1. OSI MODEL AND DATA LINK
LAYER
2ND UNIT
COMPUTER NETWORK AND
SECURITY

2. Chapter…….
• Network Models
• Error Detection and Correction
• Flow and Error Control

3. Network Models
• Protocol
Set of rules that enable two device to
connect and transmit data to one another.
• Service
Set of opration that layer provides to
the layer above or below it.

4. Key Elements of Protocol
• Syntax
Structure or format of the data.
• Semantics
meaning of each Section of bit.
• Timing
When data should be sent and how
fast they can be sent.

5. Standards
• De facto
De facto standards are often
established originally by manufacturers who
request to define the functionality of a new
product or technology.
• De jure
Thos standards that have legislated
by an officially recognized body are de jure
standards.

6. OSI Model
• Developed by the International standard
Organization(ISO).
• The model is called ISO OSI(Open System
Interconnection)
• The OSI model has Seven layers.
• H- Header Information(Source & Destination
address)
• T- Trailer Information(Error control)

7. OSI Layers
1.
2.
3.
4.
5.
6.
7.
Application layer
Presentation Layer
Session Layer
Transport layer
Network Layer
Data link layer
Physical layer

9. OSI Reference model
Application
Sending Device
Application
AH Application data
Presentation
SH
Transport
Physical
TH
DH
Application
Data Unit
Presentation
Data Unit
Session
Data Unit
PH
Session
Network
Receiving Device
Data
Transport
Data Unit
Data Unit(Bits)
DT
Network
Physical

10. Physical Layer
• The physical layer is responsible for transmitting
individual bits from one node to the next.

11. Data Link Layer
• The data link layer is responsible for
transmitting frames from one node to the
next.

12. Functions of Data link layer
• Framing
Divides the data in to smaller unit.
• Physical addressing
Hardware address is added in Header.
• Flow control
Control the flow of data between sender and
receiver
• Error control
generally error control mechanisms are added in
Data link Trailer(DT)

13. Network Layer
The network layer is responsible for the delivery
of packets from the original source to the final
destination.

14. Functions of Network Layer
• Logical addressing
The physical addressing
implemented by the data link layer
handles the addressing problem locally.
• Routing
Finding the shortest path
between source and destination.

15. Transport Layer
• The transport layer is responsible for delivery of a
message from one process to another.

16. Functions of Transport Layer
• Service point addressing
Used to identify the process
• Segmentation and reassembly
A message is divided into transmittable
segments, each having a sequence number. These
numbers enable the transport layer to reassemble
the message correctly upon arrival at the
destination.
• Flow control.
• Error control.

17. Session Layer
• Allow a user to log into a remote system or to
transfer a file between two machines.
Functions of Session Layer:
• Dialogue control
Control the conversation between two
system
• Synchronization
Coortinate the interaction among
communication system.

18. Presentation Layer
• It concerned with the syntax and semantics of the
information transmitted.
Functions of Session Layer:
• Translation
Own format information is exchange between two
different system
• Encryption
Original message into secret form
• Compression
Reduce the number of bit s contained in the
information

19. Application layer
• The application layer is responsible for providing
services to the user.

20. Functions of Application Layer
• Network Virtual Terminal
It allow a user to log on to the remote
computer.
• File Transfer, Access and Management
Allow user to access file from remote
computer.
• Mail Service
It provide the basic for E-Mail forwarding
and Storage.

21. Error detection and Correction
• Error:
unpredictable changes of bits from
1->0 or 0->1
• Types:
Single bit error
– Burst error(Multiple)
–

22. Single bit error
• A 0 is changed to 1 or a 1 is changed to 0
Received
Sent
10011100101
1 0 0 1 0 1 0 0 1 0 1
Error

23. Burst Error(Multiple)
• 2 or more bits in the data unit have changed from 1
to 0 or from 0 to 1
Length of burst error (8 bits)
0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Corrupted Bits
0 1 0 1 1 1 0 0 1 1 1 1 0 0 1 1

24. CRC
• Cyclic codes are special block codes with one extra
property.
• A codeword is cyclically shifted (rotated) the result is
another codeword.
MSB
1
0
LSB
1
1
0
0
0
MSB
0 1
1
LSB
0
0
0
1

25. CRC Encoder & Decoder
Data word - 1001(data to be sent)
Divisor - 1011(predetermined constant)
Code word- Data word + Remainder
1 0
0
1 0
0
1 0
1 0
1011
1
1
1
0
0
Quotient
0
0 0
0 0
1
Code word-100110
1
0
0 0 0
1
0 1
0
1
0 1 1 0
0 0 0 0
1 1 0
Remainder

26. CRC Encoder & Decoder
Data word - 1001(data to be sent)
Divisor - 1011(predetermined constant)
Code word- Data word + Remainder-1001110
1 0
0
1 0
0
1 1
1 0
1011
1
1
1
0
0
Quotient
1
0 0
0 0
1
Code word-100110
1
0
0 0 0
1
0 1
0
1
0 0 0 0
0 0 0 0
0 0 0
Remainder

27. Parity Check
• Parity is a system in which each transmitted
character contain one additional bit.
• Two system of parity are normally used:
odd parity
even parity
• Odd parity means the total number of binary 1’s is in
the character,including the parity bit is odd.
• Even parity means that the number of binary 1’s bit
in the character,including the parity bit is even.
• This technique only detects the single bit error.

28. Checksum
• Several protocols still use the checksum for error
detection.
• For example, if the set of number
is(7,10,3,11,5),we send(7,10,3,11,5,36),where 36
is the sum of (7+10+3+11+5) the original number.
• The receiver adds the five numbers and compares
the result with the sum.
• If the two are the same,the receiver assumes no
error,accepts the five numbers and omits the sum.

29. Check sum
• There is an error somewhere and the data are not
accepted
• The traditional checksum uses a small number of
bits(16) to detect error in a massage of any size.
• It is not strong as CRC in error checking capability.
• For example,If the value of one word is increamented
and the value of another word is decreamented by
the same amount,the two error cannot be detected
because the sum remains the same.

30. Flow control
• Flow control
The management of data flow
between computers or device or between
nodes in a network so that the data can be
handled at an efficient pace
• Types of flow control
Stop – and – wait flow control
Sliding window flow control

31. Stop and Wait Flow control
• Sender sends a frame and wait for acknowledgement
from the receiver.
• After receiving the acknowledgement from the
receiver,sender sends next frame.

32. Stop and wait flow control
• Advantage
Gurantee for the delivery of
every frame.Because sender sends the
next frame only after receiving the
acknowledgement of previous frame.
• Disadvantage
It is time consuming.

33. Sliding window flow control
• The flow control technique that allows multiple
frames to be in transit on the line at one time is
called sliding window .

34. Error control
• Error control is a method that can be
used to recover the corrupted data
whenever possible.
• Types of error control
backward error control
forward error control

35. Damaged frame
• A recognizable frame does arrive,but some of
the bit are in error.

36. Lost frame
• A frame fail to arrive at the other side.

37. Lost acknowledgement
• An acknowledgement fails at the source.the sender is
not aware that acknowledgement has been
transmitted from the receiver.

38. ARQ
• Full form --Automatic Repeat Request
• The purpose of ARQ is to change an
unreliable data link into a reliable one.
• Version of ARQ
Stop-and –wait ARQ
Go-Back-N ARQ
Selective-repeat ARQ

39. Stop-and-wait ARQ
• Sender transmits a single frame and then
twaits for acknowledgement(ACK).

40. Go back N ARQ
• A station may send a series of frames sequentially
numbered with some maximum value.
• It give a NACK from the receiver.

41. Selective Repeat ARQ