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Transmission Modes

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Isochronous. REGULAR gaps between blocks. telephone PCM ... Isochronous. Different arrival rates of bytes. Alternating Interactions. Device 1. Device 2 ... – PowerPoint PPT presentation

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Title: Transmission Modes


1
Transmission Modes
  • Different ways of characterizing the transmission

2
Timing of the transmission of the data bits
  • Serial
  • Data bits transmitted at different times
  • One bit after the other
  • Parallel
  • Multiple bits transmitted simultaneously (same
    time)
  • Typically with different data lines for each bit

0 1 0 1 1
1 1 0 1 0
0 1 0 1 1
0 1 0 1 1
3
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

4
Serial (asynchronous) Encoding
5
Idle -gt No information on the line Start Bit -gt
Defines the beginning of the byte Data -gt
Information (number of bits varies) Parity -gt A
check digit for correct reception (more later)
Even/Odd/None Stop Bit-gt A check
for correct detection of start bit
1/1.5/2 bits long
6
Start Bit Timing
Bit Centers
Clock -gt 4 times faster that bit rate
2 ticks from beginning is bit center
4 ticks from there is next bit center
7
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

8
Parity examples
PARITY (even)
DATA
0 0 1 0 0 1 0 0 1 1 1 0 0 0 1 1
0 1
0 2 2 1 5 6
Use Second example and assume errors
1 0 1 0 0 0 1 1 1 0 0 0 0 0 1 1
1 1
1 4 5 ERROR 1 3 4 ???????
One cant detect multiple bit errors properly!
9
Serial TransmissionMany concepts in one byte
  • Synchronization on a byte level
  • Framing with start and stop bit
  • Error detection with parity
  • What does this cost us?

10
Efficiency
Data
8
8
Efficiency


.7272
Data Overhead
1 1 8 1
11
1200 bps line modem 1200 .7272 872 bps
ignoring idle!
11
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 cant tell when the data
    will be transmitted from one byte to the next

12
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

13
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
14
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.

15
Isochronous
  • Asynchronous
  • irregular gaps between bytes
  • Synchronous
  • no gaps between bytes
  • gaps between blocks
  • Isochronous
  • REGULAR gaps between blocks
  • telephone PCM
  • 4000Hz -gt 8000 samples/sec -gt 8 bits/sample-gt
    64000 bps
  • What if on 1.5 Mpbs line?

16
AsynchronousSynchronousIsochronous
  • Different arrival rates of bytes

17
Alternating Interactions
18
Simplex - one way (tv,radio, weather satellite)
Device 1
Device 2
time
data
19
Half Duplex - alternate each way (telephone, cb,
ham radio)
Device 1
Device 2
time
data
20
Full Duplex - both ways same time (computer
serial)
Device 1
Device 2
time
data
21
Sharing the medium
  • Many users
  • One channel

22
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

23
Space division
User 5
User 1
User 6
User 2
User 7
User 3
User 8
User 4
24
Time and Frequency division
User 5
User 1
User 6
User 2
Medium
User 7
User 3
User 8
User 4
25
Time Division
time
frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
26
Frequency Division
time
frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
27
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!

28
Data Transmission ExampleTDM
A4A3A2A1
..A2A1
S I T E 1
S I T E 2
B4B3B2B1
..B2B1
D3C3B3A3
D4C4B4A4
...
C4C3C2C1
..C2C1
D4D3D2D1
..D2D1
Fully Utilized!
29
Data Transmission ExampleNOT Fully Utilized
(9/16)
A4A3A1
..A1
S I T E 1
S I T E 2
B4.B2B1
..B2B1
.A3
...C4B4A4
...
C4.C1
..C1
.D1
..D1
How Do We Make This More Efficient?
30
Under-Allocate
A4A3A1
..A1
S I T E 1
S I T E 2
B4.B2B1
..B2B1
A30001
C4B4A40111
...
C4.C1
..C1
.D1
..D1
Overhead
4 bits overhead per frame saves wasted
slots. Less capacity required. Unable to meet
Maximum Demand.
31
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)

32
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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