1) Errors can occur during data transmission due to interference that changes signal shapes. There are two types of errors: single-bit errors affecting one bit, and burst errors affecting multiple consecutive bits. 2) Error detection allows a receiver to detect errors without correcting them, using redundancy by adding extra bits. Error correction detects and corrects errors using even more redundancy. 3) Flow control regulates the amount of data sent before waiting for acknowledgment. Error control detects and retransmits damaged frames using automatic repeat request. 4) Transmission media are either guided (wired) using twisted pair, coaxial cable and fiber optics, or unguided (wireless) using radio waves, microwaves and infrared.

1. ERROR DETECTION AND CORRECTION,
FLOW AND ERROR CONTROL AND
TRASMISSION MEDIA

2. WHAT IS ERROR?
Whenever bits flow from one point to another, they are subject to unpredictable changes
because of interference. This interference can change the shape of the signal.

3. There are two types of errors
 Single –bit error
 Burst error
TYPES OF ERROR

4.  Single-Bit Error:-The term single-bit error means that only 1 bit of a given data unit (such as a byte,
character, or packet) is changed from 1 to 0 or from 0 to 1.
In single bit error, only 1 bit the data unit has changed.

5.  Burst Error:-The term burst error means that 2 or more bits in the data unit have changed from 1to 0 or
from 0 to 1.
A burst error means that 2 or more bits in the data unit have changed.

6. ERROR DETECTION AND CORRECTION
Data can be corrupted during transmission. Some applications require that errors
be detected and corrected.

7. Redundancy
The central concept in detecting or correcting errors is redundancy. To be able to detect or correct errors, we need
to send some extra bits with our data. These redundant bits are added by the sender and removed by the receiver.
To detect or correct errors, we need to send extra (redundant) bits with data.

8. BLOCK CODING
In block coding, we divide our message into blocks, each of k bits, called datawords. We add r redundant bits
to each block to make the length n = k + r. The resulting n-bit blocks are called codewords.
For the moment, it is important to know that we have a set of datawords, each of size k, and a set of
codewords, each of size of n.
With k bits, we can create a combination of 2 𝑘
datawords; with n bits, we can create a combination of
2 𝑛codewords. The block coding process is one-to-one; the same dataword is always encoded as the same
codeword.

9. 1) Error detection means to decide whether
the received data is correct or not without
having a copy of the original message.
2) Error detection uses the concept of
redundancy, which means adding extra
bits for detecting errors at the destination.

10. Error Detection:-How can errors be detected by using block coding? If the following two conditions are met,
the receiver can detect a change in the original codeword.
1. The receiver has (or can find) a list of valid codewords.
2. The original codeword has changed to an invalid one.

11. It can be handled in two ways:
1) receiver can have the sender retransmit the entire data
unit.
2) The receiver can use an error-correcting code, which
automatically corrects certain errors.

12. Error correction:-Error correction is much more difficult than error detection. In error detection, the receiver
needs to know only that the received codeword is invalid; in error correction the receiver needs to find (or guess)
the original codeword sent. We can say that we need more redundant bits for error correction than for error
detection.

13. To correct an error, the receiver reverses the
value of the altered bit. To do so, it must know
which bit is in error.
Number of redundancy bits needed
 Let data bits = m
 Redundancy bits = r
Total message sent = m+r
The value of r must satisfy the following
relation:
2r ≥ m+r+1

14. Error Correction

15. 1. One of the most effective codes for error-recovery
2. Used in situations where random errors are likely to occur
3. Error detection and correction increases in proportion to the number of
parity
bits (error-checking bits) added to the end of the information bits
code word = information bits + parity bits
Hamming distance: the number of bit positions in which two code words
differ.
10001001
10110001
* * *
Minimum Hamming distance or D(min) : determines its error detecting
and
correcting capability.

16. Hamming Code

17. Example of Hamming Code

18. FLOW AND ERROR CONTROL
Data communication requires at least two devices working together, one to send and the other to receive. The
most important responsibilities of the data link layer are flow control and error control.
 Flow Control:-Flow control coordinates the amount of data that can be sent before receiving an
acknowledgment and is one of the most important duties of the data link layer. In most protocols,
flow control is a set of procedures that tells the sender how much data it can transmit before it must
wait for an acknowledgment from the receiver.
Flow control refers to a set of procedures used to restrict the amount of data that the sender can send
before waiting for acknowledgment.

