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Transmission Modes

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					   Transmission Modes

Different ways of characterizing the
           transmission
     Timing of the transmission
          of the data bits
0            • Serial
1              – Data bits transmitted at
0   11010
1
                 different times
1              – One bit after the other

      0      • Parallel
0     1        – Multiple bits transmitted
1     0
0
                 simultaneously (same time)
      1
1     1        – Typically with different data
1                “lines” for each bit
            Timing between
        transmitter and receiver
            ASYNCHRONOUS
• All transmissions are synchronized somehow
  – once per bit (Manchester)
  – once per byte
  – once per frame …..
• Asynchronous (means without synchronization)
  but DOES synchronize once per BYTE.
• Awful name
     Serial (asynchronous) Encoding

        1   0    1   1   0    0     1   1   0    0    0




                                                StartBit       Idle
Idle        Parity           Data
   Stop Bit
 LAST                                                      FIRST
        1     0     1    1    0    0     1   1    0     0    0




                                                      StartBit      Idle
Idle          Parity              Data
   Stop Bit
       Idle ->          No information on the line
       Start Bit ->     Defines the beginning of the byte
       Data ->          Information (number of bits varies)
       Parity ->        A check digit for correct reception (more later)
                        Even/Odd/None
       Stop Bit->       A check for correct detection of start bit
                        1/1.5/2 bits long
          Start Bit Timing




                              Bit Centers
Clock -> 4 times faster that bit rate
         2 ticks from beginning is bit center
         4 ticks from there is next bit center
                        Parity
• Counts number of ones in DATA
• Sets the parity bit to 1/0
    – Even or
    – Odd
•   May not choose to use at all (None)
•   Not a good means of error detection
•   Error in one bit 10-6 … Error in 2 bits 10-12
•   Assumes independence of bit errors … not
    always true
           Parity examples

                   PARITY (even)          DATA

0+2=2                    0             00100100

1+5=6                    1             11100011

        Use Second example and assume errors
1 + 4 = 5 ERROR            1             10100011

1 + 3 = 4 ???????           1               10000011
 One can’t detect multiple bit errors properly!
         Serial Transmission
          Many concepts in one byte


• Synchronization on a byte level
• Framing with start and stop bit
• Error detection with parity

• What does this cost us?
                          Efficiency
       1    0     1   1      0    0     1    1       0    0       0




                                                         StartBit        Idle
Idle        Parity               Data
   Stop Bit
                      Data                       8                 8
   Efficiency =                         =                     =        = .7272
                  Data + Overhead           1+1+8+1               11

  1200 bps line modem = 1200 * .7272 = 872 bps             ignoring idle!
      Where would you see it?
• On a PC it is the COM1, COM2 .. Port
• Typically RS232 interface
  – 9 pin
  – 25 pin
  – or others
• Modem, mouse, keyboard
• ASYNCHRONOUS because one can’t tell
  when the data will be transmitted from one
  byte to the next
            Serial Summary
• Same name (asynchronous) used for two
  concepts
  – lack of timing
  – Serial (byte transmission)
     • NOTHING in the name imples BYTE transmission
       but that is how it is used
• Synchronizes once per byte
  – assumes clocks will remain synchronized until
    the end of the byte
• Illustrates OVERHEAD
      So what is Synchronous?
• Synchronizes
   – once per block of data not per byte
• Typically faster rates
• USB ports on a PC (find rates on www)
   – see www.pcs.cnu.edu/~dgame/cs335/topics/usb.ppt
   – easier to understand after protocols
• More complex framing (each of these are bytes typically)
  (end) errordetect      DATA        control sync sync
            Sync byte/string
• A pattern with which receiver can
  established synchronization
• The longer it is (to a point) the greater the
  reliability of the synchronization
• Like a start bit
• 010101010101
• No idle times between bytes(bits) in the
  frame.
                Isochronous
• Asynchronous
  – irregular gaps between bytes
• Synchronous
  – no gaps between bytes
  – gaps between blocks
• Isochronous
  – REGULAR gaps between blocks
  – telephone PCM
  – 4000Hz -> 8000 samples/sec -> 8 bits/sample->
    64000 bps
  – What if on 1.5 Mpbs line?
     Asynchronous
     Synchronous
      Isochronous
Different arrival rates of bytes
Alternating Interactions
           Simplex - one way
           (tv,radio, weather satellite)

Device 1                     Device 2



                                           time




                                        data
Half Duplex - alternate each way
           (telephone, cb, ham radio)

Device 1                   Device 2



                                        time




                                      data
Full Duplex - both ways same time
            (computer serial)

 Device 1               Device 2



                                     time




                                   data
Sharing the medium

    Many users
    One channel
               Multiplexing
• Space - division
  – physically separate channels (wires)
• Time - division
  – sharing a CPU in multiprogramming OSs
  – telephone connections to a switching station
• Frequency - division
  – tv channels on a cable line
  – telephone conversations on a TRUNK line
  – radio stations sharing the airwave
         Space division

User 1                    User 5


User 2                    User 6


User 3                    User 7


User 4                    User 8
Time and Frequency division

  User 1             User 5


  User 2             User 6
           Medium
  User 3             User 7


  User 4             User 8
                Time Division




         time
                                frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
          Frequency Division




         time
                               frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
       Statistical Multiplexing
• Making the use of the medium more efficient
• Examples
  – cars on the highway
  – seats reserved on an airline flight
  – lines for making phone calls
• All overbook. Do not provide sufficient
  capacity to meet maximum demand.
• Provide less capacity. Save money.
  Usually good enough!
     Data Transmission Example
               TDM

A4A3A2A1   S                        S   ..A2A1
B4B3B2B1   I                        I   ..B2B1
           T ... D C B A D C B A    T
                  4 4 4 4 3 3 3 3
C4C3C2C1   E                        E   ..C2C1
D4D3D2D1                                ..D2D1
           1                        2




                 Fully Utilized!
     Data Transmission Example
     NOT Fully Utilized (9/16)

A4A3…A1     S                           S    …..A1
B4….B2B1    I                           I        ..B2B1
            T ... ...C B A    ……….A3    T
                      4 4 4
C4…….C1     E                           E        …..C1
……….D1                                           …..D1
            1                           2




           How Do We Make This More Efficient?
               Under-Allocate

A4A3…A1    S                            S     …..A1
B4….B2B1   I                            I     ..B2B1
           T ... C B A 0111    A30001   T
                  4 4 4
C4…….C1    E                            E     …..C1

……….D1              Overhead                  …..D1
           1                            2




        4 bits overhead per frame saves wasted slots.
 Less capacity required. Unable to meet Maximum Demand.
                 Queueing
• Statistical multiplexing generates a whole
  new science
• Underallocating generates potential waiting
  lines
  –   gas station
  –   bank tellers
  –   on-ramps at interstate
  –   your personal “to-do” list …………….
• Computer simulation
  – when to change resource amount (more tellers)
             Multiplexing
          a subtle distinction
• Users trying to make calls
  – Statistical
  – Some users have to wait to gain access
• Calls actually on the line
  – Not Statistical
  – Once on, you consume the line as long as you
    are connected

				
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