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 modes Types
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9. Transmission modes can be divided into two fundamental categories:
A transmission mode is the manner in which data is sent over the underlying medium.
Transmission modes can be divided into two fundamental categories:
Serial — one bit is sent at a time
Parallel — multiple bits are sent at the same time
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10. Parallel transmission
Parallel transmission allows transfers of multiple data bits at the same time over separate media.
It is used with a wired medium
The signals on all wires are synchronized so that a bit travels across each of the wires at precisely the same time
n wires are used to send n bits at one time
Advantage: speed
Disadvantage: cost; limited to short distances
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11. Parallel transmissions:
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12. Serial transmissions:
Serial transmission sends one bit at a time.
Most communication systems use serial mode, because:
serial networks can be extended over long distances at less cost
using only one physical wire means that there is never a timing problem caused by one wire being slightly longer than another
Advantage: reduced cost
Disadvantage: requires conversion devices
Sender and receiver must contain a hardware that converts data from the parallel form used in the device to the serial form used on the wire.
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13. Serial transmissions
In serial mode, when sending bits, which bit should be sent across the medium first?
 Consider an integer: Should a sender transmit
the Most Significant Bit (MSB) first?
the Least Significant Bit (LSB) first?
 Either form can be used, but the sender and receiver must agree.
Serial transmission mechanisms can be divided into two broad categories (depending on how transmissions are spaced in time):
Asynchronous transmission can occur at any time
Synchronous transmission occurs continuously
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14. Serial transmissions:
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15. Asynchronous Transmission
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16. Asynchronous Transmission
Asynchronous transmission allows the physical medium to be idle for an arbitrary amount of time between two transmissions.
It is well-suited to applications that generate data at random time intervals.
For example:
a user typing on a keyboard
a user that clicks on a hyperlink
Asynchronous disadvantage:
While the medium is idle, a receiver cannot know how long the medium will remain idle before more data arrives.
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17. 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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18. 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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19. Asynchronous Transmission characteristics:
Serial transmissions
Cheap and effective
Ideal for low-speed communication when gaps may occur during transmission (ex: keyboard)
Asynchronous disadvantage
Asynchronous technologies usually require the sender to transmit a few extra bits before each data item:
to inform the receiver that a data transfer is starting
extra bits (preamble or start bits) allow the receiver to synchronize with the incoming signal
Slower
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20. Synchronous Transmission
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21. 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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22. 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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23. 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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24. Transmissions Types:
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25. References: Chapter 6 ( 6.1) 22-Sep-18
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