1. Data Communications & Networking
Lecture-10
Course Instructor : Sehrish Rafiq
Department Of Computer Science
University Of Peshawar

2. Lecture overview
Transmission mode
 Parallel and serial transmission
 Synchronous and asynchronous
transmission

3. Transmission mode
 A given transmission on a communications
channel between two machines can occur in
several different ways.
 The transmission is characterized by:
 the direction of the exchanges.
 the transmission mode: the number of bits sent
simultaneously.
 synchronization between the transmitter and receiver.

4. The direction of the exchanges
 There are 3 different transmission modes
characterized according to the direction of the
exchanges.
 Simplex
 Half Duplex
 Full Duplex

5. Simplex Mode
 In simplex mode the communication is uni-directional as on a one-way
street.
 Only one of the two stations on a link can transmit, the other can only
receive.
 Examples: Keyboard, Monitor, Mouse

6. Half Duplex
 In half-duplex mode, each station can both transmit and receive but not at
the same time.
 When one device is sending, the other can only receive.
 The half-duplex mode is like a one-lane road with two-directional traffic.
 The entire capacity of the communication channel is taken over by
whichever of the two devices is transmitting at the time.
 e.g. walkie-talkie

7. Full duplex
 In full-duplex mode both stations can transmit and receive
simultaneously.
 The full duplex mode is like a two-way street with traffic flowing in
both directions at the same time.
 In full duplex mode signals going in either direction share the capacity
of the link.
 This sharing can occur in two ways either the link must contain two
physically separate transmission paths, one for sending and the other
for receiving or the capacity of the channel is divided between signals
traveling in opposite directions.

8. Data Transmission
 The number of bits sent simultaneously
 When we talk about transmission of data from one
device to another the important issue is wiring.
 When we consider wiring the important thing is
how a data stream is sent.
 Parallel Transmission
 Serial Transmission

9. Parallel Transmission
 Binary data, consisting of 1’s and 0’s,may be
organized in to groups of n bits each.
 Computers produce and consume data in groups of
bits.
 By grouping we can send data n bits at one time.
 Each bit has its own wire.
 N bits of one group can be transmitted with each
clock tick from one device to another.

10. Parallel Transmission

11. Serial Transmission
 In serial transmission one bit follows another, so we
need only one communication channel instead of n to
send n bits.
 The advantage of serial over parallel transmission is
that with only one communication channel.
 Serial transmission reduces the cost of transmission
over parallel roughly by a factor of n.
 Communication within devices is parallel, conversion
devices are required at the interface between the
sender(parallel-to-serial) and receiver (serial-to-
parallel).

12. Serial Transmission

13. Serial Transmission
 Serial transmission can occur in one of two
ways.
 Synchronous
 Asynchronous

14. Asynchronous Transmission
 Asynchronous is so named because the timing of a
signal is unimportant.
 Information is received and translated by agreed
upon patterns.
 The receiving device can retrieve the information
without regard to the rhythm in which it is sent.
 Patterns:
 Patterns are based on grouping the bit stream in to bytes .
.

15. Asynchronous Transmission
continued…
 Each group usually 8 bits is sent along the link as
a unit.
 The sending system handles each group
independently, relaying it to the link whenever the
link is ready, without regarding to a timer.
 Without synchronization, the receiver cannot use
timing to predict when the next group will
arrive???
 Answer: Use Start & Stop bits

16. Start & Stop bits
 Start bit
 To alert the receiver to the arrival of a new group, therefore an
extra bit is added to the beginning of each byte.
 This bit is usually a 0,is called the start bit.
 Stop bit
 To let the receiver know that the byte is finished, one or more
additional bits are appended to the end of the byte.
 These bits usually 1’s are called stop bits.
 In addition, the transmission of each byte may be followed
by a gap of varying duration.
 This gap can be represented either by an idle channel or by
a stream of additional stop bits.

17. Asynchronous Transmission

18. Why called asynchronous???
 The synchronization is at the byte level.
 The sender and receiver do not have to be
synchronized all the time.
 When the receiver detects a start bit,it sets a timer and
 begins counting bits as they come in.
 After n bits,the receiver looks for a stop bit.
 As soon as it detects the stop bit,it waits until it detects
 the next start bit.

19. Synchronous Transmission
 In synchronous transmission, the bit stream is
combined in to longer frames which may contain
multiple bytes.
 Each byte is introduced on to the transmission link
without a gap between it and the next one.
 It is left to the receiver to separate the bit stream in
to bytes for decoding purposes.
 Timing is important in synchronous transmission

20. Synchronous Transmission

21. Advantages of synchronous transmission
The advantage of synchronous transmission
is speed.
Byte synchronization is accomplished in the
data link layer.

22. Wish You best of Luck
for
Your paper