1. 2.1
Chapter 2
Network Models
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2. 2-1 LAYERED TASKS
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TTooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn::
Sender, Receiver, and Carrier
Hierarchy
2.2

3. Figure 2.1 Tasks involved in sending a letter
2.3

4. 2-2 THE OSI MODEL
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TTooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn::
Layered Architecture
Peer-to-Peer Processes
Encapsulation
2.4

5. 2.5
ISO is the organization.
OSI is the model.
Note

6. Figure 2.2 Seven layers of the OSI model
2.6

7. Main benefits of layered network
The process of breaking up the functions or tasks of
networking into layers reduces complexity.
Each layer provides a service to the layer below it in the
protocol specification.
Each layer communicates with the same layer’s software
or hardware on other computers.
The lower 4 layers (transport, network, data link and
physical —Layers 4, 3, 2, and 1) are concerned with the
flow of data from end to end through the network.
The upper four layers of the OSI model (application,
presentation and session—Layers 7, 6 and 5) are
orientated more toward services to the applications.
Data is Encapsulated with the necessary protocol
information as it moves down the layers before network
transit.
2.7

8. Figure 2.3 The interaction between layers in the OSI model
2.8

9. This interface defines
what information and
services a layer must
provide for the layer
above it.
 Between machines, layer
x on one machine
communicates with layer
x on another machine, by
using a protocol (this is
Peer-to-Peer Process).
2.9

10. Figure 2.4 An exchange using the OSI model
2.10

11. 2-3 LAYERS IN THE OSI MODEL
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TTooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn::
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
2.11

12. Figure 2.5 Physical layer
2.12

13. 2.13
Note
The physical layer is responsible for movements of
individual bits from one hop (node) to the next.

14. Physical Layer
 The physical layer coordinates the functions required to transmit a
bit stream over a physical medium.
 Physical characteristics of interfaces and media: The physical layer
defines the characteristics of the interface between devices and the
transmission media, including its type.
 Representation of the bits: Transmitted, bits must be encoded into
signals –electrical or optical-. The physical layer defines the type of
encoding.
 Data rate: The physical layer defines the transmission rate, the
number of bits sent each second.
 Line configuration: the physical layer is concerned with the
connection of devices to the medium.
 Synchronization of bits
 Physical topology
2.14 Transmission Mode

15. Figure 2.6 Data link layer
2.15

16. 2.16
The data link layer is responsible for moving
frames from one hop (node) to the next.
Note

17. Figure 2.7 Hop-to-hop delivery
2.17

18. Datalink Layer Fuctions
 Framing. The data link layer divides the stream of bits received
from the network layer into data units called frames.
 Physical addressing. The data link layer adds a header to the
frame to define the physical address of the sender (source
address) and/or receiver (destination address) of the frame.
 Flow Control
 Error Control :- Retransmit damaged or lost frame, recognize
duplicate frmae
 Access control :- In multi-point connection, which device has
control over medium
 If the frame is intended for a system outside the sender’s network,
the receiver address is the address of the device that connects
one network to the next.
2.18

19. Figure 2.8 Network layer
2.19

20. 2.20
The network layer is responsible for the
delivery of individual packets from
the source host to the destination host.
Note

21. Figure 2.9 Source-to-destination delivery
2.21

22. Network Layer Functions
Routing:- Defines the most optimum path the
packet should take from the source to the
destination
Logical addressing so that any endpoint can be
identified.
Facilitates interconnection between
heterogeneous networks (Internetworking).
2.22

23. Figure 2.10 Transport layer
2.23

24. 2.24
The transport layer is responsible for the delivery
of a message from one process to another.
Note

25.  Segmentation and reassembly
 Connection control
 Flow control
 Error Control (process to process rather
than across a single link)
 Service point addressing(port addressing)
2.25

26. Figure 2.11 Reliable process-to-process delivery of a message
2.26

27. Figure 2.12 Session layer
2.27

28. 2.28
The session layer is responsible for dialog
control and synchronization.
Note

29. Session layer provides mechanism for controlling the
dialogue between the two end systems. It defines how to
start, control and end conversations (called sessions)
between applications.
This layer requests for a logical connection to be
established on an end-user’s request.
Any necessary log-on or password validation is also
handled by this layer.
Session layer is also responsible for terminating the
connection.
Session layer can also provide check-pointing
mechanism such that if a failure of some sort occurs
between checkpoints, all data can be retransmitted from
the last checkpoint.
2.29

