0”) and stop (“1”) bits.
Line is idle: transmits “1”.

               University of Delaware CPEG 419   60
Asynchronous Xmission 2

Example: sending “ABC” in ASCII
  0 10000010 1 0 01000010 1 0 110000 1 1111…
Timing requirements are not strict.
But problems may occur.
  Significant clock drifts + high data rate =
   reception errors.
Also, 2 or more bits for synchronization:
 overhead!
                University of Delaware CPEG 419   61
Synchronous Xmission 1

No start or stop bits.
Synchronization via:
  Separate clock signal provided by transmitter
   or receiver; doesn’t work well over long
   distances.
  Embed clocking information in data signal
   using appropriate encoding technique such as
   Manchester or Differential Manchester.

               University of Delaware CPEG 419   62
Synchronous Xmission 2

Need to identify start/end of data block.
Block starts with preamble (8-bit flag) and
 may end with postamble.
Other control information may be added
 for data link layer.

   8 -bit Control                                             8 -bit
                                 Data                 Control
   flag                                                       flag


                    University of Delaware CPEG 419                    63
Data Link Layer

So far, sending signals over transmission
 medium.
Data link layer: responsible for error-free
 (reliable) communication between
 adjacent nodes.
Functions: framing, error control, flow
 control, addressing (in multipoint
 medium).
               University of Delaware CPEG 419   64
Flow Control

What is it?
  Ensures that transmitter does not overrun
   receiver: limited receiver buffer space.
  Receiver buffers data to process before
   passing it up.
  If no flow control, receiver buffers may fill up
   and data may get dropped.



                University of Delaware CPEG 419   65
Stop-and-Wait
Simplest form of flow control.
  Transmitter sends frame and waits.
  Receiver receives frame and sends ACK.
  Transmitter gets ACK, sends other frame,
   and waits, until no more frames to send.
Good when few frames.
Problem: inefficient link utilization.
  In the case of high data rates or long
   propagation delays.
                University of Delaware CPEG 419   66
Sliding Window 1

Allows multiple frames to be in transit at
 the same time.
Receiver allocates buffer space for n
 frames.
Transmitter is allowed to send n (window
 size) frames without receiving ACK.
Frame sequence number: labels frames.

              University of Delaware CPEG 419   67
Sliding Window 2

Receiver ack’s frame by including
 sequence number of next expected frame.
Cumulative ACK: ack’s multiple frames.
Example: if receiver receives frames 2,3,
 and 4, it sends an ACK with sequence
 number 5, which ack’s receipt of 2, 3, and
 4.

              University of Delaware CPEG 419   68
Sliding Window 3

Sender maintains sequence numbers it’s
 allowed to send; receiver maintains
 sequence number it can receive. These
 lists are sender and receiver windows.
Sequence numbers are bounded; if frame
 reserves k-bit field for sequence numbers,
 then they can range from 0 … 2k -1 and
 are modulo 2k.
              University of Delaware CPEG 419   69
Sliding Window 4

Transmission window shrinks each time
 frame is sent, and grows each time an
 ACK is received.




             University of Delaware CPEG 419   70
Example: 3-bit sequence
number and window size 7
      A                                               B
0 1 2 3 4 5 6 7 0 1 2 3 4...               0123456701234
                                       0
                                           1
0123456701234                              2 0123456701234
                                RR3
                                               0123456701234
0123456701234
                                           3
                                           4 0123456701234
0123456701234                               5
                          RR4               6
0123456701234                                 0123456701234
                    University of Delaware CPEG 419        71

								
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