1. Data Communication

2. Data Communications System
Transmitter – transmit data to another medium.
Receiver – receive data from a transmitter.
Medium of transfer – the medium for transfer of data.

3. Communications Model Source Receiver Transmitter Destination
Transmission
system
Transmitter
Receiver
Destination
Source System
Destination System
Workstation
Modem
Public Telephone
Network
Server

4. Simplex Transmission
Simplex communication means that communication can only flow in one direction and never flow back the other way.
Data

5. Half-duplex Transmission
Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time.
Data

6. Full-duplex Transmission
Full-duplex data transmission means that data can be transmitted in both directions on a signal carrier at the same time.
Data

7. Real life examples Simplex transmission Half-duplex transmission
Pager
Half-duplex transmission
Telephone, facsimile
Full-duplex transmission
Dual Carriageway

8. Data Transmission Rate
Data transmission rate: bps, Kbps, Mbps
bps – bits per second
Kbps – kilo-bits per second
Mbps – mega-bits per second
Bps – bytes per second
1 Byte = 8 bits

9. Serial Transmission
The transfer of discrete signals one after another.
Bits travel sequentially along the same wire.
Send information over a single line one bit at a time, as in modem-to-modem connections.

10. Parallel Transmission
The simultaneous transmission of a group of bits over separate wires.
The transmission of 1 byte (8-bits) with computers.

11. Parallel Transmission
Relatively fast
Limited distance before data is lost
As short as possible (no longer than 15 feet)
As the length of cable increases so does the danger of cross-talk.

12. Serial Transmission Not as fast as parallel transmission
Can transmit data for longer distances

13. Asynchronous Transmission
In modem communication, a form of data transmission in which data is sent one character at a time. In addition, a parity bit is usually used for error checking.
Avoid timing problem by not sending long, uninterrupted streams of bits.
Start bit
Data bits
Parity bit
Stop bit
The coding of a typical character sent in asynchronous transmission

14. Asynchronous Transmission1
Idle state
of line
Start
bit
Stop
5 to 8 data bits
Odd, even,
or unused P
1 – 2 bit times
Remain idle
or next start bit
Character format

15. Asynchronous Transmission1
Start
bit
Stop
Unpredictable time interval
Between characters
8-bit asynchronous character stream

16. Asynchronous Transmission
Transmitter timing
Start
Stop 1 2 3 4 5 6 7 8
100
200
300
400
500
600
700
800 93
186
279
372
465
558
651
744
Receiver timing
Effect of timing error
Assumptions: data rate of 10 kbps  0.1 ms each bit.
The receiver is off by 7% or ms per bit-time
The receiver samples the incoming character every ms
(based on the transmitter’s clock).

17. Asynchronous Transmission
Advantages:
simple
cheap
Disadvantages:
requires an overhead of 2 – 3 bits per character (start and stop bits) (>=20%)
cannot send large blocks or bits between start and stop bits with great cumulative timing error

18. Synchronous Transmission
Data transfer in which information transmitted in block (frames) of bits separated by equal time intervals
A block of bits is transmitted in a steady stream without star and stop codes

19. Synchronous Transmission
Method 1:
Provide a separate clock line between transmitter and receiver
The other side uses these regular pulses as a clock
This technique works well over short distances
Method 2:
Embed the clocking information in the data signal

20. Synchronous Transmission
8-bit
flag
Control
fields
Data fields
preamble
postamble
Synchronous frame format

21. Synchronous Transmission
Advantage:
For sizable/large blocks of data, synchronous transmission is far more efficient that asynchronous.
The control information, preamble, and postamble are typically less than 100 bits.
E.g. 48 bits of control, preamble, and postamble with 1000-character block of data, each frame consists of 48 bits of overhead and 8000 bits of data, so % overhead = 48/8048 x 100% = 0.6%

22. Data Transfer Directed from PC to PC
Direct Cable Connection
A null modem cable allows you to connect your PC to another nearby PC or serial device using its modem protocol.
A null modem cable is limited to 30 feet in length.
A null modem cable is sometimes called crossover cable.

23. Crossover Cable
A crossover cable is a cable that is used to interconnect two computers by "crossing over" (reversing) their respective pin contacts.
Either an RS-232C or an registered jack (e.g. RJ-45) connection is possible.

24. Diagrams on RJ-11 and RJ-45 Interfaces

25. Data Modem Modem – Modulator and Demodulator
Modulator – convert digital signal (data in PC) to analogue signal (data via telephone line)
Demodulator – convert analogue signal to digital signal

26. Data Modem Modulation Demodulation Digital signal Analogue signal PC
Public Telephone
Network PC
Modem
Digital signal
Analogue signal
Demodulation

27. Data Modem Baud Rate Bit Rate
This refers to the number of signals per one second transmitted
Bit Rate
The bit rate is multiplied by the bits per signal

28. Sources of errors during data transmission
Attenuation
Signal grows weak over distance
White noise
Caused by molecular movement
Impulse noise
Caused by electrical interference
Cross-talk
Caused by interference from adjacent lines

29. DCE and DTE
DTE
Data Terminal Equipment which is the ultimate source or final destination of data messages
DCE
Data Circuit-Terminating Equipment which connects the DTE to the communication circuits

30. Bandwidth
The capacity at which you can transfer data is called bandwidth
Typical telephone line: 33,600 kilobits per second (33.6 Kbps)
Cable TV: 10 megabits per second (10 Mbps) – almost 300 times the capacity of the normal phone connection