EDT 4608 - PART 2 - DATA COMMUNICATION AND COMPUTER NETWORKING

WEEK 3- 1-2/14 1
EDT 4608
DATA COMMUNICATION & COMPUTER
NETWORKING
Introduction to Data
Communication

WEEK 1- 2/14 2
DATA COMPRESSION
 Storing data in a format that requires less space than usual.
 Disadvantages of Compression
 Loss of image quality
 Possible file incompatibilty with other software
 Time it takes to compress the image takes longer than an uncompressed
image
 Advantages of Compression
 Greatly reduces file size so can be easily transmitted over the web (as an
email attachmemt)
 Many graphic compression types such as GIF, JPEG and PNG are standard
file formats so can be used by many software applications

WEEK 1- 2/14 3
DATA COMPRESSION STANDARDS
There are a variety of data compression techniques, but only a few
have been standardized.
The CCITT has defined a standard data compression technique for
transmitting faxes (Group 3 standard) and a compression standard
for data communications through modems (CCITT V.42bis).
In addition, there are file compression formats, such as ARC and ZIP.
Data compression is also widely used in backup utilities,
spreadsheet applications, and database management systems.
Certain types of data, such as bit-mapped graphics, can be
compressed to a small fraction of their normal size

WEEK 1- 2/14 4
DATA COMPRESSION TECHNIQUE

WEEK 1- 2/14 5
DATA COMPRESSION TECHNIQUE
Lossless Compression:
Lossless Compression packs the data in
such a way that if you were to
decompress you would get the original
file back. So reduction in file siz but no
real reduction in the quality of the file.
Several file formats are associated
with Lossless Compression including GIF,
PNG and Zip.

WEEK 1- 2/14 6
DATA COMPRESSION
TECHNIQUE
Lossy Compression
Lossy Compression is the other form of
data compression. Lossy Compression
reduces the file size but there is usually a
loss in quality that can be noticed. Our
example is a Jpeg image which begins to
gradually show a visible lack of quality
as it is compressed more and more. In
audio files, Lossy compression usually tries
to reduce the file size by removing sounds
that normally are inaudible to the human
ear.

WEEK 1- 2/14 7
DATA ENCRYPTION
encryption is the process of encoding messages (or information) in such
a way that only authorized parties can read it.
Encryption doesn't prevent hacking but it reduces the likelihood that
the hacker will be able to read the data that is encrypted

WEEK 1- 2/14 8
TYPES OF ENCRYPTION
Symmetric-key
 the encryption and decryption keys are the same. Thus communicating parties must
agree on a secret key before they wish to communicate.
Public key encryption (asymmetric encryption)
 In public-key encryption schemes, the encryption key is published for anyone to use
and encrypt messages. However, only the receiving party has access to the decryption
key and is capable of reading the encrypted messages.[4] Public-key encryption is a
relatively recent invention: historically, all encryption schemes have been symmetric-
key (also called private-key) schemes

WEEK 1- 2/14 9
Think of it like this: You create a coded message to send to a friend in which each letter is
substituted with the letter that is two down from it in the alphabet. So "A" becomes "C," and "B"
becomes "D". You have already told a trusted friend that the code is "Shift by 2". Your friend
gets the message and decodes it. Anyone else who sees the message will see only nonsense.
The same goes for computers, but, of course, the keys are usually much longer. The first major
symmetric algorithm developed for computers in the United States was the Data Encryption
Standard (DES), approved for use in the 1970s. The DES uses a 56-bit key.
Because computers have become increasingly faster since the '70s, security experts no longer
consider DES secure -- although a 56-bit key offers more than 70 quadrillion possible
combinations (70,000,000,000,000,000), an attack of brute force (simply trying every possible
combination in order to find the right key) could easily decipher encrypted data in a short
while. DES has since been replaced by the Advanced Encryption Standard (AES), which uses
128-, 192- or 256-bit keys. Most people believe that AES will be a sufficient encryption
standard for a long time coming: A 128-bit key, for instance, can have more than
300,000,000,000,000,000,000,000,000,000,000,000 key combinations

