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
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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

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Serial (asynchronous) Encoding
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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
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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
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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

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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!
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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?

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Efficiency
Data
8
8
Efficiency


.7272
Data Overhead
1 1 8 1
11
1200 bps line modem 1200 .7272 872 bps
ignoring idle!
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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

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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

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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
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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.

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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?

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AsynchronousSynchronousIsochronous

• Different arrival rates of bytes

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Alternating Interactions
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Simplex - one way (tv,radio, weather satellite)
Device 1
Device 2
time
data
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Half Duplex - alternate each way (telephone, cb,
ham radio)
Device 1
Device 2
time
data
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Full Duplex - both ways same time (computer
serial)
Device 1
Device 2
time
data
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Sharing the medium

• Many users
• One channel

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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

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Space division
User 5
User 1
User 6
User 2
User 7
User 3
User 8
User 4
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Time and Frequency division
User 5
User 1
User 6
User 2
Medium
User 7
User 3
User 8
User 4
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Time Division
time
frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
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Frequency Division
time
frequency
User 1 and User 5
User 2 and User 6
User 3 and User 7
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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!

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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!
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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?
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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.
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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)

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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