19.  Error Control:-Error control is both error detection and error correction. It allows the receiver to inform the
sender of any frames lost or damaged in transmission and coordinates the retransmission of those frames by
the sender. In the data link layer, the term error control refers primarily to methods of error detection and
retransmission. Error control in the data link layer is often implemented simply: Any time an error is detected
in an exchange, specified frames are retransmitted. This process is called automatic repeat request (ARQ).
Error control in the data link layer is based on automatic repeat
request, which is the retransmission of data.

20. TRANSMISSION MEDIA
The transmission media that are used to convey information can be classified as guided or unguided. Guided media
provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable and optical
fiber. Unguided media employ an antenna for transmitting through air, vacuum, or water.
There are two types of transmission media:-
 Guided (wired)
 Unguided (wireless)

21. GUIDED MEDIA
Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable,
coaxial cable, and fiber - optic cable. A signal traveling along any of these media is directed and contained by the
physical limits of the medium.
Guided
Media
Twisted
Pair Cable
Coaxial
Cable
Fiber
Optics

22.  TWISTED-PAIR CABLE:- A twisted pair consists of two conductors (normally copper), each
with its own plastic insulation, twisted together.
Physical Description:- A twisted pair consists of two insulated copper wires arranged in a regular spiral
pattern. A wire pair acts as a single communication link. Over longer distances, cables may contain hundreds
of pairs. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable.
Neighbouring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk
interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have
thicknesses of from 0.4 to 0.9 mm.

23. UNSHIELDED AND SHIELDED TWISTED PAIR
Twisted pair comes in two varieties: unshielded and shielded. Unshielded twisted pair (UTP) is ordinary
telephone wire. This is the least expensive of all the transmission media commonly used for local area networks and
is easy to work with and easy to install.
Unshielded twisted pair is subject to external electromagnetic interference ,including interference from
nearby twisted pair and from noise generated in the environment. A way to improve the characteristics of this
medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference. This shielded
twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more
difficult to work with than unshielded twisted pair.

24.  COAXIAL CABLE:-Coaxial cable, like twisted pair, consists of two conductors, but is constructed
differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical
conductor that surrounds a single inner wire conductor. Coaxial cable can be used over longer distances and
support more stations on a shared line than twisted pair.

25. APPLICATIONS
Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use in a
wide variety of applications. The most important of these are:-
• Television distribution
• Long-distance telephone transmission

26.  OPTICAL FIBER:-An optical fiber is a thin (2 to 125 micro m), flexible medium capable of guiding an
optical ray. Various glasses and plastics can be used to make optical fibers. The lowest losses have been
obtained using fibers of ultrapure fused silica.

27. Five basic categories of application have become important for optical fiber:
• Long-haul trunks (average 1500 km in length)
• Metropolitan trunks(average length of 12 km)
• Rural exchange trunks (lengths ranging from 40 to 160 km)
• Local area networks

28. UNGUIDED OR WIRELESS TRANSMISSION
For unguided media, transmission and reception are achieved by means of an antenna.

29. Wireless
Transmission
Radio Wave Microwave Infrared

30.  RADIO TRANSMISSION:-Radio waves frequency are easy to generate, can travel long distances, and
can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio
waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter
and receiver do not have to be carefully aligned physically. It is used for multicast communication such as
radio and television.

31.  MICROWAVE TRANSMISSION:-Above 100 MHz, the waves travel in nearly straight lines and
can therefore be narrowly focused. Concentrating all the energy into a small beam by means of a
parabolic antenna (like the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the
transmitting and receiving antennas must be accurately aligned with each other. These are used for
unicast communication such as cellular telephones ,satellite networks and wireless LANs.

32.  INFRARED TRANSMISSION:-Infrared waves lies in between visible light spectrum and
microwaves. It has wavelength of 700 nm to 1 mm and frequency ranges from 300 GHz to 430 THz.
Infrared waves are used for very short range communication purposes such as television and it’s
remote. Infrared travels in a straight line so they are directional by nature. Because of high frequency
range, Infrared do not cross wall like obstacles.

33. Thank You
Regards :Pankaj Kohli