30. Figure 2.13 Presentation layer
2.30

31. 2.31
The presentation layer is responsible for translation,
compression, and encryption.
Note

32. Presentation layer defines the
format in which the data is to be
exchanged between the two
communicating entities.
Also handles data compression and
data encryption (cryptography).
2.32

33. Figure 2.14 Application layer
2.33

34. 2.34
The application layer is responsible for
providing services to the user.
Note

35. Application layer interacts with
application programs and is the
highest level of OSI model.
Application layer contains
management functions to support
distributed applications.
Examples of application layer are
applications such as file transfer,
electronic mail, remote login etc.
2.35

36. Figure 2.15 Summary of layers
2.36

37. 2-4 TCP/IP PROTOCOL SUITE
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TTooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn::
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
2.37

38. Figure 2.16 TCP/IP and OSI model
2.38

39. Advantages/Disadvantages of TCP/IP
 A standard, routable enterprise networking protocol that is the most
complete and accepted protocol available. All modern operating
systems support TCP/IP, and most large private networks rely on
TCP/IP for much of their traffic.
 A technology for connecting dissimilar systems. Many TCP/IP
application protocols were designed to access and transfer data
between dissimilar systems. These protocols include HTTP, FTP,
and Telnet.
 A robust, scaleable, cross-platform client/server framework.
 A method of gaining access to the Internet.
 Disadvantage :- Overhead is higher, so not suitable for video
transmission
2.39

40. 2-5 ADDRESSING
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TTooppiiccss ddiissccuusssseedd iinn tthhiiss sseeccttiioonn::
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
2.40

41. Figure 2.17 Addresses in TCP/IP
2.41

42. Figure 2.18 Relationship of layers and addresses in TCP/IP
2.42

43. In Figure 2.19 a node with physical address 10 sends a
frame to a node with physical address 87. The two nodes
are connected by a link (bus topology LAN). As the
figure shows, the computer with physical address 10 is
the sender, and the computer with physical address 87 is
the receiver.
2.43
Example 2.1

44. Figure 2.19 Physical addresses
2.44

45. As we will see in Chapter 13, most local-area networks
use a 48-bit (6-byte) physical address written as 12
hexadecimal digits; every byte (2 hexadecimal digits) is
separated by a colon, as shown below:
2.45
Example 2.2
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.

46. Figure 2.20 shows a part of an internet with two routers
connecting three LANs. Each device (computer or
router) has a pair of addresses (logical and physical) for
each connection. In this case, each computer is
connected to only one link and therefore has only one
pair of addresses. Each router, however, is connected to
three networks (only two are shown in the figure). So
each router has three pairs of addresses, one for each
connection.
2.46
Example 2.3

47. Figure 2.20 IP addresses
2.47

48. Figure 2.21 shows two computers communicating via
the Internet. The sending computer is running three
processes at this time with port addresses a, b, and c. The
receiving computer is running two processes at this time
with port addresses j and k. Process a in the sending
computer needs to communicate with process j in the
receiving computer. Note that although physical
addresses change from hop to hop, logical and port
addresses remain the same from the source to
destination.
2.48
Example 2.4

49. Figure 2.21 Port addresses
2.49

50. 2.50
Note
The physical addresses will change from hop to hop,
but the logical addresses usually remain the same.

51. 2.51
Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
address represented by one decimal number as shown.
753
A 16-bit port address represented
as one single number.

52. 2.52
Note
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.

53. Summary
1.The OSI model originally distinguishes between service,interface and
protocols.
1.The TCP/IP model doesnt clearly distinguish between service,interface
and protocol.
2.The OSI model is a reference model.
2.The TCP/IP model is an implementation of the OSI model.
3.In OSI model,the protocols came after the model was described.
3.In TCP/TP model, the protocols came first and the model was really just a
description of the
existing protocols.
4.In OSI model,the protocols are better hidden.
4.In TCP/IP model ,the protocols are not hidden.
5.The OSI model has 7 layers.
5.The TCP/IP model has only 4 layers.
6.The OSI model supports both connectionless and connection-oriented
communication in the
network layer,but only connection -oriented communication in transport
layer.
6.The TCP/IP model supports both connectionless and connection-oriented
communication in the transport layer,giving users the choice.
2.53