Achyut S.Godbole, “ Data Communications and Networks”, Tata McGraw-Hill Publishing Company Limited, Ninth reprint, 2007

1. Data Communications and Neworks
Modes of Transmission
R.Anushiya
Assstiant Professor,
P.K.R Arts College for Women, Gobi
Author: Achyut S.Godbole, “ Data Communications and Networks”, Tata McGraw-Hill
Publishing Company Limited, Ninth reprint, 2007

2. Modes of Transmission

3. Serial Comminication
• Sending data serially, characters or bytes have to be
seprated and send bit by bit.
• The shift register is used for serial transmission.
• The byte to be transmitted is the first stored in a shift
register.
• Then these bits are shifted from MSB to LSB bit by bit in
synchronization with the clock.
• As an advantage only one wire is used in serial
transmission between the transmitter and the receiver.

5. Parallel Communication
• When we transfer a word or a byte at a time, we need
many wires prallel to each other carrying a single bit.
Words ->bytes -> bits -> transmission at a single time
• Very fast method to transmit data
• Suitable for short distance

7. • Only one wire is required
• Reduction in cost due to less number of conductor wires
• It is the most used method for long distance data
transfers.
• Since there is only one line of transmission therefore
the speed of transmission is quite slow.
• If we have to increase the speed of data transfer then it
is necessary to increase the clock frequency. But there
is a limit of clock frequency.

8. • Three ways to help serial communication
- Asychronous
- Synchonous
- Isochronous

9. • It is called asynchronous because timing of the signal is
not an issue. The information can be sent & received as
per mutual agreement of the sender & receiver.
• Asynchronous transmission is at byte level only because
transmission of bits is always synchronized.
• Asynchronous transmission is eased by two bits, namely
start bits as '0' and stop bits as '1'.
• We send '0' bits for start the communication and '1' bit for
stop the communication

12. • It is a flexible data transmission method.
• Synchronization between the transmitter and receiver is
not necessary.
• It is possible to transmit signals from the sources having
different bit rates.
• The transmission can commence as soon as the data byte
to be transmitted becomes available.
• This mode of transmission is easy to implement.
• It is a cheap scheme in terms of money.

13. Disadvantages of Asynchronous Transmission
• Additional bits called start and stop bits are required to be
used.
• The timing error may take place because it is difficult to
determine synchronicity.
• It has slower transmission rate.

14. • Synchronous events means events which happen at the
same clock rate.
• Synchronous transmission is carried out under the control
of a common master clock.
• Here the bits which are being transmitted as synchronized
to the same reference clock.
• The stream of bits is combined into bigger frames which
may comprise more than one byte.

15. • Each byte is transmitted without gap between the next
byte.
• No start and stop bits are used instead the bytes are
transmitted as a block in a continuous stream of bits.
• The receivers operates at the same clock frequency as
that of transmitter.
• The data is sent in blocks ( we can call these blocks as
frames or packets) spaced by fixed time intervals.
• It is the duty of the receiver to separate the bits sent in
group

17. • The main advantage is speed. The speed of transmission
is much higher than that asynchronous transmission.
• This is due to the absence of gaps between the data units
and absence of start stop bits.
• Timing errors are reduced due to synchronization.

18. • The timing is very important. The accuracy of the received
data is dependent entirely on the ability of the receiver to
count the received bits accurately.
• The transmitter and receiver have to operate at the same
clock frequency. This requires proper synchronization
which makes the system complicated.

19. • It is a combinaion of asychronous and synchronous transmission
• The ISOCHRONOUS (ISOC) format for data transmission is a
procedure or protocol in which each information CHARACTER or
BYTE is individually synchronized or FRAMED by the use of Start
and Stop Elements, also referred to as START BITS and STOP
BITS.
• Each character or byte is framed as a separate and independent
unit of DATA that may be transmitted and received at irregular and
independent time intervals.

21. Classification of Data Transmission

22. • Data flows in only one direction on the data
communication line (medium).
• Examples are radio and television broadcasts.

23. • Data flows in both directions but only one direction at a
time on the data communication line.
• For example, a conversation on walkie-talkies is a half-
duplex data flow.
• Each person takes turns talking. If both talk at once -
nothing occurs!

24. • Data flows in both directions simultaneously.
• Modems are configured to flow data in both directions.

25. Muliplexing
• Multiplexing is the techniques that allows the simultaneous
transmission of multiple signals across a single data link
• The figure below shows the basic format of a multiplexed system.
• The lines onthe left direct their transmission streams to a
multiplexer (MUX), which combines them into asingle stream
(many-to one).

26. • At the receiving end, that stream is fed into a de-multiplexer
(DEMUX), which separates the stream back into its
component transmissions (one-to-many) and directs them
to their corresponding lines.

27. Fequency Division Multiplexing (FDM)
• Frequency-division multiplexing (FDM) is an analog technique
that can be applied when the bandwidth of a link (in hertz) is
greater than the combined bandwidths of the signals to be
transmitted.
• In FDM, signals generated by each sending device modulate
different carrier frequencies. These modulated signals are then
combined into a single composite signal that can be transported
by the link.

28. • A very common application of FDM is AM and FM radio
broadcasting and television broadcasting, Telepone
Exchange.
• Radio uses the air as the transmission medium.

29. • This technique uses digital transmission
• Here we need not divide the frequency bandwidth of a
medium into number of channels instead we divide
transmission time into number of time slices.
• Therefore, each user is allocated with different time
interval known as a Time slot at which data is to be
transmitted by the sender.
• A user takes control of the channel for a fixed amount of
time

30. • There are two types
Synchronous TDM
Asynchronous TDM / Statistical TDM
Synchronous TDM
• Time slot is pre assigned to every device.
• Each device is given some time slot irrespective of the fact that
the device contains the data or not.
• If the device does not have any data, then the slot will remain
empty.
• Signals are sent in the form of frames

32. Asynchronous TDM
• An asynchronous TDM is also known as Statistical TDM.
• Time slots are not fixed.
• Time slots are allocated to only those devices which have
the data to send.
• An asynchronous TDM technique dynamically allocates
the time slots to the devices.

34. • Wavelength-division multiplexing (WDM) is designed to
use the high-data-rate capability of fiber-optic cable.
• Using a fiber-optic cable for one single line will waste the
available bandwidth. So, multiplexing is used which allows
to combine several lines into one.
• WDM is conceptually the same as FDM, except that the
multiplexing and de-multiplexing involve optical signals
transmitted through fiber-optic channel

35. • Multiple light sources are combined into one single light at
the multiplexer and do the reverse at the de-multiplexer.
• A multiplexer can be made to combine several input
beams of light, each containing a narrow band of
frequencies, into one output beam of a wider band of
frequencies.