1. DIGITAL DATA COMMUNICATION TECHNIQUES
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Chapter 6

2. Lecture Contents Communication Types: Transmission Types: HDX. FDX.
Parallel
Serial
Synchronous.
Asynchronous.
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3. Communications Types
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4. Half Duplex (HDX) :
A half-duplex (HDX) system provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying.
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5. Full Duplex (FDX) :
A full-duplex (FDX) system, or sometimes called double-duplex, allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time.
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6. FDX vs. HDX :
There are several benefits to using full-duplex over half-duplex:
Firstly, time is not wasted, since no frames need to be retransmitted, as there are no collisions.
Secondly, the full data capacity is available in both directions because the send and receive functions are separated.
Thirdly, stations (or nodes) do not have to wait until others complete their transmission, since there is only one transmitter for each twisted pair
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7. Transmission Types
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8. Transmission Types
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9. Parallel transmissions:
Transfer of one (byte) with all bits are sent at the same time, each using a separate link
n wires are used to send n bits at one time
Advantage: speed
Disadvantage: cost; limited to short distances
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10. Parallel transmissions:
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11. Serial transmissions:
Transfer of one (byte) with one bit at a time using only one single link
Advantage: reduced cost
Disadvantage: requires conversion devices
Techniques:
Asynchronous
Synchronous
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12. Serial transmissions:
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13. Asynchronous Transmission
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14. Asynchronous Transmission
Transfer of data with start and stop bits and a variable time interval between data units.
Timing is unimportant
Start bit alerts receiver that new group of data is arriving.
Stop bit alerts receiver that byte is finished.
Synchronization achieved through start/stop bits with each byte received.
Serial transmissions
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15. Asynchronous Transmission
The beginning of a character is signaled by a start bit with a value of binary 0.
followed by the 5 to 8 bits that actually make up the character.
Then the data bits are usually followed by a parity bit, set by the transmitter ,The receiver uses this bit for error detection.
The final element is a stop element, which is a binary 1
Serial transmissions
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16. Asynchronous Transmission
Serial transmissions
Requires additional overhead (start/stop bits)
Slower
Cheap and effective
Ideal for low-speed communication when gaps may occur during transmission (ex: keyboard)
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17. Synchronous Transmission
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18. Synchronous Transmission
Serial transmissions
Requires constant timing relationship
Bit stream is combined into longer frames, possibly containing multiple bytes
Any gaps between bursts are filled in with a special sequence of 0s and 1s indicating idle
Advantage: speed, no gaps or extra bits
more efficient than asynchronous
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19. Synchronous Transmission
Serial transmissions
block of data transmitted, sent as frames in a steady stream without start and stop codes.
clocks must be synchronized
can use separate clock line
or embed clock signal in data
need to indicate start and end of block
use preamble and post-amble (flags)
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20. Transmissions Types:
In serial transmission a character is converted from parallel to serial form when transmitting and from serial to parallel form when receiving.
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21. Transmissions Types:
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22. References: Chapter 6 ( 6.1) 14-Nov-18
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