WEEK 1- 2/14 10
PUBLIC KEY AND PRIVATE KEYS
The Public and Private key pair comprise of two uniquely related
cryptographic keys (basically long random numbers). Below is an example
of a Public Key:
3048 0241 00C9 18FA CF8D EB2D EFD5 FD37 89B9 E069 EA97 FC20
5E35 F577 EE31 C4FB C6E4 4811 7D86 BC8F BAFA 362F 922B F01B
2F40 C744 2654 C0DD 2881 D673 CA2B 4003 C266 E2CD CB02
0301 0001
The Public Key is what its name suggests - Public. It is made available to
everyone via a publicly accessible repository or directory. On the other
hand, the Private Key must remain confidential to its respective owner.
Because the key pair is mathematically related, whatever is encrypted
with a Public Key may only be decrypted by its corresponding Private Key
and vice versa

WEEK 1- 2/14 11
DIGITAL SIGNATURE

WEEK 1- 2/14 12
DATA ENCRYPTION STANDARDS (DES)
symmetric-key algorithm
Developed in the early 1970s at IBM
72 quadrillion possible keys of a 56-bit DES
1997 - The DESCHALL Project breaks a message encrypted with DES for the
first time in public.
 With the software that was used, a single 200 MHz Pentium system was able to test
approximately 1 million keys/second if it was doing nothing else. At this rate it would take
around 2,285 years to search the entire key-space. The number of computers being used rose
rapidly and in the end, a total of 78,000 different IP addresses had been recorded, with a
maximum of 14,000 unique hosts in a 24 hour period. By the time the key was found, they had
searched about a quarter of the key-space and were searching about 7 billion keys per second,
but the number of participants was still increasing rapidly.
1999 – Triple DES
2002 – Advanced Encryption Standards (AES)

WEEK 1- 2/14 13
HACKING ENCRYPTED DATA
Brute Force Attack
 trying every possible key in turn. The length of the key determines the number of
possible keys, and hence the feasibility of this approach
Backdoor
 is a method of bypassing normal authentication

WEEK 1- 2/14 14
ERROR DETECTION AND
CORRECTION
 techniques that enable reliable delivery of digital data over
unreliable communication channels.
 Many communication channels are subject to channel noise, and
thus errors may be introduced during transmission from the source
to a receiver.
 Error detection techniques allow detecting such errors, while error
correction enables reconstruction of the original data.

WEEK 1- 2/14 15
THREE TYPES OF ERRORS:
 Single Bit Error
 Multiple bits error
 Bursts Error

WEEK 1- 2/14 16
ERROR DETECTION
Errors in the received frames are detected by means of
 Parity Check and
 CRC (Cyclic Redundancy Check).
In both scenario, few extra bits are sent along with actual data to
confirm that bits received at other end are same as they were sent. If
the checks at receiver’s end fails, the bits are corrupted.

WEEK 1- 2/14 17
PARITY CHECK
One extra bit is sent along with the original bits to make number of 1s
either even, in case of even parity or odd, in case of odd parity.
The sender while creating a frame counts the number of 1s in it, for
example, if even parity is used and number of 1s is even then one bit
with value 0 is added. This way number of 1s remain even. Or if the
number of 1s is odd, to make it even a bit with value 1 is added.

WEEK 1- 2/14 18
CYCLIC REDUNDANCY CHECK
CRC is a different approach to
detect if the frame received
contains valid data. This
technique involves binary division
of the data bits being sent. The
divisor is generated using
polynomials. The sender performs
a division operation on the bits
being sent and calculates the
remainder. Before sending the
actual bits, the sender adds the
remainder at the end of the
actual bits. Actual data bits plus
the remainder is called a
codeword. The sender transmits
data bits as codewords.

WEEK 1- 2/14 19
ERROR CORRECTION
In Digital world, error correction can be done in two ways:
 Backward Error Correction: When the receiver detects an error in the data received,
it requests back the sender to retransmit the data unit.
 Forward Error Correction: When the receiver detects some error in the data
received, it uses an error-correcting code, which helps it to auto-recover and correct
some kinds of errors.

WEEK 1- 2/14 20
CONNECTOR INTERFACES
Gender – Male / Female
RJ45 – LAN Connector
RJ11 – Modem Connector

WEEK 1- 2/14 21
CONNECTOR INTERFACES

EDT 4608 - PART 2 - DATA COMMUNICATION AND COMPUTER NETWORKING

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