Transmission modes
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Transmission Modes. Different ways of characterizing the transmission. Timing of the transmission of the data bits. 0 1 0 1 1. Serial Data bits transmitted at different times One bit after the other Parallel Multiple bits transmitted simultaneously ( same time )

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Transmission Modes

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Transmission modes

Transmission Modes

Different ways of characterizing the transmission


Timing of the transmission of the data bits

Timing of the transmission of the data bits

0

1

0

1

1

  • 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

1 1 0 1 0

0

1

0

1

1

0

1

0

1

1


Alternating interactions

Alternating Interactions


Transmission modes

Simplex - one way

(tv,radio, weather satellite)

Device 1

Device 2

time

data


Transmission modes

Half Duplex - alternate each way

(telephone, cb, ham radio)

Device 1

Device 2

time

data


Transmission modes

Full Duplex - both ways same time

(computer serial)

Device 1

Device 2

time

data


Timing between transmitter and receiver

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

FIRST

LAST

Serial (asynchronous) Encoding


Transmission modes

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

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


More timing details figure 3 5

More Timing Details - Figure 3.5

  • Timer must be faster than the data rate

  • Degree to which faster dictates how close to center you can get

  • 16x (Fig 3.5.c.iii) shows only off 1/16th

  • 4x (Fig 3.5.c.ii) shows only off 1/4th

  • 1x (Fig 3.5.c.i) shows problems

  • Page is misleading

    • 1200/2400/9600 note is not for the diagram

    • i -> fastest data rate, iii -> slowest data rate

    • fastest clock ratio -> slowest data rate


Maintaining synchronization figure 3 9

Maintaining SynchronizationFigure 3.9

  • DPLL digital phase locked loop

  • count for transition

  • if transition not seen as expected

    • adjust the timing interval by

      • adding 1 tick

      • subtracting 1 tick

    • on the next cycle.

  • Used in many coding techniques.

  • Book uses NRZI.. Forced change on 0, require a zero peridically.


Parity

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 10-1

  • Assumes independence of bit errors … not always true


Parity examples

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 + 4 = 5ERROR

1 + 3 = 4 ???????

1

1

One can’t detect multiple bit errors properly!


Serial transmission many concepts in one byte

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?


Efficiency

Efficiency

  • A means of evaluating the nature of the cost to implements solutions to these fundmantal communication problems

  • (data)/(data + overhead)

  • Overhead is anything not part of that intended to be communicated for the receiver

  • Be careful to pick a time which is repetitive when evaluating. Choose the right time is everything.


Transmission modes

1

0

1

1

0

0

1

1

0

0

0

StartBit

Idle

Idle

Parity

Data

Stop Bit

Efficiency

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

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


Review modem communication

Review Modem Communication

Serial transmission


Figure 2 31

Figure 2.31

  • Interface between computer (dte) and modem (dce)

  • Not all lines used

  • Higher level controls present

    • request to send

    • clear to send

  • asynchronous across xmit/recv

  • interface can also be used for synchronous

    • line 16

  • one interface … both async and sync


Figure 2 32 typical scenario

Figure 2.32Typical scenario

  • Notice the timing between dce / dte

  • Notice the timing between dce / dce

  • Scenario describes half-duplex

  • Full duplex minimizes the impact of “turnaround time”

    • revc

    • (turnaround)

    • send


Serial summary

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

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

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

  • (e.g. ) 0 1 1 1 1 1 1 0

  • No idle times between bytes(bits) in the frame.


Figure 3 12

Figure 3.12

  • SYN for character frames

  • 01111111 (idle) 01111110 (start and end)

  • synchronization maintained for frame duration


Character bit stuffing

Character/bit stuffing

  • What if timing/framing flags in beginning or ending?

  • AND they are part of the data itself!

  • Stuff extra symbols if in data

  • Stuff even if “stuffed” version is in the data


Character stuffing

Character stuffing

Data

STX

SYN

SYN

ETX

Expected End of Data

STX

SYN

ETX

SYN

ETX

ETX in data

STX

DLE

DLE

DLE

SYN

ETX

SYN

DLE

ETX

ETX in data

DLE in data

Receiver discards!


Bit stuffing

Bit Stuffing

Basically the same!

5 zeros in a row, insert extra 0


Isochronous periodic

Isochronous(periodic)

  • 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

AsynchronousSynchronousIsochronous

Different arrival rates of bytes


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